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  1. #1
    FEP User Fast_4_Eye's Avatar
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    Default High RPM stutters and bucking - 1986 GT

    Starting roughly just before I garaged the car last year, I was getting a really aggressive sputter and this bucking in the car at about 4,500rpm. It was just before the car was stored for the winter so I couldn't really do much about it. Anyways, I now have the car out since the weather has been nice and for the past month I've been trying to combat this issue.

    I don't really know whether I should call it a misfire or not, because I don't want to mislead anyone on what my issue is. I can rev the engine all the way to 6,000rpm no problem in neutral. When I'm in first, the stuttering occasionally happens at about 5,500rpm. The real bad bucking starts at the top of 2nd which is about 4,500-5,000rpm and in 3rd starts as soon as 4,000rpm. I should add this issue only persists when the engine is at operating temp/just above operating temp. If the engine is underneath that center line to any degree, the car doesn't seem to buck and sputter. As much as I hated doing it, I was able to rev the engine right out to 5,800rpm when it was "cold" and it pulled straight through with zero hesitation.

    I have replaced the cap, rotor, coil, plugs and wires. I gapped my plugs at .052" and that seemed to be terrible for blow out so I gapped down to .035" and that seemed to make it a little better. I am also running 14 degrees base timing but the problem has been the same whether I was at 10, 12 or 14. I also have a new Motorcraft TFI module I purchased from my local Ford dealership, the 5.0 manual transmission one.

    My car is an '86 but previous owner did convert to MAF so I put a BBK fender well CAI which I felt was better than the previous "hot air intake". I also know the car has an upgraded fuel pump that previous owner purchased from the Ford part catalog decades ago so I don't even know the flow rate of it, I can assume more than stock.

    I hope someone will be able to help me out with this as its driving me nuts and I want to be able to drive my car how I want without fear of blowing it up LOL. Let me know if you guys might need any more information. Thanks in advance

    EDIT: Previously mentioned new TFI module is not installed on the car, just meant to say I have it on hand if install would be a good idea.
    Last edited by Fast_4_Eye; 04-20-2022 at 12:00 AM.

  2. #2

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    I would try the new tfi, just to be certain (and it's easy). The closed loop thing is interesting. Idle and everything is fine? Cruising at highway speeds there is no problem? High rpm based stutter I would be inclined to think is either a lean miss or your dizzy is dying but if it revs fine cold I can't imagine it's the dizzy. Is it the original distributor?

    You could always try some injector cleaner. It could be a temp sensor or O2 sensor, something that kicks in on closed loop.
    1984.5 G.T.350 5.0 CFI AOD Convertible (TRX package, loaded)
    K&N filter in a stock dual snorkel, GT40 heads, Edelbrock 3721 intake, MSD 8456 Dist., MSD 8227 coil
    Comp cams XE254H, hypereutectic pistons
    Hooker Super Comp Shorty Equal Length Headers, catted BBK H-pipe, full custom duals
    Maximum Motorsports caster/camber plates and strut tower brace, 3.73 rear.
    Ford Performance Springs, Firehawk A/S 225/55r16 on LMR TRX r390 wheels (street)
    Federal 595 rs-rr 245/40r17 and 255/40r17 on OE cobra r wheels (race)
    AOD rebuilt with a 6 clutch direct drum, Koline steels stacked with 8 clutches, Kevlar band, superior shift kit, new torque converter. --Everything else stock and fully functional.

  3. #3

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    You should run the self test and see what codes are stored in memory. Also I didnt see where you have changed the fuel filter. they can cause some unusual problems.
    1978 Mercury Zephyr boxtop 5.0 EFI T5Z 8.8
    1999 Ford Contour 2.5 V6 5 speed
    2016 Ford Focus ST 2.0 Ecoboost 6 speed

  4. #4
    FEP Power Member richpet's Avatar
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    I have had a fuel filter with a lot of debris in it. Would rev with no load and cruise fine. But as soon as there was REAL demand on it THEN the debris caused an issue.
    When I changed that filter it looked like someone put carrots in there.
    Don't laugh. I didn't get it either. That's just what it looked like.

    Sent from my Pixel 6 Pro using Tapatalk
    83 5.0 GT. Quicker than it looks! 10:1 (or just over) 306, Motorsport a332 cam, 140A alt, t5 conv, 8.8 w/ 3.27's, Edel rpm, alum rad, very worked e7's, Holley SA carb, etc... SOLD IT!!!!

    Now an 1981 Granada! 9.5:1 .040 over 302, Edel E-street heads, 268 cam, T5, 8.8 with 3.55, plus all the stiffening goodies, all control arms, lowered, alum shaft, x-pipe with Outlaws, FiTech FI system...

    "Never underestimate the power of stupid people in large groups"

  5. #5
    FEP Senior Member Tigger's Avatar
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    Default

    When my 86 was bucking real bad it was the tps. I would check it and see if there is a dead spot in it.
    67 Mustang Coupe
    96 Tangerine GT
    86 Saleen #179

  6. #6
    FEP User Fast_4_Eye's Avatar
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    Quote Originally Posted by Tigger View Post
    When my 86 was bucking real bad it was the tps. I would check it and see if there is a dead spot in it.
    What was your bucking like? Mine only happens at the top end when hot and it literally feels like there is 0 power when it does it. Feels like you're hitting off a sort of rev limiter...

  7. #7
    FEP User Fast_4_Eye's Avatar
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    Quote Originally Posted by richpet View Post
    I have had a fuel filter with a lot of debris in it. Would rev with no load and cruise fine. But as soon as there was REAL demand on it THEN the debris caused an issue.
    When I changed that filter it looked like someone put carrots in there.
    Don't laugh. I didn't get it either. That's just what it looked like.

    Sent from my Pixel 6 Pro using Tapatalk
    I was going to look at getting a new filter for the $15 and maybe a pump? I'm at school and haven't dumped codes yet but could this be an O2 sensor issue? I'll have to check once I'm home and then let you guys know!

  8. #8
    FEP User Fast_4_Eye's Avatar
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    Quote Originally Posted by emerygt350 View Post
    I would try the new tfi, just to be certain (and it's easy). The closed loop thing is interesting. Idle and everything is fine? Cruising at highway speeds there is no problem? High rpm based stutter I would be inclined to think is either a lean miss or your dizzy is dying but if it revs fine cold I can't imagine it's the dizzy. Is it the original distributor?

    You could always try some injector cleaner. It could be a temp sensor or O2 sensor, something that kicks in on closed loop.
    Yeah I do have the original dizzy, MSD cap because I first speculated that the spark wasn't getting through so I bought a screamin demon and an MSD cap. I also just go through a tank of gas that I added a can of sea foam to and oddly enough, it seems to have made the sputter a bit better but it could just be a plateau in my head to make me feel better about dumping $13 into an already $75 tank of gas LOL

  9. #9

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    That would make me think more along that plugged filter, however that doesn't answer the cold vs hot issue. I have heard stories of crap draining back when off then slowly plugging the filter again, but I don't think that is it here. It could be but I suspect something closed loop related if it only happens when warm. Particularly since fuel demands will be higher when cold.
    1984.5 G.T.350 5.0 CFI AOD Convertible (TRX package, loaded)
    K&N filter in a stock dual snorkel, GT40 heads, Edelbrock 3721 intake, MSD 8456 Dist., MSD 8227 coil
    Comp cams XE254H, hypereutectic pistons
    Hooker Super Comp Shorty Equal Length Headers, catted BBK H-pipe, full custom duals
    Maximum Motorsports caster/camber plates and strut tower brace, 3.73 rear.
    Ford Performance Springs, Firehawk A/S 225/55r16 on LMR TRX r390 wheels (street)
    Federal 595 rs-rr 245/40r17 and 255/40r17 on OE cobra r wheels (race)
    AOD rebuilt with a 6 clutch direct drum, Koline steels stacked with 8 clutches, Kevlar band, superior shift kit, new torque converter. --Everything else stock and fully functional.

  10. #10
    FEP Senior Member Tigger's Avatar
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    Quote Originally Posted by Fast_4_Eye View Post
    What was your bucking like? Mine only happens at the top end when hot and it literally feels like there is 0 power when it does it. Feels like you're hitting off a sort of rev limiter...
    It has been a few years now but I remember that it ran ok when cold but started acting up when the car got warm. I want to say my dead spot was low to mid throttle.
    67 Mustang Coupe
    96 Tangerine GT
    86 Saleen #179

  11. #11
    FEP User Fast_4_Eye's Avatar
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    Quote Originally Posted by Tigger View Post
    It has been a few years now but I remember that it ran ok when cold but started acting up when the car got warm. I want to say my dead spot was low to mid throttle.
    Little update: After I got through my tank of sea foam, the stutter has easily been halved in intensity. I'm starting to think it is a fuel issue and a lot of the crap has been cleaned out of the lines, filter, injectors. I'm still going to buy a new filter and just do the pump while I'm at it. \

    About an hour ago I was merging onto the highway and WOT pulled through 1st, 2nd and 3rd and had 2 pops at the top of 3rd. By the top of 4th I was going too fast to continue lol

    I'm hoping I can narrow this issue down so that I can drive the car without any concern. also just for piece of mind that the car won't blow haha

  12. #12

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    I hope it cleans up with seafoam. That would be a nice break.
    1984.5 G.T.350 5.0 CFI AOD Convertible (TRX package, loaded)
    K&N filter in a stock dual snorkel, GT40 heads, Edelbrock 3721 intake, MSD 8456 Dist., MSD 8227 coil
    Comp cams XE254H, hypereutectic pistons
    Hooker Super Comp Shorty Equal Length Headers, catted BBK H-pipe, full custom duals
    Maximum Motorsports caster/camber plates and strut tower brace, 3.73 rear.
    Ford Performance Springs, Firehawk A/S 225/55r16 on LMR TRX r390 wheels (street)
    Federal 595 rs-rr 245/40r17 and 255/40r17 on OE cobra r wheels (race)
    AOD rebuilt with a 6 clutch direct drum, Koline steels stacked with 8 clutches, Kevlar band, superior shift kit, new torque converter. --Everything else stock and fully functional.

  13. #13
    FEP User Fast_4_Eye's Avatar
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    So I installed the new TFI module from the Ford dealership. Absolute junk. I had read that the new TFI's were pretty bad for out of box reliability but I didn't think it would be THIS bad. I started the car fine, drove it about 30 meters and then it completely died. Had to push it all the way back to my house while it seemed every single neighbour was out on their porch. TFI doesn't seem to be my issue anyways so I'll be leaving the stock one.

  14. #14
    FEP Super Member erratic50's Avatar
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    14 degrees is too much for an engine with high swirl heads. 13.5 is the absolute max I’ve gotten away with on premium fuel and usually without octane boost my car hates me for it

    Based upon symptoms I believe you have a failing fuel pump. I would make sure the lines are good, replace the sock, and replace the fuel filter.

    I’d look at my fuel pressure at the rail while driving to verify. Zip tie a gauge to your wiper

    The other thing that strikes me is the coil wire and the rotor as those have given me fits before

    Also do yourself a favor and look for oil in the bottom of your distributor. That means a failed seal which messes with the pip signal as it gets progressively wet

  15. #15
    FEP Super Member erratic50's Avatar
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    You may want to inspect your capacitors on your ECU

    Your gap is too tight which suggests a weak spark. Check voltage at the coil. Maybe try another coil

    I would also clean and inspect all of the ECU grounds

    As far as TFI modules go, I would ohm it out vs spec and also make a trip to the local u pull it place to grab a couple more distributors. Ohm those out. Use thermal paste to install. Make sure your distributor is clocked so it points mostly towards the drivers fender when it is in time. It needs the air flow to run properly long term

    Also look for vacuum leaks while you are at it

    these things don’t like much past 5300 but will do it when provoked

    Good luck

  16. #16
    FEP User Fast_4_Eye's Avatar
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    Quote Originally Posted by erratic50 View Post
    14 degrees is too much for an engine with high swirl heads. 13.5 is the absolute max Iíve gotten away with on premium fuel and usually without octane boost my car hates me for it

    Based upon symptoms I believe you have a failing fuel pump. I would make sure the lines are good, replace the sock, and replace the fuel filter.

    Iíd look at my fuel pressure at the rail while driving to verify. Zip tie a gauge to your wiper

    The other thing that strikes me is the coil wire and the rotor as those have given me fits before

    Also do yourself a favor and look for oil in the bottom of your distributor. That means a failed seal which messes with the pip signal as it gets progressively wet
    I just timed the engine down to 12 degrees about 30 minutes ago before I read this post. I'm also thinking of ordering a Walboro 255lph and a new fuel filter from LMR (K&N or Stock?)

    The only reason I don't believe this is a spark related issue is because out of all the caps, rotors, coils, plugs and wires I've gone through, the problem has stayed pretty well the same. I definitely need to gap my plugs back out to around .050" which I probably won't get to tonight but I will most likely have done over the weekend. And I did run out to the garage quick and check the distributor, there is no oil anywhere around or inside it.

    Quote Originally Posted by erratic50 View Post
    You may want to inspect your capacitors on your ECU


    Your gap is too tight which suggests a weak spark. Check voltage at the coil. Maybe try another coil


    I would also clean and inspect all of the ECU grounds


    As far as TFI modules go, I would ohm it out vs spec and also make a trip to the local u pull it place to grab a couple more distributors. Ohm those out. Use thermal paste to install. Make sure your distributor is clocked so it points mostly towards the drivers fender when it is in time. It needs the air flow to run properly long term


    Also look for vacuum leaks while you are at it


    these things donít like much past 5300 but will do it when provoked


    Good luck
    How and where should I check the capacitors on my ECU, would I need to clean anything which a special liquid or compound?

    I was also planning on going to a pick n' pull with one of my buddies in the weeks to come and was going to just grab a TFI if I could find one, would it be worth getting an entirely new distributor? And how would I clock it so that it faces drivers fender in time, right now it goes just over the thermostat housing which I know does not help the heat situation.

    Thank you so much for the insight and I will get on everything when I get a spare minute away from school and work. At least I can drive the car around fine under 5,000rpm which is all I really need to do.

    Final question, would the plug gap affect anything such as timing and gas mileage? I only seem to get 200kms (124 miles) to a tank which seems scarily low and after I had gapped my plugs down it changed my timing somehow, unless the computer was compensating for the new gap? It just seems so odd that I'm having to fill up twice a week unless that's just a usual thing for these cars being daily driven.

  17. #17

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    Don't go overboard yet. The car apparently runs fine open loop. Put it all to stock (gap, timing, etc) and start there. Try a cool run to rpm again. If there is no miss your fuel supply is fine unless your pump starts failing after warm up, which they don't really do. They just die. Tfi modules are hit or miss. I have an msd one now (works great) but I still have the original 84 and it is rock solid (and probably going back on).

    Verify the motor is in fact free revving cold. That will clear some of the noise and focus the troubleshooting.

    And mine rarely hits 200 miles on a tank of gas. You would have to be dead driving one of these and get better than 12 mpg. Yeah, I can get 21 mpg highway but town kills me.
    Last edited by emerygt350; 04-21-2022 at 07:42 PM.
    1984.5 G.T.350 5.0 CFI AOD Convertible (TRX package, loaded)
    K&N filter in a stock dual snorkel, GT40 heads, Edelbrock 3721 intake, MSD 8456 Dist., MSD 8227 coil
    Comp cams XE254H, hypereutectic pistons
    Hooker Super Comp Shorty Equal Length Headers, catted BBK H-pipe, full custom duals
    Maximum Motorsports caster/camber plates and strut tower brace, 3.73 rear.
    Ford Performance Springs, Firehawk A/S 225/55r16 on LMR TRX r390 wheels (street)
    Federal 595 rs-rr 245/40r17 and 255/40r17 on OE cobra r wheels (race)
    AOD rebuilt with a 6 clutch direct drum, Koline steels stacked with 8 clutches, Kevlar band, superior shift kit, new torque converter. --Everything else stock and fully functional.

  18. #18
    FEP User Fast_4_Eye's Avatar
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    Quote Originally Posted by emerygt350 View Post
    Don't go overboard yet. The car apparently runs fine open loop. Put it all to stock (gap, timing, etc) and start there. Try a cool run to rpm again. If there is no miss your fuel supply is fine unless your pump starts failing after warm up, which they don't really do. They just die. Tfi modules are hit or miss. I have an msd one now (works great) but I still have the original 84 and it is rock solid (and probably going back on).

    Verify the motor is in fact free revving cold. That will clear some of the noise and focus the troubleshooting.

    And mine rarely hits 200 miles on a tank of gas. You would have to be dead driving one of these and get better than 12 mpg. Yeah, I can get 21 mpg highway but town kills me.
    I just have to ask since I can't find anything on google, what do you mean exactly when you refer to open loop vs closed loop. I'm sure its not important just curious.

    I wish I could narrow this issue down to just one system (fuel, spark or air). I also wish I actually had the money to be buying a bunch of parts to "just check" but the worst is when they don't even work like the TFI I got. I'm seriously thinking that my car had sat for about 17 years before I bought it, there has to be some sediment in the gas tank that was getting into the pump/filter and fuel system. I can honestly say that I remember this problem starting maybe a month after I had the car and it also seems that the use of Seafoam in my tank has been the only thing to make a noticeable difference in the sputtering. I was so happy last night that I could rev 3 gears out without the stupid pops and stuttering.

    And about the mileage, I normally get ~250kms to a tank but I did do a highway trip yesterday that had me revving at about 2600rpm for a majority of, and I also have been ringing each gear out more than normally trying to diagnose the bucking issue so hopefully after everything is resolved my mileage will go back up a bit, even though I bought this car knowing it wouldn't be efficient.

    Thank you everyone for the help so far as I honestly don't have much of an idea as to what I'm doing so the guidance from all you experienced guys is greatly appreciated.

    EDIT: should I buy another can of Seafoam and try doing a second run and see if that makes it any better? I need to fill up anyways I don't know if that could be worth it.

  19. #19
    FEP Power Member richpet's Avatar
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    Closed loop is like choke on.
    Open loop is like choke open.
    During closed loop it is in warm up mode.

    My higher compression, .040 over, moderately cammed, AL headed motor gets 17-22 mpg with normal driving/some play with a T5 and 3.55 gears

    Essentially 12 mpg seems low unless you are in the throttle a lot.

    If one can of SF seemed to help I would run a tank with two cans in it. It won't hurt the motor any. And for $15 or so it is an inexpensive try.

    My 2000 Goldwing, last of the carbed models. VERY small idle passages. Very intricate carbs. Ran one can of SF annually, rode it hard, let it idle. Never had a problem even after 130000 miles. Many had issues a lot sooner. It's one snake oil product that actually seems to work in many cases

    Sent from my Pixel 6 Pro using Tapatalk
    Last edited by richpet; 04-21-2022 at 10:42 PM.
    83 5.0 GT. Quicker than it looks! 10:1 (or just over) 306, Motorsport a332 cam, 140A alt, t5 conv, 8.8 w/ 3.27's, Edel rpm, alum rad, very worked e7's, Holley SA carb, etc... SOLD IT!!!!

    Now an 1981 Granada! 9.5:1 .040 over 302, Edel E-street heads, 268 cam, T5, 8.8 with 3.55, plus all the stiffening goodies, all control arms, lowered, alum shaft, x-pipe with Outlaws, FiTech FI system...

    "Never underestimate the power of stupid people in large groups"

  20. #20

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    Other way around richpet (I always have to check, I mix them up all the time). Closed loop is when the motor is paying attention to all the info coming in (coolant emp, baro, O2 sensor, charge temp, etc) open loop is during warm up or in my case for about 30 seconds after any start. Open loop the computer runs things entirely off tables and throttle position.

    So, if you have a problem only occuring in closed loop then it is likely something to do with a sensor being out of range etc.
    1984.5 G.T.350 5.0 CFI AOD Convertible (TRX package, loaded)
    K&N filter in a stock dual snorkel, GT40 heads, Edelbrock 3721 intake, MSD 8456 Dist., MSD 8227 coil
    Comp cams XE254H, hypereutectic pistons
    Hooker Super Comp Shorty Equal Length Headers, catted BBK H-pipe, full custom duals
    Maximum Motorsports caster/camber plates and strut tower brace, 3.73 rear.
    Ford Performance Springs, Firehawk A/S 225/55r16 on LMR TRX r390 wheels (street)
    Federal 595 rs-rr 245/40r17 and 255/40r17 on OE cobra r wheels (race)
    AOD rebuilt with a 6 clutch direct drum, Koline steels stacked with 8 clutches, Kevlar band, superior shift kit, new torque converter. --Everything else stock and fully functional.

  21. #21
    FEP Power Member richpet's Avatar
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    Crap! Thanks for correcting me! Apparently I need to double check every time too.

    Sent from my Pixel 6 Pro using Tapatalk
    83 5.0 GT. Quicker than it looks! 10:1 (or just over) 306, Motorsport a332 cam, 140A alt, t5 conv, 8.8 w/ 3.27's, Edel rpm, alum rad, very worked e7's, Holley SA carb, etc... SOLD IT!!!!

    Now an 1981 Granada! 9.5:1 .040 over 302, Edel E-street heads, 268 cam, T5, 8.8 with 3.55, plus all the stiffening goodies, all control arms, lowered, alum shaft, x-pipe with Outlaws, FiTech FI system...

    "Never underestimate the power of stupid people in large groups"

  22. #22

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    The TFI module is not the problem. The symptoms your car has, are nothing like what happens when a TFI module fails.

    Open versus closed loop probably has nothing to do with this also. The problem only happens when the engine is at high load. Under high load, the engine needs more fuel and it needs less spark advance. If the spark voltage is too low, under high load there won't be a spark because the increased cylinder pressure will stop the spark from happening. This is not your problem.

    Most likely, there is a fuel delivery problem. This is because as the car is used at WOT in higher and higher gears, the fuel starvation starts happening at lower and lower rpm. That means that the problem is time based, which means fuel flow.

    Either the fuel filter is clogged or the rubber hose inside the tank has collapsed. Don't waste time and money. Put a fuel gauge on a hose, which is attached to the fuel rail. Put the gauge on the cowl and drive the car. Watch what it does under WOT conditions.

    If the car is really getting 12mpg, something is REALLY wrong with it.
    Jack Hidley
    Maximum Motorsports Tech Support

  23. #23

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    Jack, he drove it cold and was able to climb through gears without stutters.
    1984.5 G.T.350 5.0 CFI AOD Convertible (TRX package, loaded)
    K&N filter in a stock dual snorkel, GT40 heads, Edelbrock 3721 intake, MSD 8456 Dist., MSD 8227 coil
    Comp cams XE254H, hypereutectic pistons
    Hooker Super Comp Shorty Equal Length Headers, catted BBK H-pipe, full custom duals
    Maximum Motorsports caster/camber plates and strut tower brace, 3.73 rear.
    Ford Performance Springs, Firehawk A/S 225/55r16 on LMR TRX r390 wheels (street)
    Federal 595 rs-rr 245/40r17 and 255/40r17 on OE cobra r wheels (race)
    AOD rebuilt with a 6 clutch direct drum, Koline steels stacked with 8 clutches, Kevlar band, superior shift kit, new torque converter. --Everything else stock and fully functional.

  24. #24

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    Emery,

    That may prove my point.

    When the engine is cold, it is going to run much richer, meaning that the injector pulsewidth is wider. This is allowing enough fuel to flow (fuel mass = pressure x time) into the runner so that combustion can still occur at WOT. The target a/f ratio may be 12:1, but with a clogged filter it ends up at 15:1.

    When the engine is warmed up, the pulsewidth goes down, so the engine is so lean that no combustion can happen. The target a/f ratio is 15:1, but with a clogged filter it ends up at 18:1.

    No matter what, it is an absolute waste of time to do anything else other than check the fuel pressure under load.
    Jack Hidley
    Maximum Motorsports Tech Support

  25. #25

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    One other thing.

    Don't put a 255lph fuel pump in it. Use the smallest pump that you can find. The stock fuel pump was 88lph and was good enough for 225hp. Do you plan on the engine making 652hp ((255lph/88lph*225hp)? Probably not.

    Too large a fuel pump creates all kinds of problems, almost all of them happen at part throttle, which is where the car is driven 99% of the time.
    Jack Hidley
    Maximum Motorsports Tech Support

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