High RPM stutters and bucking - 1986 GT

[emerygt350]

Not to second guess you, but if he can get enough fuel to run it rich and cold he can get enough to run it lean and hot.

[Jack Hidley]

I don't understand the point of your previous comment.

[emerygt350]

The car will use more fuel cold revving to redline in gear (under equal load) than it will hot. Therefore it is getting enough fuel.

[Jack Hidley]

Your last statement, "The car will use more fuel cold revving to redline in gear (under equal load) than it will hot." is 100% correct, but you need to look at why this is so.

The amount of fuel that the pump can deliver, in a given time period, is a function of the load that it sees, the blocked pressure at the fuel rail and the percent of time that the injectors are open. The load that the pump sees is not a simple thing to explain. Some of the load is external (metal line running from front of the car and to the back), but some of the load is internal (This is the size of the internal passages, sock, pickup hose). If the internal load were zero, and the pump outlet connected to free air, when it was turned on, the pump would move an infinite volume of fuel. This is just like shorting the + and - terminals on a battery with a thick bar. The current coming out of the battery isn't infinite. It is limited by the internal resistance of the battery.

When the pump "fails", (sock is clogged, hose collapses, etc.), the pump then has an output flow versus pressure curve which tapers off at a lower volume.

When the injector pulsewidth is say 20% greater at low engine temperatures, 20% more fuel is going to flow, because the load on the pump has not changed. This is enough fuel to allow the engine to not misfire under more or less all conditions. It is important to realize that there will probably be 20% less fuel flow than the EEC expects, but this will still be higher for the engine to have normal combustion. This 20% reduction is from the higher internal load (output impedance) of the failing pump.

When the engine is at higher temperatures the injector pulsewidth returns to a normal 0% amount, but the 20% increased internal load on the pump reduces the volume of fuel by that much. Because the warm engine a/f ratio is 15:1, this 20% flow reduction pushes the engine to a 18:1 a/f ratio where lean misfire happens.

[emerygt350]

I guess I just come from the physics side of it, if it can pull strong cold in a rich condition, it should have more than enough fuel for a lean hot pull. It doesn't matter what else is going on. 5600 rpm under full load accelerating cold requires more fuel than the same operation hot. If the pump can supply the fuel cold it will be able to supply it hot. That increased duration cold just proves the point. If the closed loop operation isn't asking for enough fuel I doubt that is a fuel pump issue. Seems more like a temp sensor issue. I mean at least he isn't fighting cfi.

[Fast_4_Eye]

Thank you guys for the in depth explanations on how the pump and injectors communicate at cold and hot. I will get a fuel pressure gauge asap and check the pressure at the rails.

I was on LMR looking in the engine cooling section and saw there was a sensor for coolant temp (not sending unit for the gauge). It says that it can fix symptoms of lean conditions and incorrect AFR which just made me realize, this car had sat for 17 years and I've had to replace atleast 10 sensors on this car already. Would it be likely that this coolant sensor that communicates with the computer could be messed up? If it really is a matter of it being open loop and the car only pays attention to the sensors then... it would make sense that this sensor is throwing it off.

I could be very mistaken so please go easy on me because I don't have nearly as much knowledge on this.

Lastly, the pump in the car right now is a 120lph I believe and it was circled in the Ford performance part magazine that the original owner gave me with the car. Should I get another 120? I do plan on doing heads and cam whatnot eventually but not for the next year I would think so maybe I'll stick with the 88 idk. I will also order a fuel filter tonight as that seems to be what this has all boiled down to. I did one can of SF and it fixed the stutter about 50% I would say, I will run another can or 2 tomorrow and drive it for the week to come and let you guys all know.

Also I think my fuel mileage on my last tank was atrocious since I had just done a long highway drive and I've been revving the balls out of my car trying to trouble shoot this problem. Hopefully efficiency goes up when I drive it more tame. Also I gapped the plugs back out to .050" and it seems to be smoother (??) if thats even possible I'm not quite sure. I did it at school and it sure didn't smell as bad out the tail pipes after so I have no idea what could be going on there. I also timed the engine down to 12°.

Let me know if you guys need any more info.

[emerygt350]

My car sat for quite a while and I ended up replacing most of the sensors. Do you have a code reader? Along with seafoam you should check to see what codes it's throwing. If a sensor is failing the computer would tell you.

I am hoping it's just something like dirty injectors etc.

[Jack Hidley]

Sensors generally don't go bad from sitting. The EEC in your car uses OBDI diagnostics. These are very crude. Many of the sensors on the car can be giving very poor data and the EEC will not catch this and set a code. The only reliable way to test the sensors on the car is to use a DVOM (digital volt ohm meter). This is easy to do.

I would purchase a copy of the book linked below. It explains how the entire system works. Has schematics. Has data and instructions for testing all of the sensors on the car.

https://www.amazon.com/Ford-Injectio.../dp/0837603013

The only sensors that the car ignores when in open loop are the O2 sensors.

You can use the math I posted above to determine how large of a fuel pump that you need for the engine, based on how much power it is going to make normally aspirated. I'm 95% certain that the fuel pump that was in the old SVO catalog was 110lph. The next normal aftermarket step above that was a 155lph pump. If you change the heads, cam and intake, the most power you will make under the 6,250rpm limit and with a 10:1 CR is maybe 380hp, with everything perfect.

[Bryan Knebworth]

Question; why would you bring your Fox above 3,000 RPM? That seems cray-. I did that ONCE in 30 years, at 5500 RPM, never again!

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.

[emerygt350]

I have found my computer is more than willing to tell me about even the slightest deviation from expectations (punk a$$ computer). But it has been quite useful in getting my car running. You just have to be very smart about what the codes mean and not just replace everything. Often it's ground issues, vacuum leaks, stupid things done by previous owners etc.

[Fast_4_Eye]

Question; why would you bring your Fox above 3,000 RPM? That seems cray-. I did that ONCE in 30 years, at 5500 RPM, never again!

Well I don't have an answer for you as to why I'm bringing it past 3,000 for any rational reason other than hearing the exhaust and going fast. I'm sure all of you guys were 16 at some point lol ��

[Fast_4_Eye]

Sensors generally don't go bad from sitting. The EEC in your car uses OBDI diagnostics. These are very crude. Many of the sensors on the car can be giving very poor data and the EEC will not catch this and set a code. The only reliable way to test the sensors on the car is to use a DVOM (digital volt ohm meter). This is easy to do.

I would purchase a copy of the book linked below. It explains how the entire system works. Has schematics. Has data and instructions for testing all of the sensors on the car.

https://www.amazon.com/Ford-Injectio.../dp/0837603013

The only sensors that the car ignores when in open loop are the O2 sensors.

You can use the math I posted above to determine how large of a fuel pump that you need for the engine, based on how much power it is going to make normally aspirated. I'm 95% certain that the fuel pump that was in the old SVO catalog was 110lph. The next normal aftermarket step above that was a 155lph pump. If you change the heads, cam and intake, the most power you will make under the 6,250rpm limit and with a 10:1 CR is maybe 380hp, with everything perfect.

I don't have enough money to do a head, cam and intake so realistically I just want my car running 100% at stock as there really isn't much of a reason I should have a bunch of issues on a stock, NA fox but here we are lol. I'll probably get 110 or whatever slightly above stock is just for the head room. It wouldn't hurt the car to have a bit more flow would it?

[Fast_4_Eye]

I have found my computer is more than willing to tell me about even the slightest deviation from expectations (punk a$$ computer). But it has been quite useful in getting my car running. You just have to be very smart about what the codes mean and not just replace everything. Often it's ground issues, vacuum leaks, stupid things done by previous owners etc.

I still need to get a test light from local part store and dump codes, I've spent the past 2 days fixing my leaking thermostat, installing new cobra pulleys (previously some crappy underdrives), and lastly cleaning up any mess of coolant and RTV on the front of the engine best I can. Also cleaned up the fan shroud, doesn't look half bad lol.

I'm not going to lie computer stuff really scares me on these older cars even though I would say I'm more computer literate than the average person. It's merely the fact that everything is so old I'm worried it might break lol

[Bryan Knebworth]

You take care of a foxbody, a foxbody will take care of you (Love you're in Canada, we need to hear more from our Foxers in the Great White North!) But really, if you like getting your vehicle to 5000-plus between shifts, buy something foreign, V8s don't like that kind of stuff

Well I don't have an answer for you as to why I'm bringing it past 3,000 for any rational reason other than hearing the exhaust and going fast. I'm sure all of you guys were 16 at some point lol ��

[emerygt350]

I picked one up for 35 dollars at a flaps 7 years ago, probably a hair more now. The Ford code reader and book OBD2 actually is quite helpful. I have OBD1 and even that space shuttle tech is great resource. Between that and some of the people on here I was able to troubleshoot all the little gremlins out of my cfi (and there were many). Definitely the first thing you should do. You will throw codes on any car and on these early fuel injection systems they can and will save you so much time.

[richpet]

I still need to get a test light from local part storel

If you were close I'd give you one. Think I've got three of em.

Just a question - is this early car OBD-1? If so, can you use a paper clip and count engine light flash pattern to pull codes and look them up online? That's what I did with my '90 Bronco before I bought a code reader for it.

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[teal95]

I have seen a TFI do a somewhat similar failure. In my 95 GT it wouldn't pull over 4k rpm. Was fine up to that point and then just went soft and wouldn't pull.

However I'm betting your fuel sock and/or filter are plugged. Putting a fuel pressure gauge on it is how you can test this. To fix it you're going to have to drop the tank, pull the fuel pump and clean the tank out.

steve

[Fast_4_Eye]

If you were close I'd give you one. Think I've got three of em.

Just a question - is this early car OBD-1? If so, can you use a paper clip and count engine light flash pattern to pull codes and look them up online? That's what I did with my '90 Bronco before I bought a code reader for it.

Sent from my Pixel 6 Pro using Tapatalk

'86 doesn't have a CEL anywhere on the dash unfortunately, I heard you can use a multimeter but idk if thats actually possible but I need to go and get a test light to check everything. Along with a new belt, more coolant, fuel filter, fuel pump, fuel pressure gauge and then whatever parts needed from the engine codes lol. This is turning out to be quite the project now I just want to be able to drive my car haha

[Fast_4_Eye]

I have seen a TFI do a somewhat similar failure. In my 95 GT it wouldn't pull over 4k rpm. Was fine up to that point and then just went soft and wouldn't pull.

However I'm betting your fuel sock and/or filter are plugged. Putting a fuel pressure gauge on it is how you can test this. To fix it you're going to have to drop the tank, pull the fuel pump and clean the tank out.

steve

I already knew I'd have to drop the tank and whatnot, I was going to finish this tank of gas since I have seafoam in it, then i was gonna go further with filter and pump. The problem is that there's all these things I need to do and places to go for these parts, and not enough time lol. I still don't know where a good pick n pull would be for me to get a new dizzy/tfi

[emerygt350]

The original stuff is built really well. I wouldn't replace it unless you are certain it is gone.

[Jack Hidley]

A 110lph pump will not cause any problems. There will be a little bit more heating of the fuel.

I can not stress enough the importance of getting and reading the book I linked above.

You can test for codes with a test light and a paper clip.

[emerygt350]

Or buy a reader. Not as macgyver but worth it if it means you can check them more often and accurately.

[Fast_4_Eye]

I'll see where this tank of SF gets me as the first tank seemed to clean up the issue a good 50% so I'm hoping this concentration of SF will wipe it clean. Still getting the filter, iffy on the pump. Going to find a nice gauge and whatnot so that my car doesn't look like a mess with gauges everywhere. I'm a bit of a perfectionist so I don't want to start zip tying and screwing gauges everywhere, maybe a nice trim piece that accepts gauges?

I also don't think my dizzy or tfi are the issue tbh so I'm leaving them alone, I don't know if it is worth finding a remote mount kit for the TFI since its already been exposed tp heat but I may as well try.

[emerygt350]

I wouldn't worry about tfi heat yet. That has very obvious symptoms. If it happens, go for it, but the engine looks cleaner without the remote tfi. I printed a little three gauge cluster that velcros under the radio. No damage, can remove when needed.

[KevinVarnes]

I'll see where this tank of SF gets me as the first tank seemed to clean up the issue a good 50% so I'm hoping this concentration of SF will wipe it clean. Still getting the filter, iffy on the pump. Going to find a nice gauge and whatnot so that my car doesn't look like a mess with gauges everywhere. I'm a bit of a perfectionist so I don't want to start zip tying and screwing gauges everywhere, maybe a nice trim piece that accepts gauges?

You don't need a permanent fuel pressure gauge. Just go down to the parts store and rent one. The hose is usually long enough that you can stick it up on the windshield so you can see it while driving. It is simple, fast, and easy and would help point you in a direction.