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  1. #1

    Default 1984 5.0 HO Firing Order Question

    Hey!
    Hope I'm in the right place for this.

    About 2 months ago I bought a 1984 5.0, lately it's been idling pretty rough and intermittently stalling so I figured I'd change the spark plugs to see if that would fix the issue. While I was at it why not change the wires, rotor and distributor cap since they're pretty cheap anyways, but I ran into a bit of an issue when it came time to replace the wires...
    I have reason to believe whoever changed the plugs last might have messed up the firing order:
    1) The plugs I pulled weren't stocked and weren't gapped right, all had different gaps
    2) Wire lengths didn't make much sense, i.e. plugs closer to the distributor should have shorter wires, plugs farther away should have longer ones, they seemed mismatched.
    This makes me think that whoever did it last wasn't too certain on what they were doing and might not have done it properly, especially with regards to firing order.

    Here's my issue, firing order for a 1984 5.0 is 15426378, firing order for a 5.0 HO is 13726548, and the plugs were wired in the non HO configuration, however my air cleaner housing says the engine is an HO, and according to about half of the VIN decoders I've looked at, my engine code (M) says it's an HO engine as well. As of right now, I'm hesitant to run the car as is, and I'm also hesitant to re-wire the plugs to the HO configuration as they weren't in that order when I got the car.

    Hopefully someone with a little more knowledge and experience with these cars can help me out since I'm sort of at a loss of what to do here and this is my first time doing more than super basic maintenance.

    Thanks!

  2. #2
    FEP Power Member Ourobos's Avatar
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    If it's a H.O, it has the 351w firing order, 1-3-7-2-6-5-4-8. Won't hurt to try it and see if it helps (correct plug gap first of course)
    1986 CHP SSP Coupe

  3. #3
    FEP Senior Member qikgts's Avatar
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    Welcome to the site!

    Ourobos has you on the right track with the firing order. Follow his advice it's right on.

    I'd add that you may want to get a can of carb cleaner and a paper clip and clean up the carburetor some. Spray it down liberally making sure any grime and grease is removed from the outside and also blast down the bores letting the can do the work. That will help you see later if there are any leaks that need to be addressed.

    Then, straighten out the paperclip and poke it down the air bleeds that run around the perimeter of the top of the carb. Also spray the carb cleaner in them with the straw. Be careful, carb cleaner in the moth and eyes is not good. I think there are 6 total between the primary and secondary sides. These carbs are notorious for having those bleeds obstructed and causing idle issues especially when the car hasn't been driven regularly.

    Here is an example of the air bleeds. They are the 4 little holes running vertically.

    Name:  4180 air bleeds.jpg
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    Last edited by qikgts; 01-14-2018 at 11:21 AM.
    '85 GT

    The other one... 2016 F-150 XLT Supercab Ecoboosted with 3.73's...

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    FEP Senior Member cb650's Avatar
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    Anyone before swap out the cam? My 83 HO has the non firing order too. But there was a cam swap before I got it. Who knows what was put in. It does run very well so leaving it.

  5. #5
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    Easy to swap the wires around either way if the cam was swapped; only 2 choices. Gotta be sure you're on #1 compression stroke when reinserting the dist. if ever it was removed, double check with timing pointer to be sure.

  6. #6

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    84 5.0 Mustangs only came with the H.O. engine. 1-3-7-2-6-5-4-8 is correct. But if a PO changed the cam, it could be the old sequence.

  7. #7

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    Thanks for the replies gents!
    Doing some more research I think dynodon64 is right, the 84's only came with the HO, but just to be sure I'm going to manually crank the engine this weekend and see if there has been a cam swap, or if the muppet who changed the plugs last wired them in the wrong order.

    I'm pretty sure it's the latter, like I said the plugs weren't stock and they didn't seem THAT old, but they weren't gapped properly, plus the wire lengths didn't make much sense so it it almost seems like whoever did it last wasn't too keen on doing it 100% proper.

    Cheers!

  8. #8
    FEP Senior Member cb650's Avatar
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    Does it look like any new untrimmed gasket material or silicon under the intake or timing cover?

  9. #9

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    It will still run wired up either way without hurting anything, obviously not as much power. Couldn't tell ya how many crown Vic and f-150's I've seen with the h.o. wires because its got the "mustang 5.0".
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

  10. #10

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    Quote Originally Posted by Haystack View Post
    It will still run wired up either way without hurting anything, obviously not as much power. Couldn't tell ya how many crown Vic and f-150's I've seen with the h.o. wires because its got the "mustang 5.0".
    Four of eight cylinders firing on the exhaust stroke instead of the compression stroke should make for an entertaining version of "It will still run...", IF it will... though if so, tremendous hilarity like gatlin gunfire or a howitzer, great balls of fire out the exhaust, followed shortly thereafter by fireworks and flames out the intake... probably blowing the exhaust system and induction off and across town, LOL!
    Last edited by Walking-Tall; 01-17-2018 at 04:24 AM.
    Mike
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  11. #11

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    Its good way to blow flames out of the cat. Don't ask 16 year old me how I know...

    Idled perfectly though. Only shot flames on throttle. Sounded pretty cool...
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

  12. #12
    FEP Power Member Ourobos's Avatar
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    It will run with the wrong order, but you'll think it's a Ford tractor vs. a Mustang
    1986 CHP SSP Coupe

  13. #13
    FEP Power Member erratic50's Avatar
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    The firing order is literally the main thing that makes a 5.0 HO an HO.

    Back in the day one of the magazines did dyno comparisons standard order vs HO. Same cam specs, same install degreed in, same ignition advance, same carb, etc. HO made more torque and more horsepower.

    sounds SO stupid.....

  14. #14

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    http://sbftech.com/index.php?topic=7747.0;wap2

    Supposedly that article found a 12hp difference.

    As much as I like car magazines and dyno sheets, I take it as a grain of salt. Magazine has to show an improvement or they lose a sponsor. If the "old" firing order is wrong, guys might just swap to the "new" firing order. They sell more cams and previous customers feel better about the cam they already bought.

    If the 351 firing order was that much better, then why did they keep the old firing order during the fuel injected hydraulic roller lifters revamp in Lincolns and other luxury cars and only carry it over to the mustang and the few other performance motors?

    Ford sometimes does stupid things that don't make sense, but I'd think that they would only make one firing order if one was better all around.
    2 1986 cougars (both 4 eyed and 5.0)
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  15. #15
    FEP Power Member erratic50's Avatar
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    HOs produce more vibration?
    Last edited by erratic50; 01-18-2018 at 03:30 AM.

  16. #16

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    Don't forget that they chiseled beef (strength) off of the crankshafts, requiring the 50oz-in imbalance factor as well as the firing order (and necessary differently indexed lobes for four cylinders camshaft for that ignition firing order, a supposed less stressing order of cylinder firing...) change at the same time... something tells me they were mostly interested in the cost savings of the chiseling, worth the cost of a casting revision... pretty much like chiseling for every nickel using four lugs per wheel after ions of five, lol... and now things are $80K+ so there's no worries about an extra four or more wheel nuts anymore, LOL!
    Last edited by Walking-Tall; 01-18-2018 at 04:42 AM.
    Mike
    1986 Mustang convertible 3.8L ---> BUILD THREAD
    1983 Mercury Cougar 3.8L LS
    1986 Ford Thunderbird 5.0L ELAN
    1966 Ford Fairlane sedan 200-6
    1966 Ford Fairlane GT (390ci 4spd)
    1981 Mercury Marquis Brougham
    1980 Capri RS Turbo
    1971 Mustang Fastback
    1974 Pontiac Luxury LeMans

  17. #17

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    I don't see how changing up the firing order is going to increase horsepower, and it's not like it's a completely new pattern that reduces stress on the engine. I've drawn it out, it's the same pattern just oriented differently. Name:  firing_orders.jpg
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  18. #18

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    Imbalance factor changed from 28.2 oz to 50 oz for the 1981 and later model years, and for all 5.0L engines. So not only was the change one full model year before the H.O. was introduced to production but it was used in H.O. 5.0L engines with the 351W firing order and in S.O. 5.0L engines with the "old" 302 firing order. In other words, the imbalance factor had nothing to do with the firing order.
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  19. #19
    FEP Member brianj's Avatar
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    Ford wanted to re use an existing 351 cam in the HO motor, in a typical "we've got it already, so let's use it" manufacturer methodology. Hence, 351 firing order.
    1983 Mustang G.T. No-option stripper- I like strippers.
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  20. #20

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    They used the 351 cam in 302's since the 351 came to be. Not always in performance motors either.
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

  21. #21

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    Which 302s, besides the 1982-95 H.O. 5.0L, was the 351W cam/firing order used in?
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  22. #22

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    The main reason for the firing order change from the old 302 to the 351, in the H.O. motor, was to take some of the load off of the crankshaft bearings. I don't remember which bearings though.

  23. #23

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    The main reason for using a 1974 351W 4V Torino camshaft in the H.O. 5.0L in 1982 was because that's what cam they had available 'off-the-shelf' to make the power they were looking for. Yes, it really was that simple.
    Last edited by FoxChassis; 01-20-2018 at 05:48 PM.
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  24. #24
    FEP Member brianj's Avatar
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    Supposedly #1. However, if this was a problem, you'd think they would have done it on all of the 50oz balanced motors, and not just the H.O. motors.
    1983 Mustang G.T. No-option stripper- I like strippers.
    5.0, GT40P heads, Comp Cams XE270HR-12 on 1.6 rockers, TFI spring kit, Weiand 174 blower, Holley 750 mechanical secondarys, Mishimoto radiator, Edelbrock street performer mechanical pump, BBK shortys, T-5 conversion, 8.8 rear, 3.73 gears, carbon fiber clutches, SS Machine lowers, Maximum Motorsport XL subframes, "B" springs.

  25. #25

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    I started looking into it, I couldn't find any factory 351 cams stuffed into 302s.

    Couple marine cams guys did for more power, but that was it.
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

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