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Discussion Starter · #1 ·
I've been reading other threads here and elsewhere without any luck so.. this thread is bound to be at least as useful!

The problem I'm encountering is misfire/bucking at part throttle, basically the car is undriveable. Idles well and runs smooth and hard at >50% throttle

Here's the hardware:

1994 GT
94 Cobra intake manifold
All emissions/driveability hardware in place
BBK catted midpipe + Flowmaster mufflers/tailpipes
Scorpion 1.7 Endurance rockers on stock E7 heads
Stock headers
Stock fuel injectors (19 lb)
Stock throttle body
Stock MAF (new)
New timing assembly
New distributor
New TFI module
New fuel pressure regulator
New fuel filter
New oxygen sensors
New IAC
New TPS
Timing set to 10° BTDC (with SPOUT out)
Every vacuum hose under hood new. Smoke tested no vacuum leaks.
No CEL and no stored codes with OBD1 reader.

I ordered a SCT/BAMA tune from American Muscle to suit the intake/exhaust/rockers, thinking it was maybe just lean because of the increased air flow. Installed that and it did not affect the behavior at all.

Engine does not overheat.

The only other potential symptom is that it does "hang" idle, for example if I push in the clutch while driving and release the gas pedal it will continue to run at ~1200rpm or so - not sure if that is normal.

Any random guesses?

Vehicle Hood Car Motor vehicle Automotive air manifold
 

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Idle hanging and bucking at cruise sounds like dashpot settings needing to be tuned. Unfortunately, no off the shelf tune is going to nail these.

If you indeed have a misfire (what are the symptoms to make you believe its misfiring?), then thats a another route of troubleshooting.
 

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Discussion Starter · #4 ·
The car jerks violently when cruising at part throttle. It is a manual transmission car and if I put it into any appropriate forward gear at any normal road speed in which holding the throttle steady would usually produce steady cruise at that speed, instead it jerks violently and constantly. There is no "shotgun blast" backfire sound, just failure to smoothly deliver power. At idle there is no stumbling and at wide throttle there is no stumbling. I drive it as little as possible for diagnosis only because that violent jerking cannot possibly be good for the transmission, driveline, axle, or the engine or anything else.

The car had a minor misfire when I first bought it because the timing assembly was original and the distributor was a cheapie with some play of its own. I replaced the timing assembly and distributor which made it run smoothly until I later increased the available air to the engine with a Cobra intake, high ratio rockers and a higher flowing exhaust/midpipe set. Then it began bucking/misfiring/being undriveable at part throttle.

After running through as much analysis as I could and disassembling it repeatedly I concluded it is likely running very lean so I ordered a twEECer-RT, only to have that order cancelled because of supply chain hardship the guy who makes them can't get some specific parts. So as an attempt to salvage my summer/fall I ordered a SCT chip from American Muscle with their canned tunes on it. However I get the same results with or without that installed.

I am trying to rule out if this is a tuning issue or some hardware/mechanical/whatever issue.

This summer is basically over so I guess now I have until spring to fix it but I sure would feel better if I could put it away running properly and then actually enjoy the car next summer instead of just looking at it every time I get in my 4 cylinder Jeep.
 

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1967 Mercury Cougar, Restomod, 5.0L, AOD
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So, I'm a noob here, but have a few decades of experience (esp when that is what u get when u don't get what u want) with cars having owned 40+. If you really want to examine / alter the EEC-IV tuning and log, get a Moates Quarterhorse and TunerPro RT software. It isn't quite OBD-2 but pretty amazing given the generation. I was going to get the twEECer-RT, but after reading up a bit more, went with the Quarterhorse. When you get your tune set, you can write it to a module and remove the QH. IIRC you can also have 4 tunes loaded to the module and switch between them on the fly.
 

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I second getting a quarterhorse as it will be essential for tuning and just as important datalogging if you plan to get into that and modify further. I'm not sure if they have the same supply issues though.

Some things you can try are to get the car warmed up, and try to run it with the IAC unplugged to see if it makes youre cruise response better with less bucking. Do this on an empty road if you can if it dies due to poor idle management you wont be stuck in traffic. If this helps, you need to adjust/tune idle air and dashpot.

If it doesn't help I'm always suspicious of stock reman MAF sensors and how they are calibrated. That is another item that controls the load your ecm is calculating. If its off for any reason it can cause these symptoms.
 

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You could also try locking the timing at a certain ° and going for a test ride to see if the violent bucking goes away.

but just to clarify, is it a misfire or is it bucking at light load? Does the CEL come on?

stock cam?
Has anyone turned the throttle stop screw for any reason?
 

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Discussion Starter · #8 ·
That's a fair suggestion to lock the timing, easy enough to pull the SPOUT for a test.

I'm not sure of where one draws a distinction between bucking and misfiring, it's not backfiring or combusting in the exhaust system.

The car jerks violently at speeds between idle and ~50% throttle while in gear and traveling down the road. It climbs hills at low RPM/heavy throttle without difficulty. Partially revving or holding any randomly chosen RPM while in neutral with no load it runs smoothly. It idles smoothly and it accelerates briskly at 0 throttle and >50% throttle, respectively.
If I knew another way to describe that with other words I would assuredly do so.

The cam is stock. The rockers are 1.72:1 ratio. The cam timing is correct both because I am at least capable of following instructions :) and because it ran very smoothly during the period of time between replacing the timing assembly and installing the aspiration improvements.

The throttle body is stock but the throttle stop screw has clearly been replaced by a previous owner, because it is an actual philips screw. As part of my diagnostic measures I have adjusted it by starting the car with the IAC unplugged and all electrical loads turned off, and turning it down until the car just idles. I do not know if the previous owner touched the bypass adjuster under that little cap.
 

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Does it have a warm idle of 672rpm?
Your problem could be in the IAC duty cycle if not.
 

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Discussion Starter · #11 ·
Does it have a warm idle of 672rpm?
Your problem could be in the IAC duty cycle if not.
I don't have a digital tach but warm idle was higher than that, just below 1000. So I found the instructions for the Professional Products throttle body and saw that their initial installation procedure, decided to follow it despite the fact that I have a stock throttle body.

I did this because I saw that the previous owner had adjusted it (by replacing the screw..) . The instructions for the Professional Products throttle body includes fully closing the idle bypass . So I used a hex wrench and counted turns clockwise until it was fully closed, 4.5 turns.

Then I left it fully closed and adjusted the throttle stop by turning the screw first until I could just pass a piece of paper between the screw and the throttle arm, then clockwise one full turn.

After this I started the car and drove around the block to get it warmed up, then let it idle and it seems like probably around 700 based on needle position. Notably lower than before. No difficulty idling.

With high hopes I took it for a drive and it actually did seem less violent, but it still bucks a lot under the same circumstances as before.

If it's lean vacuum leak. Have the intake smoke tested to verify. Almost a must any time you replace an intake just to verify.
I bought a smoke tester and smoke tested it after replacing the intake, mentioned that above :) But it's a good suggestion.
 

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Okay well I always hear about people buying the smoke testers or making their own but have they performed the test the right way building up 4psi-6psi of smoke pressure in the system? That is the only way to do it right. And taking the dipstick out and putting your finger over the dipstick tube when smoke starts pumping out the tube is how you can build up the psi and watch the psi gauge on the smoke machine it goes up when you plug it with your finger. And before performing the smoke test going over any place for the smoke to escape covering all the places with plastic quart size baggies that the smoke can escape like the throttle body and or mass air meter so the system can build up the smoke pressure. Takes an experienced smoke test technician to do it right that has many many successful smoke tests under his belt.
Just sayin.
 

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I don't have a digital tach but warm idle was higher than that, just below 1000. So I found the instructions for the Professional Products throttle body and saw that their initial installation procedure, decided to follow it despite the fact that I have a stock throttle body.

I did this because I saw that the previous owner had adjusted it (by replacing the screw..)

After this I started the car and drove around the block to get it warmed up, then let it idle and it seems like probably around 700 based on needle position. Notably lower than before.
neither of those are adjustable without a competent tuner datalogging the values.
Some of the OEM throttle stop screws do have a phillips head on them.

And, when a tuner does adjust those, the car should be fully warmed up. With the IAC disconnected, the car should barely idle at around 500rpm (warm idle).

im fairly certain this is where your issues are coming from now. 1,000 rpm was too high, affecting the IAC duty cycle. The IAC also acts as a dashpot, stall prevention, cold idle increase, and idle load increase.

it has a transfer function of its own. Turning the bleed screw has completely skewed those airflow values.

that’s under the assumption that your 1,000 rpm wasn’t from a vacuum leak.
 

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Well, they weren’t correct to begin with if you had a 1,000 rpm idle 🤷🏻‍♂️
 

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Discussion Starter · #18 ·
I agree, but if it was incorrect before I touched it and it's incorrect after I touched it, that doesn't leave me with a lot to go on. I feel like my question has lead to nothing but a tense thread and that's not something any of us needs so I'll go back to working on this by myself. Thanks for the suggestions.
 

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What makes you think it’s “tense”?

I didn’t really see where anyone made it that way, IMO

it’s really difficult to get those parameters back in spec if you can’t see the data, best you can hope for is “close enough” to still allow the EEC enough room to do its thing.

at least you clarified that it wasn’t a misfire, which would take you down an entirely different path.
 

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I guess interpreting the thread as tense is all about how you read text and translate that to a tone.

What I see is good suggestions being offered and none of them being tried yet.
 
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