Ford Mustang Forums banner
1 - 6 of 6 Posts

·
Registered
Joined
·
203 Posts
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
 

·
Registered
Joined
·
203 Posts
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.
 

·
Registered
Joined
·
203 Posts
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.
 

·
Registered
Joined
·
203 Posts
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.
 

·
Registered
Joined
·
203 Posts
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.
 
1 - 6 of 6 Posts
Top