ottos

I am new here but I have been scanning for a while looking for a cure to these symptoms. Most said to check the hardware like ignition and injectors, fuel filter and stuff but mine was running fine before the winter, i didnt think anything went bad. I checked injectors, disconnected the battery, checked for spark, all were ok. So here is what I did.

Car: 1999 Mustang GT, 135k miles, dynomax mufflers

Issue:
Hesitation or stuttering or bucking on acceleration. Idle is rough, exhibits a popping sound, and smells like fuel or something burning. Worse when car is warmed up. During idle I could see the engine shake. Isolated the shake to cyl 2 by disconnecting the injector and observing no change in vibration. Car still put out full power at high RPM (this rules out a clogged fuel/air filter).

Cause:
Bucking, hesitation, etc, during acceleration is usually caused by misfires. A misfiring cylinder sends fuel and air or partially burnt fuel out the exhaust and tends to smell bad. The engine will be out of balance if one cylinder misfires, exhibited by shaking and bucking.

Verify hardware:
First ensure the following hardware is working properly:
- no large air leaks from MAF to throttle body and no vacuum leaks past the throttle body (check all connections to induction system)
- MAF (visual inspection)
- spark plugs (check each cylinder... disconnect the injector, take out spark plug, re-connect it to COP, connect the threaded part of plug to an electrical ground, start engine and verify that plug generates spark)
- coils (these rarely fail, use above method to check function)
- injectors (rarely fail, use stethoscope to verify ticking sound)

** My car was running fine at one time, then the issue suddenly started after I disconnected the battery. So I ruled out any hardware malfunctions.

Resolution:
If all hardware issues are ruled out, obtain the new PCM calibration ($80) at the dealership (TSB 01-01-09 http://www.stangnet.com/images/stori...s/01-01-09.pdf) and drive around until it learns the correct fuel trim.

** Immediately after the reflash, you may still have a slight stumble. Continue driving for ~20 miles until the PCM learns the correct fuel trim.


Explanation:
The PCM has a learning mode that adjusts the air/fuel mixture lean or rich to compensate for differences in air mass compared to what is measured directly by the MAF. If components start to degrade or leak air, measured air mass by the MAF might be more/less than the actual air mass entering the engine. When the PCM senses that the engine is misfiring, it feeds back to the learning program to lean out or enrich the fuel on a permanent basis.

** If the required fuel trim is more than a certain limit, the engine MIL is lit.
** The short term fuel trims are stored in the parameter SHRTFTx, long terms fuel trims are stored in LONGFTx, with x being the cylinder bank number.
** The learning program might also be sensitive to large, sudden changes in air density (like when air temperature plummets) and it seems to take its time to learn the lean/rich fuel trim the engine needs (like several miles).

ZW99GT

Well, that's a pretty cool fix, but what if you are running a custom dyno tune?

ottos

It should still be ok. I think the new PCM calibration just fine-tuned the learning program. But the basic learning program is on all PCMs and it should eventually learn the needed fuel trim even with modified PCM programming. Unless the dyno tune adjusted the actual adaptive fuel parameters... then nevermind.

dtkikols

so did this reflash cost you anything? after doing some mods to my car (2000) sometimes in engines studders and so on but its nothign serious. my guess is that the A/F ratio is off or whatever, and like ur car the PCM has to be reflashed b/c i have never got my car tuned.

joshafmil

i just posted this in another thread... i think theres like 3-4 of them up right now on this problem... (provided they are all the same problem)

ottos

Yeah its a mix, not sure which is which. I'd say the idle problems could be the IAC valve or my explanation with the PCM learning. The stuttering on acceleration problems are probably curable with having the PCM learn the fuel trim. Then if that dont work, they can start taking things apart and checking the hardware.

ottos

Reflash was $80 at the dealer. I mean i would try just driving around for like 20 miles first. If you already did that and no change, then check your injectors (listen with a screwdriver or stethoscope) and spark plugs, make sure they arent fouled. If you have problems at high rpm then it might be the fuel filter. If you think the A/F is just off, then its probably the PCM needing to learn more.

Mine would run fine before warmup then bang, misfire city. So i knew my hardware was worked fine, just something wack with the fuel control after warmup. And thus I arrived at my PCM needing a reflash.

Brute03

i've been having all these symptoms for a while and nobody seems to be able to pinpoint the cause, the only difference is mine is an '03. where there any tsb's for later years?

ottos

There were no TSBs for later years, but there may still be an updated calibration available for the later cars. Try calling the dealer and ask them to obtain the calibration level for your car (an OASIS report will show it). Then see if they have a newer calibration.