99 GT still missing......

ab 281 gt · Dec 13, 2006

Good luck bro... All of my misfire problems have been ignition coil related.. I've changed out about 3, but I just recently picked up some low mileage Motorcrafts for $30, so I replaced them all!! It got rid of my slight miss at idle.

jstreet0204 · Dec 13, 2006

Are there any mods done to this car? Going back through your posts it doesn't look like you have had it very long. Do you possibly have a timing adjuster on it. Timing adjusters can cause some bucking during light load like that. If your test don't work out, post back with all the mods if any that have been done to the car.

Mike97gt · Dec 13, 2006

Smokeurhonda said:
Well call up your local dealer and see. Maybe there is an extended recall for PCM flashes or something like that.

There is none.

Kappa2006 said:
Yes, I followed some DC's and it directed me towards fuel and ignition tests. I just finished the MAF ones and they checked out ok. Now I'm going to do the fuel tests tonight (fuel pressure, injector balance test, injector resistance tests). and if I have time I'll so some tests on the coils. One thing I forgot to mention is that I found a ford TSB stating that some pcms need to be reprogrammed due to hesistation when accelerating. I kind of pushed this to the side since the hesitation is pretty bad, but if everything else checks out, I might have to look into pcm programming options.

Check this TSB out.. might not be your problem but it's worth a shot.

http://www.stangnet.com/images/stories/docs/sn95_TSBs/98-23-10.pdf

bgrd351 · Dec 13, 2006

svttech76: it is is interesting that that TSB you provided doesnt say to clean the MAF at all but instead to replace it. Does Ford not allow the cleaning or just not on this TSB?

Mike97gt · Dec 13, 2006

bgrd351 said:
svttech76: it is is interesting that that TSB you provided doesnt say to clean the MAF at all but instead to replace it. Does Ford not allow the cleaning or just not on this TSB?

Ford has never suggested cleaning the MAF to us.. They want us to replace them. That does not mean it's a bad idea to clean them, it's just not ford suggested practice.

Kappa2006 · Dec 14, 2006

Ok, the search continues. I did the injector balance test yesterday, each injector
lowered the starting fuel pressure the same amount. so no clogged injectors. I ohmed out each coil, no coil was over 6 K ohms resistance. The primary circuit resistances were all within spec. Hooked it up to an oscilliscope to monitor the coils while the car was running, nothing abnormal. Checked the maf readings again, according to a volumetric efficiency program I have, the efficiency of my intake and maf sensor is about 78-83%. This tue I'm going to put the car on a dyno and monitor EVERYTHING. This way I can make the car misfire and see if anything shows up. and I will look through that tsb you supplied me. Thanks for the help so far. I'll keep everyone posted.

Smokeurhonda · Dec 14, 2006

Man, it's gotta be your pcm.

Kappa2006 · Dec 14, 2006

Well I finally have some results here....or should I say, I finally found something thats off. Checked fuel pressures today while running. with the vacuum hose attatched to the fuel press. regulator, I got barely 32 psi. With the vacuum hose removed, I got 41 psi. So my regulator is working fine, but according to my trusty Haynes manual, I don't have enough fuel pressure. The specs I have are 35 psi at idle, vacuum hose attached, and 45-55 psi at idle vacuum hose removed. I did replace my fuel filter too. I know how to test the output of a fuel pump on a return fuel system, but I'm not sure on a returnless system. and for the record, the guy I bought this car from said he had it sitting for a few months. He claims he'd go out and start it up once in a while, but who knows. So there is a possibility its a fuel pump issue. anyone know how to test the max output of the fuel pump?

Smokeurhonda · Dec 14, 2006

Well here are some numbers for comparison. I tested my fuel system a few months ago in an engine performance class. Here are some of the results. 02 Mustang GT, with bolt on's. Stock fuel delivery.
Idle: 28-30 psi,
2500rpm: 29psi,
wot rev:55psi.
I also have results from putting vacuum on the regulator.
0"Hg 40psi,
5"Hg 37psi,
10"Hg 35psi,
15"Hg 32 psi,
20"Hg 30 psi,
25"Hg 29psi.
I have absolutely no drivability issues what so ever. Sounds like your car is right in the ball park compared to mine. And remeber, the pcm alters the amount of pulseing current to the fuel pump. Mine operated @ 7.1 volts and about 2.4 amps at idle, and will go up as you add load to the engine. Hope this helps.

Kappa2006 · Dec 14, 2006

that does help. everything seems to be the same except for my wot figure. mine definately didn't get up to 55 psi. i think the max i saw was 45. But I don't think its a fuel problem anymore. After thinking about it, if lacking fuel was causing the mis, it wouldn't pull normally at high rpms, when the fuel demand is higher than lower rpms. It would mis throughout the rpm range. Mine mostly does it at the lower rpms. I guess I gotta wait it out till I can get the car on the dyno, make the problem happen, and monitor every possible sensor on the car.

Smokeurhonda · Dec 15, 2006

I mean, you've pretty much checked every sensor, or major component. I truely do beleive it is your pcm. But, it's good that you are actually taking the time to diagnose, instead of throwing parts at the car. Stay with it and you'll find the problem.

Mike97gt · Dec 15, 2006

Kappa2006 said:
that does help. everything seems to be the same except for my wot figure. mine definately didn't get up to 55 psi. i think the max i saw was 45. But I don't think its a fuel problem anymore. After thinking about it, if lacking fuel was causing the mis, it wouldn't pull normally at high rpms, when the fuel demand is higher than lower rpms. It would mis throughout the rpm range. Mine mostly does it at the lower rpms. I guess I gotta wait it out till I can get the car on the dyno, make the problem happen, and monitor every possible sensor on the car.

I agree with you that it's not a lack of fuel issue, that would be more noticeable at high load/high rpm...

I would be looking more at maf contamination or a week COP

Sounds like you have access to scan equipment, what are you using?

Kappa2006 · Dec 16, 2006

I've been using a snap on scan tool. Its been pretty helpful in checked sensors, but it won't let you monitor the ignition system, or misfire recognition. Basically useful for looking at PIDS. Anyways...

I hooked it up to an oscilliscope one more time and found some strange readings at my ignition coils. I saw a few of them spiking inconsistently. So I went out to Flood Ford and picked up three coils for 180 bucks. I took my best guess at which cylinders to install them in, and got lucky. The misfire is completely gone. So after all of this, it was weak cop's.

All in all, never go by the resistance values in your coils to determine whether they are bad or not. All of my readings were within specs, and it still turned out to be the coils.

Well at least I know every other component in my vehicle is working perectly haha.

Thanks for all of your help guys. Hopefully this thread can help other people out now. and now I can finally start modifying the car!

Brian

Brute03 · Dec 20, 2006

this thread helps a ton man, i've been having the same problems and i did the new-coil-swap also and got no results but i didn't consider it could be multiple coils. oh, and all my coils were supposedly in spec so i guess i'll check e-bay for a cheap set

Mike97gt · Dec 21, 2006

Kappa2006 said:
I've been using a snap on scan tool. Its been pretty helpful in checked sensors, but it won't let you monitor the ignition system, or misfire recognition. Basically useful for looking at PIDS. Anyways...

I hooked it up to an oscilliscope one more time and found some strange readings at my ignition coils. I saw a few of them spiking inconsistently. So I went out to Flood Ford and picked up three coils for 180 bucks. I took my best guess at which cylinders to install them in, and got lucky. The misfire is completely gone. So after all of this, it was weak cop's.

All in all, never go by the resistance values in your coils to determine whether they are bad or not. All of my readings were within specs, and it still turned out to be the coils.

Well at least I know every other component in my vehicle is working perectly haha.

Thanks for all of your help guys. Hopefully this thread can help other people out now. and now I can finally start modifying the car!

Brian

GOOD JOB, glad you fixed it!! thanks for posting what you found after you fixed it. These threads are a lot more helpfull that way.

ab 281 gt · Dec 21, 2006

Told ya!! These ignition coils are a pain in the ass...

99 GT still missing......

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