Miss/exhaust backfire

O

OrLackThereof

New Member
Mar 9, 2003
560
0
0
Palm Bay, FL
#1
It's been forever and a day since I've been on the boards...forgot that I was even a member. Anyways I've been having persistent issues with this combination since it went together, its an intermittent misfire (all operating states except WOT) combined with random, severe power-loss and exhaust backfiring.

I have swapped/tested so far: Plugs, plugwires, cap/rotor, distributor (complete), computer, ignition coil, 6al box, mass air meter, barometric pressure sensor, TPS sensor, ohmed all injectors, adjusted the valves, checked for vacuum leaks, checked every ground I know of including O2 ground, swapped O2 sensors.... nothing is affecting this misfire.

I have not had the heads off...they were tanked and decked slightly after they came off the old motor, which snapped an intake pushrod at 5k rpms wide-open...machine shop said there was nothing wrong with them so they went back on. Im running a breather, no PCV system, and the heads are non-EGR so EGR is disabled. Motor is very tight and has almost zero blowby and does not smoke out the exhaust.


Just looking for anything I might have missed? I've gone down the list and the only thing I can think of left is maybe a sticking exhaust valve? :shrug:

Thanks for any help guys :nice:
 
#2
are you running a straight breather with no line hooked up the intake or air intake tube? if so that could be part of the problem. you could be getting unmetered air into the engine with the breather setup. does the computer throw any codes besides egr?
 
#3
dragnazz5.0 said:
are you running a straight breather with no line hooked up the intake or air intake tube? if so that could be part of the problem. you could be getting unmetered air into the engine with the breather setup. does the computer throw any codes besides egr?
Click to expand...

Computer is chipped, throws no codes.

Its a breather off of the oil-fill location on the passenger side. The line to the intake, where it used to go to, is capped, and the PCV valve on the back of the manifold is capped.
 
#5
dragnazz5.0 said:
hmm...guess there isnt gonna be unmetered air then. and you said you replaced the tps sensor or just checked it? what did the plugs look like when you pulled them out?
Click to expand...

Yep, TPS is .98 at idle and a steady sweep to like 4.5 or 4.7volts.

Thats the thing, the plugs look fine! Light tan, slight carbon at the base of the plug. Last time I pulled them, #1 was completely fouled, looked like oil...but that's only happened one time, and every time since theyve all come out clean and equal, with slight carbon, which is what I dont understand.
 
#6
that is wierd. i was thinking it could be a collapsed lifter but that would affect all conditions from idle to wot. whats your timing set at? to check for the sticking exhaust valve you would have to do a compression check but i dont think thats your problem. just a few more questions....does it do it all the time unless the pedal is at wot? or does it clear up at the rpms increase? that might help us determine what it is. a mechanical misfire will get better with rpm and primary ignition misfire will get worse with rpm and a secondary ignition misfire will stay the same with rpms
 
#7
dragnazz5.0 said:
that is wierd. i was thinking it could be a collapsed lifter but that would affect all conditions from idle to wot. whats your timing set at? to check for the sticking exhaust valve you would have to do a compression check but i dont think thats your problem. just a few more questions....does it do it all the time unless the pedal is at wot? or does it clear up at the rpms increase? that might help us determine what it is. a mechanical misfire will get better with rpm and primary ignition misfire will get worse with rpm and a secondary ignition misfire will stay the same with rpms
Click to expand...

Timing is 15 degrees base advance.

And it's an intermittent miss. Example, cold leaving the house, it runs fine for 30 seconds, then begins to miss randomly, clear up for a few seconds, start missing again. Wide open throttle it runs fine, all other throttle positions it misses. It only backfires when it bogs real bad. And it gets better with increased rpms, seems to clear up, in most cases, over 3000 rpms but sometimes it carries on up to 4000+.
 
#8
maybe an iat or ect sensor is bad. check the resistance of both of those. i know jrichker has a list of what the resistance should be a different temps. but thats almost what it sounds like
 
#9
dragnazz5.0 said:
maybe an iat or ect sensor is bad. check the resistance of both of those. i know jrichker has a list of what the resistance should be a different temps. but thats almost what it sounds like
Click to expand...

Thanks, I'll try to look up the charts. Im assuming that IAT sensor is the one mounted in the intake, on the driver-side, in one of the runners?

I've tried disconnecting both that one, and the ECT sensor, but driveability didnt change.

By the way I really appreciate your help man. Basically, everybody I talk to says the same thing, "hmm that's weird." I've baffled everyone I know with this car, which is why it's up for sale dirt cheap now.
 
#10
nah, dont sell it. i went through the same thing with my car and ended up switching it to carb and it got rid of the problem and than my buddy tried my maf on his car and found out it was bad. i just wish i could actually see the car and work on it. its really hard to figure out something like that. the only problem with watching the tps on a multimeter is the refresh rate isnt very fast on those things and it could skip over a bad spot. but i guess as long as it doesnt peg out or hit 0 anytime you are sweeping it slowly it should be fine. well if the iat is bad at its telling the computer its -40 degrees outside unplugging it will just send the same signal to the computer so that could be part of the problem. i would just ohm out the ect and the iat and see what kind of resistance you get. as long as its not some rediculous amount of resistance you should be pretty good. im starting to wonder if it has something to do with the chip. is it just a generic chip or is it a custom burned chip for your car. those generic chips bump the timing so if you have the chip in there and than bumped the timing manually it could be way too much timing.
 
#11
dragnazz5.0 said:
nah, dont sell it. i went through the same thing with my car and ended up switching it to carb and it got rid of the problem and than my buddy tried my maf on his car and found out it was bad. i just wish i could actually see the car and work on it. its really hard to figure out something like that. the only problem with watching the tps on a multimeter is the refresh rate isnt very fast on those things and it could skip over a bad spot. but i guess as long as it doesnt peg out or hit 0 anytime you are sweeping it slowly it should be fine. well if the iat is bad at its telling the computer its -40 degrees outside unplugging it will just send the same signal to the computer so that could be part of the problem. i would just ohm out the ect and the iat and see what kind of resistance you get. as long as its not some rediculous amount of resistance you should be pretty good. im starting to wonder if it has something to do with the chip. is it just a generic chip or is it a custom burned chip for your car. those generic chips bump the timing so if you have the chip in there and than bumped the timing manually it could be way too much timing.
Click to expand...

It was chipped on the dyno and I had some input in the tuning. It has 2 degrees of timing added across the entire advance curve, and 10% of the fuel pulled out from idle to 2000 rpms because it was idling pig rich to get to a good mixture up top. A:F ratio was at about 12.9:1 on the dyno and powerband was rock-solid repeatable and smooth, never made a LOT of power but it runs faster than what the dyno says...but for some reason it runs like **** on the street. I'll definitely ohm those out and try to find a chart, thanks again.

And Ive actually been trying to sell the car for a while, I just cant afford it anymore....I need a more reliable economical daily, right now this is my daily and its miserable!
 
#13
dragnazz5.0 said:
well with a custom chip it should be fine....maybe its too lean when the motor is cold. hard to tell. good luck with it
Click to expand...

Thanks...the fact that it doesnt do it for the first 30-60 seconds of startup, but then always afterward, makes me think it's a sensor problem, but I cant think of a sensor that the computer starts reading that quickly, its not even CLOSE to closed-loop operation at that point and probably hasn't risen more than 10-15 degrees coolant temp.
 
#15
dragnazz5.0 said:
does the problem go away once the car is warmed up or is it still there and just not as bad?
Click to expand...

Ya got it backwards, ICE-COLD the problem isnt there at all. After 30-60 secs of driving it starts, and after that it's unaffected by engine temperature and continues to happen until the car sits for a few hours, then same deal....30-60 seconds of clean operation.

This made me think, failing ignition module, but I already tried a second dizzy and this is an aftermarket module, no dice.
 
#17
To be honest, this sort of sounds like how my MAF problem started. It just progressively got worse until I had to replace it. Ice cold was perfect, but then it got warm and would start missing, eventually leading into no power at all, like it was running on 4 of 8 cylinders. Do you have anyone you could borrow a MAF from to rule that out by chance?
 
#20
dragnazz5.0 said:
well he said he had replaced it. maybe the wires to the computer are bad. could be open circuit or something
Click to expand...

Yep replaced it today, to no effect..... Wish I were more of a wiring guru but honestly I suck as an electrician, I hate doing electrical...Im the type of guy that *barely* knows how to use a DVM and used to ground at the battery :nonono:
 
You must log in or register to reply here.

Similar threads

M
Help! 2003 GT misfire when hot
Replies
4
Views
310
SN95 4.6L Mustang Tech
Mustang5L5
Mustang5L5
Creomod
Engine Fuel not pumping
Replies
43
Views
695
Fox 5.0 Mustang Tech
Shakerhood
Shakerhood
S
Fuel ‘88 GT Runs Rich Cold — Fixes Itself When I Unplug Sensors (ECT, MAF, O2, etc.)
Replies
12
Views
711
Fox 5.0 Mustang Tech
Mustang5L5
Mustang5L5
T
Engine Need help figuring out misfire and backfire
Replies
8
Views
268
Fox 5.0 Mustang Tech
Noobz347
Noobz347
R
Misfire at high rpm! Help
Replies
9
Views
880
2015 - 2023 (S550) Mustang -General/Talk
highplace11
H
Top Bottom
Back
Forums
Menu