WOT Misfire Causing CEL in '99 2.5

[McMadCow]

Hi everyone, I've been reading the RT Forums for a while but now I'm finally registered in order to try and track down an issue I've been having.  I've searched for related issues through the forums already, but nothing is showing up which is a match for mine.  

 

'99 2.5, 5 Speed, 99K Miles

Only upgrades are headers and catback.

 

I've owned the car since March of this year and it's been great so far.  I've even taken it to the local PCA Autocross.

 

A couple of weeks ago I was cruising on the freeway when my CEL came on steady, no blink.  There was no noise or indication to me that anything had gone wrong, the car kept running completely as usual.  When I got it home I plugged in my code reader and got a cylinder 5 misfire.  I had been planning on a spark plug job anyway, so I got new spark plug tubes the next day as well as 6 plugs and did the job.  I reset the CEL for the drive to my garage space, but on the way there I got a real misfire under heavy acceleration that felt and sounded like the engine was really struggling.  It did go away after a few seconds, though.  

Whomever was the last person/garage to do the plugs on the car was a complete idiot and should never touch a car again.  None of the spark plugs were torqued, and were all finger tight.  The cylinder 5 plug was covered in dirty oil and had BOTH of its electrodes snapped off.  God only knows where they ended up, but it didn't seem to be in the engine, so that's good I guess.  Once I replaced all the the plug tubes and actually torqued down the plugs themselves, the engine seemed to fire up fine, plus I was no longer getting a ticking on idle that I thought might be a valve issue.  I guess things sound a lot better when the spark plugs are tightened.

 

Since I thought that cleared things up, I drove it home, and on the way found space to put my foot down.  As soon as the pedal reached the floor, the misfiring started again, with the engine again getting super rough and struggling.  Once I got home I was able to recreate the issue in my driveway by just pinning the throttle and letting the misfires happen again.  Just to be sure the computer didn't still have some wonky settings in it, I disconnected the battery for 30 minutes to let everything reset.  

Since then I've been pretty scared to go WOT, but a few days ago I did and the exact same thing happened again.  That code indicated cylinders 2 and 3.  Last night I did it again and the code showed cylinder 2.  Unfortunately, the miss didn't go away immediately like it usually does, so it followed me home.  Still, it went back to normal as soon as I turned off the engine and restarted.

 

Long story short:  I'm getting a rough misfire when I go to full throttle, happens pretty much the instant the pedal hits the floor.  Usually goes away within a few seconds.  No other symptoms, idles and run completely as normal at all other times.  No CEL if I don't trigger the misfire with the gas pedal.  

 

Any ideas?

 

 

EDIT: Also, it doesn't appear to be a symptom of revs.  The miss starts when the pedal goes to WOT, not when the revs climb past a certain mark.  I've revved it quite high at partial throttle with no issue.  My code reader says the miss last night started at 3800 rpm, but full throttle.

Edited August 21, 2015 by McMadCow

[JFP in PA]

Hi everyone, I've been reading the RT Forums for a while but now I'm finally registered in order to try and track down an issue I've been having.  I've searched for related issues through the forums already, but nothing is showing up which is a match for mine.  

 

'99 2.5, 5 Speed, 99K Miles

Only upgrades are headers and catback.

 

I've owned the car since March of this year and it's been great so far.  I've even taken it to the local PCA Autocross.

 

A couple of weeks ago I was cruising on the freeway when my CEL came on steady, no blink.  There was no noise or indication to me that anything had gone wrong, the car kept running completely as usual.  When I got it home I plugged in my code reader and got a cylinder 5 misfire.  I had been planning on a spark plug job anyway, so I got new spark plug tubes the next day as well as 6 plugs and did the job.  I reset the CEL for the drive to my garage space, but on the way there I got a real misfire under heavy acceleration that felt and sounded like the engine was really struggling.  It did go away after a few seconds, though.  

Whomever was the last person/garage to do the plugs on the car was a complete idiot and should never touch a car again.  None of the spark plugs were torqued, and were all finger tight.  The cylinder 5 plug was covered in dirty oil and had BOTH of its electrodes snapped off.  God only knows where they ended up, but it didn't seem to be in the engine, so that's good I guess.  Once I replaced all the the plug tubes and actually torqued down the plugs themselves, the engine seemed to fire up fine, plus I was no longer getting a ticking on idle that I thought might be a valve issue.  I guess things sound a lot better when the spark plugs are tightened.

 

Since I thought that cleared things up, I drove it home, and on the way found space to put my foot down.  As soon as the pedal reached the floor, the misfiring started again, with the engine again getting super rough and struggling.  Once I got home I was able to recreate the issue in my driveway by just pinning the throttle and letting the misfires happen again.  Just to be sure the computer didn't still have some wonky settings in it, I disconnected the battery for 30 minutes to let everything reset.  

Since then I've been pretty scared to go WOT, but a few days ago I did and the exact same thing happened again.  That code indicated cylinders 2 and 3.  Last night I did it again and the code showed cylinder 2.  Unfortunately, the miss didn't go away immediately like it usually does, so it followed me home.  Still, it went back to normal as soon as I turned off the engine and restarted.

 

Long story short:  I'm getting a rough misfire when I go to full throttle, happens pretty much the instant the pedal hits the floor.  Usually goes away within a few seconds.  No other symptoms, idles and run completely as normal at all other times.  No CEL if I don't trigger the misfire with the gas pedal.  

 

Any ideas?

 

 

 

Welcome to RennTech :welcome:

 

OK, let's start with a couple of "obvious" places to start:  How old are the coil packs and have you checked them all to make sure the connectors are properly seated?  If the shop that worked on the car was dense enough to not torque the plugs, you can pretty much bet they did not properly check the coil connections.  Look at the coil packs and check them for signs of cracking, while completely normal, it a sign they are getting old and need to be replaced.

 

If the car was in my shop, I would be bore scoping the cylinder where the plug electrodes were missing as they had to go somewhere.  Same thing applies to the current cylinders that are misfiring.  I would also be running a compression test at a minimum, preferably a leak down on the same cylinders, just to make sure these is nothing physically wrong with them.  I would also be checking the fuel pressure and delivery rate, just to be sure that the system is not leaning out at higher revs.  When was the last time the fuel filter was changed? 

[McMadCow]

Hey, thanks for the fast reply!  

I don't know if the coil packs have ever been changed, but I'll look through the giant pile of receipts I have from the PO.  When we did the job it didn't look as though they were.  I also went back the next day to make sure everything was connected properly and remained torqued down.  I'm planning on going into the garage today, I'll check and see if any of the coil packs- especially on the passenger side bank- are cracked.  I don't have a compression tester in my shop... maybe I need to find one.

 

I changed the fuel filter in April, probably about 6K miles ago.  

 

My crank and cam sensors should be giving error codes if they were at fault, no?  The problem seems to be somehow related to throttle position, as I've had it almost to redline on partial throttle, but the miss happens anywhere in the rev range the instant the pedal maxes out.

[JFP in PA]

My crank and cam sensors should be giving error codes if they were at fault, no?  The problem seems to be somehow related to throttle position, as I've had it almost to redline on partial throttle, but the miss happens anywhere in the rev range the instant the pedal maxes out.

 

While the crank position sensor might not code, the car would not even start if it was bad.  The cam sensors would throw codes.

 

I would still suggest checking fuel pressure and delivery just to be on the safe side.

 

If you could get access to the Durametric software on a laptop, you could record real time data while actually driving the car, which could throw some more light on what exactly is happening when the car starts to miss, including the fuel trims.

[McMadCow]

My Bosch Mobile Scan software on my tablet shows the real-time fuel info as the engine is running.  Is there something I should be looking for?  Should I be noting differences before and after a misfire occurs when I floor the throttle?

[JFP in PA]

My Bosch Mobile Scan software on my tablet shows the real-time fuel info as the engine is running.  Is there something I should be looking for?  Should I be noting differences before and after a misfire occurs when I floor the throttle?

 

If the system can record fuel trim data, try making a pass in the car with it recording and then look to see if the fuel trims drop off lean when the misfire occurs.  if it does, the car is basically running out of gas at high revs, which would probably be a fuel delivery issue (pump, pressure regulator).

[McMadCow]

Ok, I'll check it and report back, thanks!

 

Also, just to be sure... the misses have never occurred at high revs, just at open throttle.  I've had the engine revved pretty high at partial throttle with no issue.  The misses have always happened when I stomp the pedal all the way down, before even having a chance to rev high.  Are we still talking about the same thing, fuel-flow-wise?

[JFP in PA]

Ok, I'll check it and report back, thanks!

 

Also, just to be sure... the misses have never occurred at high revs, just at open throttle.  I've had the engine revved pretty high at partial throttle with no issue.  The misses have always happened when I stomp the pedal all the way down, before even having a chance to rev high.  Are we still talking about the same thing, fuel-flow-wise?

 

Yes, at WOT the engine is inhaling all the air it can get, and the fuel system has to deliver enough fuel to keep it happy, if it does not, the engine leans out and starts to miss.  I would also look at the MAF values both at idle, part throttle and WOT.

[McMadCow]

Thanks, I'll report back on all of that.  I'll be doing this in my driveway in neutral since I don't want to take it to WOT on the road while looking at DME numbers.  Is it safe to let it sit there and bang off the limiter while going WOT and letting the info register on my tablet?

[JFP in PA]

Thanks, I'll report back on all of that.  I'll be doing this in my driveway in neutral since I don't want to take it to WOT on the road while looking at DME numbers.  Is it safe to let it sit there and bang off the limiter while going WOT and letting the info register on my tablet?

 

Sorry, but I would never do this in that manner, bouncing an engine off the rev limiter while standing still is a recipe for disaster.  You need a system that logs (read records) the data over time, and then drive the car and collect the data in a "real world" (read engine under load, not free wheeling) manner; then you take the car out for a brief run, let it record the data, and review it afterwards.  With a decent system, like the Durametric, you can collect data from multiple system simultaneously so you see the systems interacting.  

[McMadCow]

Ok, my software isn't Durametric, but it still does pretty extensive data logging.  I'll let it run for a couple of days and see what it has to say.  I'll report back when I have some good varied data.  I'll even try to make it miss out on the road by being a leadfooted nutjob.  

[pkust]

I had a coil pack failure and feather footed the car home changed them all as they were all original and eighteen years old. Knew what the failure was as I had it happen to me in my wife Audi A4, lost two at once, factory recall. Now my '97 runs like new, what a difference. If you have your original packs, bite the bullet and just change them all at once, you will be glad you did.