I ahve this posted all over the net so whats one more site



it's a 01 firebird with a 3.8 Totaly stock when purchesed, He notices soon after getting the car that he had what he called a "miss" after talking about it I desided to take a look for him over the weekends and get it fixed. Since he still had a warrenty he took it to teh dealer first. They condemed 4 bad plug wires. replaced them. No help. went back, they said one coil, replaced that and no help. Then hey changed the PCM (gota love dealers) no help. So he got sick of that and let me take a look.


after drivng the car and feeling what it was doing right away it didn;t feel like a miss to me. it felt like someone had a switch to cut off the motor and right about at 4,000 RPM while acelerating would turn the motor off for a split second and turn it back on. I would more like to call it a "surge".

My first thoughts were fuel pressure. checked fine. inspected the vacuum and ran a test based on MAP BARO and MAP KPA and came up with 20 inches of vacuum so we were good there. Check all of teh computer systems. Everything nominal. I tied into the injector control with a graphing meter to look for a drop out while driving, nothing.

The more that I heard it the more it didn;t make sence. I then was listening closly and figure something out. everytime it "surged" I heard something. it was an audible "ping" My thoughts then went to detonation. So I looked at the knock retard. while driving I randomly got 2-4 degrees of KR. when we felt the surge I could see anywhere from 6-14 degrees of KR. I knew we had something.

I have tried everything that I could think of so far. everything looks good. WE played with the knock sensors trying to emiminate a faulty one. to no avail. it can not be false knock since you can hear it ping. he also to emiminate false KR unhooked both knock sensors (bad idea I know) and still it did the same.

he recently changed plugs and wires and ran 93 octane. still no help.



This weekend my plans are to run a back pressure test to ruel out a converter going bad. The catylist is posibly shifting and causeing a blackage.

I will also be doing a good top engine clean to try and eliminate any carbon biuldup that could be causing this. But I would still welcome some more sugestions.

It's defenitly connnected to combustion chamber temps. it doesn;t really do it when the motor is cold. we even installed a 160 thermo and a fan switch to try and keep things cool.

 

anyone wanna take a stab at it?

 

Yes do a back pressure check. That is one place I would have looked first.
What do the plugs look like?
Make sure the EGR isn't open at wide open throttle.
I bet the 02's are ok, but you never know, can you plug a o2 meter in there some where?
Is it burning oil? oil on the combustion chamber can cause detonation. but you would see it on the plugs it would have to get in from the rings or maybe the intake valve seal.
Is the heat range on the plugs right?
Pull plugs and look in the chamber and see what the piston looks like, see if there is tons of carbon on top of the piston there will be a lot more on the chamber walls.
If you are running lean it could have burned the exhaust valves and it could be sucking in contamination but it would have to be real bad burned valves, like real low compression and bad leak down test.
Do an EGT probe if you can on the cylinders or banks if you can.

 

did you try what was suggested in the stickied thread in this forum?


http://web.camaross.com/forums/showthread.php?t=78028

 

It's not a missfire. It's a sevear detonation. Not involving the ign system other then a possible ICM which has not been totaly rueld out yet.

 

anyone else?

 

update.

so far the detonation is gone.

My theory.

ever since I did the top engine clean it ran like crap worse and worse till it was just plain bad. Then one day like magic.. poof. My best guess is that the cleaner broke up some carbon that took a few days to finaly clear out. The carbon was getting super heated and igniting the gases in the combistion chamber prematurely.

Thats my best guess. The other one is bad contact on the 02 sensor that I pulled to perform the back presure test but I doubt it. We also swaped 2 of the 3 coils with known good ones. And after all that. it;s perfect for now. I doubt it was the coil since if it was an actual missfire the crank sensor would have picked it up, set a miss code and loged a missfire history for that cylinder or cylinders


So far for 4 days now the car has ran great. That has not happened since the car started having this issue. Now the only thing that is left is figureing out the open loop issue that we are having which I posted about in depth here

http://web.camaross.com/forums/showthread.php?t=298359


He is going back to the track tonight. It;s a little hot today but I think that now we can get some good testing in. He is testing a new mod that I have made and before with the "miss" his best was a 15.6 . without the mod his best was a 16.3. I think that now that the "miss" is gone he should be sub 16's stock and with my mod well under 15.6

If anyone wants to chime in on the open loop isue from the link above pease do so even though I ahve pretty much already desided on the course of action I will have him take.