Trying to help a friend.. Your Thoughts. (Misfires after tune up)

[TurboLS1Camaro]

I'm trying to help a friend out on his 2001 Buick Century. He had a mobile mechanic come do plugs, wires, changed filter and fluid on the trans. Car has a little over 80k on the clock.

After doing the work the car started missing at load only, not off idle. The service tech told Tony he needed a new computer because it was not feeding info back Well A-tap gives me a ton of info back and I think the tech was just trying to make a few more bucks..


I have hooked up my laptop and found the car missing across most cyl's (Using A-tap).

I have re-routed the wires to make sure they were not arking with no luck. They all also seem to be set well on the plugs and on the coils.

I double checked the firing order against some posts here and everything looks ok (firewall being odd's 1,3,5 going back and radiator side 2,4,6 going back). Everything lines up based on the numbering on the coil packs.

No SES light not DTC's either.

I'm ruling out injectors because the car ran perfect before the work was done on it. I'll start checking things like AIT sensor, MAF Sensor and some other things but I really don't think those are casing the miss.



Any thoughts?

[TurboLS1Camaro]

Few more things to add.

The car was sitting for almost 1 month with less than 1/4 tank of gas. We went and filled it up with 91 octane (Best we can get at the pump in CA) and still has issues..

Notice the car is pulling 8-9* timing at WOT (KR) so I really think it has something to do with a loose sensor or something like that..

I know with my 01 Camaro I could pull a couple fuses with the key in the on position for 15 minutes to reset the PCM. Is there an option for that on this car?

MAF, AIT, Coil pack, O2 and all other sensors that I could see are all plugged in solidly..

The car really wants to bog at WOT then it will kick in but again with a lot of KR.

Any suggestions would be great.

[Flatrater]

Turbo

First of all these mobile mechanics are hacks, they come take your oney and are gone.

The car had no miss before his attempt of a tune-up and now has a miss. Kinda easy to figure out what is wrong.

A miss under load is usually an ignition miss. Take off the wire at the coil and start the car up, if you have spark the coil is fine. Next remove the wire from the plug, get the boot close to the engine and start it up. If you see spark the wire is fine. That would then leave the spark plugs. Is it possible he put junk plugs on the car?

[BNaylor]

Originally Posted by Flatrater

Turbo

First of all these mobile mechanics are hacks, they come take your oney and are gone.

The car had no miss before his attempt of a tune-up and now has a miss. Kinda easy to figure out what is wrong.

A miss under load is usually an ignition miss. Take off the wire at the coil and start the car up, if you have spark the coil is fine. Next remove the wire from the plug, get the boot close to the engine and start it up. If you see spark the wire is fine. That would then leave the spark plugs. Is it possible he put junk plugs on the car?

I agree with Flatrater. But I wonder what brand wire and sparks plugs the mobile mechanic put in. From my experience its best to stay with the OEM GM Packard wires and AC Delco Rapidfire platinum plugs, IMO.

[TurboLS1Camaro]

Good thing is he has an extra vehicle to drive around..

I was going to see if I can pull the plugs and wires on Wed and see if that fixes the problem. I kinda figured going that direction since everything else checked out and the mosses were across all cyl's.

Thanks for confirming my thoughts.

[TurboLS1Camaro]

Ok.. More info on this..

Changed the plug wires back to the originals. Same issue. Put the original plugs back in.. Same issue. Put the new wires on the original plugs, same issue.

I am assuming GM has this set up kind of like my 2001 Camaro in regards to timing maps and fuel cells..

At cold fire up, it should be in open loop right? I think my Camaro is in open loop under 145* F. Once above 145* it goes in to closed loop and wont enter open loop until at WOT.

With the Century, at cold, open loop it runs great. No miss and no bog. As soon as it gets warm and goes in to closed loop it starts to miss under load. Perhaps it is seeing some false knock and slipping into limp mode?? Again, it is yanking a ton of timing (9* at WOT). It should be going from closed loop to open loop over 70% throttle if like my Camaro.

Any more suggestions? What could be causing this false KR if that's what it is..

I can log with A-tap and post it in to an excel spread sheet if any of you would like to view it. Give me the perameters you would like to see and I'll post a link to it. I would assume you would want to see:

KR
Timing
TPS%
O2
RPM
Gear
MAF
AIT

Anything else?

[BNaylor]

Originally Posted by TurboLS1Camaro

Ok.. More info on this..

Changed the plug wires back to the originals. Same issue. Put the original plugs back in.. Same issue. Put the new wires on the original plugs, same issue.

I am assuming GM has this set up kind of like my 2001 Camaro in regards to timing maps and fuel cells..

At cold fire up, it should be in open loop right? I think my Camaro is in open loop under 145* F. Once above 145* it goes in to closed loop and wont enter open loop until at WOT.

With the Century, at cold, open loop it runs great. No miss and no bog. As soon as it gets warm and goes in to closed loop it starts to miss under load. Perhaps it is seeing some false knock and slipping into limp mode?? Again, it is yanking a ton of timing (9* at WOT). It should be going from closed loop to open loop over 70% throttle if like my Camaro.

Any more suggestions? What could be causing this false KR if that's what it is..

I can log with A-tap and post it in to an excel spread sheet if any of you would like to view it. Give me the perameters you would like to see and I'll post a link to it. I would assume you would want to see:

KR
Timing
TPS%
O2
RPM
Gear
MAF
AIT

Anything else?

I agree with your analysis of open and closed loop ECU operation. Approx. at what rpm does the miss start? In closed loop FI system is dependent on 02 sensors primarily the one before the CAT. Is it possible the mechanic damaged the 02 sensor when changing the aft plugs and wires?

How much load is on the engine when the miss starts? For example how close to WOT. Did you check the MAF sensor. Is it clean? Have you tried disconnecting it to see if there is any difference for troubleshooting purposes. Hows the fuel system pressure at the rails? Other possibility ICM/coil packs.

[TurboLS1Camaro]

In regards to some of you suggestions and questions..

The miss is at any RPM with load on the motor. Off idle from a stop or at 3500 rpm.. Doesn't matter.

It seems to miss with anything above TPS above 30-35%. It also does not want to kick down from 2nd or 3rd for a good 4-5 seconds at WOT. At cruise (15% or so TPS), coasting or idle, there is 0 miss and 0 misfires.

In regards to the O2 sensor. I double checked the wires thinking it may have a pinched wire from the plug change. I'm not sure thats it.. I see .550mv at idle and closer to WOT I see .85-.92 ish. That was about right on my Camaro. I would always tune for about .89-.91 on te Camaro(12.7-13.0 on the dyno with a wideband). So I am seeing a good reading here.

I did not pull the MAF out but, I disconnected the MAF and I could see the 0 reading via A-tap. I also disconnected the AIT sensor and TPS sensor. All of them give me no reading when disconnected. So I know the computer is seeing them. I am hesitant to think it is a dirty MAF as before the "tune up" (If you want to call it that) it ran great. He only did the tune up as a preventative matter. Nothing was wrong.

I have not checked the fuel pressure at the rails, but I am really leaning away from fuel as the problem. Again, I am seeing misfires on all cyls. I loged my last around the block (really like a 1 mile ride) I had over 2k misfires on cyl 1 and well over 750 on all the rest. I think 5 had like 2k as well. However, this time I saw 0 KR..

Coils packs go bad sure, but all 6 at once.. Man that would suck

Again, this miss does not show me a SES light r a DTC. I know my Camaro would get a flashing SES light for misses.

Please keep the suggestions and Ideas coming.


Shaun

[TurboLS1Camaro]

Oh yeah.. One more thing. When the tech filled the trans up with fluid, it was 4 1/2 quarts low on fluid for the test ride that intiated the miss. Could it have caused something to go haywire and be causing this miss?

[BNaylor]

Originally Posted by TurboLS1Camaro

Oh yeah.. One more thing. When the tech filled the trans up with fluid, it was 4 1/2 quarts low on fluid for the test ride that intiated the miss. Could it have caused something to go haywire and be causing this miss?

Its possible, you never know but highly unlikely on the tranny issue.
There is over 80K on the Century, right? One last suggestion, hows the flow out of the exhaust. Is it consistent with engine rpms?

I worked on a friend's '01 Chevy Impala LS a few months ago with a similar issue. Missing was detected on all cylinders and the 4T65E tranny started acting up. Shifting weirdly especially under load or going up a hill.
No DTCs or SES light.

We found out that the CAT convertor was clogged once the engine was fully warmed up. Evidently it was still letting exhaust flow when cold.

After it was changed out everything went back to normal.

[bignoisey]

Since it sat for a long time you could have bad gas in the tank or water has contaminated the gas. Even though you filled it with Hi-octane, the water would still go to the bottom of the tank. You might try a gas additive that absorbs water. May not help, but won't hurt. And is a cheap thing to try.

[BNaylor]

Originally Posted by bignoisey

Since it sat for a long time you could have bad gas in the tank or water has contaminated the gas. Even though you filled it with Hi-octane, the water would still go to the bottom of the tank. You might try a gas additive that absorbs water. May not help, but won't hurt. And is a cheap thing to try.

Good suggestion. I agree. Stuff called "Heet" works. Won't hurt to try it along with possibly a new fuel filter.