Hi all, I have a 2003 3.4 liter Monte, this morning on my way to work a new problem developed. The symptoms developed like this: the car behaved perfectly for the first half of the drive, I stopped at a red light and the engine randomly stumbled (misfire?) then returned to normal. It bugged me a bit but the car pulled away from the light normally and I thought nothing of it. As I pulled onto a highway onramp I floored it and dropped to second gear, while the car was shifting to second there was a rapid fire of pops from the exhaust but as soon as the trans fully engaged second gear the pops were gone and the car accelerated normally. this happened a couple more times on my trip as I accelerated to pass slower traffic.

At work I hooked up my scanner and noticed a few random misfires but nothing that worried me. If I slowly rev the engine to 2k while in park everything behaves normally, and letting off the throttle results in a couple pops (as has been normal with my car for 45k miles now). However, rapid application of throttle with the car in park results in the engine stumbling as it accelerates (accompanied by the exhaust burbling and popping) then a few normal pops as the engine returns to idle.

Potentially important details about the car:
head gaskets were replaced at 96k and I'm at about 100k now
(head gasket job was very through: decked heads, new spark plugs, wires, coils, upper/lower IM gaskets, fuel injectors ect.)
filled the tank at my usual station last night
catalytic converter was replaced with a u-bend delete version during the head gasket job
after the converter is a glasspack, and the exhaust is cut right behind the passenger door making the burbles and pops very noticeable.

 

Could possibly be a MAF sensor issue

 

Interesting, I've had a couple friends point towards air issues and a couple have pointed towards fuel issues. Looks like I'll start with a can of maf cleaner, check for vacuum leaks and go from there.

 

When a MAF starts to fail, it could cause stumbling, stalling, running lean or rich... If you can reproduce the issue with the car in park by just revving the engine, unplug the MAF then rev it up and see if it still has the issue.

 

Oh that's neat, if the MAF fails it is basically running on airflow assumptions which would work fine at idle/low rpm but with higher throttle positions it becomes inaccurate to the point of running rough?

 

Eh. I've had a bad MAF make my car stall at idle once fully warmed up, and stall upon letting off the gas while cruising. A buddy of mine had a failing MAF and the only real symptom was running very rich, with black smoke coming from the tailpipe upon hard acceleration. That being said, his had a check engine code for the MAF, and mine did not, but in both cases replacing the MAF resolved the issue. Hopefully yours is something simple.

 

Thanks for the advise! I'll try the trick you mentioned and see if cleaning the maf helps, I'll run codes as well, maybe the bulb has gone out since the last issue lol.

 

Since you have access to a scanner - go out and log the data while recreating the issue. A MAF failure is very obvious on a data log as you'll have a value or two not make any sense at all compared to the data around it- ie light acceleration - 1500, 1590, 1710, 1800, 1860, 0, 0, 1920, 1970 - etc.

I had a failure on my 99 gtp similar to what was mentioned above (stalling). Mine wasn't very frequent, but would occasionally drop to 0. At low rpm / light throttle, it always killed the engine. With more rpm + throttle, it would majorly stumble but recover. They dont always necessarily fail to 0 though as was pointed out above.

In my case cleaning the MAF didn't help. I ran a K&N at the time and cleaned my MAF fairly often (out of worry about an over oiled filter building up on the wires). The failure was somewhere internal on the sensor.


Even if its not a MAF, getting a scan log of the main sensors, timing, fuel trims, O2s, etc should get you to the culprit pretty quickly.

 

The MAF is the primary driver, but the system does have a MAP sensor / VE fueling tables in the case of a complete MAF system failure. The downside is that MAF failures aren't always permanent - its not uncommon for them to have internal connection issues that cause them to fail for a split second, but them resume normal operation.

In that case, the pcm doesn't know its bad and continues to use its data as the primary driver for fueling, and by using inaccurate readings for a few cells - it can cause momentary oddball fueling issues.

That's why it was mentioned that unplugging the MAF is a simple test for this. Because that forces the pcm to use MAP / VE mode solely (so if it runs ok like that, then the MAF circuit is the confirmed culprit).

 

I unplugged the MAF and the issue persisted, does that mean the MAP sensor is bad instead then?



Does this help at all?


Unplugging the MAF and or charge temperature sensor doesn't change the way the vehicle runs, after I messed with both of them I got two codes: airflow or charge sensor low voltage (duh I was unplugging them) and a cylinder 6 misfire. I'm getting ready to pull the spark plug right now.