Hi all I'm a Monte owner trying to help out a good friend who is also a fellow 5th gen Monte owner. He has a 1999 3.8l z34 and he said when he hits the gas just a little harder than normal that the car sputters and has no power. He has taken it to the shop and they are at a loss. He's checked the throttle position sensor, the crankshaft position sensor, installed new spark plugs, checked the MAF sensor and also the oil filter for good measure. I know I've seen this same issue on here before so I'd be super grateful for any help I could get. He's thinking of selling the Monte for a Subaru and I'm trying to keep him on the right side as he really does love his monte except for this issue.

 

Sounds like a possible clogged fuel filter...
Maybe the catalytic converter is plugged.
Bad gas could also be the culprit! Try some fuel treatment... (Gummed up clogged injectors)
Could also possibly be a the fuel pump is failing... (How many miles is on it)

How did you test the Maf?

 

Hey Stump not sure how he tested the MAF so I'll ask him that. Also his monte is very low mileage. Only around 41,000 miles.

 

You can disconnect the MAF and it will throw a code, but the computer works around it. That's a simple test to see if the MAF is the problem.

My money is on a catalytic converter. Have that tested (any exhaust shop should be able to assist.

If the MAF is not it and the cat is not it, based on what's been done so far, my next step would inspect the O2 sensor on the rear manifold (the upstream sensor).

 

Unplugging the MAF is not going to tell you if it is bad or not.
Loose/Dirty connectors are a common cause for a MAF to malfunction that will not be detected if you unplug the MAF sensor. I don't understand why people keep repeating this on the forum. When you unplug it the signal loss from the MAF will put the PCM into open loop mode. The engine is now running on default parameters so it may appear to perform better, but that does not mean that the MAF is faulty.

The first thing you need to do is check the wiring harness and connector for continuity. If the harness checks out okay then you need to connect a multimeter to the connector and observe the frequency reading while accelerating/decelerating. It should be a rapid and smooth rise/fall transition throughout the rpm range. If not, remove and clean the MAF sensor. Test again and if the readings are still erratic then replace the MAF sensor.

Refer to a repair manual or the 'net for proper testing procedures.

 

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When was the last time he changed his PCV, the Fuel filter could cause this too. It and the Cat will not through any codes on their own. His Sensors should But I've seen cases where a "Going Bad" sensor is not bad enough ti through a code. You didn't mention his air filter? Or TPS? Hope this helps!

 

I can say I've seen first hand where a 3800 kept stalling daily. Unplugging the MAF, as a quick road side test and the car appeared to run fine. Attempted to clean the MAF with MAF cleaner, no change. Replaced the MAF and the car never stalled again.

Disconnecting the MAF is not a 100%, but it's a simple "test" to see if the problem goes away, isolating to something related to what the MAF is doing with the PCM. It the MAF harness had a broken connection, a CEL code would light immediately (unless the break was intermittent).
Many problems resolved by cleaning or replacing the MAF tend to not throw a CEL code.

Even though GM has other troubleshooting steps to check for MAF problems, disconnecting it is a quick and easy way to isolate if the problem is in that location. The amount of times disconnecting the MAF suggested a MAF problem and MAF replacement resolved it is far more common then the other way around.

That is why it's recommended a lot with these cars.

 

Thanks for all the great info. I'll pass along the advice.

 

The thing is you have no idea what actually resolved the problem. You assume replacing the MAF did, but you cannot say that with 100% certainty. When you unplugged the MAF then plugged the new one in you may have resolved the problem simply by making good contact at the connector or bending a wire so enough of the strands allowed for proper signal transfer. How many times on this forum have people followed the advice to 'unplug the MAF and if the car runs better replace it' only to follow up with a post stating the problem has returned shortly thereafter? Chances are there was nothing wrong with the MAF to begin with. The car ran fine because a bad connection was 'fixed' when they manipulated the connector/wiring.

GM recommends the procedure I posted because diagnostics follow a logical flow chart that narrows down and pinpoints the problem through a process of elimination so it is corrected the first time the car is in the shop.

Yes, it only takes a minute to unplug the MAF and see if the car runs better. But, even if the MAF is functioning properly you are still going to see a difference in the way the car runs because you have forced the PCM to operate in open loop mode. So in reality this 'test procedure' has done absolutely nothing to help pinpoint the problem. The test procedure I suggested takes all of 10 minutes to carry out. Personally, I'd rather spend 10 minutes of my time to verify the circuit is in good condition then to guess and spend hundreds of dollars hoping it fixes the problem.

 

I agree, that by doing the unplug the MAF trick has a gamble to it. Most of the time that method is recommended, the person complains that the problem occurs on a frequent/daily basis. If you unplug it, and it goes away, but you plug it back in and the problems re-surface, you are at least narrowing down to something related to the MAF. But if the wire on the MAF harness is broken inside and not allowing the MAF to communicate to the PCM, the PCM will throw a code for that. But I don't know how the PCM responds if the wire intermittently makes/breaks connection within less then a second.
If the person attempts cleaning the MAF and no success, then decides to replace the MAF and the problem goes away, then the gamble worked (I agree, what I just said is a gamble).

This reminded me of a funny moment troubleshooting a wheel speed sensor on my '94 Grand Am. Following GM's chart to test the wiring, when running through the steps a 2nd time, I found a broken wire (barely held on by the sheathing). What makes it funny was as working on that harness, when prepping to do the diag step, I originally, unknowingly was re-connecting the broken connection. Probably a rare situation, but once found was funny at the time.

Open loop or closed loop, if there are other non-MAF related problems (such as plug, injector, ICM), most these items should still cause there problem symptoms with or without a MAF.

As for how many forum members come back having the same symptoms again shortly after replacing the MAF and performing the MAF unplug, few if any from what I recall. BUT, we have had members who in their first post about their problem listed things they changed and the MAF was one of the things they changed. No idea if they tried the unplug trick (I am certain they did not do the GM diagnostic chart).

Again, I will not dispute, the GM diagnostic steps you posted are going to be the definitive answer where as unplugging the MAF trick has a level of gamble to it.
One thing I would be curious of (and I lack the resources for this case study) is to compare GM diags to unplugging the MAF/reconnecting the MAF. Gather comparison data to determine how often the unplug trick lines up to the GM diagnostics vs how often it does not.

Again, I do support the GM diagnostics as being the best method, they run through a lot of good and logical steps (all for a reason). Even with the MAF, those are superior steps. But, the unplugging the MAF method usually gets people into the ball park of if it's something tied to/around the MAF (wiring or the MAF itself).