Hey everyone. Thank you in advance for any help you can offer.

So, I have a 98 ls1 trans am with just under 100k miles. When I start the car, it seems to run fine until I drive it...

I have a substantial loss of power and after a few minutes the exhaust note will COMPLETELY change (it sounds like an old truck). If I put it to the floor, it will down shift and I'll get power back for a moment before it goes back into the previously mentioned state...

I already tried a new mass airflow sensor and it didn't help at all. I've checked for vacuum leaks, but I can't seem to find one (unless I'm just not checking hard enough).

Can anyone give me any advice? I've heard a couple people suggest it's a PCM issue? I'm stumped...

Thank you again for any responses

 

Is it throwing any codes?

 

o2 sensors faulty possibly. you need to read your fault codes

 

You didn't mention if you had a service engine light, but if so, as stated above, you need to start by pulling the codes and go from there.

The car is 20 years old with 100k. Are the plugs and wires still original?

 

Besides what's posted above it could also be a partially plugged catalytic converter.

 

I would not suspect any "PCM issue" based on what you've posted so far.

As others have asked: are you getting an SES light? If so, I would start with scanning that and then decide on a course of action based on the code.

Assuming there is no SES light (since you didn't mention one), and since you've already replaced the MAF, then I would check fuel pressure (under load) next.

 

+1 @LLLosingit Posted ,"Besides what's posted above it could also be a partially plugged catalytic converter."
If you have a live data stream scanner available you could look at the mass air flow data and see if the normal amount of air is flowing though the engine at specific throttle positions. If flow is low at WOT could w be clogged cat.

 

Sorry for the late response and thank you for the replies. Had to get a new key for the car (long story). Plugs and wires are new. I'll check the code as soon as possible. The previous owner also installed an air fuel Gauge. Maybe the wire has been compromised?

 

Update! I'm getting a mass airflow low output code. I changed the mass airflow sensor a while ago when I first had this issue and it didn't help. Did I maybbget a bad one? Could o2 sensors cause this problem?

 

That's what it sounds like to me

"changed exhaust note" also indicates that

You can take the oxygen sensors out before the cat and if it revs fine it's the cats. Usually with plugged cats it wont even rev up well while parked.

 

I should have mentioned, I don't have cats.

 

The very 1st thing you should do is replace the fuel filter then test the fuel pump under driving conditions. Then if all that checks out ok move forward from there.

 

How do I test the fuel pump? I have an obd2 Bluetooth scanner, if 5hat helps

Thanks for the reply.

 

Have you tried unplugging the MAF (forcing SD mode) to see if it acts any different/better?

Has it ever triggered any O2 codes, or just the one for the MAF?

I had mentioned this as something to check in my reply two months ago. You'll want to check fuel pressure under load for most accurate information (sometimes they show fine pressure at idle or free rev but indicate unsatisfactory pressure in a loaded state). A fuel pressure gauge connected to the fuel rail and taped to the windshield is a common temporary test method for this.