Vehicle: 1998 Pontiac Firebird Trans Am Convertible LS1
Mods: SLP Lid, Cold Air Scoop, K&N Air Filter
Symptoms: Car is a little sluggish, big loss of power after 3000 rpms; car takes about 5 seconds to start
Engine is throwing 4 codes: P0131, P0137, P0151, P0157
which I believe is saying all four oxygen sensors are reporting that it's running lean

So, before I checked my codes, I thought maybe the tranny was slipping. I took it to a shop and got it checked out and the tranny is just fine, fluid is right and everything.

Since it wasn't the tranny, I narrowed it down to the engine's fuel system. I decided to check the fuel pressure, thinking that may be why it's running lean. The fuel pressure when the car was running was right around 42, and dropped down to around 32 at redline. According to specs, this seems right around normal, maybe a little low but not low enough to cause this much power loss.

So then I was thinking maybe the injectors are clogged. I put some cleaner in it and its still working its way through at the moment. But the thing that confuses me is that sometimes the car will randomly have its power back for like 20 seconds, but this happens rarely. So I doubt the injectors are clogging and unclogging that quick.

So what I'm down to is maybe the fuel filter is clogged or there's a vacuum leak somewhere, maybe the intake manifold. There's no suction noise around the intake manifold so if there was a leak, it probably isn't a big one. Also, it idles just fine. The engine is a little shakey but not much more than normal. The RPMs don't fluctuate at all and it idles around 900 cold and 600 warm. The fuel pump is an ACDelco and its only 2 months old.

I also cleaned the MAF and air filter today, to try to solve the problem, but didn't help.

Anybody have any suggestions on where to go from here or what you may think the problem is?

Thanks to all in advance.

- John

 

Do you have the front O2's in the front and rear O2's in the rear? If you used extensions they can be accidently swapped.

 

Yes, they must be fine. These codes just came up last week and nothing has changed. The car ran great before last week.

 

that fuel pressure seems really low... i run 60 at idle and only fall to around 55 at the very top of my dyno pull. i am on a bone stock fuel system

 

hmm, if so then it could possibly be the regulator. Do you know where it's located?

 

Yeah, fuel pressure should be 58 at idle and 55 at WOT. The regulator is in the tank on the sending unit.

 

i agree mine stock was 56 psi that is very low.

 

So then, what is this part that sits in the middle of the fuel rail?
Note: This isn't mine, i just found it online

 

its not the regulator its kind of a damper in the sytem.the regulator is in the tank

 

Replace your fuel pump for starters... If you're going to do more to the car, you'll need it anyways. Racetronix or Walbro 255.