This car has been great to me, no engine lights, no problems of any kind so far. Today coming home from work the car dumped to 3-400RPM at closed throttle coming to idle/stop for a stop light. (A4 Transmission.) The car nearly shut off, and I threw it into neutral (I had to do this 24/7 in my old Camaro so it was pretty natural) and opened it up just a hair to stop it from stalling out. After putting it back into Drive, the car ran just fine, no noticeable changes in performance anywhere in the throttle position. Every stop the idle was much lower than normal though, probably about 700 instead of 900 that I'm used to seeing, and it kind of bounced around (nothing super noticeable, like 690-710) but didn't stall out.

The only recent modification I've done to my car is remove the muffler and driver-side tailpipe of my Borla catback temporarily until I can afford an electric cutout. I know that changes the back pressure of the car, but can it really throw my idle off that much? I also know my air-filter is a little dirty, so I'm going to be putting an airlid with a filter in it soon... but I didn't want to until I figure out this problem first.

I hooked up the pressure gauge, and it read about 49PSI prime, and about 55PSI idle. Revving it gently made no noticeable change in fuel pressure, although I couldn't drive around with it (the hose on mine isn't long enough.) I thought maybe the fuel filter, which I'll take care of in the next day or two.

The only other thing I can think of, a few weeks ago I hit the brakes too hard in the rain and spun out. I didn't hit anybody but I hopped my wheels up on a sidewalk. There was no noticeable damage, (and I had a shop do a quick check), but sometimes when I drive the car for a while I can hear a whine coming from what sounds like my fuel pump. I've never had a problem starting or driving the car, even with the whine, but it concerns me. Could I have shocked the fuel pump into destruction (or something?) What about the FPR, is that accessible under the hood like it is on the LT1?

(PS: Car is 1998 LS1 TransAm WS6. A4. Stock except for Borla cat-back and free ram-air mod.)

 

Fuel pressure sounds a bit low (should be about 60psi at all times), and that combined with the whine might be an indication of a pump on the way out. I'd keep an eye on it. The FPR is part of the pump module on these cars, there is no way to adjust or change it from the engine bay.

Just an FYI, if the car still has stock tuning then the idle speed is factory commanded to 550rpm in gear and 650rpm in P/N (A4). The stock tach often isn't accurate, of the four I've owned they've all been off by 50-150rpm or so. Due to the tach inaccuracy, you can't really know the true idle speed without a scanner.

Not sure how many miles are on your car, but it's 18 years old now so unless it was a garage queen I suspect it's got plenty of mileage. As such, you may have a vacuum leak in the PCV system (these are known to deteriorate) causing some idle weirdness. This would cause a lean condition (as seen by the O2 sensors), resulting in elevated LTFTs (you would need a scanner to verify this as well.) I would also want to look at IAC counts, especially if you can catch them while the issue is occurring.

As you mentioned the car is stock, I assume that the EGR system is still in place. There was a recall on the EGR valve for '98s; if the pintle is stuck it would cause a big vacuum leak and result in idle weirdness as well. You should see a code for this if it were the problem, but the code might still be pending (again, something you'd need a scanner to see.) Yours was likely replaced during the recall years ago, but you could have a GM dealer run the VIN to make sure there are no open recalls.

If a K&N filter has ever been in use, the MAF could probably use a good cleaning. Some people like to use MAF spray cleaner but IMO this is not good enough unless administered with a Q-tip. Gentle pressure is needed to really get the wires clean if they have a bunch of gummed up K&N oil on them.

Changing the fuel filter isn't a bad idea either. Perhaps pressure will come up a bit with the new filter. Some fresh plugs certainly wouldn't hurt, assuming the car has plenty of miles and that the last plug change is unknown.

 

In addition to the above check your IAC (idle air control) on your throttle body.
I have a 98 and i have blown through 2 of them.

 

Another good thought, not sure why I didn't think to mention it above.

Having said that, the IAC motor used for '98 is not specific to only that year so nothing about the model year really makes this any more or less likely, but it's definitely a possible cause of idle issues for any model year.

 

It's 40,000 miles, one owner before me. I just replaced the fuel filter, and I'm definitely getting a lot better throttle response and it hasn't stumbled in idle yet. I haven't been able to hook up the fuel pressure gauge, but hopefully the numbers are better.

 

Pressure is 50PSI at prime and 62~PSI while running. This is after the fuel filter change, so it did improve. It seems to be running OK, but I know I need to get my gas cap replaced (does this have a huge effect on the car, other than triggering a light?)