'91 Olds 98--- If the car is sitting still idling, after a while it begins to run rough and the check engine light comes on. When the accelerator is pushed and you move, the light goes off and does not come back on. In pulling the codes it comes back to the TPS showing a low voltage. Bad TPS? What other issue could this be?

The 91 TPS has adjustable screw fittings. Low voltage means that the TPS is a little off center. Ideally you'd be checking this with a scantool hooked up so you can watch the voltage, but you need to adjust it back into place. Someone with the 91 FSM will stick their nose in and give you the correct voltage it should be at idle I'm sure.

Once you get the idle voltage taken care of, you'll want to check the voltage at all throttle positions and make sure you're getting a steady increase to 100% throttle. You can do that by hooking up a scantool and bringing up TPS voltage, then walking the throttle slowly while the ignition is turned on (but not in start). If you see any dead patches or sudden deceases in voltage, you need a new TPS.

Thanks, I've got an AutoXray....it does a wonderful job. As tight as these screws are I can't imagine how it got off.....I'll check it out. These screws are hard to get to under the TBI....I've had to take the TBI off to loosen the screws.

I used a low profile ratcheting screwdriver with a drill bit style phillips head to get at them. And it still was a pain.

The earlier models had bolts and they were sooo much easier to get at. I've resorted to pulling the TB off, which makes it much easier BUT on the later model 3800'* the bolts on the TB were screwed into studs on the back side that always turns when you are trying to get at the bolts....and the studs are low profile without much meat to grab on to to hold it from turning..........then you round off the devil and then.......so on and so on..........these easy hour jobs take all blasted day long to do!

My TPS is showing a range of 0.41 to 4.07, and the throttle angle is 0-99% which appears to be in specs. Now my engine is loping on idle when engine is completely warms and showing a code of low O2 sensor voltage. I replaced the O2 sensor late last summer.....could it be time for a new one already?

Whens the last time you checked & cleaned your battery, cables, posts, and grounds?

I clean mine up every spring after having some 'strange' voltage problems occur the first winter we had our olds 98. No problems since.