91 regal stalling/wavering idle

It idles erratically, first fast then slow in a cycle. It does this worse when it's warm and as it gets warmer the problem gets worse to the point of dying. At normal speeds I don't notice any power loss, but once in a blue moon it'll hesitate when giving it gas. This has just started, although the hesitation has been happening since I got the car a year ago.

When I got the car, the transmission had just been rebuilt and it had a blown water pump gasket. While investigating that, I discovered the cooling fans wouldn't come on. Since the dash temp light came on, I ruled out the sensor. Voltage checks around the fan relays looked normal, so I installed a switch across the relays and the fans came on. Have been driving it with this switch ever since. My crap haynes manual didn't tell me what your threads told me about a second sensor in the thermostat housing. I've just now tested this sensor by unplugging it, and it's bad(fans come on with relays reinstalled).

Have replaced the fuel pump and filter. pressure at fuel rail is 50 while fuel pump is running, then drops to 45 instantly when pump cuts off. pressure then drops fairly rapidly to 0. Pressure is steady 39 while engine runs but drops a couple of pounds when the engine speed drops. The fuel pressure gauge needle used to wiggle rapidly between 3 pound markings on the gauge before I replaced the pump but after I replaced the filter. The old filter was clogged, couldn't even blow through it.

unplugging the MAF sensor (or is it a MAP sensor?) located right above the TPS on the throttle body doesn't affect the problem, although I can't start the engine (only tried when warm) with it unplugged.

There is a little gunk inside the throttle body before the butterfly valve from an improperly attached air cleaner hose that I discovered while investigating this problem. I figure when I take the plenum off to install the new fuel pressure regulator I'll be able to clean the throttle body thoroughly.

I fixed a vacuum leak at the cruise control (now my cruise works again), but it only minorly affected the problem. I don't notice any vacuum leaks on the lines coming out of the throttle body, especially the one going to the fuel pressure regulator. Then again, unplugging the lines from the throttle body didn't affect the engine running either.

Does anyone know of anything I've missed?? I haven't yet installed the pressure regulator because I'll need new plenum gaskets. I'll also replace the bad CTS while replacing the regulator.

Any ideas??

I'm totally stumped and would be grateful for any advice anyone can give me. My haynes manual is worthless for locating sensors and parts and I don't intuitively know the computer/motor relationship to effectively diagnose what's the computer's fault and what's mechanical. BTW, no trouble codes are set in the computer.

It would help, especially in such a long detailed post, to include which engine your car has.

That being said, your problem is most likely the IAC valve, as both available engines have one and it will cause an irratic idle. The fact that your T/B is gummed up would increase the probability of the IAC being your issue. THe IAC is an electronic stepper motor that varies the amt of air allowed in the engine at idle and part throttle.

It would help, especially in such a long detailed post, to include which engine your car has.

That being said, your problem is most likely the IAC valve, as both available engines have one and it will cause an irratic idle. The fact that your T/B is gummed up would increase the probability of the IAC being your issue. THe IAC is an electronic stepper motor that varies the amt of air allowed in the engine at idle and part throttle.

:banghead: DOH!! It's a 3.1L engine.. And it turns out that I mistook the IAC valve for the MAF sensor. So when I unplugged the IAC valve, the engine wouldn't start.. I'll try that this weekend and will post back the results. And I thank you for your reply..

Here are the results of the repair. :)

Took off the throttle body and cleaned it thoroughly and replaced the IAC, which was gummed up horribly.. When I started the engine, it ran fine as usual while cool but started doing the crazy idle/dying bit after it warmed up.. After it died, I was fiddling around with the vacuum lines at the throttle body because the connector wouldn't fit at the left line, trying to make it go in fully. That's when I noticed a bit of gasoline in that vacuum line (it's the one from the regulator, although there is a T'd off line going somewhere I can't see beneath the plenum).

Have fuel pressure regulators been known to be thermally leaky?? Once the engine cools again, I will install the new regulator and post back..

Again, I want to thank you for the information you provide to us out here. It's a lifesaver and has saved me countless in labor!

AFter installing the new fuel pressure regulator, the car ran fine. In fact, it ran better than it ever did before! I took it on a 30 minute test spin across town and back. Even stopped for 5 minutes at a store and turned it off. It restarted warm just fine, and drove it back home..

An hour or so later, I went back out to fetch some tools I remembered leaving in it and tried to start it again.

It lopes and misses. Sometimes it backfires into the intake. The brake boost which worked before isn't working.

Vacuum leak?? What gives? I'm stumped again!

While running at idle, the service light comes on but I can't get any codes out of it.. Sometimes when I short the diagnostic, it flashes 11 and other times it will just flash rapidly. Other times it will simply go out untill I remove the short. Giving it gas will clear the light but it still runs like crap.

When revving it up, it makes a sound in the back of the motor (at first I thought it was the egr valve because it sounded like a shaft rattling as it spun too fast), that sounds a bit lower than whistling paper.

Vacuum leak??