I have a '91 vert which is essentially stock and here is my problem (or at least one of the big ones.) When the temperature outside dips below about 55 degrees I get the follwoing behavior:

1. Idles like crap. Rough, does not rev to the ~3K RPMs you usually get at startup. Fan Clutch test does not engage and idle will vary depending on how cold it is from 2K to 750 RPMs. The colder it is the lower and rougher the idle.

2. Check engine light flashes on and off when I hit the accelerator and try to accelerate at any pace other than smooth and slow.

3. ECU codes show Intake Pressure Sensor, Throttle Position Sensor and Crank Angle (NE Signal)

4. Gas mileage horrible (we're talking less that 10 mpg)

5. Backfiring at high RPM (not that odd) and at idle (a little odd)

6. After the car warms up, it runs fine, but I still get the check engine light flashing.

7. I can turn the car off, let it sit for about an hour, start it and it idles and runs fine and the check engine light never flickers once.

What I have done:

1. The car had a tune up about 3500 miles ago, regular oil changes, fresh coolant, all regular maintenance done to it since I bought it.

2. I have replaced the vacuum hose running from the Intake Pressure Sensor to the Intake Manifold thinking maybe a crack in the vacuum hose was opening up during cold weather.

3. I used some electronic contact cleaner/lubricant on the TPS in case it was just some dirt in it gunking it up.

4. Resoldered the idiot light cluster to ensure that it wasn't bad cold solder joints there causing the lights to flicker.

5. Made sure that all the plugs are securely connected to all of these sensors, etc.

I feel certain I have a sensor or something bad somewhere, but I don't believe all three listed in the ECU are bad based on how it is so clearly related to the temperature outside.

Any thoughts? I have the FSM and have looked at what it says to do for these things, but crappy weather has prevented me from doing much of what it said to check everything.

Any help would be appreciated.

Thanks
Darren

 

Bad water temp sensor (at the rear of the water pump). Or possibly a damaged wiring harness that is cracked, which then gets irritated by the cold.

 

Thanks for the reply. I'll add the water temp sensor to my list of things to check. I've thought about the wiring harness. Could it possibly be bad cold solder points on the ECU?

 

bumpity

 

Have you set your TPS? I didn't see it mentioned in that list. Since the error codes are pointing to it...

http://fc3s-pro.com/TECH/HOWTO/TPS/tps.html

The write-up features a TII, but the method is the same for an NA.

 

It was adjusted at tune up and I didn't think it should go out of spec so quickly, but I'll add that to the list as well.

 

bump

 

When checking connectors, look inside the harness side to check if the contact clips have backed up.
It is not unusual for the brass clips to push back and not make good contact.
If you spot one pushed back, click it back in place and hold it there with a BB size dab of Bondo Marine Epoxy stick in the back of the connector.

 

Last bump.

 

I am completing this thread with my results.

I replaced my Intake Pressure Sensor (n350) and adjusted my TPS until I am blue in the face. The flashing light and the codes are now gone.

 

How is the car otherwise?

 

Still hunts for idle when cold (jumps between 1K and 1.5K RPMS until warm - and yes the TPS is set correctly using both resistance and light methods), idles where I set it (~1K RPM) but will occasionally drop and idle at 750RPM when warm and you blip the throttle. Whatever is causing it to idle down below where I set the idle at is probably the problem I have had from day one. If I were to set the idle to 750 RPMs (where it idles 95% of the time) it would stall when it does this drop. Going after Water Temp Sensor now...

 

My thoughts turn to vacuum leak. Remember that vac leaks can also happen at joints (including at the fuel injectors) and not just in hoses...


My preferred, and cost-free method for finding vac leaks is to use a garden hose with a light-but-steady stream of water. Let the water run over everything, one bit at a time. If/when the motor bogs, you know that the water is reaching a leak. Close in from there.