4.3L V6 TBI will not cold start, idles too fast

I have a 1991 S10 with a 4.3L V6 TBI that recently began having trouble cold starting, meaning that at a mere 40F I can crank it over and over and it will not catch. If I plug the block heater in for at least an hour, it usually will start, but at a very low, very rough idle. Once it has about 30 seconds or so to warm up, it will then idle TOO FAST at 1500 rpm, and stay that way for the first few miles of driving, which makes it rather awkward to drive, being a 5-speed. Once its been driven awhile and fully warmed up, it runs normally. (I'm running a 180F t-stat.)

The IAC valve, block temp sensor, and TPS sensor, were all replaced last year, so I'd find it hard to believe that one of those could be the problem. I put Heet treatment (red bottle) in the fuel, changed fuel filter and reset the computer. (Although it's never given a trouble code.) The problem is still there.

My local mechanic says replace the ECM, but he hasn't actually diagnosed that as the problem, so I think he may just be taking a wild guess. (He hates OBD1 systems with a passion!)

Any help would be much appreciated!

High idle after a Cold start really isn't that unusual, especially in old vehicles. Just let the engine warm up before you take off.
As far as the rough low idle, make sure the fuel injectors are squeaky clean at all times. The TBI system in general is an inefficient dinosaur, so in order for for your engine to run properly, every single component has to be working just right.

It's never idled over 1000 rpm when cold in the past, until this problem started. I've even started it outside at 17F a few winters ago with no problem! 1500 is way too fast, and it sometimes spikes while idling in traffic too.

I've never messed with the injectors -- I wonder if that could be the problem. I've put about 30K miles on this truck since I bought it. Just rolled 193K.

Although verifying correct fuel pressure (11-13 PSIG) and making sure the injector pods are not leaking would be a good idea, injectors themselves should not cause a high idle. The engine is in open loop when first started and is solely under ECM control based upon coolant and intake air temperature and time from start. Leaking or clogged injectors would only create a lean or rich condition but not control idle air (the thing causing the higher RPM).

It would be good to verify that the CTS and IAT are reporting correctly to the ECM, and that the IAC is both clean and lubricated so that it can step properly. It is also important for the throttle body bore and throttle plate to be clean so it cannot stick. While cleaning out the TB, make sure the throttle shaft is not loose. Worn bushings can allow the throttle plate to become wedged in the bore and bind in a partially open position.

17°? That's a heat wave! I had an '88 Astro TBI that used to routinely start at -30°F, and started as low as -40° once.

You must be in Alaska? I'm in NW Oregon, so I don't think I've ever seen it lower than 15F in my lifetime.

Where is the IAT located on this engine? I've never checked it.

I'll check the throttle shaft and pull the IAC again -- I'm sure its still quite clean though. I've never heard of lubing an IAC before. I'd thought about it, but was a little leery being as its an electronic component. Any idea what kind of lube I should use?

Between a place by the lake in northern Wisconsin and daily life in northern Illinois I've had my share of cold, heat, rain, snow, ice, tornadoes, floods, etc. When the wind comes off Lake Superior in January it can get a wee bit brisk. If you've been on a sled at 40 MPH in -45° you would probably remember it.

The IAT is probably clipped into the air cleaner housing or the air inlet snorkel if my memory serves me - It's been a while since I've seen a TBI up close. It should have a tan and a black wire connected to it. You can disconnect and test the resistance of both the IAC and CTS sensors. At a given temperature their resistance should compare closely to this:

http://www.wwdsltd.com/files/CTS-MAT.gif

The IAC is the only thing controlling the inlet air it is responsible for controlling idle speed. Since the idle eventually reaches a normal range we can presume that there are no other significant vacuum leaks which would cause a high idle.

In order to operate properly and follow the stepper commands from the ECM, the IAC needs to be clean, along with its air passages. Varnish and gum tend to accumulate on the IAC valve pintle, in the air passages in the TB, and on the rack gear of the IAC itself. It only takes about this much contamination to "kill" an IAC and cause the kind of problems you are experiencing:

http://www.wwdsltd.com/files/IACCompare.jpg


The IAC can be removed, cleaned, and lubricated to restore its operation. Remove the IAC from the TB, clean off the end of the pintle and exposed parts with a carburetor spray cleaner and paper towels. Carefully apply light to moderate pressure to extend the pintle while rocking it side to side. This will force the pintle rack gear to slowly "walk" out of the corresponding rack gears in the IAC housing stepper. Close observation will reveal that the IAC pintle is NOT threaded but is a sinusoidal rack gear. If the pintle does not slowly and incrementally walk out of the housing, change the direction you are rocking the pintle by about 90° and continue trying to work it out of the housing. When you find the proper direction to wobble the pintle side to side the stem will slowly come out of the housing on each wobble. It should come out completely in about 60-90 seconds of working it side to side.

http://www.wwdsltd.com/files/IACDisassembled.jpg


The only thing behind the pintle is the spring, so don't fear losing any small parts. When the pintle and spring are freed, clean the spring, pintle stem, and interior of the IAC stepper using the spray carburetor cleaner. Allow everything to dry thoroughly since carburetor cleaner left inside the IAC housing can have a detrimental effect on the insulation of the solenoid coils of the stepper motor. Blow it out if necessary to be sure it is dried.



Apply a couple drops of clean engine oil or light machine oil to the interior of the stepper motor through the pintle stem hole. Check the free length of the return spring. It should be at least 2-1/8" long (50mm) to have the proper tension.

http://www.wwdsltd.com/files/IACReCleaningSpringShort.jpg


If it is short, as many are, the spring can be carefully stretched and re-checked until it has a free length of about 2¼ inches.

http://www.wwdsltd.com/files/IACReCleaningSpringAdjusted.jpg


When the spring is correct, everything is clean, and the oil has been applied the IAC can be reassembled. Align the flats of the IAC pintle stem with the key in the stepper motor, apply a bit of moderate pressure, and wobble the pintle side to side again to walk the stem back into the housing. Position the pintle so that the spring is nearly fully compressed before installing the IAC back into the TB. If the pintle is extended too far it can be forced back into the IAC when the unit is installed, breaking the rack gears in the unit.

http://www.wwdsltd.com/files/IACExternalCleaning.jpg


Make sure the O ring is clean, intact, and has a bit of oil on it to allow proper seating and reinstall/reconnect the IAC. Within a few second of starting it should extend to the proper position and control idle air just where the ECM commands it.

Wow Blue Bowtie, that was pretty in-depth! I appreciate the effort to post all the details! I'll try it out in a few days and see what I can come up with. I also have the original IAC that I may try swapping and see if it changes things.