Pontiac Repair: rough idle when cold, chevron techron, crank sensor

Question
QUESTION: Hello Todd, thanks in advance for your advice.  I have a '92 Buick LeSabre with the 3.8L V6 and 80K miles.  The problem is that when the temp drops below about 20 degrees, the idle is very rough and it even sounds like it is backfiring, making kind of a sputtering noise with some "pop" sounds.  Also the idle is erratic and changes RPM up and down some.  If I hold the gas pedal in a bit to get the RPM up, it smooths out.  If it's warmer out or once the engine warms up, it runs well, although when I'm at speed and not accelerating/decelerating, it still seems to miss because I can feel a slight hesitation when I'm driving.  It recently got a new set of plugs and wires at 75K and a new crank sensor in September that fixed the stalling-when-hot issues it was having then.  The "check engine" light does not stay illuminated past startup.  I disconnected the neg battery cable for a couple of minutes to reset the computer, but no change (it's been well over 100 miles since I did that).  Please tell me any thing you think might be the cause.  I really appreciate you taking the time.  Thanks much, Matt

ANSWER: Hi Matt:

A few things. depending on which ignition system is on this car that could be the cause. If this has 1 big coil that has all 6 cylinder on it that could be the cause those coils were made by motorola and when they would go bad when it's cold out they would not deliver a hot enough spark and you get the back fire sound and it will labor to start.. If it has the 3 individual coils that is not the issue... Fuel quality would be another thing. try a different brand of fuel onl 87 octane and get a bottle of Chevron techron and add the proper amount to the tank per the directions on the bottle. This stuff works great for dirty injector and fuel system cleaning. How about the fuel filter has that been changed recently? Lastly a dirty throttle plate and or idle control valve will cause an erratic idle. Or the very last thing could be a vacuum leak check all the hoses for cracks or splits. Good luck :)


---------- FOLLOW-UP ----------

QUESTION: Hi Todd, thanks so much for the really fast and knowledgeable response.  My car has the 3 individual coils, so as you said, it's not that.  In MN where I am, during the winter months all gas has ethanol added, so fuel quality as far as water in the line would not be a problem.  I'm not sure when the filter was changed, but wouldn't my problem be occurring all the time, not just when it's cold out?  As far as the dirty throttle plate, I recently took the air snorkel off in front of the MAF sensor and sprayed everything with carb cleaner.  It looked clean before I did that, but I gave it a shot anyway - no difference.  I did check for vacuum leaks and could not find anything with that either.  Can you tell me more about the idle control valve?  What does it do and how does it do it?  How is it controlled?  I know where it is on the throttle body, but is it hard to check or repair (or replace)?  Any suggestions will again be greatly appreciated.  Thanks and have a great new year.  Matt

Answer
Well here are a few other things. The idle control valve is controlled by the computer it's job is a calibrated vacuum leak in runs a pintel in and out and it gives the engine more air to breath or less thus the idle speed. The pintel and bore carbons up and it can't give the engine enough air to run when it's cold out or due to the fact it's dirty the valve moves slowly or not in and out enough. The fuel quality is an issue too. We have that same mix here in NY but a max of 10% to gas if it's much more than 10% it WILL run rough/stall when cold The ethanol doesn't burn when cold like gas does it has less BTU's and is harder to fire. As far as a fuel filer on cold starts after the car sits the fuel pressure bleeds back into the tank and on a cold start it needs to run rich and if the filter is restricted it takes too long for the pressure build up and the car will run lean and stall on the initial start. One last thing being that this car is old and more than likely has a lot of miles on it. it could have a carbon build up on the valves and the carbon acts like a sponge on a cold start it wil absorb the fuel before it goes down into the cylinders and then the engine will run lean again and stall. You can buy a product called "top engine cleaner" from any GM dealer and run a few cans of it through the engine. Let the car stall out on the last can of it and let it sit overnight. Start it up in the am and let it clear out the cleaner It will START HARD after sitting with that stuff in the engine. It works wounders and it's cheap. use it along with the injector cleaner I mentioned before. The dealer sells it as well it's called fuel system treatment and it's the same stuff but at about 2X the price.