Question
I have an annoying automotive problem that I hope you can help me with. I have been trying to fix it myself without sucess. Here's the problem: When starting the car for the first time of the day, the engine starts out to idle at about 2000 rpm and slowly drops to about 600-800 within about 5 to 10 seconds. With my foot on the brake, I then put the transmission into reverse and the engine begins to stall. Some times it will actually stall after about 3 or 4 seconds of a slow and rough idle. Other times it will stumble for about 3 or 4 seconds and then pick up a smooth and faster idle again allowing me to back up without the need for the throttle. If it stalls, it will start right back up, but this time it won't stumble nearly as bad when I put it into reverse again as long as I do it immediately and at a fast idle. If it doesn't stall and I then shift back to park the idle will be about 2500 rpm but will slowly drop down again. After either case, when accelerating forward after backing out of my driveway, the engine will stumble for 1-2 seconds before accelerating normally. This stumbling will get better as the engine warms up but never really goes away completely. It will almost always hesitate a little for 1 second or less from a stop light or stop sign even when warm. I have found that if I start the engine when cold, allow the idle to drop from a fast idle to a more normal one (5 - 10 seconds) and then pump the power brakes 4 or 5 times the engine will die. At normal driving speeds the engine runs smoothly and has normal power at any engine temperature. This car is a 1988 Pontiac 6000 LE with a 2.8L V6 MPFI engine, automatic transmission. It has a little over 100,000 miles. I have done the following in attempting to fix this problem: - No "Check Engine" light when driving - no trouble codes stored in ECM - New air filter - New spark plugs (replacing originals which showed no signs of mechanical engine trouble - plugs showed normal wear for high miles) - Visually inspected spark plug wires (no oil on or cracks in them) - Measured cylinder compressions (4 @ 165, 1 @ 160, 1 @ 150 psig w/engine cool - don't know what it should be but seems OK) - Checked fuel presure (44 psi and holds without engine or fuel pump running - 31 to 36 psi with engine running at idle - rises when throttle is opened to rev engine - fuel presure regulator and fuel pump seem to be working OK) - PCV valve is clear, operates freely and seems to be working OK - MAP sensor checks OK - Coolant temp sensor checks OK - TPS sensor checks OK - MAF sensor checks OK - EGR valve seems OK - is completely closed at idle - Evap. purge valve seems OK - energized to be closed at idle - All vacuum hoses and connections check good. No vacuum leaks through any of the off-engine components including brake booster. - Engine vacuum at MAP sensor at idle is smooth at 17" Hg I have done all of the above myself with the help of auto repair manuals. The only things I haven't checked are the idle air control motor (which seems to be controling the idle) and the fuel injectors both of which seem to require expensive test equipment to check accurately. I do, however, question the MAF sensor condition. Even though it checks OK by me, I read somewhere that if the MAF is disconnected and the driveability problem gets better (which it does a little) that the sensor is defective. The only way for me to rule it out is to purchase a new one for about $150 which I can't return if the problem remains. I am an electronics/electrical professional and consider myself to be reasonably good with auto mechanics as I have been doing it for myself for 20 years. Yet, I can't seem to solve this problem. Please HELP! Thanks. P.S.: I do not have access to a "scantool" but would like to build one of my own. Do you know where I can find very detailed information about the data communications protocol of General Motors engine computers for little or no fee? The SAE will sell this information to me, but the cost to gather all the documents that I need would be excessive.
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