92 Bravada (Blazer) stalling on shift

I have another old thread on this problem, but I think some of the info was misleading, so after a lot of messing around I'll try to describe this problem again.
This is a W (Vortec) engine and it is now tuned well with no codes. When the transmission (auto) is shifted from park or neutral to either drive or reverse about every third or forth time the engine will lose power and stall. I do not believe at this time that the problem is with the engine, but I may be wrong. Their seems to be a connection to the brakes at times but not sure. one thing that has been mentioned a few times to me is torque converter lock-up. I don't know that this fits the description. Another symptom is that the engine suddenly will surge up to 1500 RPM.

Any Ideas?

GL

Try cleaning the Idle Air Control (IAC) with some carbretor cleaned sprayed on a rag...don't spray the IAC or move the pintle....the little plug thing sticking out. Spray some cleaner in the intake manifold hole where the IAC screws in too.

I had a 4 banger Cutlass act the same way...turned out to be a bad pickup module in the distributor.
The pickup not only tells the coil to light the plugs, it also tells the computer the crankshaft position & engine RPM so the computer can kick open the Idle Air Control Valve (IAC) when you pull it in gear or the AC compressor kicks in.

Good advice about cleaning the IAC. You should also consider cleaning the throttle body.

However, I would advocate removing the IAC and disassembling it. You'll get a lot more thorough cleaning, and will be able to re-lubricate the pintle and potentially restore proper operation.

The stepper motor that controls the IAC pintle position consists of a pair of rack gears that rest against springs. The rack gears are slightly staggered in their positions. The pintle has a mating set of teeth along its length. The two rack gears are actuated outward by the solenoid coils in the motor, and returned by the springs. If the coils operate alternately, the pintle is moved in a direction depending upon which coil/rack gear is operated first. To reverse the direction, the coils are operated in the opposite order. This is what the ECM does to control the IAC pintle position, and therefore, the idle RPM.

You can move the pintle by simulating the alternate operation of the stepper coils. If you are trying to extend the pintle, pull very lightly while you move the pintle side to side. This will alternately engage and disengage the rack gears and allow the pintle to "walk" out over them. It takes about 3-4 minutes to remove a completely retracted pintle that way.

http://mysite.verizon.net/strey/files/IACDisassembled.jpg

Clean everything with lint-free cloths and a mild solvent. Harsh solvents can affect the insulation of the stepper motor coils. It's generally the dirt and buildup on this worm shaft that causes sluggish IAC operation. Set the pintle aside and allow it to dry while you clean the spring. Make sure the spring is at least 110mm long (about 2¼”. If not, you can usually carefully stretch the spring to the correct length.

When the worm gear on the pintle shaft is clean and dry, apply one or two drops only of clean light oil to the pintle shaft, then work the pintle back into the rack gears of the motor by the same rocking motion. It takes a while to get the pintle back into the worm gears, but you'll get it. It is important to get the pintle fully retracted into the housing so that the pintle is not forced against the gears when reinstalling the IAC unit in the throttle body.

The difference between a completely "dead" IAC and fully active one is only a little carbon and varnish:

http://mysite.verizon.net/strey/files/IACCompare.jpg

While the IAC is still out, clean the air passages in the throttle body. The orifice in the TB where the IAC resides is the seat that the IAC valve closes against, and it can accumulate a lot of carbon, dirt, and debris. The easy way to do this is with carburetor spray cleaner and a small stiff brush or cotton swabs.

Try cleaning the Idle Air Control

The IAC is new and I have no codes. I'm no longer sure it's an engine problem. I have the neutral safety switch out right now and I'm going to clean it up and get it adjusted properly and then run it again. EGR also checks out. This is a tough one and all the auto mechanics (3) in town have had a shot at it before it was given to me. The free car is getting expensive.

GL

I just put the neutral safety switch back in with no change. Another thing that I should mention that does seem like an engine problem is that it doesn't start to happen until the engine is warmed up.

How is your charging system, are you getting the proper voltage when the engine is running? Is the battery good and properly charged? You mention brakes, does the brake pedal go hard/stiff? The EGR is commanded only when the engine is warm(closed loop) and also when the shift lever is not in either park or neutral. Check and make sure the connections at the ECM are good, also make sure your grounding straps to the engine and chasis are good too. If that all checks out, I would be looking at a possible ECM problem. Its not that uncommon for the ECM to still keep the truck running when its going out. Check those other things first though and narrow it down from there.

Are the brakes being applied when the engine speed suddenly increases to 1500 rpm?

Are the brakes being applied when the engine speed suddenly increases to 1500 rpm?

I believe that I've always had my foot on the brake at the time it surges.

How is your charging system, are you getting the proper voltage when the engine is running? Is the battery good and properly charged? You mention brakes, does the brake pedal go hard/stiff? The EGR is commanded only when the engine is warm(closed loop) and also when the shift lever is not in either park or neutral.

The charging system new and working well. The brakes seem hard at times, but I'm not sure they're too hard. The EGR was taken out and found to be sticking open when pushed in all the way. I cleaned it up and it seemed fine so It went back on.

Although we'd normally see an O2 sensor sensing rich condition (code 45), the fuel regulator may have a little leak.
Applying the brake relieves some vacuum which is replinished by the motor by pulling the air out of the diaphram. The shot of air and a little fuel leak gave mine a slight increase in engine rpm which kinda caught me off guard as the car is supposed to slow down with the application of brakes instead of pulling ahead for a split second.
After warming the engine then shutting the engine off, you might try taking the rubber intake snout off the throttle, open the throttle wide open then check for raw gasoline fumes at the throttle butterfly. The poppet injectors squirt in the intake runners, not inside the plenum.

I have some update info after trying all of the above. I talked to a transmission shop today and the guy listened to my problem and immediately said I had a torque converter lock up problem and to disconnect the wires to the tranny and the problem would go away. It didn't. I did notice that all I have to do is move the shift handle around a quarter inch towards reverse and the engine will sputter and die, so I'm back to square one. As soon as the snow storm here quits I'm going to get under it and pull the mechanical linkage off the side of the transmission and then mess with it and see if I can narrow it down to the transmission.

If anyone has any more Ideas let me know as this is a perplexing, frustrating problem.

GL

I have new info on this and have it narrowed down to the ECM and the Park/Neutral switch. I disconnected the transmission mechanically and electrically and the problem continued, so the transmission guy is wrong. I found that the ECM is supposed to be sending a signal to ground when the switch is in park or neutral and is open when in any gear. If the ECM does not sense the ground in park or neutral the engine idle will drop and cause the stall. I guess the only question I have now is how do I verify this. I have a good ground but I really don't know what kind of signal the ECM is putting out or how I measure it or is it just shot?

any Ideas?

GL

Ground the orange/black wire going to the Neutral safety switch.

Guess what else is in that circuit between the ECM and gear selector? The Endgate(tailgate) Window Release Relay...looks like a safety item to not allow things to happen if the tailgate window is open.
Back inna minute.

Well mine starts in Neutral/Park & idles good in Drive, with or without the endgate and or back glass open or shut...will keep thinking.

The keyswitch won't turn to the start position unless the gearselector is in Park or Neutral too.