Idle problem

I have an ongoing problem with my truck, appreciate any input. The truck is a 93 S-10 4x4 Blazer, 4.3 vin W CPI engine. Problem started quite some time back when the engine died suddenly and a mechanic nearby diagnosed it as a bad ignition module. I noticed that I had a problem with it stalling in gear, say when parking, waiting at a stop light etc. but was told to run a while to let computer relearn idle. Well that is not the problem. I have tried tune ups with proper AC delco plugs, wires, etc. cleaning EGR, checking for vacuum leaks, you name it. I have tried it. Still stalls. The truck runs well on the road, gets 21 mpg overall, does not miss while driving, BUT, when when the engine gets up to normal operating temperature it will miss, stumble a bit at idle. Basically it has a weak idle, but runs well otherwise. I have not removed the plenum as of yet, did not see any visable signs of washing using a special light although there is no way I can check all areas with it. The truck runs and idles like a champ when cold, miss comes in to play when it gets hot. During summer months it is worse, as heat has a definite affect on the problem. Perhaps it is just a fluke that this would start after the distributor was replaced (mechanic suggested that it was better to replace the entire thing given the age of the truck) that this would happen. Fuel pressure is fine, pump being replaced a few months back. Since the problem SEEMS to be hear related I wonder if the miss could be caused by something electrical, say the coil or even the igniton module itself. Perhaps the CPI unit acts up when warm? I would add the only error codes I have ever got from the truck was egr related, as I get a message once every year or so and clean the unit and all is well. Considered replacing it, but EGR does not do anything at idle as I understand and so i have not replaced it as of yet. I have replaced O2 sensor, and run seafoam thru the engine to clean out fuel assembly. I do have a OBII scan tool, but I will admit that while I see nothing out of the ordinary when using it I am no expert with it either. Any clues I could look for that might help show a vacuum leak etc., or anything else would be appreciated. I have put off doing work on this truck but need to get it right.

I will add, the truck will idle all day in park or neutral. But under a load such as in drive or with a/c engaged it won't run for longer than 15-20 seconds then is simply shuts off.. Re starts OK but I generally kick it into neutral at stop lights to avoid stalling it out. It idles at 600-575 in gear, 650 in park. The miss is not horrible but you can tell it is not quite right.

check your EGR VALVE

You might check and clean the idle air control (IAC)valve...next to the throttle cables on the throttle valve/butterfly housing.

Thanks, but I have cleaned and then later replaced the IAC , with no help whatsoever. I have cleaned the EGR three times over the last 10 months, I am not sure it is 100% though. It does have a miss I can't narrow down that appears after the engine warms up a little. You can put into drive and it will idle OK then suddenly shut off after 10,20, or 30 seconds, depending on temperature.

A bad ignition coil initially shows up as misfires when warm or hot and wide open throttle...kinda lean into it without the downshift to passing gear. Later it won't start at all.
Do you have at least 52psi fuel pressure?
Did you replace the ignition module inside the distributor? It tells the computer what RPM the motor is running and if the computer doesnt have that info, it relies on the TPS and MAP sensors to keep running but they don't react fast enough to catch the RPM drop due to pulling in gear or the AC compressor suddenly loading the motor...same thing on my '86 Cutlass Ciera w/Pontiac Iron Duke 2.5L 4 banger summer before last.

When the module failed the entire distributor assy was replaced, as the mechanic told me it was best to replace it.

Sorry, forgot to mention, fuel pressure well above 52. I would suspect injectors if this did not start with the ignition module failing and being replaced. Could just be a coincedance. Anything I can look for with the OBDII scan tool that might pin something down?

I was just thinking, when the truck was first repaired and the distributor replaced, I noticed sloppy workmanship on the distributor. The wires were smashed under the cap and one bolts was not tightened, as well as vaccuum hose left off. I wonder if perhaps the wiring was damaged, although I would ASSUME that the truck would not run at all if the module was bad or wiring was screwed up, but assuming is not always the best thing to do. Since the EGR is the only thing that has actually showed up as a possible problem as far as codes go I am thinking of replacing it, just in case. I have a good bead on a new one, should be half of a normal priced unit and it is GM. I see some posts that advise not to use solvents on these and I have to admit I have used carb cleaner on mine 2-3 times so I may have done more harn than good when cleaning.... if the ignition module can still fire the truck but not tell the computer to control firing etc. that makes sense to me except that the truck will run for a while in gear, but stalls out after a while, say sitting at a red light, or, making a sharp slow turn. I hate to throw parts at it but want to fix it once and for all. I have grown accustmomed to kicking it into neutral at stop lights but I have ignored it long enough. The fact that is misfires is a concern, although again, it only does this at idle, no hint of a miss at driving speeds or WOT. The miss is more like a stumble or random miss, not like a dead cylinder. Makes it hard to diagnose. The mechainic who worked on it before looked at it a few months ago and thought it was running like a normal 4.3, and his guess was a dirty EGR, although cleaning it did little to help.

The "star wheel" thing on the distributor shaft that signals the module can develop a crack then wobble on the shaft sending all kinds of kooky signals to the computer...I saw it on a 305 powered Monte Carlo. Replacing the distributor fixed the problem.

I will d/c so see if I can find any apparent problems with it, although it was new so i would hope it is not bad. I want to d/c the wires too, just to be sure the one was not smashed worse than I thought. Am wondering if that could be a problem. Did not look broken but with wiring I guess you can't be too sure. Since I have had some issues with the EGR, I have been lucky enough to find a NEW one for $78 on Ebay, actual GM part. Even if this is not the major problem I hope it will still help some. Is there some way to adjust the idle speed on these engines? If this thing idled at 700-750 it would never stall. I saw a vague mention of this in the GM manual I have but it was not explained at all. BTW, thanks to you folks for your advice and thoughts, always appreciated! PS, the Blazer site is set up very nice, very well organized. By far the best forum on here.

You might've just hit on something. If I put a blade of my pocket knife between the throttle stop and the throttle tang at the passenger side of the throttle plate behind the cables, the motor will speed up briefly until the computer-reading the engine RPM with the module inside the distributor- tells the Inlet Air Control (IAC) valve to close more to bring the engine RPM back to the normal 700 rpm. Now if yours doesn't slow back down, the computer can't be getting the RPM signal from the module....

I will try that and see what happens. I wish mine DID idle at 700, but does not. It does at startup, but drops to 650 or so, and then a bit lower. Goes to 600 in gear. Jumps between 575 and 600 until it shuts down, which can be 10 seconds or 30. Thanks.

Here is an update. I replaced the EGR today and the coil too. Am getting ready to take it for a drive to see how it runs on the road. One thing I noticed was that after it warmed up it stalled once as I held it in drive at idle, after about 10-15 seconds. So, the idle problem seemed to be a issue still. I noticed a small adjustment screw I had never seen before that actually let me open the TB a tiny bit. The truck now idles at 750 rpm and in gear idles at 675-700. That seems OK to me and it would not stall even when i gear and me turning the wheels over and over so I may have the stalling under control, but not ready to celebrate yet. I notice the computer shows the idle set at 10 x 65 , or , 650 rpm so i am over riding the settings to do this, and unsure if it will stay at this speed. The computer so far has not re adjusted the idle back down, so this is similar to the thing done with the knife to hold idle open a bit. Maybe I do have an issue with the distributor as we discussed. I am going to work with it further but have to put is aside for a day or two as my son's old Corsica has a problem now I must deal with (possible ignition coil going bad) so I will advise further later. I can live with the truck as is if the idle is OK but also do not want to ignore a possible problem.