1995 Blazer 4WD - 4.3 L (vin W & PCM) - 230,000 miles

I've really been struggling with this intermittent rough idle condition for a very long time. Only recently has it been getting worse along with the occasional "run-on" or "dieseling". The "run-on" is a very new symptom as this has never happened before the last couple months. The exhaust smells very strong but it's not smoky. It seems like it's running on the rich side.

- Rough idle, intermittent becoming more frequent
- Spongy pedal on moderate acceleration, intermittent becoming more frequent
- Seems good during hard acceleration
- Strong exhaust smell, no visible smoke, little/no gas small, no sulphur smell
- Very occasional "run-on" when turning off ignition
- Little/no trouble starting
- No Error Codes

Last Summer I changed the two exhaust manifolds and, at the time, thought it would be a good idea to change the cap, rotor, plugs & wires. Then I went on a 2-week trip and one of the spark plugs must have been defective as a chunk of porcelain from a plug ended up inside the EGR valve. What a miserable time. I had the EGR jammed open on my drive during the beginning of the trip, it cleared for about 10 days, then it got stuck open for the entire drive home. Got home, took off the EGR, cleaned it out, and installed a screen gasket.

Then during last winter it just started idling rougher. Lots of vibrations. Occasionally it would throw a Code 43, but not very often and not during last few months. This is the same code I got when the EGR was stuck open last summer but this time, the EGR seemed to be fine.

During the last several months after winter, I stopped getting the Code 43, did not get any codes at all, but it's still not running right. I think it might be running rich as the spark plugs from last summer were black & sooty (only 5000 miles).

Now I'm finally digging into this and not having any luck. Here's what's happened in the last week or so...

- New AC/Delco spark plugs
- New Bosch wires
- New MSD Ignition Coil

Previously, in the dark I could see some sparking around one cylinder and corona around the coil. After changing plugs, wires and coil, I could no longer see sparks or corona. The idle seemed to smooth out a bit, but it was only temporary. It's idling pretty rough again today. Perhaps worse after the engine is warm.

The cap & rotor are Duralast (AutoZone) brand that I changed a year ago (5000 miles) and still appear like new, no burning, tracking or cracks. I even put my old cap & rotor back on as a quick test and there was no noticeable difference.

Vacuum Leaks - I've visually inspected all vac lines/hoses and sprayed a little carb cleaner looking for leaks. Could not find any problems.

Air Filter - changed 1 year ago (5000 miles).

Fuel Delivery - No leaks. Pumping and holding specified pressure and passed all tests as per GM manual. Spider was changed sometime within last several years. The fuel filter was changed 5000 miles ago.

EGR (Exhaust Gas Recirculation) - inspected and tested good as per this procedure. Intake manifold is also not clogged as per this procedure.

TPS (Throttle Position Sensor) - inspected and tested good as per this procedure.

MAP (Manifold Air Pressure) - inspected and tested good as per this procedure.

IAC (Idle Air Control) - visually inspected and wiped off black grime but could not find a test to perform.

IAT (Intake Air Temperature) - could not find a specific test, but resistance reading goes up as temperature goes down.

MAF (Mass Air Flow) - this model year/engine does not have one.

PCV Valve - inspected and working.

Exhaust Restrictions - exhaust system is clear from front to back.

ICM (Ignition Control Module) - removed and tested good at AutoZone.

Pickup Coil - tested good. Putting out 1.0 VAC when cranking. Reading 800 Ohms across and infinity to ground.

Could there be a timing issue? I've never messed with it and I'm the original owner.

I have not performed any cylinder compression tests. Since the problem is intermittent, I didn't think it could be something mechanical.

Any helpful suggestions would be very welcome. Thank-you!

 

Well, first off u need to only use Ac Delco cap and rotor......as well as Ac plugs....then u can go with either ac deco wires or auto lite pro series wires...

 

I put the original cap & rotor back onto it. Runs the same. I've only used AC/Delco plugs. Bosch wires are good, but runs the same with AC/Delco wires.

 

the original cap?????? seriously.......how many miles are it? 230k......i can see where it wouldn't make any difference in the way it runs......if u want it to run correctly u need ac deco cap,rotor,plugs........

 

NOT the cap/rotor from when I bought the truck in 1995! ... what cap/rotor could even last 230K miles? Sorry if I wasn't clear enough.

The "original" simply meant the one that was on there just before all these issues started. I figured I'd see if it got better, worse, or stayed the same. It ran the same, so either there's nothing at all wrong with them, or they're both degraded in the exact same way.

It's probably a moot point... I believe I found two serious issues unrelated to cap/rotor/wires. After both issues are fixed, and once I've confirmed the new condition via a live scan tool, I'll post the resolution here. I appreciate your input!

 

well, u know some people run them till they're dead...... therefore u can't assume anything!

 

Changed..

1. O2 sensor.
2. Passenger-side knock sensor.

Those parts tested bad, but changing them did not make much difference.

3. Distributor assembly.

This was it. Something was wrong with the bearings and it had moved CW (corresponding to 10 degrees retarded timing). Set the timing back to factory spec of 0 TDC (with timing bypass wire disconnected) and replaced the distributor assembly. Problem solved.

 

I was going to start to write my own post about the exact same problem or at least looks about the same from my 1995 Blazer until I saw this one by accident, I have had this issue for a long time and haven't found the cause after replacing some most obvious parts.

Thanks to this I will take a look to the distributor assembly, but first I would like to know, how did you found that this was working incorrectly, did you saw some odd behavior in the live scan tool, maybe a weird behavior on the ignition timing or something else?

 

The live scan tool showed no errors except running rich.

It was a process of elimination. I found the bad timing first and when the issue remained, I inspected the slop in the rotor. I thought the slop was excessive so I pulled out the distributor. Turns out the slop was normal but the old rotor's bearings seized up about two days after I removed it. They must have been sticking as it was running.

 

I too suffer from a weak idle, engine sounds like it wants to die but it doesn't. RPM stay steady and no CEL but I don't like the hesitation. I have only 52,000 miles. Could this be the problem?


puttster