If the radiator hose remains soft, and no smoke from the tail pipe, I'd say the head gasket is OK. Really rare for them to be a problem on the 4.3L engine. Most of the time they're replaced due to a misdiagnosed leaky intake gasket, which gets replaced when doing the heads anyway, presenting the question: was it head gaskets or just an intake gasket?

I was just going to suggest a "wet" compression test, (using oil). The oil helps seal the rings during the test... good job Compared to the other cylinders, 120 is still a little low. Could be any number of things causing that, but if it runs smooth cold, I'd look for something other than the compression issue causing the rough running when warm. During cold start ups the PCM operates in closed loop. It ignores oxygen sensor data and applies a fuel mixture based on data from MAP, MAF, ECT, & TPS sensors. When the PCM switches to closed loop operation, fuel mixture leans out. If there's a vacuum leak affecting cylinder #6, it will run even leaner and could easily have a lean misfire, and NOT show up as a lean DTC. It's a stretch, but worth looking in to. Could be a leaky intake gasket around #6 intake port. Could be internal or external. Using an analog tachometer and propane is probably the easiest way to detect it. For an external leak, direct the propane along the intake gasket and watch for an RPM increase on the tach. To detect an internal leak, remove the PCV valve from the rocker arm cover and insert the propane nozzle into where you pulled the PCV valve out. It will take a few seconds for the propane to arrive at the leak, if there is one, and the RPM will increase. SO, for the sake of safety, put a 4 or 5 second shot of propane in and then plug the hole with your finger. Don't fill it up with propane!



As for oil viscosity making a difference: Oil does thin out as it warms up, and the oil pressure drops slightly as a result, but it shouldn't make a difference in how the engine runs.

 

I know the MAF sensor is okay, or at least it was before the incident. The ECT was changed out last Saturday as well as the MAP sensor. This leaves the TPS sensor. I'm pretty sure I have over looked this sensor.

As far as testing with propane, I'm letting the vehicle warm up now. Then, once it's at operating temperature, I CAREFULLY follow your recommendations.

 

Ahhh, just had another idea...... The MAP sensor plugs into the plenum right above the #6 intake runner.... if the seal/gromet is leaking, it could cause #6 to run lean. Again, just a thought

 

Okay. I

So, I did the propane test. Nothing. I even had the mrs help me out but I think I may have found a temporary fix.

I was doing some research on cylinder compression and found a few websites which raved over the results they had using Restore. Well, after reading all the opinions I decided 10 bucks couldn't hurt.

After driving around town today with a 20 oz bottle of Engine Restore poured into the crankcase, the truck ran very well. In fact, there was almost no rough idle at all while I would sit at a traffic light waiting for my turn to go. I haven't checked the compression again but I plan on doing that tomorrow.

Why would a product like Engine Restore work? Does it just temporarily apply a thin sealant against the cylinder walls? Is there a product like it which is made for a more permanent solution?

 

Difficult to say what caused the improvement. There's always the possibility that a valve lifter, (#6 cyl) got pumped up and couldn't bleed down when the engine overheated. If that happened, it would explain the low compression, and the Restore may have freed it up. Typically I'm not a believer in additives, but if it works, cool. Put some miles on it and see what happens.

 

Well, after about 300 miles I'm getting the same code, again. The Restore must have done something for awhile but I guess it was just temporary. I haven't checked the compression on cylinder #6 yet but I'm also getting P0102 as well.

I checked the MAF sensor out yesterday per the guide below and ended up replacing it last night. Now, I'm still getting the P0102 code and P0306.

Part 1 -GM MAF Sensor Test (P0101, P0102, P0103). BAD MAF Symptoms.


Would a bad O2 sensor have anything to do with this problem? I replaced the one down stream of the catalytic converter a few months ago but the other two look like they could be original sensors.

What about a faulty ECM? If I replace this, do I need to have it programmed?

Does anyone have any connections with a St. Louis auto shop? I think it's time to take this truck in for a looksy. I've spent alot of money over the past several months replacing sensors and parts just to get stuck on these two codes.

 

Not sure where you found that test procedure, but it obviously didn't find or fix the problem This is straight out of the GM factory shop manual, follow the flow chart exactly as written, don't skip any steps, and it will take you to the problem. https://blazerforum.com/forum/diagno...plained-28157/

 

I followed the instructions you provided and this time the sensor and the harness checked out okay. I'm going to reset the DTC and see if I get another code later on tomorrow.

If I take off the MAP sensor, will I ruin the gasket? This was another sensor I recently replaced but I just wanted to check the sealed connection.

I'm starting to think it maybe be a small vacuum leak near the EVAP canister purge valve. That I know is original.

 

YES. If the MAP sensor is removed, the seal needs to be replaced. Same goes for the fuel tank pressure sensor, (same type of sensor and seal). Use it once and throw it away.

 

Okay, so I got a friend to help me tear into the truck this past weekend to see if we could find anything under the valve cover.

Well, the intake valve isn't completely closing during combustion. After looking through the spark plug hole we could see a large amount of carbon build-up on the valves seat. Thinking this was a possible cause we decided to clean it as best as possible without taking the head off. It did help some but there was still a small leak after doing another compression test.

Later this week, we may go ahead and check out the lifters. From what we where able to see and measure, the stems to the valves checked out okay as well as the push rods.

I guess my question is this - which is more likely to fail, a lifter or compression rings?