My recommendation still stands as described in posts 56 and 65. But this is the last time I will bring it up.

 

I hear you, Les. I have a set of gauges coming to to do a compression test then follow up with a leak-down if necessary. I'm hoping I don't have to pull the heads.

What's the best procedure for a compression test? My plan was to unplug the ignition coil and fuel injection spider so no attempt at combustion is made, then use the starter to turn over the crank with the gauge attached. Does that sound like the safe/correct way to do it?

 

Yes, for cranking compression unplug the coil and spider, prop throttle wide open while cranking. Do the same number of pulses for each cylinder. Record values.

Then put plugs back in and do running compression tests (snap and running) on each cylinder per link previouly posted This checks volumetric efficiency of each cylinder and in conjunction with cranking tests can ID individual cylinder valvetrain problems as well as restricted exhaust. Its all in the link and yes its a pain!! However I suspect you will find the cause of your low MAF sensor reading.

Dont try to jump past the running compression testing into leak down testing as that will not check for restricted exhaust or some other things. But the leakdown testing can be used as a final confirmation of valve or ring sealing as a problem if you like. But so can just pressurizing the cylinder with air without the leakdown tool.

 

Completed a dry compression test with excellent results.

185 psi ① ② 193 psi
175 psi ③ ④ 176 psi
185 psi ⑤ ⑥ 187 psi

There is more of a discrepancy with ③ and ④ being lower than the others. I'm guessing this is normal those two being in the middle.
Other than that, my intrepretation if correct seems to indicate that there's nothing wrong with the cylinders or heads and this is a prefectly healthy running engine.

So the misfire and loss in coolant has to be because of LIM gaskets.

I skipped the wet compression test because the dry results seem so promising that I don't suspect bad piston rings. If it's absolutely necessary to do before pulling the manifold off, then I'll go back and do it.
I also skipped the running compression test because I don't suspect valvetrain issues (at least not any more). The low MAF reading and restricted exhaust issue has been resolved and she's been running at full power until this misfire started.

 

I have an update on this misfire mystery.

I left off last time trying to figure out this rough idle that came back a week later after seeing a mechanic and internal coolant loss. I've determined this to be bad LIM gaskets so I replaced those. The old gasket definately showed a lot of wear around the coolant passages, so that the coolant loss problem solved. Didn't really see any wear around the intake passages so I thought no air leak there. I did find that the gasket for the upper part of the spider injector was broken, so that may have been the air leak causing the rough idle.

It cranked up fine after putting it all back together (once all the air was blead out of the fuel lines) and ran a little smoother than it did before. Drove it around for a while following the mechanic's advice by ""flushing"" the fuel tank: driving it until E then putting clean fuel in. A couple weeks of this and it still has a rough idle and a SES light that comes on for P0300-6. I can't quite figure out the data to trace this one down.

Here's the output from my Autel showing the freeze frame data below. I'm currently researching what all these values mean to figure out if anything is in an undesirable range. So far the one value I resarched is MAP sensor voltage at 1.91v. I've read that this value shouldn't exceed 1.5v and if so would indicate pressure pulses in the intake. I read this from Underhood Service.

I haven't done a road test or static tests watching the live data yet. Will plan to soon. I just want to drive my Blazer for a while without constantly obessing over diagnostics. But the rough idle, slight hesitation, and flashing SES gets pretty hard to ignore.

 

Another update.

I've put a few hundred miles on the Blazer since the last update. Still idling a bit rough, slight hesitation when coming to a stop, and occasional throws a P0300 code that sometimes goes away on its own but comes back. I can't quite nail down the symtopms on this. I had another mechanic run some diagnostics and he found nothing wrong except for the O2 sensor graphs. He claims that they're acting "lazt". Which means "they're not moving all the way up or down on each oscillation" paraphrasing what he said. I should have taken photos on what the graphs looked like on his scan tool, but anyway, they looked very similar to what I record using my BlueDriver. I'll pull up a graph I recorded several months ago when I was diagnosing the catalytic converter.

This O2 graph was recorded on the factory OEM sensors. S1B2 was very similar. I have since then installed brand new Walker O2 sensors after dealing with an obstructed catalyst, and the mechanic I visited last week recorded a very similar graph on his $2,300 SnapOn with the Walker sensors.

Something is causing O2s to read like this, and I'm not sure why. There's no intake leaks; I rebuilt all the upper and lower bits with new gaskets, o-rings, etc. Fuel injection manifold: new. Whole ignition system: new. Fuel trims are running normal, ignition advance in spec, cam shaft position in spec, fuel is clean running on regular 87 octane gasoline from popular Wawa locations.

I did give my Blazer the Sea Foam treatment this past weekend as recommened by that mechanic (was really the only solution he had given all the other work I've already done). I'm currently running the high milage cleaner in the fuel tank and I did the air intake cleaner spray. I noticed positive results with the Sea Foam when driving it after the "hot soak" period. However it still idles rough. Hesitation went away which is definately a result. Maybe it just needs another round of Sea Foam? Has anyone else done Sea Foam on a 4.3L before? Is there suppose to be lots of white smoke during the process and after the "hot soak". I bearly got any smoke so I'm wondering if I did the procedure right.

What ever is causing O2s to read like this is having on impact on fuel delivery which is causing the misfire. I'm going to run another test drive with my Autel and use it to force the fuel status in open loop so the O2 sensors are ignored and see if it rough idles then. If this comes back confirmed, what would be causing the O2 sensors to be getting bad readings? Exhause leak at the manifold? I don't really smell exhaust under the hood. Perhaps its small enough to not smell, but large enough to cause a sensor misread.