What is your MAF reading at idle in drive hot when you feel your problem? Intake air temp? coolant temp? throttle position? Would like to eliminate these sensors as possibilities. Did you fix exhaust leak?

 

This might be interesting to you on the MAF sensor. Also another way to get at volumetric efficiency in general for the engine (snap test checks each cylinder)
https://blazerforum.com/forum/2nd-ge.../#&gid=1&pid=1

 

Thanks George and Les, the fuel trims indeed seem goofy. The reason I asked about idle RPM was that with this rough idle problem when the A/C is on at idle it makes the rough idle vibration a lot worse and the RPMs drop down near 500, I usually just turn the A/C off when I'm idling because it's that bad. To add to this, there is also a notable rpm drop when I turn my wheels when stopped, it's like the engine is struggling to handle any extra load? I have a video here of me idling and turning the A/C on and off and seeing the difference in RPM's and IAC counts, also have the MAF reading with that. You can also hear in the video the power steering pump when I'm turning the wheels and see the RPMs drop with it. At a time in the video I had the A/C on, wheels turning, and in drive just trying to make it stall, and it didn't, only very large RPM dips. But I'm sure this RPM dip is not normal, right? I noticed today I had the A/C on and I was pretty much home (in my ally about 10ft from the parking spot) so I left it on to see what happens, and right when I stopped the RPMs dropped a lot, I'm sure it was very close to stalling, then the RPMs went back up to the normal rough idle speed. Let me know what you guys think about this. So I have taken these days here to further confirm this is not a fuel delivery problem, it will cold start in the morning first start of the day with no prime, it will start most of the time instantly, it's just sometimes (rarely) it doesn't start and needs a long crank to get going? And I can't seem to narrow it down to anything in particular happening. It will always start with a long crank if I just keep cranking for a few seconds, but something is still not right here. My exhaust leak is not fixed yet, I have all my bets on cylinder 1 right now and my OTC compression tester is here so tomorrow marks a big day, I will see one way or another what is going on with cylinder 1. Hot and cold, static and running test on cylinder 1, maybe even rainy and dry test too LOL, I will not be leaving any stone unturned here, including cylinder 3. I really want to finally find what's going on with my Blazer here, whatever it is, it was like this before it bent a valve as well. All the same stuff, its just now I have the set screw turned out a bit so it doesn't set P0507 anymore and has some IAC counts. If I continue to not find anything I will slap the gasket in the exhaust and see what that does. That is an interesting picture about the MAF sensor and volumetric efficiency Les, however I am a bit lost with it, to really do that test, do you need to block the pcv valve? I looked at my MAF readings and compared them with that picture and they seem in check... I think?

 

I don't think there is any need to block the PCV to check MAF sensor readings.

I'll do a video with the A/C on/off and steering turned against the lock for your comparison - will do same Dash Command screen and PIDs. Yours does seem to drop quite a bit in RPM. But we can compare directly.

 

Here is mine - AC on/off - turning against steering lock. Note my LTFTs! They were near zero when I first started the vehicle and warmed it up.

Actually your IAC counts and MAF with AC on are pretty close to mine.

My Mass Air Flow was 155 at 4900RPM when trans shifted under full throttle.

I'm not seeing any smoking gun here. You may want to check your full throttle at shift MAF reading for comparison. That will also check volumetric efficiency in general.

 

Thanks Les, I will compare your video to mine more later. I just finished with the compression testing, the short, nothing was found. While the idle and snap measurements are different from my other tester they are all the same pretty much. So the top tests in the picture are with the engine cooler, while I did cylinder 1 at the top of the picture it was halfway between cold and operating temp, I did cylinder 5 right after then waited for some time to do 1 again (this is the second #1 on the picture), then I did cylinder 4 and 6. Then I brought the engine to operating temp and held it there by running it in between tests and did the rest of my testing, these tests are denoted with an 'H" beside the cylinder number. Right after sending this I will go test cylinder 1 cold and also cylinder 3 all around, if I still don't find anything I can be sure in saying my engine is working fine. What about the tick then if this is the case? You can see on 1H and 4H tests I had 90 and 120 written for idle and snap then crossed them out. This is because I went back and tested them again, and they gave different results. For both of these, I tested another 4 times approx to make sure what I was changing the results to was the real thing. It seems the tester will read 90psi sometimes at idle because it happens right when it cranks up, I even had it saying 90psi at idle on cylinder 4 right from when I started it up then bled the pressure down with it running and it only climbed back up to 60psi idling. Adding to this I can be pretty confident in saying the cylinder 5 test with 90 idle and 120 snap was also this 'glitch' per se where the 90psi is from it cranking to start.

 

Yep, for running compression is best to get running and burp the compression tester with its valve and then let it build up again while holding to the 1200 rpm. You can even burp and repeat more than once if you like.

If it all tests OK then tick is just a noise, not the cause.
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your snap tests seem very low. Are you getting wide open full throttle and going up to about 2500?

 

Thanks Les, here are the tests from today. Again I found nothing, I feel I may end up testing more cylinders cold to see if they follow 90 idle 120 snap like #1 and 3, but I think they will. I bled the gauge many times to watch it always come back to 90psi on cold idle. And my problem is with the engine hot too and I have all the idle and snap measurements here which I feel confident about for all cylinders, and they are all the same more or less. Let me know what you think Les about the difference in measurements between hot and cold, maybe it is the difference in idle as cold idle is around 1000rpm if I'm not mistaken, and hot around 600rpm. Idle tests are being done at idle (no throttle), snap tests are done by a quick snap wide open with the throttle, watched the tach as I did the tests this morning, and it went up to around 2500rpm on the snap. Am I supposed to be doing the idle tests at 1200rpm? I guess most important is that they are all tested at the same rpm. I feel pretty confident that my engine is mechanically working correctly now with all this testing, I think I will put in the aluminum distributor today or in the next few days with the new Delco cap. After that I will be able to put to rest any ignition related concerns, I will also put the exhaust gasket on in this time and let you guys know where I'm at after that.

 

I will be interested to see if your idle fuel trims change significantly when the exhaust leak is removed.

George

 

Me too George, that is all I really have to go off now, the fuel trims on both banks being around -10% at idle. So I will be chasing this now and as you said the common culprits for negative trims are MAF sensor, Fuel injectors, exhaust leaks, Ignition, compression, O2 sensors, Other sensors. My fuel delivery system is good as can be, tested to death, ruling out fuel injectors, with all of that compression testing I'm going to rule out compression too. After I fix the exhaust leak and put a new dist in I will rule out exhaust leaks and an ignition problem. Everything left is just a sensor of some sort, hmm. Guys, what could set P0507 (0 counts on IAC), make a rough idle, and STAY after an engine rebuild? (ruling out vacuum leaks) I guess I didn't replace any sensors so they are all the same as before. Maybe I should go back over the P0507 diagnostic tree.