Been there, done that with the wheel well. Not so with the steering shaft. But, I plan to try. First thing I will do is switch #3 Plug with #1 and switch #4 with #2, to see if the misfires move.

 

Nice addition! Don't forget to read the plugs and check carefully for cracks in porcelain (especially #3) when any are out. Were these new plugs you installed before?

Steering shafts have a plastic cover over the joint at the steering box. Need to pop this off and remove the single clamp bolt - then the steering shaft pulls off and can be positioned out of the way for super-easy access to #3. When going back in watch to make sure the flat on the steering box stub shaft goes back in the right orientation. I just left my plastic cover off.

During any compression check you will want to watch the compression gauge as it builds pressure with each piston stroke (sometimes the valves in them leak and give erroneous readings) so maybe get someone to help by turning the key. I go until the gauge stops building any more pressure and doesn't leak down.

 

THINGS WE KNOW:
2002 Blazer LS, 4WD, AC, 4.3L, "W" 144,400 miles.
New head gasket & valve resurface on RH side, MAF sensor& heater, NGK TR55 spark plugs @.060gap, new plug wires, new Delco dist cap & rotor, new Y pipe & pre cat& catalytic converter, new muffler, one new O2 sensor

Compression (cold & dry with plugs removed)
#1 = 170
#2 = 150
#3 = 120
#4 = 145
#5 = 170
#6 = 150

Fuel pump pressure: (engine not running)
65 psi with ignition key
55 psi after key off
50 psi after 5 min
40 psi after 15 min
35 psi after 30 min

Still get misfire on #3 & #4 AT IDLE (after switching these twoplugs with other plugs)

N0 MISFIRES @ 800rpm & above???

THINGS WE DON'T KNOW:
Battery may be marginal. I have kept it charged, but when load tested, it drops to just above"weak" on the tester.
I don't know any history of first 144,400 miles. I have had the vehicle about 5 weeks.
I don't know which fuel injection spider.
Are these NGK spark plugs OK? Some have recommended AC CR43TSM @ .045 gap, but Haynes Book says these are for 1994 & 95 Blazers with 4.3L.I put these NGK Advanced Auto recommended ones in before I saw other suggested ones.Need an answer on this before I replace the plugs and reattach steering shaft.

I think this is all for now 10/18/2015 @ 1:30pm Can't thank the crew enough for your patience and help.

 

Couple things here, if I'm reading your post correctly your truck failed the leakdown test. Continue on using the instructions in this post https://blazerforum.com/forum/2nd-ge...eakdown-88305/ to further identify what is leaking.

It looks like 3 & 4 have significant difference in pressure. Put about 3 squirts of oil and check the cylinders again. If the pressure comes up your looking at rings probably. But either way get a cylinder pressure tester (the type with two gauges) and you can nail it down for sure.

 

I discovered a drop of gas at the fitting on the gauge. I tightened the fitting, actuated the pump again at the fuel pump prime terminal and the pressure started at 62psi then dropped to 54 when I released the 12 volts. The pressure is still at 54psi after 23 minutes. This was with a cheap Pittsburg fuel pressure tester, so it might have some error in the actual pressure.

 

Your leakdown appears to be succesful then. What did the #3 & #4 spark plugs look like when you pulled them out? Were they oily or white? One thing that is possible is that the plugs are fouling do to piston blow-by when you first start it up and the metal is working it's way to expanding once warm and a little bit more sealed and the plugs have either "cooked" the residue off...or any blow by is making making the mixture to "rich" until the rings heat up. Admittidely I'm not certain if this would register as a misfire since the power is getting to the plugs.

EDIT: Yes, the engine detects misfires from speed characteristics of the crank so even with spark getting all the way the plugs the VCM would detect misfires from fouled plugs. Don't know what I was thinking.

 

Also meant to add that I have "heard" that a weak battery can cause misfires. No personal experience with it. Just thinking out loud...I'm wondering if it is not related to warming up...but rather the time it takes for the battery to get back to an operating voltage after the power taken to start the truck. This is pure just a thought, but maybe search around the forum to see if there is any validity to that. I'll take a look in my service manual to see if I can find something that mentions tions checking the battery for strength with your type of problem.

 

I took a look at my service manual ('98), didn't see anything in the trouble shooting that SPECIFICALLY mentioned the battery state. I also went back and read your thread from the begining. Here are a couple of notes of things I would do (of course use your own judgement):

1. Utilizing the same conditions when the misfire occurs (cold at idle), I would use a propane cylinder to relase propane around the the engine compartment and around vacuum lines. If the engine changes, there's probably a vacuum leak. I noticed you have a tool that can graph real time for you. Have that running at the same time for STFT to see if you see any spikes and drops that you can't hear or notice.

2. I noticed that your taking a look at your STFT but I didn't see where you mentioned your Bank 1 & 2 LTFT. That's numbers you definitely need to take a look at. If the STFT has or is "dragged/dragging" the LTFT to a new standard, your STFT and O2 volts are going to be off also. Optimally your LTFT should be within a few points of 0%. (+/-). If there is a lean or rich condition in your engine, the STFT is going to adjust. After a bit of time the STFT is going to "drag" the LTFT to a new normal. After that happens the STFT and O2 sensors will be within the new normal and will not indicate and issue. EXAMPLE: If the VCM directs the STFT to adjust to let's say -15% based off the info it got from the O2...after a bit of time the LTFT is going to move from it's normal of 0% to a new normal of -15%. Then if the STFT goes to -18% for example, it will look as if it's only -3% beacuse it's using that new -15% LTFT as if it was set at 0%. Of course the O2 sensors will read normal also because they too are using the new "normal" of the LTFT. Use that new tool of yours and graph out both trims and see what happens when you recreate the conditions when you get no misfires.

3. Now I know some are going to disagree with me, but I'm looking at my service manual and it flat out states "If there is no current misfire performing the CKP System Variation Learn procedure may correct an intermittent condition." as step #4 in the "DTC 0300 Engine Misfire Detected" troubleshooting steps. If you haven't done it yet, I would do that. Yes, I know there is disagreement...but I look at it this way....where's the harm? Worse case scenario is...?

Apologies for spelling....on my tablet.

EDIT: Just thought of an analogy on the fuel trims...the Short Term Fuel Trim (bad kid) can be a bad influence on the Long Term Fuel Trim (gullable kid). OK...maybe not a great analogy...

Also, get a cylinder leakage tester and test the 3 and 4 cylinders (test them all actually). Something tells me that your misfires lie there if it's not vacuum or CKP relearn. Meaning leaking rings, valves, etc. The tester will help you pinpoint the compression issue. You can do it without the tester, but drop your air compessor regulator to about 15lbs. If you hear air coming out of the oil dipstick, bad rings/cylinder walls...air out out of the exhaust, exhaust valves not sealing...air out of the throttle body, intake valves. If you need to know how to do this test, let me know. But I really recommend borrowing the tester from a parts store first.

 

Joe, the NGK TR55 are for your vehicle per NGK. How did the old spark plugs look? I'll write more tomorrow.

Les

 

Don't remember anything special. The old plugs just looked like they had been in there a long time. The new NGK TR55 plugs was one of the first things I replaced. when I removed them to do compression checks, they all looked about equally used. Not wet or burned.