4.3 runs lean - long!

weeoh5 · February 24, 2005

Hey...I'm new here but in reading some of what y'all have been through with the 4.3, I can feel your pain!

I've been losing my mind with a problem with my 4.3 CPFI Blazer. I've spent more money on fixing this truck than I care to think about...but the problem has persisted over several YEARS. My g/f has been driving it for the past few years and finally bought a new car b/c she is tired of how it runs. The initial problem was the usual fuel injection spider problem...a couple of the injectors leaked badly and it ran WAY rich. I changed out the injector, cap, rotor, plugs wires, o2, fuel filter. It ran better but still surged a lot. The next year it failed the emissions test for high NOx. So it had problem #2, cleaned the EGR passage and valve, and it passed on retest. NOx was still high...but it passed. fast forward two more years to the next test, failed again for NOx, cleaned the passage and replaced EGR valve, passed quite well... shortly after that it started to act up again...stalling at lights, no code, supplement the idle and it was fine. Thats when she bought the new car...and the blazer became MY problem (as she put it!) I did find that advancing the timing 8 degrees made it idle more acceptable.

So the first thing I did was go through the basics, compression, cam lift, look for vacuum leaks, pull the upper plenum. I saw that a lot of people recommended changing the fuel lines at the same time as the spider so I changed out the fuel lines (nut kit). It has a few symptoms that confuse me. First things first, it runs LEAN...the plugs have absolutely no black on them. They were black when I changed the fuel lines and now they are white with lots of rust colored deposits on them. There is no high idle on cold start up...but cold is when it runs best. When it operates in open loop, it drives great with no discernable power loss. Once it hits closed loop, the INT and BLM numbers drop to under 100, which indicates that it is leaning out the mixture to "compensate" for a rich signal. I thought that the O2/02 heater might be bad so I put another o2 in it...with no change. If I disconnect the ECM and let it forget it will drive okay until it relearns its stupidity. As far as how it runs, it has sufficient power, but if you hold it at a constant speed, the engine surges up and down. idle at lights is about 500.

So...I have an OTC scan tool, access to Mitchell, service manuals. I'm determined not to let it beat me...what am I missing? I'd like to think I got it all but I had to have missed it cause it ain't fixed!!

Oh...and fuel pressure is 51-52 while running/driving.... and it doesn't set any codes...

Thanks for any help...

edit: Its hard to say if it detonates...i assume it does. I happen to be lucky enough to have one of the first balance shaft motors that has the needle bearings in the rear of the block instead of the oiled bearings...so the motor makes noise anyway.

Haulin' 8.1 · February 25, 2005

What does your O2 sensor read when cruising above 50? What does it read at idle? Is it steady or limited to a certain range?

Vernon

weeoh5 · February 28, 2005

there isn't a pattern to it...if thats what you are asking. As long as I'm cruising it is all over the board, .35, .8, etc. it isn't always lean or rich or worse, fixed. It seems to be operating correctly at idle and under cruise. I dont' understand why the computer thinks that it needs to lean it out. I think that the information that I get out of the scan tool is a little too slow to get an actual trend. I'm looking into an ALDL cable to use my laptop to datalog it. Maybe then I can see that the ECM is smoking crack!

At this point, it is less about having it run right and more about figuring out what the problem is.