Serious misfire problem with 4.3 csfi.

[jim_in_dorris]

I can see fuel pressure causing random misfires or misfires on all cylinders, but 1 cyl? We are not even sure it really is a misfire at this point. We need to do more diagnosis. Would a weak valve spring cause this? At an idle, there is misfires, but slow count. Above 1700 rpms the count just flies. The O2 sensor doesn't look like it's going lean, but it has a lean rattle. When we resealed the lower intake manifold, the distributor did not appear bad. The brand new distributor cap was replaced with another brand new one and didn't change symptoms.

[PJG1173]

yes a weak valve spring or sticking valve could cause one too. I broke a valve spring and had one heck of a missfire once. not a common occurance on these motors though. at least in my experiance.

[jameslleary]

I would throw in a compression check while your at it.

[1project2many]

Check the basics. An ignition scope would be the best way to rule out spark or electrical issues but you can make a spark tester from an old plug by opening the gap to .070". Install the test plug in series with the normal plug. If spark can jump both gaps then you don't have an issue with the secondary ignition system. A fuel pressure gauge should be installed for testing. Pressure should be at the mid to high end of spec and should not drop substantially in the first few seconds after the engine is shut off. Fuel pressure right on the borderline can do strange things with these engines. Manufacturing tolerances can be such that a few injectors will open with a weak spray while others won't crack. Also, leaking regulators tend to dump too much fuel into the manifold which will foul one or two plugs. Drain holes in the manifold funnel liquid fuel to cyls 3 and 4 when there's a problem. If the regulator's leaking you'll quickly see a nice, clean path indicating fuel has been bleeding into the manifold when the upper half is removed but a visual inspection of the plugs is a quick clue as they'll be soaked. Broken valve springs are unlikely but they will almost always show up on a vacuum gauge as a weak pulse of the needle. You need a good gauge that doesn't have a large damping factor for this test to work. Finally, the EGR valve can get stuck open which will cause all sorts of ridiculous symptoms. Problem usually affects more than one or two cyls but a quickie test / fix with a good scantool is to open valve 100% with engine rpm at 2500. Returning to idle after 35-45 seconds, you'll notice a large improvement in idle if the valve was open.

I have rarely had to do a crank relearn. But I have been doing this for over 20 years and ime when a problem crops up shortly after a repair it can frequently be traced back to the repair. If the crank signal is erratic then the pcm will have trouble firing spark at right time, correctly diagnosing specific cylinders which are misfiring, firing injectors at correct point, etc. Assuming this basic sensor is providing correct information could cause many, many wrong guesses.

[jim_in_dorris]

Okay, I talked with the mechanic today. Here is what he tried. 3 different crank sensors shimmed 3 different ways, multiple crank position relearns, 2 different cam position sensors, 2 distributor caps, changed plug on offending cylinder, changed wires on cylinder, changed poppit valve on injector spider. Nothing changed the symptoms. Still cyl 4 misfire above about 1500 rpm. I found another post on a forum today with exactly the same symptoms, but they never posted a solution. My friend who is a 30 year chevy mechanic at a chevy dealership doesn't have a clue either. So I am driving it with a flashing SES light.

[1project2many]

Well, if nothing else your guy was determined! I'd force a misfire on a different cylinder to make sure the computer is able to accurately report the offending cylinder. Then I'd pull the #4 wire and check for good strong spark above 1500 rpm. Finally a fuel pressure check to cover basics. If everything looked OK there I'd perform a vacuum, compression or leakage test. Vortec exhaust valves can sometimes hang open (guides were very tight from OEM) and once in a while they stick while a piston is coming up. Vacuum gauge would show a fluctuating needle that pulsed each time #4 came around.

[PJG1173]

I have read on other forums that its a best guess by the pcm on what cylinder when it reports a miss. my wifes car was reporting missfire on cyl 1 & 3. replaced #1 sparkplug because it was cracked and both missfire codes stopped. of course I did all this troubleshooting to get a recall from GM a week later for faulty OEM plug wires causing plugs to go bad...

[jim_in_dorris]

Okay, I went and talked to my mechanic again, it appears that the crank-cam correlation value is out of spec. It is stuck at 6 degrees, no matter what he did. Since the vortec V6's have a non-adjustable distributor, it has to fix the correlation in the PCM. The easy solution is to make it adjustable by swapping out the V6 hold-down with a V8 one. I guess I will pull the one off the 283 this weekend.