Serious misfire problem with 4.3 csfi.

[jim_in_dorris]

2002 Blazer 4.3 Yes I know about the TSB and all the problems with this unit. I had to take it to another mechanic (who I trust) because my son's work doesn't have a scanner to do a crank relearn. I took it in Thurs. for a crank relearn, and the problems began. First the SES light just came on. misfire code on Cyl 4 and 1. Cleared them and drove back to work. SES light starts blinking. Cr@p! Back to the mechanic. It it misfiring a bunch on Cyl 4, especially above 1700 RPM. replaced brand new Dist. cap with another one. no help. Left it with him to see what he could do. He informed me Sat. morning that he has no idea what's wrong, but the voice mail didn't tell me what he did, so I can't eliminate anything until I talk to him monday. I assume he tried wires and plug (both of which are brand new, and yes the plugs are the correct AC Delco plugs) You can here a lean rattle when it starts misfiring, so I don't want to drive it, but my truck has the entire front axle out of it. It is probably going to stay that way for a while, since I can't afford to fix both vehicles right now. I think that I may have to go with the upgraded spider to fix this, so I am looking for a decent price online right now. It also has a crack in the lower manifold that needs fixing, and I found a brand new entire upper and lower manifold with CSFI and throttle body for $329 plus $30 shipping, I might go that way too. I really don't know yet, I will know Monday what I am looking at.

[black dawg]

Did anybody notice what the cam/crank offset number was? I have seen several badly worn distributor gears in 4.3s lately. This causes all kinds of missfire issues

[PJG1173]

I work on these alot. common problems are dist cap going bad reguardless of age, fuel pump, FPR, injector spider/poppets bad, and worn dist gears. Occasionally we'll find the plastic dist base is egged out causing the rotor to wobble. I would first check the fuel pressure

[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.