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Thread: Serious misfire problem with 4.3 csfi.

  1. #1
    Fuel Injected! jim_in_dorris's Avatar
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    Serious misfire problem with 4.3 csfi.

    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.
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    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

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    Fuel Injected! PJG1173's Avatar
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    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
    87 4Runner, 15" spring lift, 3" body, chevy vortec 355, 5.29 gears, 44x18.5x15" TSL's, 280hr, 16168625 running $0D
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    Fuel Injected! jim_in_dorris's Avatar
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    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.
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    Fuel Injected! PJG1173's Avatar
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    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.
    87 4Runner, 15" spring lift, 3" body, chevy vortec 355, 5.29 gears, 44x18.5x15" TSL's, 280hr, 16168625 running $0D
    93 S10, 12" spring lift, 4.56 gears, F/R lockers, 36x12.5x15 TSL's, long tube headers, 266HR cam, p&p vortec heads, 16168625 running $0D.
    05 Silverado, 2' lift, 4" exhaust, Bully Dog programer,
    88 Jeep Wrangler (Daughters) 3" lift, 33x12.5x15, no gears, no lockers, no engine mods.

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    Fuel Injected! jameslleary's Avatar
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    I would throw in a compression check while your at it.

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    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.
    Last edited by 1project2many; 06-03-2012 at 11:35 AM.

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    Fuel Injected! jim_in_dorris's Avatar
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    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.
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    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.

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    Fuel Injected! PJG1173's Avatar
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    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...
    87 4Runner, 15" spring lift, 3" body, chevy vortec 355, 5.29 gears, 44x18.5x15" TSL's, 280hr, 16168625 running $0D
    93 S10, 12" spring lift, 4.56 gears, F/R lockers, 36x12.5x15 TSL's, long tube headers, 266HR cam, p&p vortec heads, 16168625 running $0D.
    05 Silverado, 2' lift, 4" exhaust, Bully Dog programer,
    88 Jeep Wrangler (Daughters) 3" lift, 33x12.5x15, no gears, no lockers, no engine mods.

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    Fuel Injected! jim_in_dorris's Avatar
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    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.
    Square body stepsides forever!!!

  12. #12
    Fuel Injected! PJG1173's Avatar
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    I think thats called rotor phasing. there was a discussion on s10forum a while back about it. didn't pay much attention to it since my distributor is adjustable. you might be able to grind the tab of the base of the distributor and use a regular hold down to get some adjustment.
    87 4Runner, 15" spring lift, 3" body, chevy vortec 355, 5.29 gears, 44x18.5x15" TSL's, 280hr, 16168625 running $0D
    93 S10, 12" spring lift, 4.56 gears, F/R lockers, 36x12.5x15 TSL's, long tube headers, 266HR cam, p&p vortec heads, 16168625 running $0D.
    05 Silverado, 2' lift, 4" exhaust, Bully Dog programer,
    88 Jeep Wrangler (Daughters) 3" lift, 33x12.5x15, no gears, no lockers, no engine mods.

  13. #13
    EFI GearHead ! EagleMark's Avatar
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    Quote Originally Posted by jim_in_dorris View Post
    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.
    I just fixed one of them for a guy who had some work done and they could not get it to run right, I pulled the distributor and moved it one tooth and it was fine!

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    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.


    This is all over the place. Misfire, change everything, problem starts after rarely needed procedure is done, do rarely needed procedure multiple times, and now the problem is claimed to be an issue which won't cause a misfire... not a very methodical approach by any means. It's starting to turn into a drama rather than a helpful thread on fixing the engine.

    I've seen cam retard values that are 10 degrees off. The engine runs well and the PCM can still identify a specific cylinder if there's a problem. The only indication of a problem is the check engine light and code 1345. PCM updates cam retard value when engine is first started or when throttle is pressed more than 70% so if that value is off, something is at the heart of it.

    My suggestion doesn't change much. Moving the distributor is not likely to be the answer. If there was no problem, then there suddenly was a problem, you should find and fix the root cause. Check the distributor shaft for free play as previously mentioned. Grasp the rotor and try to turn it to check the press fit between distributor plate and shaft. If those two tests don't reveal a failure, forget everything you've been told by the mechanic and start from scratch. Check fuel pressure. Use a spark tester or plug opened up to .070" to check for consistent spark at each cylinder. Force a misfire and ensure the VCM can report the correct cylinder has a problem then check the mechanical condition of that cylinder. I can't stress enough that you're not doing anything besides guessing if you don't follow a methodical approach and when you ask the internet people to make guesses for you they are usually wrong.

  15. #15
    Fuel Injected! jim_in_dorris's Avatar
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    I am not discounting what you have to say 1project. I am just trying to get a handle on this. I am not getting a 1345 code. That tells me something too. I will test the rotor this weekend when I can actually stop driving it to work. I will also check the shaft for free-play, but the mechanic did say he inspected the distributor because he has seen that be a problem before. My real question is why a CASE relearn doesn't change the cam retard value back to zero? The Blazer is running fine other that the codes. It isn't a real misfire, just being reported as one. You can make any cylinder misfire by pulling a plug wire and it runs like poo. I can't work on it during the week, so I haven't done any of the suggestions yet. I had to replace the Crank Position Sensor because it was throwing a P0336 code. After replacing it I was getting random SES lights (I can't remember what the code was for those) so decided to do a relearn to stop the random SES light. I am getting 17 MPG combined driving, which is what it has always gotten, so have a hard time believing that something mechanical is wrong. It may still be that something is wrong, it just doesn't run like it is. I will also check fuel pressure and look at mechanical on that cylinder if I can get some time with my son this weekend. Obviously I can't really rule anything out until I check it out completely. I only know that it is not the plug, wire, or injector on that cylinder, and that the cam retard is at 6 degrees. Everything else is mere speculation on my part. Unfortunately I do not have access to a scanner new enough to work on this truck, or my mechanic would not have even seen it as money is tight.
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