5.7 vortec random misfire HELP

[rickwolhoy]

Hey all, I've been grappling with a random misfire for a while now. I have a 2000 chevy tahoe z71 with the 5.7 vortec, currently at 322k miles. About a year ago I had a bad misfire at idle on cylinder 3 and after a leak down test I found that the exhaust valve was leaking way beyond spec. I purchased new 062 heads and installed them. After installation the misfire on cylinder 3 was gone but now i am getting a random weird misfire after warmup along with a rough idle. It's only 1 or 2 misfires here and there and it's not on any particular cylinder. I started throwing parts at it but that is not getting me anywhere. I also have slightly high LTFT on Bank 1, about 10-12 percent after warmup. Bank 1 LTFT stays below 10 and somtimes goes into the negative but only slighty 1-2 percent. Any help would be very much appreciative! Below is a list of what I have done to try to get ride of this misfire. I should also note that I have no SES light on and no diagnostic codes. I do have a bluetooth scanner with the dash command app so I can read a lot of information. PLEASE PLEASE I love this truck too mcuh and I plan on doing a minor restoration and keeping it forever.

-062 heads from jegs
-distributor is about a year old
-new cap and rotor
-plug wires about a year old
-new bosch iridium plugs
-new denso upstream 02 sensors
-spyder injector about a year old
-Delphi MAF about a year old
-All new top end gaskets
-new timing chain set
-I have smoke tested it for vacuum leak and I found the intake was leakign around the throttle body and I took care of that and smoke test again and could not find any vacuum leak at all
-gm fuel pump about a year old/ fuel pressure at 60 psi

 

[Schurkey]

What are the misfire COUNTS for each cylinder?

Cheap-junk scan tools can LIE about "random misfires"; the actual problem can be isolated to a specific cylinder but the scan tool reports it as random. I don't know if your scan tool system is able to report individual-cylinder misfires.

 

[rickwolhoy]

What are the misfire COUNTS for each cylinder?

Cheap-junk scan tools can LIE about "random misfires"; the actual problem can be isolated to a specific cylinder but the scan tool reports it as random. I don't know if your scan tool system is able to report individual-cylinder misfires.

It's usually cylinders 1,3,4, and 8. And its usually only 1 or 2 misfires, but I do in fact know its missing because the truck shakes a bit and it's running rough. And yes, my code reader does have the ability to show individual cylinders. So last night I took the distributor cap off to inspect, and test the plug wire ohms, and while I was in there I had a brand new Delphi cam sensor that I thought I would put in. I put in the new sensor and go to start it up and now it's missing bad on cylinders 1 and 4. I didn't touch the timing at all. Weird, right?

 

[Schurkey]

It's usually cylinders 1,3,4, and 8.

Interesting. 1--8--4--3 are the first four cylinders in the firing order. (6, 5, 7, 2 would follow.)

I took the distributor cap off to inspect, and test the plug wire ohms, and while I was in there I had a brand new Delphi cam sensor that I thought I would put in. I put in the new sensor and go to start it up and now it's missing bad on cylinders 1 and 4. I didn't touch the timing at all. Weird, right?

Take another look at that cap. One wonders if there's a carbon track or other defect around those four cylinders.

 

[rickwolhoy]

From what I can tell, everything looked fine? I am not sure what to look for on a distributor cap as to what makes it look bad. I am thinking one evening this week, I am going to start over and make sure it's pointing towards 1 at tdc on compression stroke and reset the cmp retard. It's just weird that when I put that cam sensor in, I didn't move the distributor at all and started having a strong misfire on 1 and 4. Is it possible that the spark is jumping? Would I need to try a new Ignition control module?

 

[Schurkey]

From what I can tell, everything looked fine?

That's a statement, not a question.

I am not sure what to look for on a distributor cap as to what makes it look bad.

Discoloration, mainly. Areas of darkened plastic where it's been "burned" by high voltage and some amperage; or "lines" eroded into the plastic from a terminal to another terminal, or to ground. General oil/dirt/carbon building-up on the inside (or outside) of the cap.

And of course, excess corrosion on the metal terminals. Aluminum terminals conduct electricity better than brass, but aluminum corrodes much more readily. I suppose that a clean brass terminal conducts better than a corroded aluminum terminal, but I've never bothered to measure the resistance.

I am thinking one evening this week, I am going to start over and make sure it's pointing towards 1 at tdc on compression stroke and reset the cmp retard.

Use a scan tool, run the engine at 1100 rpm. Enormously more precise. Trying to do this with the engine off is the baseline adjustment, the fine/final adjustment is done with the engine at fast idle.

It's just weird that when I put that cam sensor in, I didn't move the distributor at all and started having a strong misfire on 1 and 4. Is it possible that the spark is jumping?

Sure. Especially if you've turned the distributor well-away from the + or - 2 degrees of "cam retard". But that's not the only possibility when it comes to cylinder misfire. Could be something else in the ignition, could be fuel, could be valve or ring seal, etc.

Would I need to try a new Ignition control module?

Probably not. I don't see this as a module problem, which would tend to affect ALL the cylinders.

 

[rickwolhoy]

I ended up changing putting in delphi ignition control module, delphi distributor, and new plug wires. At first start up it was running great. No misfire, long term fuel trim was doing great matching on both banks. Around 2 to 4 percent when driving then 0 at idle. Then unfortunately after it warmed up (about 15 min) of driving, the LTFT went back up and I started to feel the 1 or 2 misfires. I am starting to lean towards thinking I need to do another valve adjustment, maybe even while running. I can hear a slight ticking noise on the drivers side....