7.4 Vortec misfire issue

[ken_man_1]

I know that the top thread on the forum states for 98 and older, but I am having an issue with really knowing where to post, so I hope I don't get hell in a handbasket for posting here.

I've got a 99 C3500HD truck with the 7.4 Vortec, auto trans, 81,000 miles.

Reason for post is that there is a misfire that I can't really trace down, and it happens even under very no/light load situations. If I go wide open throttle it doesn't change. I have gone through and did a full tune-up and the problem went away for a short period of time, but returns not even a month after the job. I've gone through and replaced the air filter, fuel filter, cap, rotor, wires, and plugs. Cap and Rotor are GM Dealer replacements.

I do not know much about the ignition module and wondering if that is a place to start? If anyone has any ideas, I'd certainly appreciate anything to look at. I can usually fix these problems, but this one is depressing.

[2000CAYukon]

Did you have it scanned to see where the misfire is happening?

When the cap was off, did you notice excessive movement with the rotor? There have been cases of a worn dist gear causing misfires on your engine.

//2000CAYukon

[ken_man_1]

Originally Posted by 2000CAYukon

Did you have it scanned to see where the misfire is happening?

When the cap was off, did you notice excessive movement with the rotor? There have been cases of a worn dist gear causing misfires on your engine.

//2000CAYukon

Quite honestly I did not notice excessive movement - however I hadn't thought of that and will check that. I did scan it myself, but no codes were present. Apparently this problem has been plaguing this truck for a while now. I just recently took over the job... you know how it goes. I know the mechanic before would have never checked it, just threw a new cap and rotor on it. Thanks for the speedy response. Tomorrow is another day!!

[777stickman]

My '98 Sub 7.4 had a similar missfire under light to medium load. No codes. When the intake gaskets were replaced we found bare wires to some injectors under the upper plenum Taped them up and secured all the injector wires so they wouldn't rub the upper plenum and all has been good since. That was 4 years and 75k miles ago.

As CAYukon says, the dist is another area to look at. When you check it do it gently with the motor at operating temp. Any movement of the rotor would be suspect and may mean a dist replacement.

[ken_man_1]

Great information - I'll be looking at it on Monday. Much appreciated!

[ken_man_1]

Well, so much for looking at it on that Monday - we had snow and the truck was gone. Yea, they plowed with the engine misfiring.

Anyhow, my other question on this subject--- Will replacing the cap and rotor alleviate the problem for a short period of time? That is what the previous mechanic did to solve the problem.......

Just curious - tearing into the problem today.

[ken_man_1]

I took the cap off and noticed that the rotor does have some play in it - that it not only wiggles a little bit, but that it also moves up and down. I've never had a dist out of one of these before, so is that normal? The movement from side to side is only about a 16th or an inch, but if it moves up and down while turning, it moves a good 3/8ths of an inch. Hope someone can help - we're going to get some snow today!!! Any help is much appreciated.

[2000CAYukon]

It will move up and down due to dist gear meshing with cam shaft. What you want to look for is side to side movement (if any) and excessive rotation (may be easier to pull the dist and look for a worn drive gear; however, you need a scan tool to set the cam retard when you reinstall it).

Another issue to look for is a blocked vent screen. This can cause moisture to build up and the rotor/cap will wear out faster.

How does the rotor and cap look?

//2000CAYukon

[ken_man_1]

There is no evidence of moisture under the cap. I went to Autozone and bought a reman distributor to replace the one. The gear showed a lot of wear, so for $83 I got it. I couldn't find the gear alone in stock anywhere - and the reman unit was made of aluminum instead of plastic (the oem was plastic). Also, the up/down movement is about half of what the old one was. I took close attention to the original position of the old so replacing the new should be cake. I hope that this clears up my problems!!

[777stickman]

ken_man_1: Replacing one of these newer distributors is not like the old days where you can eye-ball it and put a timing light on it and all is good. These newer ones (starting in '96) require an engine capable scan tool that can put the PCM/VCM into the "base timing mode" (cam retard/cam offset). Only then can you adjust the dist, looking at the scan tool, to achieve 0 +/- 2 degrees.

Without following the strict procedures for installing a new dist I would give you a 10-15% chance of getting it right.

Do a search for P1345 in the main forum for more detailed info as there are a bunch.

Not trying to short come your abilities, just trying to let you know what you might be getting into.

[ken_man_1]

Originally Posted by 777stickman

ken_man_1: Replacing one of these newer distributors is not like the old days where you can eye-ball it and put a timing light on it and all is good. These newer ones (starting in '96) require an engine capable scan tool that can put the PCM/VCM into the "base timing mode" (cam retard/cam offset). Only then can you adjust the dist, looking at the scan tool, to achieve 0 +/- 2 degrees.

Without following the strict procedures for installing a new dist I would give you a 10-15% chance of getting it right.

Do a search for P1345 in the main forum for more detailed info as there are a bunch.

Not trying to short come your abilities, just trying to let you know what you might be getting into.

Thanks for the heads up on the scan tool info. Unfortunately I just read it and already went through and installed the thing. Yea, I had troubles, but finally got it running. To my dismay, it still misfires, and the P1345 code came up. It does only seem to misfire on 1 cylinder now whereas before it was all over the place. The truck never gave a code before, so I don't know if I'm in the same position or getting somewhere. I've got more work to do - and a scan tool to find!

[bracketshark]

I had a 97 small block with the same issues, I changed everything, what ended up clearing up the missfire issue was running 2 tanks of fuel with chevron with techtron fuel additive through it. The missfire gradually went away. I also within the last 2 weeks replaced a 99 3500 454 due to the fuel pressure regulator rupturing and filling the motor full of gas, hydralicing the engine. I only mention this because i didn't see where fuel pressure was checked. Good luck, I can't wait to hear the outcome.

[ken_man_1]

Originally Posted by bracketshark

I had a 97 small block with the same issues, I changed everything, what ended up clearing up the missfire issue was running 2 tanks of fuel with chevron with techtron fuel additive through it. The missfire gradually went away. I also within the last 2 weeks replaced a 99 3500 454 due to the fuel pressure regulator rupturing and filling the motor full of gas, hydralicing the engine. I only mention this because i didn't see where fuel pressure was checked. Good luck, I can't wait to hear the outcome.

Just a quick reply to this post...we've run the fuel additives and injector cleaners through it a few times and still didn't cure the problem. I'm back at it again today to see if I can get it cleared up.

[ken_man_1]

Ok, final post on this problem - it's fixed. I had checked the history on this truck with the boss, and he showed me that the injectors were replaced exactly 1 year ago to the date (imagine that - does that ever happen??). I took it to the dealership and they put it on the super duper scan machine - and sure enough cylinder 1, 4, and 7 were misfiring. However, they weren't misfiring due to and electrical failure. The injectors went bad again, and were replaced under warranty. However, the company did spend $2200 last year on this same problem - but it's fixed now. I'm very thankful for everyone's help. I had no idea that the injectors were faulty because they were only 1 year old.

Again, thanks a ton for the help with the distributor knowledge and so forth. BTW, the distributor was certainly an issue - we don't have any troubles with going through caps and rotors anymore.

[usa157]

Originally Posted by ken_man_1

Ok, final post on this problem - it's fixed. I had checked the history on this truck with the boss, and he showed me that the injectors were replaced exactly 1 year ago to the date (imagine that - does that ever happen??). I took it to the dealership and they put it on the super duper scan machine - and sure enough cylinder 1, 4, and 7 were misfiring. However, they weren't misfiring due to and electrical failure. The injectors went bad again, and were replaced under warranty. However, the company did spend $2200 last year on this same problem - but it's fixed now. I'm very thankful for everyone's help. I had no idea that the injectors were faulty because they were only 1 year old.

Again, thanks a ton for the help with the distributor knowledge and so forth. BTW, the distributor was certainly an issue - we don't have any troubles with going through caps and rotors anymore.

I had similar problem on a 1988 7.4 turned out to be loose ground wire from block to firewall. good luck.