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I have a chronic misfire issue with my 1998 Chevy C3500 with a 7.4 gasoline 454, 150,000mi. My son (who is a mechanic) and his boss at his auto repair business could not diagnose the issue after tearing the top end down and trying all they could think of for three weeks. While they had it, they replaced the crank sensor but nothing else that corrected the issue. I have for at least two years now had a chronic misfire on cylinder 5 of this truck. The issue comes and the check engine light comes on and then after awhile the problem goes away and the light goes out. Until recently the problem has not reared its head but I began having another misfire one week ago on cyl. 7 (code P0307). I decided to replace the plugs, dist. cap, rotor and spark plug wires myself. I found in this instance, the rotor had been incorrectly installed (new 1 year old distributor) with one screw missing but it was in the distributor under the rotor. Installed new parts, truck ran fine...for one day towing up and down hills. Just arrive home and check engine light is again lit with a code P0308!
I am extremely tired of chasing this problem and I’m wondering if you may be familiar with a similar issue that you have encountered or if you might direct me where to look next on this rabbit hole hunt.
Update: 07/04/20 I used the truck again today after having the P0308 code. The check engine light was out again this morning and the truck was running smooth. I towed my trailer approx. 26 miles out and on the way back this afternoon the light once again comes on and the truck was running rougher this time. While driving I put my code scanner on and it was throwing a code P0300 (random multiple misfire) this time. My mechanic son suggested that this may be a computer issue so I bought a new pre-programmed computer. It has yet to arrive so I am hoping this may cure the problem. This is so aggravating. I have to use this truck for work which involves towing my trailer each weekend. I have to get this fixed as I'm worried I may damage the engine. Any help?
Don't want to discourage you but throwing a pcm at it does not convince....I am afraid this ain't over yet.
Can you elaborate a bit on the conditions when the code sets, freeze frame data maybe? Reads like it codes when hot under load? The original P0305 was true, did you feel a misfire? The P0307, P0308 and P0300 showed up after you changed the ignition parts? Are you/your son able to scope the secondary ignition? Did you do a crank relearn after the replacement of the CKP sensor?
Do you know if they tried a vacuum gauge to find a sticking valve? Which (mechanical) testings have been made?
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
The misfire generally does happen under load when towing but not always. The code P0305 was a long term code but then as I originally posted just two weeks ago or so it threw a P0307 code. After that I put the new ignition parts on and then the next day just at returning home from towing, it threw the P0308 code. It it now registering a P0300 code. I wasn’t sold on the PCM being the root cause either but they wanted me to change it and see if an updated PCM would clear the problem. It has yet to arrive.
My son may be able to scope the truck at his work. I do not know if they did a crank relearn after replacing the crank sensor. I’ll have to ask. Don’t know about the possible sticking valve but then if that were the case are you suggesting I have sticking valves on random cylinders because the code indicates misfire on random cylinders? Don’t know about other tests that may or may not have been done when they looked at it.
First thing first: without a crank relearn the computer may throw ANY misfire code, make sure a crank relearn has be done or perform it if it hasn't been done. We'll go on from there...
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
I replaced the computer in my truck two weeks ago. I’ve been waiting this long to see if the code issue would reappear. It has not. The truck is running great now for the first time in years. I wish I had done this sooner. Replacing the computer has eliminated the problem. Thank you for your time trying to solve my issue. With any luck I won’t need to post in this forum again.
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