01 duramax possible misfire
 

Hi guys, I am new to the forum so forgive my first post being a problem. I have a 2001 3500 chevy LB7 duramax. It is a 6 speed manual practically stock with 196k on it. It had injectors at 80k then again at 160k and an injector pump shortly after that. (the injector pump was bad the second time they did the injectors but they thought it was injectors bad).
I am now having a shaking issue at idle and just off idle, clears up a bit after 1200 but is still there. I thought it was the dual mass flywheel so I swapped it with a solid one to find out that it didnt fix it. I took it to a local mechanic that looked at it real quick and said right away he thought it was a misfire. He used a temp gun and # 3 cylinder was only about 65 degrees at the manifold while the others were around 145. He couldn't get to #2 as well. Here is the issue I dont understand, I plugged in a tech 2 and my balance rates are all around 0 or negative besides cylinder 2 which was generally about 1.7 to 2.0 occasionally cylinder 8 would go above 1 as well. When I ran the cylinder balance test I could disable each injector and when I did I could notice a simalar difference with every injector. some were a little less noticable BUT in no way was cylinder 3 "sticking out". cylinder 2 was definatley the oddball as far as balance rates. and if I lowered fuel rail pressure it would slowly climb in balance rate. I THINK I have an injector issue but dont want to start just replacing them. any ideas? I still have a tech 2 handy if anyone wants to help me take a shot at this...

Ok, So I went to the dealer today with it where a good buddy of mine works and we went through some things. He believes all the injectors are fine and that the shake is worse than he thinks it should be for a single misfire. did a compression test on cylinder 3 ( the one that showed to have a misfire) and it was 325. He was a bit puzzled at that point but then we noticed a bolt hanging out the front cover and he remebered he had worked on one that the reluctor on the crankshaft had come loose. Well he stuck it back together and sent me home to pull the front cover and find out but we are suspecting that its the culprit, it does seem to me like it is simply out of time as eratic as it is but yet steady. also the more I dig this seems to be not what I would call a common problem but it does happen...

\I will keep you posted if anyone cares to find out....?

at first i was thinking that it was the harmonic balancer, swapped a soild flywheel into a 6 speed truck one time thinking that was the problem and it turned out to be the balancer and it also was at a low rpm and it made the truck feel like it was missing....next since your allready taking the timing cover off check the dowl pins on the cam gear and really really check the ones on the crank gear, we have had 2 come in that the pins have broken on the crank gears and let them slip time....but they both smoked real bad because they were so far out of time so i still think that it is a balancer problem myself

wow ya that would be great to know because i have the same 6 speed and mine started doing the same sort of thing its weird it will only do it every once in a while. i have checked service records on mine and as far as i know my injectors have not been replaced unless it was done at a shop that didnt keep records and its gettin very close to 200k. i am scared that they are going to go because they are so expencive to replace. but i keep checking my oil and crossing my fingers there is no fuel in the oil

any news?

Ok started to pull front cover and found the balance bolt was tight but not tight enough. Sheared the pin and was off about an inch causing the shake. There ya go! Thanks everyone.

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Lesson to be learned .... tighten the bslancer bolt till your a**hole bleeds and a quarter turn. And make sure the guy that changes your water pump knows that too!