Would like some advice if any of you would like to offer.
I have a 1996 Chevy Tahoe (5.7L, 185k miles) that I think is misfiring. What typically happens is a shuddering at about 50-55mph while in 4th gear (though to a lesser degree, it will shudder at speeds above that too) and lightly accelerating or keeping speed on a small incline (~5-10% grade). During this time, there is no change on the tach. My check engine light is not on. Spark plugs and wires are new. The problem occurs while using 93 octane fuel. The shuddering is never noticeable in gears 1, 2 or 3, and is not noticeable at full throttle. I replaced the fuel pump about 6 years ago.
I brought it to the transmission shop, but was told that isn’t where the problem lies. Second, I took it to a mechanic. He told me that he couldn’t pin-point the cylinder, as there were no codes to unveil the problem.
My question is what are the causes I should look for, and how can I address them short of an engine overhaul? I recently read for the first time about Seafoam, and I’m intrigued. I’ve taken good care of this truck and can’t afford a new one. Thanks for any help.
If you have an engine misfire you will damn well know it…it wont go away on its own and it will certainly affect how she idles and runs. I was going to mention that it sounds more related to your Overdrive in the trans…like the torque converter doing some silly lockup stuff… i.e. locking up the TC and unlocking it…that can make you think the engine was doing something funny. But back to the engine…trust me you would surely know you had a misfire and so would your engine computer…You would have thrown a code on that no prob.
If you start the engine and let id idle…open the hood and look at it…Is it shaking? like rocking back and forth rapidly? If not …shes not missing. Rev the engine and hold it at say 2500RPM…is it smooth or do you hear and feel a miss? A “shudder” is the exact word I would use when talking about a trans issue…either with the trans clutches having difficulty locking up or the torque converter acting up. Not sure if i am heloing you much here…If I could actually see the truck I would tell you what it was in a minute…but…
Am I sure? Yes I am… I have been working on engines since I could hold a wrench. I can diagnose issues from sound as well as smell and feel at this point… So no…I dont think a misfire would slip past my examination.
What kind of vehicle do you have that the misfire is so hard to detect? Also I should add that it may not misfire all the time, which might confuse you or make it hard to tell if its got a miss. I have had some vehicles where is was difficult to notice, but was noticeable nonetheless… also if you have a compression related misfire those are the hardest to detect, but detect you can… Ever use a piece of notebook paper on the exhaust pipe end…to detect a misfire? How bout a vacuume gauge? At any rate, I havent or very rarely ever “missed” an engine misfire…
OK so maybe I shouldnt have assumed it was a very obvious type of misfire…that was wrong on my part…but I can say that I wouldnt miss the issue even if it were a difficult one to find… I just been doing this too long at this point… You get pretty good after a while.
SO yes its true that sometimes they can be difficult to detect…my bad in saying that you would damn well know it was misfiring… I would… Not everyone would. So yes I was wrong in saying that you would damn well know it… My apologies.
But a misfire should be able to be sussed out when under load or other conditions… One way or another you can find it… SOmetimes YES>…they are a bitch to find… I take it back when I said you would damn well know it. LOL…sorry
But on this vehicle…the engine ECU should tell you about the misfire…which is the other way you would know it…(and was somewhat the primary reason that I said he would damn well know it, rather than damn well feel it) OBDII systems are very sensitive and not shy about telling you about a misfire in a modern engine.
I still think its his transmission to be honest. As you should feel the miss one way or the other…and the ecu would definitely detect it…which is why I dont think he has a miss…the ecu would be a big tattle tale in this case.
Honda, I tend to agree with you. I would suspect a misfire (and I can damn well feel it) wouldn’t discriminate between gears. I’m going to look into the torque converter as a possible cause, because it makes sense that, under a load, that would be involved. Thanks for your input.
Circuitsmith, the plugs are new. The EGR valve sounds like a very possible cause, however. Thank you for your insight.
Do any of you three have an opinion on Seafoam?
Thanks again to all of you. I think I’ll have a good idea of what I’m getting into now.
As a follow up, after I did a lot of reading on several message boards, I ended up using a can of Seafoam as directed and the problem disappeared. I was shocked that the shuddering went away, but it’s been about a month since using it and my Tahoe is still without any shuddering and still no check engine light.
Hey thanks for the update and glad you are running smooth again. Which Seafoam product did you use…they make a ton of stuff actually. Did you add it to your fuel or your crankcase… Which one will tell me what fixed it. Couldve been a lifter? A semi stuck valve? etc… Let us know and happy Tahoe-ing