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Shuddering during idle - Engine Problem for 2005 GMC Canyon?

I have a 2005 GMC Canyon pickup with ~43,000 miles. This January 2012 it started shuddering while stopped or idling, shaking down from approximately 900RPMs to 5-600RPMs. It felt like it would shut down, but then jumped back up to normal. I had the fuel system cleaned out, which included a chemical clean of the throttle body. It ran fine for a few weeks but then came back with a vengeance, now also shuddering while accelerating on the freeway. After the check engine light came on (and then went off), I brought it to the dealership and they told me it was definitely something internal to the engine but couldn’t tell me what. Before I start shelling out $500+ to them just to take the engine apart, does anyone have any other hypotheses or experience with this type of situation? Thanks in advance for any help!!

The check engine light comes with error codes. You need to find out if the dealer pulled any error codes and if so post the exact error codes (format: P1234).

Other than that the only advice I’d give you is to find a good, local, independent mechanic to take it to rather than a dealer. One red flag is up already - the claim that this is an internal engine problem. What was the basis for them saying that? It sounds to me like something rather routine, like a spark plug or other ignition component issue, or even just an intake leak or something. I smell a dealer getting ready to take you for a ride.

If, by chance, you find a good local mechanic they should pull the plugs to have a look. Ask them to check the compression when they do that. If that happens you can write down the exact numbers in psi by cylinder and post them.

Is it related to times you have your foot on the brake? if so a bad vacuum brake hose or brake booster should be checked.

It is typically when I have my foot on the brake, but it can also sometimes occur when gently rolling without applying any gas.

I just checked the service sheet from the dealership and the code/bulletin# is PIP3935D. I’m not sure if the tech wrote this or it is auto generated, but it then states “Due to VIN breakpoint, may need head replacement”. I have an appointment with a local mechanic on Thursday and will ask him for the compression numbers and have him check the brake hose. One other potential piece is that it does not happen as bad or frequently when it is dry and fairly warm (as opposed to cold and snowy/rainy)…

I brought it to a good, honest, local mechanic who took care of it today. The code essentially translated into an intermittent misfire. He checked the brake vacuum hose and the cylinders and found cylinder #3 wasn’t firing. He checked and replaced the coil and spark plug for #3 and also recleaned the throttle body. All is going well so for… Thanks for the advice!!

Thanks for the response, and in comparison to alternatives it sounds like good news for you!

As I understand it (though I just had to go learn this out of my own curiosity), the “PI” stands for “preliminary information.” I can’t figure out what the last “P” is (program? protocol? pixy sticks?). Its seems to be something of a pre-TSB thing. (TSB is technical service bulletin). A PI would only be circulated internally and not yet published in basic recall/TSB bulletin data bases.

The info I found on the 3935D is about potential head issues with theses engines. Its obvious, then, why they would want to hold off on a TSB.

I think most people who deal with cars are happy about TSBs, in general. Its information sharing about common problems and remedies backed up by manufacturer evaluation. It can be useful.

But it can also result in things like this - where hasty diagnoses are made without full investigation. I hope that this does put your problem to rest. However, if it does not I’d say to do some research on that PIP # and try to get GM to recognize a potential design/manufacturing issue and help you out.