P0300 and P0332 code help - Chevy Suburban

[cbarn]

I have a 2004 Chevy Suburban 1500 with the 5.3L V8, about 180,000 miles. Several few weeks ago it started running rough at idle especially after warmup ... RPM will vary between 300-600, sometimes threatens to stall but hasn't actually done so. No apparent loss of power or change to fuel economy, and it runs perfectly at speed. After this started it began throwing code P0300 (multiple misfires) and lit the warning light - however, it will sometimes clear the light on its own, and if I clear it myself with the code reader it may remain off for several days.

 

Yesterday the warning light came on, and when I checked codes this morning I had an additional code P0332 (knock sensor). Cleared the light and it remained off through this morning's 60 mile drive. I've read other threads regarding P0332 and with the age of the car it could certainly be a failed sensor, but given the weeks of P0300 cropping up it seems (to me anyway) that the sensor may just be doing its job and that I should clear up the misfire issue before worrying too much about P0332.

 

I haven't had a chance to experiment myself or get it to someone with a better reader, so I don't know if this is a spark plug issue, vacuum leak, sensor issue, etc. yet - I'm quite willing to have a mechanic do the diagnostics but would like to have an idea which direction to point him rather than have it turn into a massive fishing expedition.

 

Any suggestions or recommendations for what to check (or warnings that I'm not treating this with sufficient urgency!) would be appreciated!

[txab]

If you have to "point" your mechanic in the right direction, he's not much of a mechanic.

 

I'd check intake and intake gaskets for leaks. Clean the MAF and throttle body

[ChevyTech77]

Get yourself a technician to replace the mechanic and you won't have those problems.

[gmtech4]

never tell a technician what to do ,,its on you if it doesnt work !!

 

broken record off !!!!

[Gregs]

First clean the Maf sensor with Maf cleaner (Not carb clean or anything else). If that doesn't fix it then I woul dbe putting my money on leaking intake gaskets, which is a good thing because you have to take the intake off to replace the knock sensors anyways. For the knock sensors replace them both and also replace the knock sensor harness and use RTV silicon to make a "dam" around the knock sensor's holes so you don't have to worry about getting in them. FYI water in the knock sensor's holes are the main reason they go bad.

[cbarn]

Thanks for the help fellas ... I've cleaned the MAF sensor and throttle body with the appropriate cleaners and while that seems to have made a slight improvement it's still noticably rough at idle - hasn't thrown a code yet but I'm pretty sure it will by the end of the weekend. Gregs & txab, I think you're right, probably an intake leak ... is there an easy way to test that here at home, or is it an "only the technician will do it right" job?

 

I'm no technician (or mechanic) but I'm handy enough with a toolbox - I can change my own oil, do my own brakes, and I managed to keep a '75 MGB running OK back in the early 90's (granted, when something went wrong with that it was usually blatantly obvious what had broken). I'd like to do more of the maintenance on the Suburban myself - I've read the procedures for replacing the knock sensors and intake gaskets and I'm confident I can do that myself without screwing it up, but want to be fairly sure that's actually the problem before investing that time & effort. :-)

[txab]

One way to check intake gaskets is to spray something like carb cleaner around the area where the intake an heads meet. See if there is a change in the engine sound/speed.

 

When, if ever, have the plugs/wires been replaced?

[ChevyTech77]

Pull the intake and do the appropriate knock sensor repairs. Replace the intake gaskets when going back together.

[Gregs]

i would also try spraying the carb clean around the intake gaskets and look for rpm changes. Another thing i just remembered and its really common, bad fuel pressure regulator. To test this you can turn the ignition key on for a few seconds to get rail pressure up then pull the vacuum line to it and see if it has fuel in it. If there is fuel then the diaphram is leaking. I've had ones that would only leak when the engine was cold and would seal up as it got hotter. If you have access to a scantool you can watch your long term fuel trims and see if they are out of whack.

[txab]

'04 pressure regulator is part of the fuel pump assembly in the tank

[Gregs]

oops, didn't see it was an 04, nevermind then on the fpr

[cbarn]

I've tried spraying carb cleaner around the intake gaskets and didn't notice an RPM change, but I'm also not convinced I did a thorough enough job so I'll try it again this weekend. Also trying to get my hands on a better scanner so I can see the fuel trims, and as it's been a LONG time since plugs were replaced I'll do that and check their condition to see if there are signs any particular cylinders are worse than the others.

 

I'll wind up replacing the intake gaskets anyway since the knock sensors need their service, but I'd like to make sure I'm not missing something else that I could be fixing at the same time - don't want to take it all apart more than once!