kimmer71

Hi all!

New member...love forums like this because I'm a DIY'er.

Yet another P0300 code post...cringe right? I bought a retired fleet van with 225k miles. Excellent condition and I got a great deal...I think.

Engine has a rough idle and CEL (check engine light) flashes at idle. When driving, it runs great and CEL is off. Code is a random misfire. I can see that a mechanic has been over it already trying to resolve the problem. Apparently he gave up so the company sold the van...to me. New plugs and wires already installed. I checked each cyl. for ignition with a timing light and it did seem like I could see the miss on 3 of em. Coil packs? Truthfully, I'm not positive I could identify a misfilre with a timing light so I dismissed it. Should the timing flashes be a consistent pattern? I assume so but you know what they say about assumptions.

After reading several posts on this site I was betting on a vac leak through the intake manifold causing a lean misfire. I swear I can almost hear a hissing but can't locate it when its running. I sprayed intake with ether with no results. Performed a crude smoke test with no leaks spotted. Now im trying to figure my next move. I've read that it could be the egr...I suppose it could be the coil packs, injectors or even the PCM.

I'm not sure how to test the EGR and I hate to replace it without knowing for sure that it's the problem. Injectors and PCM seem unlikely but I'm not ruling them out.

Anybody have any suggestions, opinions? Thanks.

kevinkpk

It probably is an injector issue. P0300 is misfire, but I'd think it would be more specific as to what cylinder it is. I washed out the accumulator coil on my mazda yesterday, almost didn't make it home as I got something wet. The cel was flashing on it. Got it home 5 &6 cylinder misfire. It cleared up after it got dry.

kimmer71

I removed the idle air valve and it was caked with carbon. I cleaned it up and it idles better now but still flashing the CEL and still isnt quite right. I found that the throttle plate was sticking so I cleaned it up too. I wonder if the idle valve is damaged from sticking and cycling maybe...like popping open and closed. Not ruling out the the throttle position sensor either but am hesitant to drop the $30 to $40 for a new one from a local auto parts store. If I had a scanner I guess I could tell if that was a problem. I expect I would see the position value bouncing?

I'm stumped...have to think on it a bit again. I'm wondering if it could be the o2 sensors or even restricted cats with the milage thats on this thing. Seems like o2 sensors would throw their own code and the cats too for that matter. I would think the thing would run like hell at cruise speeds, CEL would be on and PCM would throw codes if it were cats or o2 were issues though. Argh!

PitCrew

Don't know if this will help, but you mentioned an O2 sensor.

I had received a fault code of P0155 and ordered a 'new' OEM sensor. After replacing the sensor (passenger forward - #2 Bank Fwd) I experienced the P0300 code and the symptoms you're experiencing (but no P0155 code any longer).

Stumped, I re-installed the 'old' sensor and the engine ran fine although it popped the P0155 again. No P0300 code.

Having another separate issue with the van, I took it to the local Chevy dealer where they fixed the separate issue, but then notified me of the sensor failure. I gave them the go ahead and they replaced it. This time everything ran fine.

I checked the sensors visually and they were identical. I checked the installation, the same there. So I dunno. Batch of 'new' OEM sensors that are bad?

Whatever the problem, it's now gone and it's been 7,000 smooth running miles since.

kimmer71

Bought an Autotap scanner on ebay...nice unit. Scanned the engine while running and found cylinders #1 and #3 misfiring. Hmmmm...still seems like a vaccuum leak or possibly head gasket? Burnt valves? On to the compresssion test...