Join Date: Sep 2009
Location: Oroville, CA
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Frustrated and annoyed I spent more time last night diagnosing my problem. I did manage to get a stethoscope up on injector #6 and what would you know the injector is ticking like a time bomb. So I guess I ruled out another problem area it does not appear to be the injector.
I then noticed a small 5mm vacuum line at the back of the intake manifold that was ported into the runner tube for cylinder #6 and immediately thought this could be my problem all along. An air leak unique to cylinder #6 I promptly pulled the hose and plugged the port. My excitement was short lived no change.
After all the diagnostics I have performed I sat down and tried to record the exact moments the issue occurs, I noticed a pattern if anyone can shed more light I would appreciate it.
1) The rough idle and stumbling only appears to happen after a long period of sitting with the engine off, for example over night in my garage. When I first start the truck up in the morning and allow it to idle the engine stumbles and runs rough and then throws the misfire code, engine check light is on. However if I clear the light and start the engine cold and then pop the gas 2 or 3 times racing the engine to 3000rpm and then letting it idle again the engine does not stumble and does not throw the code.
2) Second it appears that the code is only thrown when at cold start if I drive the truck stop start though the day on and off, only stopping for an hour or so I don’t get the code.
Something appears to be happening while it is standing for long periods, fuel pressure and drain down does come to mind. Either pump or fuel pressure regulator, however I would expect a more random misfire and again its always on #6.
The investigation continues.
Finally solved the mystery, after weeks of diagnosis which I am glad I did instead of throwing money at the problem.
I ended up loaning a Radiator leak test kit from a parts store and performed a leak test on the cooling system. Well no immediate visual leaks and the pressure remained pretty constant. I left it to sit for about 30 minutes and noticed the pressure had dropped slightly, the longer I left it the more it dropped. I had found a small leak in the cooling system, the question was where?
No visual leaks at all on the outside of the engine, and the fact that the engine idled rough meant it was leaking into the cylinder. This would mean either the head gasket or the intake plenum gasket had failed. The fact that I had consistent cylinder #6 misfire and nothing else would say it could be the intake intake plenum. A quick compression test between cylinder #6 and the adjacent cylinder I think #4 confirmed no leaks between the two cylinders. The compression test also showed as before that the pressure was within manufacturing specifications. This meant the intake plenum was definitely the culprit here.
Also check the codes again and I picked up a new misfire on cylinder #5 which is on the opposite bank to cylinder #6 which definitely confirms unless I have both head gaskets blown the coolant passage between #5 and #6 is leaking in the intake plenum. This is causing the misfire code and the rough idling as the engine tries to burn the coolant, this also explains why it was rough at start but disappeared after a few seconds. Once the coolant that has pooled in the cylinder from standing has been burned up the engine performs normally.
Good excuse to fix my knock sensor while i am down there, well I will update all on the progress once I have replaced the gaskets. Thanks to everyone for input on the topic and again it pays to diagnose the hell of a problem before throwing money at it. I am planning on documenting the repair with detailed pictures, as I have noticed many people on the forum requesting information on knock sensor replacement.
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