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Old 12-02-2013, 10:02 AM   #1
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Default GMC Sierra C1500 4.3L Vortec cylinder 5 misfiring

Hello everyone. I am a novice at this so bear with me. I had a leaking manifold/Head gasket and replaced them both. After I restarted, my engine was running very rough at idle when in gear, but smoothed out after getting up speed. After an engine diagnostic, I verified that it was my Number 5 cylinder that was misfiring. I ran the dry and wet compression test and the outcome indicated that I have bad piston rings on the number 5 piston. My highest compression during the test was 162 and after doing the math, my lowest cylinder could have been 138. My compression in number 5 was 130. After the wet test it was 170. My question is this. Would the 130lbs in cylinder 5 cause my engine to run as rough as it is. I replaced plugs, wires, distributor, rotor, ignition control module (this went bad sometime during the compression check) and cannot think of anything else that may be causing it to be so rough. Could my timing being off just a little cause it to do this. My father-in-law feels that the compression being at 130 should not cause my truck to shake the way that it does. He is much more automotive literate than I am and he suggested that I try to find out more in this forum. Any help would be greatly appreciated.
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Old 12-02-2013, 11:20 AM   #2
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The low compression you have on #5 could definitely be the cause of the misfire. Based on your description, I would also take a look at all the vacuum lines and connections. A vacuum leak could also give you some of the symtoms you are experiencing.
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Old 12-02-2013, 12:48 PM   #3
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You had to pull the distributor when you pulled the lower intake manifold on the 4.3. Any chance when the distibutor was reinstalled it is off a notch?
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Old 12-02-2013, 05:25 PM   #4
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I would think 130psi shouldn't make a bad misfire. I'm thinking you have some plugged injector nozzles. If it has the original injectors in it they were a bad design and were replaced by a updated version that cured many drivability problems.
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Old 12-03-2013, 08:06 AM   #5
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It also seems to be getting some intermittent MAF or VAF low input failures. I was never having these problems until I replaced my manifold and head gaskets. Also, there as a part that, for the life of me, could not figure out where it goes. It is the vacuum control switch. It is a small module with one electrical connectors and 3 hose connections. I included a picture of it. Currently, the only thing connected to it is the electrical connector and I do not remember disconnecting any hoses. Is this a functional piece, and if so, where would the hose(*) be at that connects to it. I could not find any.
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Old 12-03-2013, 10:20 AM   #6
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the vacuum lines have to be somewhere, i had one fall down under the wiring harness on the pass side when i did mine. do you have the diagram on your truck?
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Old 12-03-2013, 07:29 PM   #7
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OK, so I have some more information on my vacuum issue. My canister purge solenoid is bolted in place over a hole on the manifold like it should. But, there is a plastic hose that is a part of the solenoid that does not have a hose on it. Should there be? Secondly, the piece that I did not really know what it was ( and called it the vacuum control switch) is actually called the canister purge vacuum switch (CPVS). What does this have to do with my first question above. Should they be connected somehow with a vacuum hose, and if so, which one? The CPVS has 3 different places to either place a hose on or to place over a hose. I hope I at least narrowing down one of my problems. I don't think the low compression will go away, but maybe the truck shaking badly when at idle will. Thanks in advance for the help.
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Old 12-03-2013, 09:28 PM   #8
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Firebird, I only have the Haynes repair manual and it only has wiring diagrams in it.
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Old 12-04-2013, 07:33 AM   #9
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the truck should have a sticker on it.


(1) EVAP Purge Vacuum Switch
(2) Positive Crankcase Ventilation Valve
(3) To Fuel Tank
(4) EVAP Canister Vent
(5) EVAP Canister Purge Hose
(6) Throttle Body
(7) Manifold Absolute Pressure (MAP) Sensor
( EVAP Canister Purge Solenoid Valve


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Old 12-04-2013, 08:09 AM   #10
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Firebird, that really helps. The fact that my neither my purge canister or vacuum switch is not connected (other than electrically), could that be the cause of the rough idling and/or the #5 cylinder misfiring?
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