GMC Envoy Cylinder Misfire

fire93fire93 Member Posts: 1
I have a 2005 Envoy 4.2 I6. About 2 years ago we had the ECM replaced with our extended warranty because it was throwing codes. Just recently it started to throw codes. I hooked the computer up to it, and it is reading P0305-cylinder 5 misfire. I replaced all the spark plugs with Bosch Platinum 4's and cleared the codes. The same day, it threw the same code. So I got a new coil and replaced the one in cylinder 5. Again, cleared the code and came back. Took the new coil and moved it to a different cylinder to see if it was the coil and still getting the same code. I have tested the battery, which is sitting within normal range. Hooked a ground wire from the ECU to a different ground to make sure that there was an adequate ground and got nothing. I have looked at the graph on the computer while the car is running and all injectors are pushing the same amount of fuel through them. I am at a loss of what to do. Can anyone help steer me in the right direction?


  • mhagemhage Member Posts: 1
    I brought my Envoy in and they came back later and said that I had the same problem. Without my permission, they also ran a leakdown test and claim that I have something going on with that same cylinder (stuck valve, broken valve spring, scored cylinder).
  • tmrut0211tmrut0211 Member Posts: 2
    Had a similar problem on an '02 Envoy. I racked my brain trying to figure out why I had a misfire on cylinder 4. Nothing would make it go away for long. One day, I had the intake plenum off and had the car running. I wiggled the wire at the connector to the coil. It would make it fire or misfire. As it turned out, it was a broken wire going into the connector. Bought a connector with 6" pigtails at the wrecking yard, replaced it and the problem went away.
  • jmes_rodjmes_rod Member Posts: 3
    the coil is new the spark plugs is new... check all the wiring harness, check for the loose connector or cut wire... some of the problem is from the pin connector of the computer its loose.... or check the injector. try to remove the no. 5 injector and exchange to the other injector and check the data again.... if the misfire is same cylinder.. make a compression check....
  • dalebranchdalebranch Member Posts: 3
    edited November 2010
    I was very interested by the postings on this forum. I have a 2003 envoy with the 4.2 straight six engine. its a 4 wheel drive and the #5 cylinder developed a serious misfire. I replaced all the plugs and it did not change. I took it to the dealer $100 later they told me that cylinder #5 had a compression rating of 25psi while the others were at 175 psi. then they told me it would cost an additional $800 dollars for them to take it apart to find out what the exact problem is. I turned them down and (perhaps mistakenly) decided to work on it myself. after discussing with several mechanics that are good friends or relatives, we decided that it was most likely a head gasket or a valve. ( i'm just shy of 100,000 miles). we began taking everything apart and found that the intake manifold port on cylinder #5 was strangely very clean? We finally went to remove the front engine cover only to discover that you can't do that without dropping the oil pan. But because it's a 4wd, the front drive assembly runs through the middle of the oil pan, so I can't drop it without taking apart the drive assemblies on the front of the car. AAAAAAAAGH!

    Does any one here have any answers?
  • dalebranchdalebranch Member Posts: 3
    after great suffering and a good deal of cussing, I finally found the problem to the bad misfire and low to no compression in my 5th cylinder. both exhaust valves were badly burnt and one looked like a cookie that someone had taken a bite out of. Literally a small chunk out of the side big enough to stick a carpenters pencil through.
    MInd you, I still have to replace the valves ( I just picked them up today.) and completely reassemble the front end of my car. The only consolation in this is that I will have spent roughly $600 or so instead of over $2500 by doing it myself.
  • wikster1wikster1 Member Posts: 1
    2005 Envoy here with the exact same problem. how long did it take to do it yourself? dont suppose you live in MN??? not very good with cars and dont have 2500 to fix it at a garage...plz help! what do i do?
  • ridgelineauto1ridgelineauto1 Member Posts: 2
    these are common for head issues fisrt thing I would check valve seats and burnt valves due to driving with misfire do a cylinder compression and cylinder leak down test . its probibally a bad valve seat good luck I hope I helped you out
  • ridgelineauto1ridgelineauto1 Member Posts: 2
    you probibally have a bad valve seat these are more common than u think head replacement is usually the answer . sometimes they even chip apart and I find them in cylinders after removing head . your intake is clean cuz num. 5 is fuel washed . I hope this helps u
  • GordyGordy Member Posts: 4
    edited November 2018
    I have a 2004 envoy with the 4.2 I replaced the plugs and coil packs 6 months ago and it has run perfect. I went to start it up las week and it ran like crap. I put my code reader on it and got codes for the cam position sensor and the vvt actuator solenoid. I replaced both. Now I onlt get the p0300 code and it runs horrible. Any ideas? Anyone? This has been a great vehicle and I can't afford to replace it.
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