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2007 gmc envoy_slt
1 Posts
Discussion Starter #1
First, I would like to say hello. I've been a member for several years, but have never posted before. I know there are several forums discussing the P0300 code, but none seem to be similar to my issue.

Vehicle: 2007 Envoy SLT with the 4.2l engine. OD: 188,000

About a month ago I decided to do some maintenance on my wife's Envoy since it was finally warming up. Here is a list of what I did:
New spark plugs (bosch platinum +4)
Dropped tran pan, cleaned, new filter/seal
Changed oil/filter
Drained coolant, circulated distilled water, refilled with approx 70/30 coolant
Replaced failed thermostat
New idler and tensioner pulley
New serpentine belt
Replaced gas cap to clear the fuel cap code
New k & n air filter
Cleaned throttle body

I'd like to mention all fluids I use are Amsoil

A week later my wife calls me panicking because her truck is idling horribly and the engine light is flashing. The errors were P0300 and P0302.

Did a compression test on all cylinders, all were good except cylinder 2. Here is how the dry test went:
170/0/190/0/0 - Most cylinders averaged 195
The next day I did another dry then wet: D - 30, W - 90.

After reading several forums here, I decided to remove the Bosch and install AC Delco plugs, replaced cylinder 2 coil (since I had already ordered it as a precaution) and replaced all remaining coil boots using OEM parts. I've learned a few things about this issue. When the truck is cold, it misfires and I can hear a hissing sound coming from the exhaust side. When the truck is warmed up and on the road, I noticed if I get the RPM's over 5k, the truck normalizes and runs fine. If I start it back up 20 minutes later, it runs fine since it's still warm. I ended up draining the oil again :( and refilled with a quart of MMO to (hopefully) clean up a stuck valve, and poured 2/3 of sea foam into the gas tank, and 1/3 into the vacuum line from the brake booster? that others have done on here.

I have a new OEM fuel pump to install, but I'm almost positive that isn't the problem. I bought a new o2 upstream and will install that tomorrow morning. I may also do another compression test, leak down test, fuel test, and vacuum test if someone thinks it may be helpful although I only know what the compression test p.s.i should be.

Does anyone have any suggestions, or agree that it may be a stuck valve, or bad valve seal? I want to get as many opinions as possible before tearing into the engine. I tried to rent a smoke machine to check for vacuum leaks, but NO ONE has them :sadcry:

I know most think I'm crazy for wanting to fix an issue with a truck with this many miles, but the fluids looked really good when drained, and 90% of the miles were highway. Plus I just bought a new diesel so money will be tighter than normal for a while. Sorry for the long post, hopefully it was detailed enough.

*P.S. the only code I receive now is the P0300
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