"Backfire" - Downhill Only / Not Under Load?
Lead Driver
Joined: Oct 2005
Posts: 6,129
Likes: 1,919
From: Ponca City, OK
Have you seen any noticable change in mpg, or fluid consumption? The PCM uses the IAT, MAT, EOT, MAP, and BARO for fuel strategies plus CPS for injection timing. So if no faults can be found in the electrical side, then we look at mechanical. Pushrods, lifters, and valve springs are what comes to mind with the symptoms you describe.
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Cross-Country
Joined: May 2012
Posts: 98
Likes: 3
From: Fort Collins, CO
Have you seen any noticable change in mpg, or fluid consumption? The PCM uses the IAT, MAT, EOT, MAP, and BARO for fuel strategies plus CPS for injection timing. So if no faults can be found in the electrical side, then we look at mechanical. Pushrods, lifters, and valve springs are what comes to mind with the symptoms you describe.
Only thing I could think of offhand would somehow be all the oil that was getting into my fuel system (think was smoking like a freight train prior to pulling the injector), would be akin to a bend pushrod, bad valve seal, etc.
I could at some point see about a compression test, then also a leak down test to see if any intake/exhaust valves aren't fully closing.
Cargo Master
Joined: May 2018
Posts: 2,442
Likes: 993
I have a 95 OBS that did this for years. Coasting down a hill @2100, letting off and then getting back into the throttle it would back fire, pop and throw blue smoke. Would also pop and smoke when free revving @2100-2300rpm. Drove it for years because it didn't change how it drove. One day I changed out the CPS and it never popped or smoked again.
*edit*
The truck would also smoke like crazy when it was cold , fill the whole yard with blue/white smoke. That changed when I swapped the CPS as well.
*edit*
The truck would also smoke like crazy when it was cold , fill the whole yard with blue/white smoke. That changed when I swapped the CPS as well.
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Cross-Country
Joined: May 2012
Posts: 98
Likes: 3
From: Fort Collins, CO
I have a 95 OBS that did this for years. Coasting down a hill @2100, letting off and then getting back into the throttle it would back fire, pop and throw blue smoke. Would also pop and smoke when free revving @2100-2300rpm. Drove it for years because it didn't change how it drove. One day I changed out the CPS and it never popped or smoked again.
Ironically, I did change the CPS out when troubleshooting the bad running condition (what turned out to be a bad injector o-ring) from the one in there (both OE). Went from Gray to Blue I believe.
I'll throw the old CPS in, since I didn't have this problem prior to the CPS changeout...worth a shot!
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