96 Tahoe Random Misfire P0300

[laredo1307]

Just wanted to follow-up. I hate when posters never give a final update

It was indeed the fuel pressure regulator. I had replaced the old injection system with MPFI set-up about 4yrs ago so I just clean up the system best I could and put on a new regulator. Runs great and was even getting over 17mpg on the highway this morning.

Pretty sure it's been failing for a better part of 2yrs. Even though it just started causing misfires my truck has had an issue with RPMs surging randomly, especially sitting in traffic. Still it was weird that it would only misfire in the mornings and run normal the rest of the day.

Hope this helps some others.

 

[Hoesgottaeat2]

Just wanted to follow-up. I hate when posters never give a final update

It was indeed the fuel pressure regulator. I had replaced the old injection system with MPFI set-up about 4yrs ago so I just clean up the system best I could and put on a new regulator. Runs great and was even getting over 17mpg on the highway this morning.

Pretty sure it's been failing for a better part of 2yrs. Even though it just started causing misfires my truck has had an issue with RPMs surging randomly, especially sitting in traffic. Still it was weird that it would only misfire in the mornings and run normal the rest of the day.

Hope this helps some others.

Thanks a ton for following up with that. I'm adding that to my list of "look what problems are in store for me down the road" lol
Good stuff man

 

[Larryjb]

I'm curious, what was cold cranking like? Did it improve after changing the FPR? Often a leaking FPR will cause extended cold cranking because the pressure has bled down, and intake air is saturated with fuel vapour as the fuel escaped through the vacuum line. (A quick check of this is to pull the vacuum hose off the FPR and sniff for fuel smell.)

 

[laredo1307]

No cold cranking issues on first start-up. sometimes it would make a few extra cranks if I had turned it off and then started again. Would misfire in cylinders #5 & 7 during idle and mainly #1 during driving. I don't think it was bleeding fuel but rather not letting fuel through during operation. But like I said, it would only do it during my morning drive. At lunch time or afternoon drive home it would drive fine.

Even after I fixed it I was still having some idle issues and RPM surging but no more misfires at all. Discovered the O-ring on my AIC was pinched and starting to let air through. Replaced that and the truck runs great now.

 

[Larryjb]

No cold cranking issues on first start-up. sometimes it would make a few extra cranks if I had turned it off and then started again. Would misfire in cylinders #5 & 7 during idle and mainly #1 during driving. I don't think it was bleeding fuel but rather not letting fuel through during operation. But like I said, it would only do it during my morning drive. At lunch time or afternoon drive home it would drive fine.

Even after I fixed it I was still having some idle issues and RPM surging but no more misfires at all. Discovered the O-ring on my AIC was pinched and starting to let air through. Replaced that and the truck runs great now.

The extra cranks after turning off and restarting is also a sign of a failing FPR. Fuel vapour escapes through the FPR into the vacuum line and enriches the intake air, causing a hard warm start. If left overnight, much of that vapour dissipates so the air is no longer enriched.

A cold engine usually runs rich for awhile. Perhaps the FPR couldn't produce enough pressure for cylinders 5 and 7 when cold?

 

[laredo1307]

I don't know about cold engine. My commute is 30 miles and it would run like that all way to work. Even if I stopped and turned the engine off for a few minutes. But once I got to work I could go out at lunch and it would run fine the rest of the day.


I bought an A/C delco electronic fuel injection 2nd hand with low miles because the prices had gone up so much on the OEM models. It ran fine for a couple years but the one thing I notice when I changed out the FPR was the one installed had (2) little holes on the outside, one at 12'oclock and the other at 6'oclock. All the other FPR's I've seen and like the new one I installed have the elbow coming down at the 6'oclock with one hole.