Hey guys I a problem I trying to diagnose with my 2000 Tahoe 4.8L

It started getting very hard to start over the course of about a week. I'm talking like 20 5+ seconds of cranking. Computer threw a P0300 code. Random misfire, so it's more than one cylinder.

1. I put new plugs and wires on it.
2. Checked Fuel pressure, it's at 50.
3. New EGR valve.
4. PCV is good.
5. No vacuum leaks.
6. All coils have spark.

After the EGR valve, the CEL went off and it started running better, but there is still the hard start.

The new plugs are already black.

If it idles too long, the CEL will come back on and even start to blink after a few minutes. When I start to drive, though, it will go back off.

I unhooked the battery for several hours hoping that it would cause the computer to relearn some stuff after the new EGR, but that didn't work.

There are no other codes than the P0300 in memory.

I'm pretty good at troubleshooting, but I don't know what to do with this and I really don't want to just start throwing parts at it.

 

thomastedder
Welcome to the forum... when you shut off the engine does the fuel pressure stay or go down quickly?
Have you cleaned the throttle body and plate?
Clean the IAC (Idle Air Control valve)?

 

The fuel pressure should be 55-62 psi with key on, engine off. It should drop by 10-15 psi from there with the engine idling. Maybe the fuel pressure regulator is leaking fuel into the intake through its vacuum port (need to check that first, given the age of the vehicle). Also, the PCM needs accurate input from the MAF/IAT and pre-catalyst O2 sensors to manage the air fuel ratio. The freeze frame data may shed light on something amiss.

 

The pressure stays when the engine is turned off.

I haven't cleaned the throttle body or the IAC.

I'll try these next.

 

Umm, that vacuum port on the fuel pressure regulator has fuel in it. Guess I need a new one.

 

I just disconnected the fuel pressure regulator vacuum line and it runs great, but there is fuel leaking out of the regulator vacuum port while it is running. So I'm going to replace that and hopefully I'm good.

 

Yup, it sounds like you found the culprit.

 

Yep, that fixed it. It was the fuel pressure regulator. Thanks guys. I really appreciate the help.

It runs like a brand new truck now. Even with 170,000 miles.

Now I'm going to go get the catalytic converters checked to make sure I didn't do any damage to them while it was running that rich.

 

The cats are fine.

 

thanks for the follow up