2002 Tahoe has a misfire at idle

[JohnnyK]

My 2002 Tahoe misfires at idle. I've replaced everything from plugs, wires,coils, injectors, purge valve and MAP sensor. Nothing has changed. All it's giving me is a. misfire code. It's a inexpensive test but I can't spend a fortune trying to get this thing running

 

[mattbta]

P0300 or P030x where x is a number 1-8?

 

[JohnnyK]

My scanner was only giving me a PO300. I've also done a fuel pressure test on it and when I hit the ignition it's at 58 pounds

 

[mattbta]

So random misfire. Does your scanner have the ability to show counts on each cylinder? If so, is one more than the others by a large margin?

No upgraded camshaft in there?

 

[JohnnyK]

It doesn't have that ability and it has the factory cam in it

 

[rockola1971]

Pop intake snorkel off and with engine running, spray mists of water from a spray bottle into throttle body. Keep doing it without killing the engine for 5-10mins. This will cause any carbon buildup in combustion chamber and piston crowns to dislodge and shoot out the exhaust. Put it all back together, clear codes and see how if misfire comes back. High mileage engines typically have alot of carbon buildup.

 

[JohnnyK]

Pop intake snorkel off and with engine running, spray mists of water from a spray bottle into throttle body. Keep doing it without killing the engine for 5-10mins. This will cause any carbon buildup in combustion chamber and piston crowns to dislodge and shoot out the exhaust. Put it all back together, clear codes and see how if misfire comes back. High mileage engines typically have alot of carbon buildup.

Pop intake snorkel off and with engine running, spray mists of water from a spray bottle into throttle body. Keep doing it without killing the engine for 5-10mins. This will cause any carbon buildup in combustion chamber and piston crowns to dislodge and shoot out the exhaust. Put it all back together, clear codes and see how if misfire comes back. High mileage engines typically have alot of carbon buildup.

The misfire is so bad that it won't stay running after startup. The Tahoe was running great up until I stopped at a red light. Then all of a sudden it died. No signs of any problems. I have to put my foot on the throttle for it to start.

 

[afpj]

If you pull the vacuum line off the fuel pressure regulator, does any gas drip out?

 

[JohnnyK]

No fuel comes out of the vacuum line.. I put my finger over the hole and I can feel a small suction while it is running

 

[Fless]

What's the fuel pressure when you make it run?

When was the throttle body last cleaned, and how does it look now?

 

[JohnnyK]

What's the fuel pressure when you make it run?

When was the throttle body last cleaned, and how does it look

What's the fuel pressure when you make it run?

When was the throttle body last cleaned, and how does it look now?

The fuel pressure is at between 50 and 55 when it's running. Hitting the the throttle makes it go up a little more. I have not cleaned the throttle body. It does have 2800,000 on it but it looks clean. I put Seafoam in it every other full tank.

 

[Fless]

So the KOEO fuel pressure is 58psi and the regulator is bringing it down to between 50 and 55 -- that seems like the fuel pressure regulator isn't bringing it down far enough. Others may chime in, but I think the running psi should be around 45.

 

[drdave81]

This code can be set by just about anything. Fuel, spark, compression or timing. You really need a scan tool that can read live data, or you're in the dark. You need to see misfire monitors to know what cylinders are misfiring and how bad. You need to see fuel trims to understand what kind of a misfire it is. If you can't see data, you can throw parts at it til you're broke. I'd suggest either buying a scan tool or just have it diagnosed. Once diagnosed, you can decide if the repair is something you want to take on, or have a shop do. But using a parts cannon is just a waste of money.