02 Xtreme light throttle misfire....

[Booch]

Have an 02 Xtreme, 4.3 liter, automatic, 120,000 miles.

Started having a random misfire under part throttle, especially noticable when wet outside.

Last summer, I replaced plugs, rotor, and cap (AC), as well as Taylor Made 8 mm wires; problem went away.

A few days ago, car dies on the side of the road; bad fuel pump, replaced with a new AC item. Happened too far from home so had to eat the $750 repair bill (including towing, but still sucked!).

Ran fine for a day or two after the new fuel pump......but just recently began having a pretty severe misfire under part throttle....idle and WOT are fine. Actually threw the codes for lean on left bank, lean on right bank, and 0300 misfire.

Looking at posts, seemed to sound like either an MAP or stuck EGR valve. Since I didn't get a code for the MAP, and the EGR had never been cleaned, decided to go down that road and purchases some cleaner and a new EGR gasket.

Got home and started looking for the EGR..........and looking, and looking.....either I'm blind, or I don't have an EGR valve (even though the carry a valve and a gasket for the vehicle).....

Any thoughts? Am I going down the wrong road with the EGR? Don't think it's fuel pump related, since running at idle and WOT does not cause a misfire.

Thanks in advance.....

[MT-2500]

If it was a EGR valve open it would no idle good.
If not a bad secandary ign system.
Damp weather miss points to secandary ign?
Got good hot blue spark to all plugs?
Maf sensor or low fuel pressure is leading cause of missfire codes.
Have you cleaned or checked MAF sensor.

What was the code no you are getting?

I would Start with fuel pressure readings.
The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

[Booch]

The codes I pulled were P0174 (left bank too lean), P0171 (right bank too lean), and P0300 (misfire).

Forgot to mention that I had pulled a code for the MAF sensor a few months back, and that has been replaced as well.

I'm planning on checking the fuel pressure this weekend (need to borrow a gauge), and I'll also be calling the shop that installed the new fuel pump as well (that won't be until Monday).

[MT-2500]

Post back all fuel pressure readings.
And recheck for any history or present MAF sensor codes.

[blazes9395]

Early '02 and up do not EGR valves, thats why you couldn't find one, but yes I do see some parts places do carry a valve for these even though they don't have one.

[Booch]

You know when they say always start with the basics?

Wasn't fuel related at all; distributor rotor had a hole burned through the center, and the cap was corroded to heck.

Replaced both, runs fine with no misfires.....a GM technical service bulletin (included with rotor) warned of a ventilation issue inside the cap, leading to premature wear and failure......they recommend that you make sure the vents in the distributor housing are clean, and if you have a screen over the vents, remove them!

Also, if you have an AC line near the distributor, they recommend you cover it with a 9 inch long piece of foam to keep condensation from getting into the distributor.

Lastly, found my next project....at some point, the front cap bolt had been overtightened, and the mounting boss on the distributor (that holds the cap in place) was cracked in half and broken off; I'm hoping some JB MetalWeld will work to fix this!

MT-2500, thanks for your help!

[MT-2500]

You are welcome.
Let us know how it goes.