Fuel Pump / P0300 Engine Misfire

[JpJr]

Hi all, hoping for some good advice here, 3 mechanics don't know what to do, and i'm trying to avoid the dealer...

Background: I purchased this 1996 Yukon about 8 months ago, has 100,000 miles on it.

After driving it a bit, realized that the exhaust manifold gasket was shot, so replaced that. Then replaced two of the four O2 sensors. Long drive later, misfiring on #2 and #6, so did a full tune up, air/fuel filters, OE parts (AC/delco plugs, cap, rotor, wires). Ran better after that.

I have a P0300 check engine light coming on sporadically. Seems like it happens when the tank is less than half full, and usually on while accelerating (softly, if I gun it, truck runs well). Haven't been able to figure it out yet. Transmission went on me last month, so I fully replaced the tranny, planetary, all that happy stuff. After dropping $3000 at AAMCO, I want a perfect running truck. The P0300 Random Engine misfire code keeps coming up. So I replaced the ignition coil, and had the injectors cleaned at NAPA (for $100), and ignition module. Ran better for 2 or 3 days, then tank went below half full again and i'm back at square one. I had the fuel pressure checked, and it seems like it is a LITTLE low, off by maybe 2 or 3 PSI, but nothing drastic.

I have a 'gut' feeling that it is the fuel pump 'starting' to go on me, but I don't want to spend the money to have it fixed professionally for a 'maybe' problem. Is the fuel pump a possibility for all my problems, or should I look elsewhere? And -- if it is the fuel pump, and I attempt the fix myself, should I replace just the pump (around $100 for an AC/Delco part) or the whole module/assembly which includes the fuel gauge sender, etc, which is like $300???)

Sorry for length, but I'm losing it here and a new (to me) truck!

[MT-2500]

What engine and what are the fuel pressure readings?
Key on eng off and engine starting and engine running and engine running on the road pressures?
Any fast leak down on the fuel pressure?
Also block off return line and give us the full fuel pressure with the return blocked off.
MT

[transgta]

AC DELCO are made in mexico now my ta pump only lasted 6 mos.Now i got an american pump at carquest runs better than ever. Have you checked for fuel pressure regulator. I went through all this with my ta pontiac said nothing wrong. I didnt get any codes just running problems. The new pump after everything else fixed it.

[GMMerlin]

3 mechanics don't know what to do, and i'm trying to avoid the dealer...

Why? A properly trained GM Technician can diagnose and repair your vehicle right the first time.

had the injectors cleaned at NAPA (for $100),

You wasted 100 bucks on that.

[JpJr]

OK, back to square one again. 4th mechanic, he kept it for a week, and he can't figure it out. I really don't want to get raked at the dealer, but I guess that's my last choice if no one has any ideas.

He doesn't think it's the fuel pump, he was considering the distributor, but then that seemed fine too. Now he can't get the miss to happen at all anymore. That's another clue for this mystery: It seems like the miss will go away when the tank is REALLY empty (less than 1/8th of a tank) and it also seems to run a lot smoother when it is cold weather. Also, it seems like it runs a lot worse on mornings after it rains heavily out.

Any thoughts before I go drop another $1000 into this hunk of junk? (I imagine the dealer diagnostic fee is going to kill my holiday spirit, and probably turn me away from being a lifelong Chevy enthusiast.)

[MT-2500]

No more info than you gave us we can not help you.
Only to tell you what you should done.
You should have went to the dealer or a known good mechanic to start with.
A good mechanic/tech is priceless.
Find one and stay with them. Make them your buddie for life.
Mt

[JpJr]

Originally Posted by MT-2500

No more info than you gave us we can not help you.
Only to tell you what you should done.
You should have went to the dealer or a known good mechanic to start with.
A good mechanic/tech is priceless.
Find one and stay with them. Make them your buddie for life.
Mt

Thanks, but I swear to G-d if I go to the dealer, and they clear the P0300 and tell me "Well, we cleared the code, and we can't make it come back on" like the past 4 mechanics have done, I'm gonna burn that place down.

(At least two of the mechanics had the decency to not charge me $50 to tell me what the code was that I had already known and told THEM what it was)

[scribblespreschool]

Hey JPJR!
I have to know what you found out. We are going through the exact same thing with our 1999 Yukon. Did you take it to the dealer? That's our next step I guess. The last thig we did was the $100 to have the fuel injectors cleaned and then when that didn't work we just got all new ones $350!)....surprise that didn't work either!!! How fustrating is this. SO WHAT DID YOU FIND OUT?!?!?!

Thanks

[JpJr]

I haven't gone to the dealer yet... I replaced (again) the fuel filter, but it didn't make much of a difference... it's a little better, but the miss is still there. The check engine light (P0300) only comes on after about two weeks of driving, and usually when:

It rained the previous day, so something is getting wet?
The tank is less than half full
It's warm outside

So, for now, I'm living with it. I do not want to pay for the dealer, because I have no faith in mechanics now (I've been to 4 already, 2 of them highly renowned in their field and suggested by many friends/shops/stores).

Also, for christmas I got a code reader, so now I can just reset that stupid light whenever it comes on. Funny thing is: It seems like that check engine light is TELLING the car to run poorly. It runs ok for a few weeks, then that light comes on, and the truck runs BAD BAD BAD. I reset the codes, and the truck just starts running fine (for the most part) again until the code comes back.

Very strange. I will eventually get the problem figured out, once I pay off my Christmas bills. But for now it's a problem I've learned to live with. (Unhappily. I'm afraid to say this may be my last Chevy purchase).

[scribblespreschool]

This is just crazy. Word for word it is the exact same as ours. We have a friend that is a mech. and he resets the codes and then it runs fine for about a week. Since I read your thread I have been watching the gas and it has yet to make a difference on how it is running but the rain might have something to do with it. And living in the great NW this could be a big problem!

Thanks, we will keep you informed and hope you will do the same on any changes or "cures" you find.

[JpJr]

Well, I got a great christmas present. I got one of those handheld orange code readers, like they use at AutoZone or Advance Auto.

So now, when my P0300 code comes on, I can clear it myself, in an instant. (I leave the code reader in the truck). Not the best fix in the world, but it's cheaper than the dealer.

Also noticed that if I drive less than 70 mph, the light doesn't come on often or at all. When I am driving leadfoot (75+) the engine misfire becomes a little more apparent, and the light will come on (P0300 Random Engine misfire).