P0300 error code

BALDEAGLE02

MEMBER
2004 CHEVROLET TAHOE
5.3L
V8
4WD
AUTOMATIC
75,000 MILES

Son took this vehicle to a GM service shop due to running rough, very poor gas mileage and gas smell after operating. Shop checked coil packs and replaced plugs and plug wires. Son picked up and drove a short time, and the same problems re-occurred. Took it back to the GM shop and they said the catalytic converters must be replaced. They wanted too much, so I had son pick it up again. Did not get home with it when it behaved worse than ever. He parked it on the curb where it shut off. I trailer-ed it to a muffler shop a few miles from my house and had the converters replaced, along with the muffler and tail pipe (rotted). I picked it up and drove it ten plus miles, and it ran great until I got a mile from home. Then same old stuff: chugging, misfiring and stunk of gas when I got home. I used a cheap code reader and pulled a P0300 code (listed twice). Went to town and got a replacement fuel pressure control valve, and installed. Drove it about ten miles after idling up to normal operating temperature (210) and it ran perfect. Left for work this morning and drove it twenty miles before it started chugging. Turned around to head home, drove about ten miles and it ran perfect. Turned around again and drove about twenty miles before it started chugging again, only this time it really got nasty. I chugged it the last ten miles with the warning lamp flashing, etc. Running on maybe six cylinders. I studied up on the code P0300 and the list of possibilities seems endless. Already dumped $2,000.00 into this problem. Any ideas or do I just junk it? Desperate here! Thanks.

1 Reply

CARADIODOC

MECHANIC
33,916 POSTS

First thing I would do is rent or borrow a fuel pressure gauge. Attach it to the fuel rail test port, then run it under the rear of the hood and clip it under a wiper arm so you can watch it while driving. I had this same set of symptoms four times on three of my Chrysler products. The first two had carbureted engines, and it was easy to figure out they were starving for fuel. The first time this happened to my 1988 Grand Caravan, it took me four hours to get through Minneapolis with all three interstate bypasses down to two lanes for road construction. Once through the city, the engine ran fine for two hours. The clue, as I figured out later, was the stalling always occurred when the largest volume of fuel was being pumped, which is during coasting. That is why the engine ran fine when accelerating or cruising at highway speed, and stalled when slowing down for turns.

The cause all four times was a plugged pick-up screen on the supply tube in the gas tank. The additional clue is once the stalling occurs, the screen will stretch out and pass fuel again after sitting about five minutes. Had I known it at the time, I suspect I could have made it through the city easier if I had unplugged the vacuum hose from the fuel pressure regulator, and plugged it. That would cause fuel pressure to go too high, with black exhaust smoke and bad fuel mileage, but with the higher return restriction, less volume of fuel would flow back into the tank, and the pump would be able to keep up with demand.

The second time this happened to my Caravan, it only acted up when dragging a huge tandem-axle enclosed trailer. I had been chasing the problem for almost a year with the fuel pressure gauge attached to the radio antenna. Normal fuel pressure is around 45 to 50 psi. Some engines, GM's in particular, won't start or run with fuel pressure just a few pounds low. My engine ran fine down to 20 psi, and would start to surge and sputter at 15 psi. I found that lifting the accelerator pedal for an instant would cause the pressure to pop back up to 45 psi, then it would slowly drop again over about twenty seconds. I nursed it over fifty miles by doing that.

That screen costs $3.00 for the older cars, and around $12.00 for my Caravan. Running problems are never solved on Chrysler vehicles with gas engines by replacing the fuel filter, but that is not true on other brands. Since it is easier to replace than the in-tank screen, you might try that first if fuel pressure is the issue.