Mechanics

CHEVY RUNNING ROUGH

1999 Chevrolet Astro

Engine Performance problem
1999 Chevy Astro 6 cyl Two Wheel Drive Automatic 96000 miles

Hello,

My Astro is running rough, with an intermittant stumble off idle. If it sits for a few days, it is VERY hard to start, especially if it is wet and rainy out. I am also getting a engine check light, and code P0300, which I understand means " multiple misfires on varioius cylinders. In other words, the engine is running crappy. If I clean the fuel injectors with a pressure cannister ( runs off the air compressor ) it fixes it right up and the car runs smooth for a few days. This has been going on for about two years,
and now I have to fix it ASAP because otherwise it won't pass smog.

This is what I have done so far:

* Replaced sparkplugs, wires, distributor cap, rotor.
. After this, I discovered that the problem was fuel-related, even though my oscilloscope showed variations in spark voltage - seems a lean mixture takes more voltage for the plugs to fire.

* Cleaned the fuel injectors with cleaner in the tank. No difference.

* Cleaned the fuel injectors with a compressor-operated cannister cleaner. This got the car running great for maybe a week.

* Replaced the (SCPI ) injectors with the complete MFI upgrade kit from GM. Also replaced the spider bracket and upper intake manifold gaskets. Referred to GM Special Policy Bulletin 99066G, service bulletin 00-06-04-003B,
and the 1999 M/L factory service manual.

( I really expected this to fix it. Was VERY disappointed when it did not. It still starts hard ( 2 or 3 minutes of cranking ) after sitting a few days. And it runs rough. The second start of the day is
always easy. )

* Replaced the fuel filter.

Put a fuel pressure gauge on the schrader test port. It reads 61PSI with the ignition key on but not running or cranking. It goes down to 52 PSI at idle, and when you stomp on it, it goes up to 58
PSI.

Now I'm pretty much at my wits end with this thing. I did see some mention of replacing the gas tank fill pipe - apparently California gas eats it - and have ordered a new one. But I have little faith in that.

Is there anything common that I'm just overlooking?
Avatar
Jerryk1234
March 2, 2009.



Hi: I read your post and was checking things out for you. The only thing I found that could cause a problem is the fuel pressure. According to Mitchell on line, with the engine off, fuel pressure should be between 41 and 47 PSI. With the engine running, it shows 31 to 44PSI. Have you checked the fuel pressure regulator? Also, with the engine off, the pressure shouldn't drop more than 5 PSI in 10 minutes.

Joe

I believe that a brand new fuel pressure regulator is part of the MFI upgrade kit (gm p/n pk12568332 ). It's mounted on the back of the spider.

According to the shop manual, the pump is rated at 61.5 PSI. I think it's so high because the original SCPI system required a minimum of 53PSI to open the poppet valves against their springs.

The next time I have the doghouse open, I'll check
that 10-minute pressure drop.

- Jerry

Tiny
Jerryk1234
Mar 2, 2009.
Pressure after 10 minutes sitting is 45PSI.

Tiny
Jerryk1234
Mar 12, 2009.
Jerry, you have me on this one too. You have done everything. I'm going to pass it over to another tech who may be able to help.

Joe

Further thinking leads me to wonder about fuel *VOLUME*.
It's possible to get good pressure in a static situation, and yet intermittantly not have needed volume.

There's a sock filter on the fuel pump. There's also a little hose between the actual pump and its outlet. I'm told that the sock filter can get clogged, and the hose can deteriorate internally.

I also have sourced a replacement filler pipe per GM's service bulletin, although as I said before, that seems farfetched.

Unfortunately, to get at the fuel pump, you have to pull the tank. Fortunately, it doesn't seem to be that big a deal. Undo the filler pipe, undo four bolts, lower the tank on a jack, undo the hoses and wires.

I have fabricated a special jig for lowering the tank, consisting of a piece of plywood screwed onto a metal plate with a piece of threaded rod that mates to my trollley jack. So it can't slosh and tip over under the car.

- Jerry Kaidor

Tiny
Jerryk1234
Mar 26, 2009.