Ramdom misfiring....mystery

[nightwalks]

Hi All, have a 98 Astro AWD. It misfires under partial (only) throttle. From idle or while driving. Replaced ALL sensors, plugs, wires, cap and rotor.
Someone said its the fuel injector unit under the air-intake manifold, but the problem isn't all the time, goes away sometimes, but always at the exact same throttle position just off idle. Only code 300 displayed (ramdom cylinder misfire) Any ideas?????

[maxwedge]

Originally Posted by nightwalks

Hi All, have a 98 Astro AWD. It misfires under partial (only) throttle. From idle or while driving. Replaced ALL sensors, plugs, wires, cap and rotor.
Someone said its the fuel injector unit under the air-intake manifold, but the problem isn't all the time, goes away sometimes, but always at the exact same throttle position just off idle. Only code 300 displayed (ramdom cylinder misfire) Any ideas?????

You should check the fuel pressure as this is happening, 60-65 is normal.

[nightwalks]

Originally Posted by maxwedge

You should check the fuel pressure as this is happening, 60-65 is normal.

Checked fuel pressure, forgot to mention....58# when running, badly or smoothly. Good according to my GM book. Any other ideas?

[maxwedge]

Originally Posted by nightwalks

Checked fuel pressure, forgot to mention....58# when running, badly or smoothly. Good according to my GM book. Any other ideas?

Try disconnecting the egr valve as a test only and/ or pull it off and make sure it is not carboned up causing a lean mixture. A full diagnostic scan can show fuel trims that may lead you in some diection, this all assumes a plug or wire ,even though new, is not at fault. Could try running some Chevron Techron thu it also, pretty good injector cleaner.

[nightwalks]

Originally Posted by maxwedge

Try disconnecting the egr valve as a test only and/ or pull it off and make sure it is not carboned up causing a lean mixture. A full diagnostic scan can show fuel trims that may lead you in some diection, this all assumes a plug or wire ,even though new, is not at fault. Could try running some Chevron Techron thu it also, pretty good injector cleaner.

I replaced the EGR valve assembly (and cleaned the ports) already, along with pretty much all the sensors. Already ran enough fuel-injector cleaner to make the Ghetto shine. Will try my buddy's scanner tool later this week.

[maxwedge]

Originally Posted by nightwalks

I replaced the EGR valve assembly (and cleaned the ports) already, along with pretty much all the sensors. Already ran enough fuel-injector cleaner to make the Ghetto shine. Will try my buddy's scanner tool later this week.

If this is a CSFI engine, the fuel pressure is 60-66, double check the specs and retest if so.

[nightwalks]

Originally Posted by maxwedge

If this is a CSFI engine, the fuel pressure is 60-66, double check the specs and retest if so.

Pressure is 58# running and 58# under load, holds pressure fine. GM spec book says: 55-65 PSI with minus up to 10# allowable.

[maxwedge]

Originally Posted by nightwalks

Pressure is 58# running and 58# under load, holds pressure fine. GM spec book says: 55-65 PSI with minus up to 10# allowable.

Is this a GM specific factory manual, and the engine id is the same as yours (8th digit in the vin), both my books show 60-65 as I said and the csfi is very sensitive to the proper fuel pressure.

[nightwalks]

Originally Posted by maxwedge

Is this a GM specific factory manual, and the engine id is the same as yours (8th digit in the vin), both my books show 60-65 as I said and the csfi is very sensitive to the proper fuel pressure.

Actually, its a Haines Service Manual which uses the exerpt from the GM manual. My 8th digit vin is:W Give me your specifics and I will recheck tomorrow. Thanks

[maxwedge]

Originally Posted by nightwalks

Actually, its a Haines Service Manual which uses the exerpt from the GM manual. My 8th digit vin is:W Give me your specifics and I will recheck tomorrow. Thanks

My specs call for the central sequential fi sytem to have 60-65, this apparently is the std engine for the astro and blazer.

[sierrap615]

i just diagnosised a simular missfire at work on a 2000 astro 4.3 vortec. the results being a arcing coil which inturn also trashed the wires. finally found when i drove the van with the doghouse off to check fuel pressure and i heard the arcing.

if you replaced the crank sensor, did you remember the crankshaft relearn procedure?

[nightwalks]

Hey Guys, did the water-spray trick late last night (total darkness) and the distibutor cap started glowing like a lightning strike, especially when it was misfiring. Guess Ill replace it....again. PS It's a year old, including the rotor, and yes guys, it IS a GM Delco unit.

[MT-2500]

Originally Posted by nightwalks

Hey Guys, did the water-spray trick late last night (total darkness) and the distibutor cap started glowing like a lightning strike, especially when it was misfiring. Guess Ill replace it....again. PS It's a year old, including the rotor, and yes guys, it IS a GM Delco unit.

When you repalace the cap and rotor chech the cam retart seting.
If not within - or + 1 or 2 degrees of 0 it will eat it upthe cap for breakfest.
MT

[nightwalks]

Originally Posted by MT-2500

When you repalace the cap and rotor chech the cam retart seting.
If not within - or + 1 or 2 degrees of 0 it will eat it upthe cap for breakfest.
MT

I assume this would show in the old cap, correct?

[MT-2500]

Originally Posted by nightwalks

I assume this would show in the old cap, correct?

Might or might not show on old cap or with a new cap it may need reset.
When you get the old cap off look for the rotor and cap terminals burned up more on one side.
But always check and adjust the cam retard if needed when replacing the cap and rotor.
MT