Ramdom misfiring....mystery

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?????

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.

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?

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.

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.

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.

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.

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.

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

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.

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?

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.

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

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?

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

Do I assume its the spring-loaded unit beneath the rotor? Either way, please detail what I need to look at and how to check and adjust it if you would be so kind.....I am at work till 7PM, then going home to do it. Thanks!

Do I assume its the spring-loaded unit beneath the rotor? Either way, please detail what I need to look at and how to check and adjust it if you would be so kind.....I am at work till 7PM, then going home to do it. Thanks!

Just change your cap and rotor and check plug wires.
The cam retard settings requires a good engine capable scanner.
Find a good repair shop with a engine capable scanner and ask them to check/reset your cam retard.
Or you can read more about it and get more infor here.
http://www.alldata.com/products/diy/index.html
Good luck
MT

Thanks, my buddy has a snap-on/sonex scanner, Im certain he can check it.....

The Distributor cap did it! Runs like brand new! Learned my lesson about checking even the obvious things.....The "old" cap was a Delco and was only 13 months old with about 3000 miles on it. Thanks all for your help!

The Distributor cap did it! Runs like brand new! Learned my lesson about checking even the obvious things.....The "old" cap was a Delco and was only 13 months old with about 3000 miles on it. Thanks all for your help!

You are welcome and glad you got it going and.
Thanks for posting back and letting us know how it went.
But you need to figure out what ate the cap?
Is the wires and plugs good?
How was the cam retart setting?
MT