P-0304 Code

[S10Fan]

Greetings,

1999 Silverado 1500, 4.3 V6, 5-Speed
156K Miles
Truck sat for 7-8 years with a no start condition.
Had fuel pressure, would only fire after a shot of starting fluid.
New AC Delco Spider installed and the truck fired right up.
New fuel filter, 3 tanks of fuel so far.
New spark plugs, wires, cap and rotor.
Replaced all 4 O2 sensors with NTK brand sensors.
New Delphi coil.
Sprayed starting fluid all around the intake area, no vacuum leaks.
Unplugged MAF sensor, no difference.
Air filter is clean.
EGR gasket is new.
Temperature gauge seems normal as does the heater output.
I have NOT checked the compression on number 4 or any other cylinders as of yet.
I'm not smelling any fuel under the hood or anywhere else.

Motor starts quickly and seems to idle fine. The miss can be felt but doesn't seem to harm the performance too badly.

I'm out of ideas as to what may be causing the number 4 misfire code.

Help!!

[homemade87]

I do know those injectors are bad to get gummed up when sitting for a long time . I would pull them and clean them using a homemade cleaner . I have done this many times and works pretty good . Look on utube and you will see what I am talking about using a syringe and carb cleaner .

Would not hurt to do this anyway as long as it was sitting .

[mattfranklin]

Fuel, spark, and compression.
You've isolated it pretty well.
Compression is a very good thing to check.

But before that, try a new plug in #4
Newer spark plugs seem to be more fragile.
Once or twice I had a new one go bad.
The insulator was broken inside the metal shell.
I knew that for two reasons.
When we replaced it the performance cleared right up.
AND because it occasionally felt loose in my hand -- I just wiggled it in the right way and I could feel that the insulator and shell were not solid together.
I did that once on a 3.1L dyno engine and once on my '84 S10 with the 2.8L.
The 2.8/3.1/3.4 was hard to change the plugs on without breaking. Plugs are probably easier with your 4.3L, but it may be an easy fix.

It's quicker to change a plug on #4 than to check compression on all six, so try that first.

Also, for long term health of your new CPI unit, I'd use Chevron gas and if you you really want to be safe put in a bottle of Techron Concentrate twice a year. Many years ago that plugging issue was discovered in the CPI's younger brother, the SPCI, by GM and Chevron had a cure. Others may have found a solution later, but I know that we were first to solve it. Both the "Chevron with Techron" pump gas and the bottled "Techron Concentrate" are designed to keep these guys clean. Sure, I'm biased, but I've also done some of the testing and seen the results. It's good stuff.

[S10Fan]

Originally Posted by homemade87

I do know those injectors are bad to get gummed up when sitting for a long time . I would pull them and clean them using a homemade cleaner . I have done this many times and works pretty good . Look on utube and you will see what I am talking about using a syringe and carb cleaner .

Would not hurt to do this anyway as long as it was sitting .

Thanks, the injectors are new.

[S10Fan]

Originally Posted by mattfranklin

Fuel, spark, and compression.
You've isolated it pretty well.
Compression is a very good thing to check.

But before that, try a new plug in #4
Newer spark plugs seem to be more fragile.
Once or twice I had a new one go bad.
The insulator was broken inside the metal shell.
I knew that for two reasons.
When we replaced it the performance cleared right up.
AND because it occasionally felt loose in my hand -- I just wiggled it in the right way and I could feel that the insulator and shell were not solid together.
I did that once on a 3.1L dyno engine and once on my '84 S10 with the 2.8L.
The 2.8/3.1/3.4 was hard to change the plugs on without breaking. Plugs are probably easier with your 4.3L, but it may be an easy fix.

It's quicker to change a plug on #4 than to check compression on all six, so try that first.

Also, for long term health of your new CPI unit, I'd use Chevron gas and if you you really want to be safe put in a bottle of Techron Concentrate twice a year. Many years ago that plugging issue was discovered in the CPI's younger brother, the SPCI, by GM and Chevron had a cure. Others may have found a solution later, but I know that we were first to solve it. Both the "Chevron with Techron" pump gas and the bottled "Techron Concentrate" are designed to keep these guys clean. Sure, I'm biased, but I've also done some of the testing and seen the results. It's good stuff.

Thanks, I'll check the plug this weekend.

[sweetk30]

also try swapping positions for that plug and then that wire and see if the problem moves . this trick helps pin point bad parts with known good parts .

might need a full on scanner like a shop would have to see live data and could possibly be a ecm/computer going bad and not firing off cyl #4 .

also run it for just a little bit and pull that plug and see if it smells of fuel .

what brand of spark plug ?

them vortec engines only like the specific acdelco brand and model plug . and they HATE bosch platinum plugs . fixed a lot of skips on them with bosch plugs in them .

[S10Fan]

Originally Posted by sweetk30

what brand of spark plug ?

ACDelco Iridium Spark Plugs

[S10Fan]

OK,

Moved plugs and wires around, no change.

For test purposes I installed a new out of the box spark plug in number 4. After the truck ran for 5-10 minutes, the new plug was removed and was completely dry with no fuel odor. The dry plug makes me think fuel is an issue, as I stated above, all the injectors/regulator are new. New parts out of the box do fail. Compression on #4 read 130lbs.

Thoughts?

[homemade87]

Originally Posted by S10Fan

OK,

Moved plugs and wires around, no change.

For test purposes I installed a new out of the box spark plug in number 4. After the truck ran for 5-10 minutes, the new plug was removed and was completely dry with no fuel odor. The dry plug makes me think fuel is an issue, as I stated above, all the injectors/regulator are new. New parts out of the box do fail. Compression on #4 read 130lbs.

Thoughts?

Take a good look at the injector and connector to be sure it has no damage like a bent pin or one that has been pushed out of the connector . Could also check the voltage at the pink wire . It should be 12v key on .

If that checks out if you have one of those sound stethoscopes see if you can here the injector cycling . If it is cycling most likely the injector is stopped up .

Could always move the injector to another hole to see if it follows as well .

[sweetk30]

Originally Posted by homemade87

Take a good look at the injector and connector to be sure it has no damage like a bent pin or one that has been pushed out of the connector . Could also check the voltage at the pink wire . It should be 12v key on .

If that checks out if you have one of those sound stethoscopes see if you can here the injector cycling . If it is cycling most likely the injector is stopped up .

Could always move the injector to another hole to see if it follows as well .

vortec engines injectors are the spider unit under the upper intake cover . hard to hear and move around .

but yes check the connector for bent pin and or start tracing out the wiring to the #4 injector for a bad wire or computer not firing off the injector .

[S10Fan]

FIXED!!!!!!!!!!!!!!!!!!!!!!

Now you guys mention a bent pin! LOL

Yes, a bent pin at the connector was straightened out, problem solved.

After my last post I took a break, grabbed a cup of coffee and went back to square one, that's when I found the bent pin.

Thanks for your help.

[mattfranklin]

Originally Posted by S10Fan

FIXED!!!!!!!!!!!!!!!!!!!!!!

Now you guys mention a bent pin! LOL

Yes, a bent pin at the connector was straightened out, problem solved.

After my last post I took a break, grabbed a cup of coffee and went back to square one, that's when I found the bent pin.

Thanks for your help.

Congrats!