Astro 4.3 idle misfire

[Alex_V]

Van is an '02 with 379K on it (ex-Pepsi truck) but engine was replaced not too many miles ago with what appears to be a used one. Seems to run quiet and smooth except a misfire on #3 - only at idle, when warm. Cool it doesn't so it, and when up to temp the misfire count on my scanner slows and stops by 1,200 RPM but the lope at idle says it's a dead miss when idling. The plug on the right is the offending cylinder - a little darker but nothing alarming. I've seen the exact same symptom in a Vortec 350 with a bad intake gasket, but it generated massive crankcase pressure because the leak was to the inside. That's not the case on this one, nor does it respond to carb. cleaner from the outside. Any advice? Head gasket or vacuum leak? Haven't done a compression check yet - that's gonna be time consuming given the vehicle it's in.

 

[clinebarger]

Could very well be a vacuum leak in the lifter valley at #3 intake port. Are Bank 1 fuel trims running more positive than Bank 2?

This is where a Smoke Machine comes in handy!

Did you check the porcelain part of the plug for cracks?

I find a Leak-Down test to be more effective as you don't necessarily have to compare the readings of other cylinders like you do with a cranking compression test.

Don't let Misfire Data fool you! If the Misfire Cycle Counter stops counting......The misfire count will stop even if the cylinder is still misfiring.

 

[jhellwig]

Could be an injector issue.

 

[clinebarger]

Originally Posted By: jhellwig
Could be an injector issue.


I thought about that as well, But....Poppet style Injectors like to act-up when their Cold. They are also not known for Electrical/Coil issues like Multec injectors.

I'm not saying it doesn't have an injector issue....Just other diagnostics needs performed.

 

[dlundblad]

Are these as much of a pain to change out the #3 spark plug as my Chevy truck? As soon as you said #3, my mind instantly went to that being an original plug while all the others are new(er). With my truck, that plug is a royal pain.

If that's the case, try a new plug first. The OE plugs are $9.50 or so. Correct me if I am wrong, but something this new shouldn't have the intake gasket issue right? I also want to say this would have the updated injector system?

 

[HonorAndHonesty]

Those "Spider" injectors have poppet valves that like to stick IIRC. Id try some chevron techtron. Also remember that those NEED (53psi? Dont remember) Of fuel pressure to spray fuel.
If you have a fuel filter restricting flow or a weak fuel pump you can start seeing issues from that. Although since its only on #3 cyl, I doubt thats it.

Whats the cap/rotor/wires look like?

Those plugs look like they have been in there a while. Start with the basics,

 

[Red91]

Five acres makes the point I was thinking. If I come across a skipping vortec 4.3/5.0/5.7, just about the first thing I do is pull the cap and look at it and the button. Things are bad to burn a hole through the metal tab and plastic beneath. Or those and a hole in the cap. Can cause a single cylinder misfire.

 

[artificialist]

First replace the ignition components mentioned by the others. Spark plugs, plug wires, distributor cap, and distributor rotor. As you remove the distributor rotor, feel if the distributor shaft has free play. That can add to the problem.

Does this engine use a PCV valve? Sometimes they don't function correctly and the engine acts like it has a vacuum leak. It could affect the nearest cylinder to the PCV hose, causing a problem in that cylinder and not others. Don't trust the shake test and don't try to clean it because a failed spring is often the problem. Getting an OEM unit is worth the price. The hoses between the PCV and air intake might develop vacuum leaks, this is why I suggest replacing PCV valves and hoses at the same time.

 

[379KITTY]

I have a 2002 GMC Safari. It's 4.3 has the Multec injectors rather than the poppets. I think 2002 was the first year Astro/Safari 4.3's got the 6 injectors - not sure if all 02's did or if it was a transition year.

 

[Trav]

Originally Posted By: 5AcresAndAFool
Those "Spider" injectors have poppet valves that like to stick IIRC. Id try some chevron techtron. Also remember that those NEED (53psi? Dont remember) Of fuel pressure to spray fuel.
If you have a fuel filter restricting flow or a weak fuel pump you can start seeing issues from that. Although since its only on #3 cyl, I doubt thats it.

Whats the cap/rotor/wires look like?

Those plugs look like they have been in there a while. Start with the basics,


The o2 has electrically triggered injectors on the spider not the poppets. Fuel trim info would help.