I'm trying to get to the bottom of (what seems to be) a fuel delivery issue causing a misfire on cyl 6. I've been following a lot of threads to go through the diagnostic process but have hit a wall thus far. Here's the only symptoms I've found so far:

• Spark Plugs #4 and #6 are slightly wet and smell like fuel
• Misfire code for #6 cylinder; appears after ~15min of driving but misfire can be felt before CEL comes on

Before I dive into pulling the upper intake, I wanted to check if there's anything else to test first. I have done the following based on a few threads:

• Checked all ignition components (all have been replaced); spark is good on all cylinders, and switching plugs and wires between cylinders does not change misfire location
• Replaced catalytic converter (old one was rattling)
• Compression tested all cylinders; all comfortably in spec
• Checked fuel pressure; runs at 62psi, passed leakdown test when key off

Does anyone have recommendations for other areas to inspect before I pull the upper intake? And if the fuel system is providing correct pressure and not losing pressure when key is turned off, can the issue still be fuel? Fuel on the spark plugs makes me think it could be. Thanks for any/all suggestions!
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What year is this Blazer?

I can't think of anything else I'd check. When I had a fuel leak on my 92, I was getting O2 sensor errors that was because there was an over-rich mixture being burned.

Fortunately, removing the upper intake plenum is much less involved than removing the lower. That said, it is still a hassle.

One possible sign of fuel leakage is that the metal will look very clean directly below the leak. This will be because the fresh fuel will have dissolved the any soot/varnish on the surface.

 

Mine is a 98. I guess I'll pull the upper plenum tomorrow and check it out. The only thing that confuses me is I tested for fuel pressure and leakdown, so that leads me to believe the injectors and pressure regulator can't be leaking. Is there a way it could leak but still hold adequate pressure?

 

That is a good thought about the fuel pressure. I don't know the answer.

 

Given good compression it’s likely either spark, timing or excess fuel. Leaking injector is still a possibility but Given that you passed the leakdown test I would start with spark. It can be the strength of the spark or the timing. Things to check:

Mist the ignition components in the dark and look for arching

Inspect the old distr cap and look for carbon traces as a sign of timing problems

Are the spark plugs gapped properly?

Check CMP retard to be certain that you don’t have a timing issue

When you checked spark did you see a uniform 1” of strong blue spark at each cylinder?

Did you replace with AC Delco ignition components?


George

 

George always has good advice!

 

Thanks for the suggestions! I did verify spark, solid spark and all new plugs and wires. Distributor cap has been replaced as well. Everything on the spark side seems solid, and my problem is only appearing on cyl 6. I came across a write-up for a similar problem that ended up being a fried PCM. I'm hoping it's not the case for mine, but I'm going to pull the spider and take a peek. Will update soon.

 

Ok, I pulled the plenum and there are significantly more carbon deposits on the 246 side, which definitely makes it seem like the problem is excess/leaking fuel on cylinder 6. It's strange though, it passed the leakdown test just fine. I just tested for always on for the injector. Is my process here correct?

• Placed the spider back in the intake
• Hooked up the main connector
• Put fuel lines back in the spider
• Turned key to on position

RESULT: No fuel from any of the injectors, so the problem is NOT injector 6 always firing

Is my test process and conclusion correct from this?


Testing with spider connected to main harness and fuel lines reinstalled

 

So I'm thinking my next approach is to swap the #1 and #6 injectors and fully reassemble to see if my problem moves cylinders. That would tell me I could just pick up one new injector instead of the whole spider. After, if it stays at #6, multimeter to test all the pins of the connector?

 

Some things to consider, from my misfire article:

7) Injector function

At the harness connector, check for proper battery voltage at the power feed for the injector (pink for most modules). Check for pulse control with a noid light or test probe on all cylinders. Consider an injector balance test with a pulse box where you watch the pressure drop/fuel delivery of each injector. Look for less tan a 1.5 psi spread with a target around 3X psi. Consider measuring injector coil resistance and look for a target of 11-14 ohms with a range of less than 3 ohms across the injectors.


George