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Discussion Starter #1
Hi there

After some adjustment to the timing, my engine runs great when the AFM/MAF is not hooked up. As soon as I plug it in it runs fine on idle but once in gear and driving it pops like a misfire and air/fuel mixture issue.

The unit was showing 1.88v which is a bit higher than the target 1.8v. I noticed the screen is missing and not sure if that would be a factor. Im going to have the vac system smoke checked for leaks but would a vac issue cause trouble like that?

any thoughts?

thanks
 

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Hi there

After some adjustment to the timing, my engine runs great when the AFM/MAF is not hooked up. As soon as I plug it in it runs fine on idle but once in gear and driving it pops like a misfire and air/fuel mixture issue.

The unit was showing 1.88v which is a bit higher than the target 1.8v. I noticed the screen is missing and not sure if that would be a factor. Im going to have the vac system smoke checked for leaks but would a vac issue cause trouble like that?

any thoughts?

thanks
So, if I’m hearing this correctly, you can drive around with the MAF unplugged and it runs fine?
What other adjustments did you make besides timing? When youdid the timing, did you reconnect the vacuum advance hose? Leaving it unplugged can cause the issues you’re describing especially under rapid acceleration because the distributor can’t advance fast enough to keep up with the surge in RPM from a quick stab of the throttle.
With the truck in park, do you get the misfire/popping if you rev the engine quickly?

I would be more suspicious of a timing related change or vacuum leak than the MAF, especially if it was working fine before.
 

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Something to consider here is that, if you advanced the timing, it will affect both idle speed and quality. Both of these changes will affect the MAF readings. Unless you changed the timing drastically, it is possible that the timing change was necessary due to a sticking, or sluggish mechanical advance. It is possible that the timing change caused the engine to become more responsive, and the MAF does not reflect the changes in airflow. I suspect the MAF dying, especially if it has a few miles on it. Ray
 

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Discussion Starter #4
Thanks for sharing.

Correct, I did reconnect the vacuum advance hose.

When the truck is in park it does not misfire or pop, even if revved.

I checked the MAF on the mutli-meter and it read as 1.88v. I was told 1.8 was the target. I'm not sure if its dying or good or what.

I do suspect the vacuum system could have a leak. The body has 282,000 miles on it so I would be surprised if the vacuum hoses didn't have a few issues.
 

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Thanks for sharing.

Correct, I did reconnect the vacuum advance hose.

When the truck is in park it does not misfire or pop, even if revved.

I checked the MAF on the mutli-meter and it read as 1.88v. I was told 1.8 was the target. I'm not sure if its dying or good or what.

I do suspect the vacuum system could have a leak. The body has 282,000 miles on it so I would be surprised if the vacuum hoses didn't have a few issues.
I spent many months chasing erratic idle, stumbling, and generally poor running.
I ended up taking the plenum and upper intake (where the “trumpets” are) off and giving everything a good cleaning. The area around the stepper motor was gunked up and the stepper motor housing needed a new gasket as well.The old one was paper and was compromised. I fitted new hoses as well as a catch can in the PCV system to keep it from getting dirty in there again. I would double check your vacuum hoses. The emissions system needs to be airtight as that can be a source of vacuum leaks as well. Make sure you have a good amount of mesh in your PCV filter (on top of the right hand valve cover). I didn’t have enough and it was letting a lot of oil into the intake.
I also swapped my vacuum advance pipe with a smaller diameter one. Make sure there’s a really tight fit on that hose and that it’s not cracked. I have small zip ties on either end of mine to ensure there are no leaks there.
From there, it’s possible that the main intake tube from the MAF to the throttle body could have pinholes which would let air escape. I have mine wrapped in some self-sealing silicone tape to prevent leaks.

On the ignition side of the equation I found a shoddy connection where the main 12V+ wire from the loom connected to the positive side of the coil which I resolderd.
On the timing side, I would be certain that it’s set correctly. I believe it’s set with a warm engine held at exactly 800 RPM (if memory serves). Make sure the vacuum advance is off AND plugged up.
If you look with the timing light, you should see the timing advance as you rev the engine, even with the vacuum advance off. This is the mechanical advance within the distributor itself. If this is out of spec, it could cause the misfiring you’re describing.
Based on everything you’ve described I suspect the timing more than vacuum leaks or the MAF. Even with a dying MAF, there should still be injector pulses and fuel in the cylinder. Popping makes me suspect that the fuel is igniting when the exhaust valve open, which would point to a timing issue. Specifically I think either the mechanical or vacuum advance is the culprit.
When the truck misfires, does it show any codes? I would highly recommend purchasing a rovergauge cable and downloading the software. It has helped me immensely because you can see exactly what the computer is seeing.
Sorry for the long and rambling post, but I just went through all of this stuff over the course of the last 6 months or so and have unintentionally become a little obsessive over the engine and the 14cux system.
 

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Discussion Starter #6
Thanks for the thorough response. Some of the items you mentioned I have not check. I'll do some more digging around and see where we end up. Thanks!
 

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Discussion Starter #7
Good call, I cleaned the MAF/AFM sensor and have noticed significant improvement.

Also, I was able to smoke check the vacuum system and engine, and found the fuel vapor hose from behind the plenum that connects to the throttle body was super loose and was leaking out like a geyser. Unfortunately, I have not yet found that specific molded hose as the dealer indicated it was no longer in production so I clamped the ends down with two of those worm-drive clamps and that too has seemed to improve the overall situation for the motor.

While I've seen much improvement, the engine still isn't running perfectly or as good as it could be yet. The timing still seems to walk a few degrees in either direction after a while. So now I'm wondering if my distributor might be having issues since it seems to mainly advance on its own after a few days of driving.

Anyone else have thoughts on this?
 
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