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Old 08-08-2017, 02:42 PM   #1
veesix
 
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Default Stuck intake valve?

I have a misfire in cylinder 5 (no fire) and I get a backfire when decelerating. Do I have a stuck intake valve?
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Old 08-08-2017, 07:02 PM   #2
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It is really rare to have a stuck valve unless the truck has been sitting in a humid environment for a long time.
I would suggest a faulty plug, plug wire, distributor cap or bad injector.
Could be a worn cam, but even that is not likely.
At least you know what cylinder it is, so you can troubleshoot at that one.

Oh, could be a burnt valve too. Run a compression check.

Other possibilities: bad piston, although there would be pretty healthy blow-by, or cracked/stuck ring(s) - but that is also not likely.
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Old 08-09-2017, 02:09 AM   #3
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Switched wires around and checked the plug for spark, still the misfire. It could be blow-by; tried a compression test but my gauge sat for so long that it doesn't work anymore.
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Old 08-09-2017, 04:05 AM   #4
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easy way to check compression, hammer the insides out of a spark plug, weld an air fitting to it, thread into plug hole and apply compressed air. if the cylinder holds air, great. if it blows air out the intake/exhaust bad valve/seat
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Old 08-09-2017, 11:43 AM   #5
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I'd make sure the timing is correct first.

Timing good, check for codes.

Is this on your 1988? It wouldn't have separate injectors for each cylinder, so that rules out a bad injector.

Bad plug wire?

If all of that is good, then it's time for a compression test and/or leak down test.
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Old 08-09-2017, 12:06 PM   #6
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It's the 88, only 2 injectors. I switched the wires around and still have the mis-fire in #5. The way I know is, if you pull off the #5 plug wire the engine doesn't run differently. I didn't change around the plugs but I did watch it spark.

Maybe there's lots of blow by. How do I check this?

Will have to pick up a new compression gauge unless there's a way to fix the old one.
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Old 08-09-2017, 01:05 PM   #7
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Blow-by is usually smoke coming out of the oil fill with the cap off.

You can rent/borrow a compression gauge from one of the parts supermarkets. They usually take a deposit and lend them to you.

Did this develop suddenly? Or was it over time?
And the spark plug is firing?
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Old 08-09-2017, 01:16 PM   #8
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Yeah. I pulled the plug and grounded it on the block. It fires.

I've removed and replaced the distributor assembly a few times recently. It might be a tooth off, who knows. Working long shifts next few days, will have to find a compression gauge on Saturday.
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Old 08-09-2017, 02:12 PM   #9
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I bought this compression tester in 2014 for a motorcycle I used to have:

https://www.amazon.com/gp/product/B000EVU89I $16

Cheap, and works.

So, let me get this right:

If you pull the plug on Cylinder #5 while the engine is running, there is no change in the RPM.

But, if you put plug #5 up to the block while the engine is running, you get fire.

That would lead me to believe that something is preventing compression in #5.
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Old 08-09-2017, 02:43 PM   #10
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at the very least, it may be time to have the heads worked. you will probably know what you need to do after seeing results from the compression and leak down tests.
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