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I got this 94 Chevy Astro with a 4.3L vin (W) overhead valve engine. It has 257,000 miles on it, not in hatefully bad shape but it cam in with a miss.

I checked it an #4 (middle cylinder on passenger side) was not firing. Pulled plug, half of the porcelain is broken off. Stuck new plug in. No difference. Checked for spark at the plug - good spark. I also noticed the plug is damp with gas, indicating the fuel injector is working. I took compression check. Only 90 psi. Low enough to indicate an issue. I pulled rocker cover off an rockers are moving same as the other cylinders so the valves should be ok.

What am I missing?

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I'ld be suspecting a bad valve on #4 cylinder, probably an exhaust with that kind of miles on the motor.

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Run a complete compression on it. You may find a tired motor that needs to be put to pasture. On vehicles with this kind of mileage once they start to grenade it's all down hill from there. I personally think 90 wouldn't cause a dead skip and that particular injector may be full flowing and it washed the cylinder causing a low reading. I've never seen a burnt valve maintain any compression, it may create it but won't hold it.

Ok, here is an update. The owner stopped an reported that while they were driving this Astro recently they heard a noise, he described it like a stone being thrown around underneath the vehicle while the vehicle was in forward motion. It was after that the miss was noticed.

So that was either the piece of porcelin rumbling around the cylinder or a piece of valve. You have a borescope? Take a look inside there.

ricmorin wrote:You have a borescope? Take a look inside there.

Yea man I do. Used it to poke around inside an the top of the piston is kinda shot up. So, I discussed it with the customer an he says fix it. I told him we'll take the head off an see whats there then decide after that what direction to go.

If it were in my shop we would be talking R&R based simply on the amount of mileage unless the customer wants it all original.

Ok. After taking the right bank cylinder head off, I discovered the intake valve slightly bent, not much but enough to lower compression and create a miss. So, the customer says just do the minimum repair. So, a new intake valve should fix this thing back to running again.

I know what some of you are going to say. That you would not do that and various other things and each one of you is entitled to your opinion and I respect you for how you feel. This is not what I would recommend but in this case, with this customer, he is a good customer an if 6 month down the road another issue develops, he will deal with that then. Neither is he the kind of fellow that is gonna come back an blame every leak and drop and noise on me just because I was the last one to touch the vehicle. I would take more customers like this. Easy to work with, easy to communicate with and honest. Van has life in it yet an so he chose to go this route. He did ask about a Jasper replacement engine but decided against it due to much higher cost factor.