Hi everyone.

Well, made a discovery on my engine that I'm not too pleased about.


First off, a little back story for any who may have missed it. A month or so ago, I went and checked out an '86 F-150 with 300 in it and bought it for parts for $300. Before I even thought about buying it, I ran a full compression check on the engine. All six spark plugs looked perfectly healthy. (I still have all six of them in my tool box and they look great). The compression test came out as ~130psi on all six. Lowest about 125, highest about 132. I fired the truck up and drove it about 20 miles (some interstate) to my brother's house. It drove just fine and I never noticed anything off about it.

So, a few weeks ago, I swapped the entire drive train over from that truck onto mine. Transfer case, transmission, engine. I pulled my engine out so that I could rebuild it since it was clacking and making all sorts of noise.

AS SOON AS I fired it up, I noticed something odd about it. The exhaust was popping and the vacuum needle was bouncing erratically between 14 and 16psi. I took it for a drive and it was low on power, sputtered, and the harder I pressed on the gas, (and the closer to 0psi the vacuum needle would go) the more wildly it bounced, like from 2 to 7psi. On the flipside, the higher the vacuum goes (like around 18 to 22 when I let off the gas) the more steady it is.

There's no backfiring, no stalling, it starts fine, drives fine, just a little weak, the exhaust sputters, and has a dancing vacuum gauge.

Well, today, I pulled the spark plugs out and 5 of them looked great, but cylinder #5 was black. Not oily. Just like it was the richest you could get it. I grounded it and checked for spark, and it was getting spark.

Did a compression test on the cylinder and it was around 30. The odd thing was, when I'd turn the engine over, the needle would bump up once (to around 30) and then wouldn't even twitch. I would normally thing it'd slowly bump itself up to 30, or would jump to 30 and then at least blip every time I'd fire it, but no.

What would cause this? As said, compression was great when I went to look at the truck. Drove it 20 miles, pulled it, put it in mine, and as soon as I fired it up, it had a dead cylinder.

Thoughts?