I posted this over at the doge talk forum earlier because this site didn't seem to be working.
I just did a compression test on my 87 D-150 318. The previous owner told the engine was rebuilt from the crankshaft up 35,000 miles ago, but it is running kind of rough and there is an intermittent grinding/knocking noise coming from the engine. 6 cylinders tested at 170 psi, cyl 1 was about 20% off at about 140 psi, cyl 5 was almost 50% off at 90 psi!. Squirting oil in the cylinder does not change the results. I guess I will have to take the head on the driver side off and see what is going on, but wont be able to do it for a month or so. Any guesses on what it could be? valves or head gasket? are there any other checks I can do without removing the head?

 

if squirting oil in the cylinder did not help that probably means your rings are good but it it does sound like you have a top end leak. if you have a compressor the easiest thing to do is use some fittings and inject air into the cylinder to see or hear where it is leaking. it would not surprise me if the valves are leaking. just because the po got the engine rebuilt does not mean he got the heads rebuilt. head work can be expensive.

 

Valves and head gaskets don't grind or knock. The head will have to come off at the very least and unless you find something obvious the oil pan will probably have to come off also. Have you checked for water in the oil, excessively high crankcase pressure, or air bubbles in the radiator tank?

I have never met a PO that I trusted. I would suspect broken rings and scored cylinders.

 

No water in the oil. How do I check for excessive crankcase pressure?
Unfortunately any major engine work is worth more than the truck
If I had a compressor I would consider a leak down test...

 

without a compressor you may have to take the top end part to see what is wrong or you might be able to take it to a garage to see if they can do a leak test.

 

Pull the PCV valve out of the valve cover. You will know if you have excessive crankcase pressure just by watching what comes out there. There should be a slow steady output. A pulsing output is not a good sign. While you are doing that look in the radiator cap for bubbles.

With the amount of compression loss and the noise I personally would not put any money into that engine for diagnosis. (some labor is free) I would think at a bare minimum you will have to pull the head but just pulling the valve cover might be informative. A bent intake pushrod can cause low compression and a pretty loud slap but no grinding. I would make a decision here as to how much money and time I would be willing to spend based on what the truck is worth to you.

 

Thank you for all the info. It is not really a grinding or knocking noise per se but more like if there was something loose somewhere inside the engine, it is not very loud , it is irregular and intermittent, so I don't know if it is related to the loss of compression on that one cylinder.