Ok, I picked up a 78 F150 4x4 pickup with a 351M and a C6. The engine is going to need some work after doing some tests on it tonight. I thought it ran ok but after my tests it should run like poop. Engine seems to run ok with no load/in park but when you put it in gear it has a shake to it. Seems to still have good power going down the road, pulls hills ok, good oil pressure, no rod/wrist pin noise, no valvetrain noise, etc.

Basically, I was thinking about re-gasketing the engine as it is leaking oil from the pan, rear main, valve covers, intake manifold. So I decided to run a compression check on it and it has two low cylinders. After my compression test I did a leak down/cylinder differential test and the results were scary.
I will list the compression and the amount of leakage from the leak down test.
#1 115psi and 25% leakage
#2 110psi and 25% leakage
#3 120psi and 10% leakage
#4 100psi and 90% leakage...Checked this one a couple of times..
#5 120psi and 20% leakage
#6 70psi and 80% leakage...Again, checked this one a couple of times..
#7 125psi and 5% leakage
#8 120psi and 20% leakage

On #4 and #6 it is leaking out of the exhaust which tells me the exhaust valves are burnt/shot. What got me even more is #4 has 30psi more compression and yet leaks 10% worse then #6.

Anyway, so now I am debating on whether or not I should rip the heads off and do some work to them. The engine does not smoke, the plugs look good in it, nice crisp brown color like it is running in top shape. The cam is not flat but the timing chain is sloppy. I am unsure on the mileage on this motor as it came from a junkyard because the previous owner blew up the 400 and put a 351M in it instead of another 400.

Thinking about looking for a "better" engine. Might be easier to buy a used engine for a couple hundred dollars versus dumping money in to this one.

Opinions?

 

cheaper and esier to get some tmi 0 deck over 30 pistons + 400 crank, a comp cam kit, and leave the head stock...you´ll get plenty of power

 

The way I see it, you need to rip the heads off anyway. Sounds like headwork is needed at a bare minimum. If your compression went up in any cylinders when adding oil, then you will have to re-ring, but at that point you should do it all anyway including bearings and hopefully machine work.

 

Are you planning on keeping this truck? If so and it ran well enough, just run it and start collecting parts for a 400 build. This is what I did. Or if you'e plannin on flippin it then just get a used 351m/400 if you're not happy with the current one.

 

If you're going to do anything to the heads, and it sound like you need to, you just as well check the bore and the crankshaft. If you tighten up the heads, in no time at all oil will be blowing past the rings.

 

Well I found a guy that has a 400 for sale and he wants $250obo for it and all it needs is a carb. Said it has been rebuilt with 60-70k miles on it, has an rv cam and it is complete with all accesories. Thinking about offering him $200 for it.

Now I am wondering what is the life (mileage) span of a 335 series engine? Think that is a good deal? I also like the thought of having a 400 (so he says its a 400).

 

Gee, I don't know... I have 540,000 on my truck with 351M, all stock 100%... Still runs smooth as can be.

 

I'd sure ask to do a compression test. Even $250 is cheaper than new valves???

If you look hard enough this truck could become a parts donor for something else. LOL

 

I would give the engine, and the whole truck it is in to Glass from RMFTC.COM.

PM me if you need the address to drop it off at.

 

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
I don't care who ya are, that right thar is funny!!!

 

Did you do anything to it yet? I've never had the opportunity to do it but I always wanted to try some of the "magic" additives that would clean up an engine inside after doing tests like you did and then redoing the tests to see if they made a difference but yours might be too far gone...