So I did a compression test and the rear cylinders on both sides have no compression and it blows air out the carb. I put some oil in the spark plug holes and did the compression test again and still no compression.

What do you think the cause is ?

 

sounds like intake valves are not sealing...

 

What would you do ?

1. Pull the whole motor out and rebuild.

2. Pull the heads with the block still in the truck and see if its the heads.




Edit : I would hate to pull the heads and it not be the problem.

 

i hear ya. my 400 also has low compression in #7 cylinder. have yet to a do leak-down test to determine if intake or exhaust valve is the problem. might try shimming the rocker pedestal in case it's just not closing completely, but without a borescope handy to take a peek thru the spark plug hole, i don't see any other way to figure out what's going on and fix it without taking the head off...

 

I think its weird that its the rear 2. If all other cylinders are reading ok, then why the REAR two? What's common about them? They share the rear camshaft section, the last conncting rod mount to the crank. Of course, a coincidence is always possible, like two dead valves or two breached head gaskets at the rear.

I really don't want to pull the whole motor, but I would hate to pull the heads off and find out the heads look ok and end up needing to pull the whole motor anyway.

 

Remove the valve covers and observe the valves while cranking the engine to see if correct action(similar to other cylinders).

 

is this a fresh assembly or an old running engine?

if it's a fresh assembly check your lifter preload, if it's an old running engine pull the heads.

 

i wouldnt start to shim the lifters if this engine has ran fine and just started to act up. to me it sounds like you might have too much build up on the valve and they are not closing all the way. maybe a bent valve

 

the 2 rear cylinders share the vacuum tree in the intake manifold. a vacuum leak would cause the 2 rear cylinders to run lean and might torch the valves.

 

Its an old motor in a truck I just bought.

 

Thats sort of what I was thinking. All the other plugs had black build up on them I wonder if its just a lot of carbon preventing the valve from closing. or a bent valve or something.

 

you should beable to get the heads mechined for like 150.00. i know i paid 75 each. if you going to pull the heads i would pull the engine and replace the oil pump, timing chain, pull the cam out and look it over, and put new gaskets on.

 

if you're just going to look the cam over don't waste your time, i guarantee that you will not like what you see. if you leave it where it is it'll serve for a while, maybe forever? when you disturb old cams they get upset. 99% of replacement cams will require new springs, not really a big deal while you have the heads off anyway but just be aware...

 

i think it's time for a good rebuilt but before check under the valve cover like he said.

 

If the wrong Tstat was used, the rear cylinders may not get enough coolant and overheat. That could cause a number of problems, like head gasket failure, but not exactly what you describe.

Pull the covers, check the valve operation through a couple of cycles, then pull the heads.