Hi,

I have a 351 engine in my '84 Bronco, it is running very roughly(obviously misfiring), I pulled all the spark plugs and they looked good and I had spark to all of them. I checked compression on all cylinders and everything was 105 to 120 compression except my 2nd and 6th cylinder(which are directly across from one another)...they had a compression of 0 , the needle on the tester did not move at all when I tested these two.

Any ideas what is causing this? Possibly a stuck valve? Or do I definitely need a full engine rebuild? Someone mentioned my pistons could have a hole in them at those spots. I'm trying to decide whether to junk this particular Bronco after taking all the parts from it for my other one but I kind of like this particular truck, if there was a way to save it without pulling the engine(beyond my budget and skills) I'd be interested. I'd really appreciate any help.

 

Zero compression...

First would be to pull the valve cover and check for the spring and valve condition, if they are still where they are supposed to be, if they move when the engine turns over.

If all looks good with the valves from the top side, then the head needs to come off. Hole in the piston or badly burned valve will be visible. Burned valve-rebuild the head(s) & away you go. Hole in the piston or dropped valve stuck in the piston tearing up the cylinder and head--rebuild time.

105 to 120 isn't the end of the world but it's pretty low methinks. IMO I would not put new heads on this engine, I'd rebuild or scrap if I couldn't rebuild and/or stand to run it any longer.

 

Thanks for the great response, I will pull the valve covers off this week and take a few pictures and hopefully get to the cause of what is going on.

I'm talking on theory here but if it is a burned valve(s) and I had a free set of heads for this particular vehicle would you think it worthwhile to install them? Could I expect maybe 30,000 to 50,000 miles out of it at that point?

 

You still have a lot to do in the way of diagnosis before condemning the heads.

Either do a cylinder leak down test, or at least use a readily available hose that will screw into the spark plug hole on one end and you can connect compressed air on the other end. With air pressure applied to the cylinder listen for where the air is escaping from. If you hear the air through the breather you have a bad piston/cylinder/ring problem, if heard through the tailpipe, it is a leaky exhaust valve, if heard through the carburetor, it is a leaky intake valve. If pressure holds high and is heard nowhere, suspect valve actuation problems.

You should try to diagnose as much as possible before exploratory surgery. If it turns out to be valve actuation, you might be wasting your time tearing down everything else.

ALWAYS diagnose as much as possible with as many methods as possible before teardown.

 

With a leakdown test, don't you have to turn the motor over to TDC of each cylinder before the leakdown? For example, if the cylinder is sitting on the intake stroke while you pump air into it, it's of course going to push air out of the carburetor. If that's the case, would the best method for that just be to rotate the motor and feel for compression with your finger over the spark plug hole? However, it seems like I remember reading somewhere about TDC not being the ideal test case because of valve overlap on some motors, I'm not really sure. That's why I'm asking.