I'm having problem. I have good compression in 1,2,and 3 cylinders but number 4 is getting nothing at all! Engine seems to be running good on top end but idles rough! Please any help will do!

 

Head Gasket? Burned Valve?

 

i'm leaning towards a stuck valve but not really sure yet. i just has a lot of power on the top end witch is throwing me off.

 

Any blow by?

 

What do you mean by blow by

 

Probably a valve, possibly head gasket. You can check for coolant in oil; oil in coolant, coolant in exhaust, but the easiest would be to find a shop that can run a leakdown test to determine whether it's valves, head gasket or rings:

http://www.geocities.com/dsmgrrrl/FAQs/leakdown.htm

You could even try to run compressed air into the cyl yourself and see if it comes out the pcv (rings), intake (intake valve), exhaust (exhaust valve), or radiator (head gasket).

Either way, I'm afraid there be major engine work in your future.

 

How much air pressure should I use to blow down?

 

I'd think that if a valve is sticking open, and the engine was running okay before, you could simply check the valve clearances on that cylinder- too much clearance and the valve is stuck open, no?
And then again, if the valves are tight, could cause the same symptoms due to valve(s) being held open. And while you're in there, make sure #4's valve springs aren't broken.

If that's all okay, then we can deal with the leakdown.

 

Should also ask if you're doing the compression test properly: no spark plugs in any cylinder and the throttle held wide open... right?

...and you should probably pull the EFI fuse too just to make sure you're not squirting fuel into the cylinders.

 

Shouldn't I at least get some kind of reading even though I don't pull a fuse or hold the throttle wide open?

 

Pulling the fuse only keeps the injectors from squirting- more of a safety thing... although you don't want gasoline getting past the rings and mixing with / diluting the oil either, if you can help it.

Holding the throttle open can make a difference since your compression test depends on the cylinder getting as much air in as possible. If the throttle is closed, it will restrict air coming into the engine and will artificially 'lower' the reading. Likewise, having spark plugs in the other cylinders will slow down the engine's rotation and can artificially 'raise' the readings.

 

Well if you're getting no pressure at all in that cyl it won't matter what pressure you use because it will leak out as fast as it goes in. But somewhere between 80 & 100 psi should give you enough flow to be able to tell where it's coming out. Leave the radiator cap off so you can see if coolant is forced out, but it'll probably be a valve. Pull a vacuum line off the plenum to see if its intake. Use a length of hose to listen at the exhaust, or at the oil cap.

Or you can pull the valve cover as suggested.

 

ok im going to try and pull the value cover today when i get off of work and check the value clearance. If all fails with that i have another head off another truck ill throw on there. The motor sat up for two years so hopefully its a stuck value.

 

Well now you didn't mention the motor sat for two years

Yeah, it's a pretty safe bet one of the valves on that cyl is stuck open. A tap or two with the persuader ought to do it - make sure the piston is not at the top.

 

ok thanks ill try it tonight. Where can i get that spray from.

 

Oops, sorry, I edited my post before seeing your reply. I deleted the reference to PB Blaster (which you can get at any auto parts store - it's the best stuff for freeing frozen parts) because it would require you to pull the spring and seal before you could really get any of the blaster into the valve guide. You can still do that if needed, but a careful tap with the hammer should probably free it.

 

ok i checked my setting on my valves. they were ok i tapped it with a hammer still nothing. i'm going to go buy some of that PB Blaster tomorrow and let it set for a day! After that i really dont know what else to do except change out heads.

 

I'm pulling the head i'm thinking it is a burnt valve on the intake! I did a blow by test and the only place the air came out was the intake! thats the only place that never stoped letting air go by.