buick 225 oddfire low compression on 1 cyl, #4

Hey all,
I am struggling with low compression on #4 of my 225 oddfire. The results are 165,170,165,30,165,170.

I did the test after a long drive so the engine was good and warm.

The numbers were the same for both the dry and wet test.

There is no smoke, blue or grey.

The jeep is not burning oil and the coolant level stays constant.

After reading a bunch of threads, it seems like the exhaust valve might not be sealing. I pulled the valve cover and rotated the engine while watching the valves. The whole bank 2,4,6 seem to move nicely with identical ranges of motion as far as i can tell. Nothing looked bent or worn or burned.

So, I am thinking a bad seal. What else should I do to test this? Leakdown test? Is this a hard repair?


thanks

 

i had the same problem last december. my #5 cylinder had 60, all the rest were 155, no change wet or dry. i pulled the head and the #5 exhaust valve was burned. i took my head in to a machine shop that does a lot of high dollar race engines and i know the guy, he's done work for me before, he told me it was from running unleaded gas they magnafluxed my heads and there were no cracks so i had them rebuilt with stainless valves and cut for valve stem seals. it ran me around 450.00.

 

i was just thinking the exhaust valve was not sealing properly based on some reading I did online. So, it sounds like the next thing to do is pull the head. I don't have dual exhaust.

 

The usual cause of a single bad cylinder compression is a burnt exhaust valve or a hole in the piston. Since you have some compression, it's unlikely that you have a hole in a piston. If you can eliminate everything else, the only way to confirm a burnt valve 100% is to pull the head and look at the valve and seat. Only way to fix it is a valve job.

 

This is what I am trying to get at. Are there others things I should/can do first that would eliminate other possible causes of this?

thanks

 

Have you done a leakdown test yet?

 

how about a sunken valve seat ?

 

I had a bad lobe on the intake side of the cam on #6 on mine, that did the same thing. I would pull the intake manifold and turn the motor by hand and watch the cam lobes.

 

That's a good suggestion, although it's not much further to pull the head from there. If a flat cam lobe were the problem, I guess you'd get a low reading because the intake valve will not open and let air into the cylinder.

You might do this after you do a leak-down test. A leak-down can be very diagnostic, but it also requires you have a compressor. You can buy a leak-down tester, or make one. There are lots of links on the 'net - here's one to get you started: http://www.xs11.com/tips/misc/misc3.shtml - with both valves closed (cylinder at TDC), you'll hear/feel air coming out of the exhaust if the exhaust valve does not seal.

I'd be happy to make comments about valve jobs, when you get to that point, but ask before you send the heads in for evaluation.

 

question about the cam lobe. I rotated the engine by hand and watched the full range of motion of the valves. Would the flat spot on the lobe be much more subtle so that it wouldn't be noticable in that motion?

i will do the leakdown today....thanks all!

 

If the lobe is really flat, you'd see it. I'd guess the lobe would have to be really flat to affect the compression ratio. Plus the worn-off cam lobe material will show up in your oil.

If you have a flat cam lobe, the best response is to tear down the engine and clean out all the metal filings.

 

introduce air into the affected cylinder (when at TDC for that cylinder)
remove sprak plug, thread air hose adpater into hole, apply air pressure.
if bad ex valve/seat, you will hear air coming out the exhaust pipe.
it's part of a leakdown test you can do on each individual cylinder

 

i am going to make a leak down tester this morning. Rather than tap an old spark plug, I am going to cut down the 14mm long adapter that my compression gauge came with. I have some air fittings to make it work.

thanks again.

 

well...that was easy. The air (@ 50 psi) was howling through the exhaust. So much so that I thought I was on the exhaust stroke. Rotated the engine and found TDC again, and repeated. Same thing, howling through the exhaust. I have to say, it really never looks like an exhaust stoke on my engine. I feel like the valve spring compresses, relaxes and then the piston comes up for both the exhaust and intake. weird.

anyway, i guess i need to pull the head. I have never done this so any advice?

 

pull the plugs on each side of the engine block and drain all your coolant out. i use cardboard boxes to hold all my parts, i mark them at the front, punch holes in them and put push rods, head bolts, intake, ect. in the holes so there layed out in the same order they are as you remove them, that way everthing goes back where it came from.

 

dang, wonder why it had low compression...


how do the others look? bad enough that I should pull the other side?

here are the pistons too...

 

They look a little carbony. Use much oil?