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  #1  
Old 12-05-2005, 07:48 PM
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Default no compression

I'm working on a brand x. (gm 3100) I have no compression in 1 cylinder. I did a leak down test only thing was hiss in the crank case. I added some oil to the cylinder and got 40psi. Ok rings right? Put in a piston and rings. Put it back together ran for 5 min. then started to miss. Tested compression 0 psi. added oil 40psi. I had the head sent out fixed and checked while it was apart nothing wrong with that. I let it set for a day with oil in the cylinder. Then checked with a bore scope and it was all gone. did a compression test 150psi. So why do I keep loosing compression in that cylinder? Someone put a pushrod in the wrong place and bent it along with a valve then I get stuck with the headache. Any help would be greatly appreciated. I'm taking the intake off tonight to dissassemble the lifters incase one of those is bad.
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  #2  
Old 12-06-2005, 12:05 AM
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Default Have you checked the cyl. for a crack?

I would check that if everything is checking. Remove head and use a dye to look for cracks.
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  #3  
Old 12-06-2005, 12:12 AM
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Are you still getting the hiss in the crank case? If not I would guess the compression could be going one of 3 places 1) out a valve from being bent or having some defect such as out-of-round or sticking open slightly 2) out a crack in the cylinder wall 3) a crack in the head...

did you try blowing air in the cylinder again after you installed the new piston and put the head back on?

Wish I could be of more help....
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Old 12-06-2005, 09:39 AM
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I had the head sent out to a machine shop and had the valves checked and checked for a crack. Nothing found. When I had it all together I did a leak down test and watched the radiator for bubbles (I've got a funnel set up that works slick for bleeding air pockets out) No bubbles. So I'm assuming not to have a cracked block. Plus that's unlikely. Yes I did a leakdown test with the new piston and rings (they really didn't need to be replaced rings were ok and I couldn't even see where the piston contacted the valve.) There was a slight hiss in the crankcase but rings don't seal perfect so I think that's normal. I hate GM. Any other idea's?
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  #5  
Old 12-06-2005, 10:27 AM
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Default no comp - stuck hydraulic lifter

if this has hydraulic lifters, consider that you might have a lifter that is pumped up to full size and will not blead back down to its correct height. this would create a slightly open valve at the comp stroke, but when the head is off, valve seems ok.

Tim
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  #6  
Old 12-06-2005, 02:53 PM
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Lifter pumps up and dumps compression would be my guess.
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  #7  
Old 12-06-2005, 03:06 PM
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The valve you replaced, was it the same length? Along with the pushrod. Crank the pedestal out a few turns and see if gets any better.
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  #8  
Old 12-06-2005, 07:49 PM
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Ok took it apart last night put in 2 new lifters for the cylinder that wasn't firing. Put it together today. Checked compression 175psi. Started it and ran it for 10 minutes started missing. Checked compression 0. Just enough to wiggle the needle on the gauge. The only thing I haven't replaced yet is the block and the connecting rod. What else could it be? The machine shop did the valve. It looks correct. I ran it and loosened the rockerarms till they clattered no difference. any other ideas?

Ok I'll add some more. It's the valves. I started checking compression again. 0 psi. So I took the rockerarms and pushrods out of that cylinder used shop air to push the piston down and then did a compression test. 125 psi. Put the pushrods and rockerarms back in 0psi. Loosen rockerarms till loose 175psi. Tightned exhaust 0psi. Loosen exhaust and tighten intake. 0psi. Loosen back up 175psi. So bad valve springs? Or too much oil pressure?
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  #9  
Old 12-06-2005, 11:40 PM
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Does that oil through the pushrods? If so, were they replaced with solid pushrods?

torch
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  #10  
Old 12-07-2005, 12:31 AM
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I dont think you tighten the rockers that tight on a chev, that could be your problem, I dont understand how you would get noise in the crank case if it is a valve staying open.
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  #11  
Old 12-07-2005, 12:35 AM
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Default Too much oil pressure

I think you said it correctly when you said "too much oil pressure." And there are 2 ways to have too much pressure. 1) pump produces too much and 2) lifter is not given enough chance to bleed down.

Way 1)

I once had a new rebuilt 318 with a new (but bad) oil pump. It was bad in that its pressure relief valve was stuck closed, so oil Pressure went high enough to pump up all the lifters, and the engine would die because all cylinders lost compression.

The only way I figured this all out was that it also blew up 2 fram oil filters. Can you say "oil all over the garage??"

Way 2)

So what about this particular lifter running up and down in its bore, is there a mechanism where the lifter's bleed port is being blocked by something foreign in the oil way passage?

I also was at a Mopar seminar once when a question was asked of Herb McCandlis (spelling) and Ronnie Sox "What is the truth behind the story of DONT USE ANY HYDRAULIC LIFTER OTHER THAN A MOPAR LIFTER". Their answer was that it had to do with aftermarket lifter makers not having the oil pressure relief area of the lifter OD not being a wide enough of a circumferential band. And thus would have this type of pump up problem. But they also claim that this problem with the aftermarket lifters had been fixed a long time ago. Those guys are Mopar gods.

But you say that this is not a mopar. However, it could be the same thing. Look at the lifter that is acting bad, and compare it to a lifter that is acting good. Compare the circumferential relief band on each. Are they the same width? At the same height from the bottom of lifter? And look at the oil passage at the location where the bad lifter came out of.

I might be all wet. It won't be the first time.

Tim
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  #12  
Old 12-07-2005, 12:35 AM
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Default Too much oil pressure

sorry delete this post.
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  #13  
Old 12-07-2005, 09:47 AM
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This GM motor has torque down rockers. There is no adjustment just torque the bolt. The pushrod was out of another running engine (same engine). I'm going to check oil pressure tonight and I hope I find something wrong.
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  #14  
Old 12-07-2005, 07:59 PM
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IMHO, the push rods are too long. Take the push rods from the next cylinder over and put them in that position. See if you have the same problem. If you don't, then it is definately the push rods.

The only other thing would be to see where the tops of the valve stems are. If they recessed the valves (while repairing the other damage), they will sit higher. That being the case, they may be open, when they shouldn't.
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  #15  
Old 12-07-2005, 11:20 PM
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What kind of Gm motor is it?
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  #16  
Old 12-08-2005, 12:28 AM
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Quote:
Originally Posted by ehostler
IMHO, the push rods are too long. Take the push rods from the next cylinder over and put them in that position. See if you have the same problem. If you don't, then it is definately the push rods.

The only other thing would be to see where the tops of the valve stems are. If they recessed the valves (while repairing the other damage), they will sit higher. That being the case, they may be open, when they shouldn't.

I was going to say the exact same thing... If you are tightening the rocker and compressing the spring in doing so you either are not on TDC or you have a push rod too long... Springs sound like they are doing their job.
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  #17  
Old 12-08-2005, 02:15 AM
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Quote:
Originally Posted by wishboneattack
I was going to say the exact same thing... If you are tightening the rocker and compressing the spring in doing so you either are not on TDC or you have a push rod too long... Springs sound like they are doing their job.
I think I did back a few posts
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  #18  
Old 12-09-2005, 09:41 AM
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What about a bent camshaft? The pushrods are the same length as all the others in the motor. (exhausts are longer than intakes). If the cam is bent couldn't that keep the valve from closing all the way?
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  #19  
Old 12-09-2005, 10:53 AM
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It also wouldn't turn in the block...
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  #20  
Old 12-09-2005, 01:16 PM
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It wont bend, but it would break.
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  #21  
Old 12-09-2005, 01:27 PM
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Biggrin

JV, correct me if I'm wrong, but you are working on someone else's vehicle, right? It also sounds like this motor was recently re-done. If that is the case, you are stuck trying to figure out what is wrong with an engine that may or may not have been fitted with the right parts. That makes it harder to pin down, although it sounds like a valve is staying open. As others have said, that could be caused by several things not quite right.
Any chance the owner could provide you with a list of what was done before?
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  #22  
Old 12-10-2005, 10:00 AM
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Ok tore it down again. The cam is not a normal cast cam with lobes ground on it. What they do is machine just the lobes with a hole in the middle. Then that take a steel rod with a hole in it. Align the lobes in the rod and then force a ball down the rod to expand it out and lock the lobes on the cam. How dumb is that. Anyways I'm going to have to get the dial indicator out and measure the lifts, but what if the lobe was shifted on the cam when the valve contacted the piston? What a nightmare. I hate GM
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  #23  
Old 12-10-2005, 11:46 AM
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WTF? That is kinda weird, like a poor mans adjustable lobe center cam..er, that cant be adjusted after you swedge it onto place. I didnt Think GM was that poor...1 Adjustable pushrod (oops, that aint gonna help if the lobe is out of time!) or a 50 buck SSI camshaft from P.A.W.,at least itll have all its lobes the same, and it is ground on a chevy pattern no matter what make you buy it for!
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  #24  
Old 12-10-2005, 12:32 PM
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[QUOTE=JVMopar]Ok tore it down again. The cam is not a normal cast cam with lobes ground on it. What they do is machine just the lobes with a hole in the middle. Then that take a steel rod with a hole in it. Align the lobes in the rod and then force a ball down the rod to expand it out and lock the lobes on the cam. How dumb is that. Anyways I'm going to have to get the dial indicator out and measure the lifts, but what if the lobe was shifted on the cam when the valve contacted the piston? What a nightmare. I hate GM[/QUOTE

That`s how Ford makes em now too on their OHC engines.
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  #25  
Old 12-10-2005, 12:46 PM
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Maybe the pedistals are backwards on that cylinder? I work at a Chev. dealership and have never ran into this. Something is holding both valves open. Will the lifter plungers move? What year 3.1? They`ve changed these engines over the years and the rocker arms have changed at least 3 times that i have seen on the FWD cars. Could be mis-matchsd parts. Check to see if all the parts on that cylinder match the others.
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  #26  
Old 12-11-2005, 01:19 AM
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I was at the local Chevy garage recently and they had one similiar to this on a 4 popper. They finally figured out what happened was the head had been shaved a bunch and then the valve stem height wasn't set. They took the head back to the machine shop and they set the valve stem height correctly and installed it and no more problem.
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