Compression?

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Compression?

Postby BeemerlessBob » Thu Jun 21, 2012 3:03 pm

My '06 patrol was crapping out after the engine got hot at high altitudes.

The right cylinder would backfire, fart and sputter until the engine died. After cool down, I could go again.

Got the ol' girl home and did a compression test.

Left cyclinder = 72 psi
right cyclinder = 110 psi

So I assume that means that I prolly have a bad ring on the left side, that correct? But it was the right site that was doing all the complaining. Does that sound right.

What compression should I expect? I note that on other threads, they were talking about reading of 120 - 125 psi. Does that mean that there is something wrong with the right cylinder as well or is 110 acceptable reading for an '06?
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Re: Compression?

Postby URAL CT » Thu Jun 21, 2012 4:00 pm

BeemerBob wrote:My '06 patrol was crapping out after the engine got hot at high altitudes.
The right cylinder would backfire, fart and sputter until the engine died. After cool down, I could go again.

...sounds like a defective coil. May have a crack. I know you have a P/A...check connections too. The motor may be overheating. hope you ain't luggin' the engine going up those high altitude "hills".
BeemerBob wrote:Got the ol' girl home and did a compression test.
Left cyclinder = 72 psi
right cyclinder = 110 psi

...drop a spoon full of oil in the offending cyl. & take a compression reading again. If compression goes up...rings would be a suspect.
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Re: Compression?

Postby rougaroo » Thu Jun 21, 2012 4:13 pm

Don't just assume rings until you check other things as well. My first guess under the conditions you describe would be a sticky or burnt valve. Before you start disassembling, check the head bolt torques and then the valve clearances, just to be sure.

The oil trick is good to check compression differences, again without disassembling. The PA coils are real good, so that wouldn't be my first suspicion.

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Re: Compression?

Postby BeemerlessBob » Thu Jun 21, 2012 4:55 pm

OK, tried the spoon full of oil thing and got some improvement is compression
Left 89
Right 120

Checked head torque and all bolts were within 30 to 35

When I first started having this problem, adjusting the valves was the first thing I did. The exhaust on both sides was still at .004 but the intake on both sides were tight (less that .002), so I adjusted all to within spec (.002 - .004)

Anything other easy checks before I pull heads?

And Oh, what is the standard compression supposed to be?
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Re: Compression?

Postby rivers » Thu Jun 21, 2012 6:20 pm

With a wet compression test follow up after a dry test and the compression came up, that's telling you the added oil is sealing your worn rings. Unless you're running an old multi cyl farm truck I'm not a big fan of just overhauling the one weak cyl. I like having both sides working equal. You want all cylinders w/in 10%. Plus it's better/easier/cheaper to do both jugs now in your garage than to just do one now and 6 months later doing the other one in a motel parking lot. Can't help with what comp should be on a new/fresh motor. Prolly have to call your dealer on that. I don't suspect even new would be much if any over 135 or so. If both your jugs were running at 120 I wouldn't worry about it till you really lose power or you're mosquito fogging the neighborhood.
Keep in mind whether you overhaul one jug or both you'll have to go through the break-in drill again till the new rings are fully seated. In any case plan on going inside.
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Re: Compression?

Postby isldtime » Thu Jun 21, 2012 6:57 pm

Don't make the same mistake I did.
Do your compression test at WOT !!
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Re: Compression?

Postby BeemerlessBob » Thu Jun 21, 2012 7:15 pm

isldtime wrote:Don't make the same mistake I did.
Do your compression test at WOT !!


WOT?
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Re: Compression?

Postby URAL CT » Thu Jun 21, 2012 7:16 pm

Hey Bob,
Do the easy checks first, like what rugaroo said. Just because you gapped the valves doesn't mean there is not a problem with 'em. Had a small metal chip lodge onto one of the valves of my 72 Beemer, same symptoms. If you pull the heads out do the leak test on it. Pull side that's showing a defect first. Also I'm concerned about the piston not matching the original letter stamp. Dunno if that has any bearing on the issue. I've always replaced pistons with the same size designation. Or I have the cylinder machined to fit the piston per spec. Dunno!
Call me if you have any questions. Good luck, bud.
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Re: Compression?

Postby isldtime » Thu Jun 21, 2012 8:01 pm

BeemerBob wrote:
isldtime wrote:Don't make the same mistake I did.
Do your compression test at WOT !!


WOT?



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Re: Compression?

Postby Jimmy » Thu Jun 21, 2012 10:54 pm

URAL CT wrote:Hey Bob,
Do the easy checks first, like what rugaroo said. Just because you gapped the valves doesn't mean there is not a problem with 'em. Had a small metal chip lodge onto one of the valves of my 72 Beemer, same symptoms. If you pull the heads out do the leak test on it. Pull side that's showing a defect first. Also I'm concerned about the piston not matching the original letter stamp. Dunno if that has any bearing on the issue. I've always replaced pistons with the same size designation. Or I have the cylinder machined to fit the piston per spec. Dunno!
Call me if you have any questions. Good luck, bud.
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what he said... check your valves first
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Re: Compression?

Postby BeemerlessBob » Thu Jun 21, 2012 11:40 pm

Ok, time to get greasy. I'll let you know my progress.
Thanks guys.
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Re: Compression?

Postby rougaroo » Fri Jun 22, 2012 1:38 pm

There's another test you can do before tearing into it, if you have the equipment or can borrow it. It's called a "cylinder leak down test". Basically you put a cylinder at TDC, then pump air into it through the spark plug hole up to the rated compression pressure. To do it you need an adapter that screws into the spark plug hole just like the compression tester (sometimes you can get one tester that does both. Most times you can *hear* where it is leaking when you do this.

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Re: Compression?

Postby URAL CT » Fri Jun 22, 2012 2:27 pm

Hey Bob,
If you were checking the compression & NOT at wide open throttle (WOT), you may want to recheck so you get a proper compression reading, befor you do anything else. Do what someone mentioned here to check compression at WOT, you may not have compression issues, may be electrical. Good luck!
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Re: Compression?

Postby BeemerlessBob » Fri Jun 22, 2012 3:36 pm

Too late. I got up early this morning so I could work in the garage before it got hot.

I removed both heads and jugs. The pistons look OK and there does not appear to be any scoring on cylinder walls.

Nothing stuck in the valves that I could see. I put spark plugs back in and filled the head cavity with gas and after about 15 min, the level had gone down about 1/4 inch on one head and about 1/8 inch on the other head. So, I assume some leakage?

I turned both heads into a machine shop to have the valves reseated. ($50 a head)

Upon examination of rings, I could not see any breaks or damage. I decided that as long as I have torn down, I should replace rings anyway. So I ordered rings and a new set of gaskets.

I guess now I wait until rings come in and put everything back together.
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Re: Compression?

Postby JohnBG » Sun Jun 24, 2012 11:25 am

If you have the heads off, here's an easy way to check for valve leaks:

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