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  #1  
Old 12-18-2006, 10:24 AM
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Changed headgaskets, now low compression!

Car is a 1985 500 sel Euro. M117.963.
Changed both headgaskets, had both heads machined (tops and bottoms).
Car started first time on reassembly. Idles poor and at 1000 RPM.
I pulled off the spark plug wires one by one, and on removing no 6 nothing changed.
I have spark, albeit a bit yellow.
Stuck the compression tester in and I have ~20psi Aghhh! put some oil in with no change. I'm hoping that I only need to adjust the valve clearance. I'm off back out there to pull the rockers to see if it makes any difference.

Any advice to a desperate man?

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  #2  
Old 12-18-2006, 10:41 AM
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Phew,
Well I pulled the rockers for that cylinder, made sure the piston was at BDC, and cranked. Immediately got 120psi on just one stroke. I think the WSM calls for 8 strokes, but without valves opening, obviously there was no point.
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  #3  
Old 12-18-2006, 11:11 AM
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Did you keep all the rockers/shims etc numbered so you could re-install them in the same place they came out of?

Are you positive your timing is spot on?

Jonathan
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  #4  
Old 12-18-2006, 11:18 AM
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Re check the cam timing.


.
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  #5  
Old 12-18-2006, 11:33 AM
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Cam timing is not bad... in the bank i'm having problems with, its 3' retarded (I have the offset keys on order).
All shims/timing gear etc is back on in exactly the same places. It must be to do with me having both surfaces of the head machined. When the head was on the bench, the cam wouldn't even rotate because the head was so warped.
I ghetto'd one of the shims with a flap disk on my angle grinder, retrying it periodically on the inlet valve, and I took it down to just over 4mm. I'm now getting a healthy 190psi! Just got to do the same on the exhaust valve shim now.
Compression otherwise looks great on all other cylinders, between 200 and 180 psi

Edit:
Just looking in the WSM and it says compression should be 8.5 bar minimum (~125psi). Is it a problem that mine's so high?
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'90 750i
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  #6  
Old 12-18-2006, 01:04 PM
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Quote:
Originally Posted by Ben12345 View Post
Cam timing is not bad... in the bank i'm having problems with, its 3' retarded (I have the offset keys on order).
All shims/timing gear etc is back on in exactly the same places. It must be to do with me having both surfaces of the head machined. When the head was on the bench, the cam wouldn't even rotate because the head was so warped.
I ghetto'd one of the shims with a flap disk on my angle grinder, retrying it periodically on the inlet valve, and I took it down to just over 4mm. I'm now getting a healthy 190psi! Just got to do the same on the exhaust valve shim now.
Compression otherwise looks great on all other cylinders, between 200 and 180 psi

Edit:
Just looking in the WSM and it says compression should be 8.5 bar minimum (~125psi). Is it a problem that mine's so high?
Nah.....just healthy...usually find 'em at 155 to 170.



.
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  #7  
Old 12-18-2006, 01:11 PM
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The closer to sea level you are the higher that will be. The higher the atmospheric pressure that day, the higher it will be. It can equate to over a 50PSI difference sometimes! I would susprct your gauge isn't that accurate, though.
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  #8  
Old 12-18-2006, 02:17 PM
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Well its running much better now, but its still a bit lumpy at idle, and idle is still at about 1000 in N or P or about 800 in Gear. ICV is operational and clean. Intake rubber is all new, as are vac connections. The bypass line between intake rubber housing and idle air pipes is blocked off.
The only thing I can think that may have caused this is all the crud I cleaned out of the throttle body and butterfly. Is there an accessable adjustment screw for this?
I think the lumpy idle must be the cam timings being out. One is advanced by 6', and the other is retarded by 3-4'. We'll see after I recieve the offset keys though.

Yeah, my compression tester isn't the best in the world, so I wouldn't be surprised if it was over reading.
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  #9  
Old 04-28-2007, 10:10 AM
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did you fix the problem? i have a similar issue.
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  #10  
Old 04-29-2007, 06:42 AM
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Did I fix the high idle? Well yeah in a way.... My engine still wouldn't stop using water. Went from not very much to about 2L every 75 miles.

So I did what I should have done in the first place... Throw a 5.6 in there.
Hey presto... Nice low ~700 idle.
I retained the upper intake manifold and all of the original FI, but used the 5.6 lower intake and throttle body. I can only guess that I must have damaged the throttle plate originally when cleaning the crap out.

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