2.7 rebuild 7k on motor one piston at 140 PSI

[Crashly]

Hi everybody I had my motor rebuilt a few months ago and it has 7k on the motor. I was putting the car away for winter and I decided to do a compression test. Five of the cylinders were in the 150-154 PSI and one of them was in the 138-140 PSI range. The re-builder is a very reputable builer that was recommended from the forum here. I did have him measure and spec the Piston and cylinders and he felt that they were still in good shape so they were reused, All of the valves and guides were replaced as well. So what do you guys think?

[cgarr]

You could do a leak down and they you can tell where the problem is, but its really not that bad but should be closer. Could just be a piece of crap under a valve or something.

[Crashly]

Possibly but I only have 7k on the motor not a whole lot of time for carbon to build up. I just got back from a spirited rive with it and tested the same cylinder and it is still 140 on a hot engine.

[Steve@Rennsport]

Craig is spot-on!!

Do a leakdown test so you know WHERE the compression loss is coming from,....

IMHO, compression testing is almost worthless due to the variables caused by differences in static compression and camshaft profiles. Further, its doesn't show you the source of any compression losses.

[herman maire]

Is it possible to do a leak down test on a cold motor? My motor is 90% ready to drop out of the car but forgot to do one.

Is there any tricks to doing a leak down test? how do you get each cylinder at top dead center? do you have to pull off the valve covers to see when the valves are closed just like when adjusting the valves?

Is a $90 leak down tester with single gauge good enough?

Thanks

[Crashly]

I believe that in order to do a leak down test you simply put each piston at tdc for that piston. One way is to mark the distributor and get rotor approaching the mark, while you are turning the nut with one hand hold a long screwdriver in the spark plug hole as you continue slowly turning the crank over by hand you can feel the screwdriver being pushed out by the piston, when it stops you are at TDC. I have used this method with great success with all of my v8 motors and I will probably do the same with the 911 when I do a leak down test. Since I have not adjusted the valves since the motor was rebuilt I will probably pulley the valve covers at the same time and that will help confirming the TDC. Isn't there a way to get TDC on the pistons using the crankshaft pulleyy?

[kodioneill]

i would use a tdc sensor pelican has them. must use the reach tool for the spark plug hole though, and it makes a compression test much easier. i would not use a screwdriver. use a straw or a tie strap if you choose to go that route. you can get tdc close on each cyl. by using the 120 deg. marks on the pulley also.

[Aurel]

Did you readjust your valves? The low compression could be caused by one valve gap out of spec.

[haycait911]

FWIW when I do a leakdown test, I just hook up the tester to air, and turn the engine until I get the lowest reading for that cylinder. when you're close just rotate back and forth a bit to get the point of lowest leakdown, TDC.

my vote is a chunk of carbon.