Compression test advice.

[optimusprime8]

Hello,

 

I have a 72 240z that the previous owner swapped an RB25 into, poorly.  The car needs a lot (rust) and the swap was done poorly or unfinished.  So I've pulled the engine out to send the body to paint and I decided to run a compression test to determine if I should have it rebuilt.

 

I'm doing a cold compression test with the engine out of the car.  I know you're supposed to have the engine at operating temperature when running a compression test, so maybe my numbers aren't valid.

 

Here's what I got:

 

1-185

2-175

3-155

4-170

5-170

6-160

 

When I squirt some oil in the cylinders, the readings don't seem to change much.  It's hard to do a lot of testing because my jump box keeps running low on juice.  Are the low #3 and #6 cylinders with a cold compression test enough to warrant a rebuild or tear down while the engine is out of the car?  Could it be a timing issue?  Should I just put the engine back in the car and drive it?

 

I'm not looking to build a monster here, I am just shooting for a nice driver with some decent power.  I am planning to run the RB25 mostly stock, unless I have to rebuild it, then I might go crazy.

[malibud]

If it were me I would run it . I have heard 10% from highest to lowest but the GM manual says no less than 70% of lowest to highest

[AkRev]

Good news, when you squirt oil in the cylinders, you are testing the rings. The oil runs down to the rings and this is an old school way to test the rings. So if you are not getting a noticeable change when you squirt oil down, that leaves your valves or head gasket. Based on the fact that two and four seem unaffected by the lower number of three, it is less likely that a head gasket is the cause. This would leave valves as your most likely suspect. 

So, the question right now, is it worth pulling the head and getting some valve work? If that is worth it to you, how much of the "wile you are at it bug" will you be influenced by?

To me, those numbers are not at the point that you "Need" to take care of it, Yes a warm engine will give you a better idea of what you are dealing with. If you are thinking about not getting the work done, before you do go down that road, get a warm cylinder compression test done first.

 

If it was me, I would crack it open and do some work, but I always like peace of mind; and I am good friends with the "wile you are at it bug." 

 

Good Luck

[NewZed]

Don't the RB's have hydraulic valve lash adjustment?  Spinning the engine at low RPM seems unlikely to pump up actuators that have leaked down after sitting for a while.  Seems like big decisions being made on iffy test procedure.

[flatrate]

It could be as simple as a piece of carbon stuck under the valve... a leakdown should be done to pinpoint the issue... or just pull the head and fresh up the engine anyways..