Low numbers on compression check

[foulplayer]

Alright, a little back story. I have a 92 240 with a S1 RB25DET. All stock with with aftermartket intercooler, walbro fp, nismo fpr, with suspension goodies. It has run perfectly on the stock KA radiator, stock boost (5 or 7 psi cant remember) for a year, driving it daily 80 miles even in 90+ degree weather, also took it to a couple autocrosses with no overheating problems. Installed a MBC, turned boost to 10 psi went to autocross and car overheated. Every since then my stock temp gauge bounces up and down and sometime will drop all the way and say there for a while then jump back up and continue to dance up and down but runs with no other noticable problems. To me that seems like it has air bubbles in the coolant. So I have bled it over 10 times and cant get the bubbles out. Almost seems like there is hole somewhere letting air into coolant. Last time i tried to bleed it the coolant began to boil.

I drained the coolant today and didnt notice any oil in it. Replacing stock rad with a mishimoto 2-core.

Today I decided to do a compression test thinking headgasket was bad. I pulled each plug, put a tablespoon of oil in each cylinder, pressed the throttle WOT and the engine was NOT at operating temp. Here is my numbers:

Back of engine
80
85
85
85
80
80
Front of engine

I know those numbers are very low. The compression tester I used was rented from Autozone, and from the videos I've seen online the gauge is supposed to stay when done cranking but this one didnt keep the pressure it dropped immediatly when I stopped cranking.

Whats your thoughts on my compression numbers? And since my tester didnt hold the pressure, does it mean that its a crappy autozone tester or is it because something else is wrong?

Thanks
Danny

[julio]

I would check the wiring on that temp sensor for corrosion or bad contacts. Even if there is air in the coolant, the needle won't go all the way down because the air is usually hotter than the coolant.

As for the compression test, did you perform a test before putting oil in the cylinders (the engine should have been at normal operating temps also)? One usually only puts oil in the cylinder to check ring conditions once a "dry" test has shown some sort low readings in some cylinders.

Perhaps you were reading the bar scale instead of the psi scale (8-8.5 bar = 116-123 psi)? It could be that the tester was fudged up. Try another one.

Bottom line is, check your coolant temp connections. Look at the wiring to ensure it is in decent shape and clean the connections. Get your hands on another tester and perform the test when the engine is hot and do not put oil in the cylinders the first time around. That will give you a better indication of your engine's condition.

[foulplayer]

Updated compression numbers per Julios advice with a tester that that actually works.

Back of engine
160
175
170
175
160
170
Front of engine

The coolant temp wiring/connections are solid, no problems from what I can tell. The car has air getting into the coolant somehow and I am thinking the problem is with the headgasket especially since two cylinders are reading 160. I am fully prepared to replace that, already got a oem gasket kit and new arp head studs. Before installing the motor originally, I replaced the head gasket but reused the stock head studs.

[USMCgetsome]

double check the hoses after u replace the headgasket. also do u have a good overflow resivour? Have the block decked and same as the head. last thing to do is check the head for cracks of anykind

[rb25det250sx]

wait. you said stock temp gauge. you still using the one off of the stock 240sx?

oh and idk how you pulled it off using a stock s13 rad. because my Rb blew holes in the three i tried.

[meet07]

why replace the headgasket? Those numbers look good...

[Kevin.A]

A small variance in compression is normal
if your numbers were like 185, 175, 90, 85, 160,170 then you replace your head gasket.

[julio]

He doesn't want to change the head gasket because of his compression numbers, he wants to change it because he is getting air in his coolant.

I would think that if you are getting air in your coolant via the headgasket, you would probably experience some leakage around the block. I would check to make sure all the 10mm head bolts are installed and torqued properly. These little guys are what usually seal up the main coolant ports to the head. I know there are 2 on an RB25, not sure on a RB25, but I presume they would be in the same location.

P.S. Since you already installed a new head gasket not too long ago, I would like to ask if you re-torqued your head after installing it after 1 good heat cycle? I know this is standard procedure with ARP studs (not sure about stock bolts), but it may be worth a shot. Who likes taking off a head when its in the car anyways!

[Xit40]

If you really are getting air in your coolant do a hydrocarbon test on your coolant and a leakdown test to see if you really are pushing air into the coolant jacket or if it's just leftover air pockets.

The hydrocarbon test should be available at any auto part store. Obviously if there are hydrocarbons in your coolant then you do have a coolant/headgasket issue because there shouldn't be any hydrocarbons in your coolant. BTW I'm assuming this is new and previously unused coolant in your system.

The leakdown test will also help determine if you getting any air into your coolant. When you do the test remove your radiator cap and check for bubbles coming to the surface. You'll need access to compressed air and a leakdown tester which you can build yourself if your into building stuff. Here's a link on how to build one that will work good. Use the hose from your compression tester and build the other part minus the regulator since you probably have one on the compressor hose.
http://www.motorcycleproject.com/motorc ... kdown.html