: Compression test for a head gasket failure

01-15-09, 11:14 AM
Can you perform a compression test to check for a head gasket failure?

01-15-09, 11:27 AM
No. Sometimes it will tell you but the Northstar has a problem with head bolts pulling up. Some bolts will release their clamping force on the head and the other bolts will hold the head down until the force of combustion pushes the head up just enough to let exhaust gasses leak into the cooling system. Rotating the engine with the starter only produces what- around 175 psi? Think about the force when that air/fuel charge ignites.

A northstar can pass a compression test and still have a head gasket problem. Rather, it's more the bolts that are the problem than the head gasket, but usually it's both.

01-15-09, 11:30 AM
The best way is to do a coolant test for exhaust gasses, or take it on the road and give it 5-10 shots of WOT going from 40MPH to 75MPH. If the temp goes up, and keeps climbing, or goes to hot right away, check the rest of the cooling system. Make sure the water pump is operative, the t-stat is good, and the rad has no blockage. If all else checks out, probably a HG/bolts on its way out. This is an easy way to test them.

I can usually tell right away if a Northstar has bad HGs. Not always, as different parts of the cooling system can fool you into thinking it's a HG too.

Also, if the cooling system has a lot of pressure in it when everything's cold, like the next morning after a drive, it's usually exhaust gasses.

01-15-09, 11:38 AM
Hey Jake, the thing is the car won't really even start. We had the HG job done at the Cadillac dealer a year ago then about a month ago it would severly bog down over 2000 rpm (To me it felt like the computer was going into some kind of limp home mode). Now it won't even start and it backfires occasionally. I have taken a bunch of t-shooting tips from Ranger (FPR, Injector leakage check) but to no avail so far.

It won't even start, does this SOUND like HG failure??

01-15-09, 11:53 AM
Nope. You have to check for codes.

01-15-09, 11:58 AM
Sounds to me like an igntion issue. No guarantees but pull the codes and start with plugs, wires, coils, ign module. Make sure it's getting fuel as well.

01-15-09, 12:57 PM
OK, I totall agree, so here is what I did. I pulled in sequence the 2,4,6,8
plug wires. I bought a spark test tool. Starting with plug wire # 2, I connect the plug wire to the top of the tool then use the tool clip to ground the tool.

The tool is nothing more than a spark plug gap. Very nice arc. I did this in sequence. 2 then 4 then 6 then 8 (Hitting all four colis in the pack). All spark very nicely. Car is getting fuel, (I can smell it). No leak in the FPR nipple.

Pulled the fuel rail with injectors. No injectors leak at 43 PSI (I have a Fuel Pressure guage), pressure does not bleed, stays rock solid.

Insert Eyes crossing at this point, right.....

01-15-09, 03:37 PM
Check the codes, check the codes, check the codes

01-15-09, 04:02 PM
I did, the only code I could get to pop was the MAP. However this was only after LOOOOOOONG cranking, which will pop the code.... strange huh?

Funny thing was, WHEN it was running (It would rev to 2000 RPM then kind of bog down like it was in limp home mode) it would also pop the MAP code.

01-15-09, 04:23 PM
What is your fuel pressure while cranking? This is starting to sound like a weak or intermittent pump.

01-15-09, 05:14 PM
Hmmmmm interesting, I have always pulled the gauge off during cranking.

I will have an answer tomorrow.