I'm currently working on a 91 300ZX 2+2.
It's not mine, and I'm working on it for a friend. I don't know much about these cars, the closest I have experience with is the VQ30DE in my infiniti.

When the car was brought to me, the top radiator hose had broken off the radiator, and likely had been driven like this for a short period of time (they drove the car to me like this). So I replaced the radiator, and thermostat. The car now overheats and runs very hot while driving (according to the temp gauge). It also lacks power and does not sound like it's running 100%.

I pulled the plugs and did a compression test, got 165-177psi on all cylinders except the middle passenger bank which came in at 125psi. The oil is not milky and does not show any sign of having coolant in it.

When I put the radiator in, I left the cap off and let the car warm up so that I could burp the system. It took awhile before the thermostat finally opened and coolant started flowing. When the thermostat finally did open, and coolant started flowing, it seemed to be blowing air quite violently out of the radiator cap, so much so that it was difficult to get the cap back on. I'm not familiar with these cars, but generally when I burp a coolant system I can leave the cap off as long as I want and it won't spit coolant at me. So this seemed suspect to me.

Now the real weird part. Initially when I did the compression test, I pulled only the front 4 plugs, left the back 2 in. When I tested the first cylinder, the car fired on the 2 cylinders and wanted to run for a few seconds on the leftover air in the system, so I shut it off after a few cranks. After doing this, I noticed the car was leaking coolant onto the floor. It peed probably a quart onto the floor after I turned it off. I put the car in the air, and it appears to be coming from around the main drive pulley, but not from as high as the thermostat housing. So this makes me think a water pump has something to do with it as well.


I'm pretty confused with the whole thing, and I'll be getting a leakdown tester so I can shed some more light on things soon, but in the mean time, does anyone have any ideas?!


Thanks in advance!

 

I'd be willing to bet HG, I'd say pressure test the coolant system as well.

 

when was the last 60 / 120k maintenance done? The FSM calls to replace the water pump when this is done, but I've seen mechanics at a Nissan dealership not do it. If anything, if the water pump is faulty, replace it, then see if your still having the same issues.

All my experience with blown head gaskets (a '00 chevy blazer and a '98ish Pontiac) there was the typical milky oil from the coolant.

When you did the compression test you did remember to do it at WOT right? I've done them a million times, but the last one I did I went through the whole thing and forgot until cylinder 5 and had to redo it.

 

What's WOT?

 

Wide Open Throttle, not sure why you left the 2 plugs in but to get accurate readings you def. need them all out. I did a bunch of porsche 944 head gaskets and on a few i was able to diagnose it by looking at the reservoir and it blowing bubbles in the coolant. There were traces of oil in the coolant too. and since they most likely overheated the hell out of it driving it that way on a turbo car thats extremely bad for everything.