Overheating! Please help!

[rosstree25]

I just purchased my first 91' 300zx a few weeks ago. I knew it had a small overheating problem to begin with but the rest of the car is in tip-top shape. only 120,000 mils on it! Ok so now to the problem....It overheats but not immediately. I can ride on the interstate for about 15-20 mile before it starts to get hot. Although, if I baby it around town sometimes it won't do it. As far as I know it has a new waterpump so that shouldnt be it. I noticed today that after it got to the top line right under H, that it dropped back down a little and then rose back up. Also last time I tried to use the heater it wouldnt blow warm air. So could the heater core be stopped up? Or could it be a faulty thermostat? Please help! I just bought it and I want to fix it so I can go cruising!

[rosstree25]

symptoms and conditions:

It can run at idle in the driveway and not overheat.
Usually overheats after 15 mins at 65-70 mph (especially after revving up past 3000 for any period of time)
Has new waterpump and new temp sensor. (not sure about thermostat)
Haven't flushed the system myself.
no steam coming from engine and tiny bit of steam coming from tailpipes and i don't think its constant.
Gurgling going on once it gets hot.
overflowing from coolant overflow tank on to the ground.
Faint hissing coming from Radiator cap.
Heater doesn't work.
Bubbles are popping up out of radiator with engine running and rad. cap off.

I think that's it.

[robomatic12]

Hopefully it's not a blown headgasket, but it sounds sort of like one. You need to try and burp the coolant system, raise the nose of the car and keep adding coolant while the engine is running up to temperature so the thermostat is open. If you can add a lot of coolant hopefully there was just a big air bubble and perhaps it never got out after waterpump was replaced.

You should grab some test strips from your local auto parts store to see if there is exhaust gases in the coolant system indicating a blown HG.

[nissanfreak12]

How is the radiator. I had a car do the same thing and the rad was pretty blocked causing it to not cool properly, this was only at highway speeds. Take it to a rad shop and have them check out the rad with a heat gun, they should be able to tell.

It does sound like headgasket like Robo said only because the bubbles with engine running and cap off. Pressure test the system.

[rosstree25]

ok so I just tried burping the system. It had a few big bubbles come out immediately after it got warm. But after that the bubbles seem to keep coming. I revved it just for a minute after the coolant started to flow and it seemed to get a little frothy...like small bubbles. The coolant also started to turn sort of a tan color but went back green after I revved it. I let it run for better than 10 mins. Is that enough? How far should the car be jacked up. I went about a foot. I'm getting no steam from my tailpipes still. Can there be exhaust gases entering the coolant and still not have steam coming from the pipes? (I still need to get the test strips and check for exhaust gases and will do that next) If it is a head gasket then how much around about would I be looking at? Oh and I almost forgot, after it ran for a while the heater started to blow warm (not hot) air out of the vents.I had the heater running about half the time the car was.

[rosstree25]

I also forgot to mention that it has been getting terrible gas mileage and I noticed a bit of a fireworks smell from the exhaust. Sorry I'm just trying to throw everything out there that could maybe contribute.

[DCaff300ZX]

Another thing to check is your heater core which seems to have been the issue with mine, I had occasional similar symptoms to your qproblem for years before I finally seem to have figured it out.
The problem surfaced about three years ago until two years ago my centrifugal fan and waterpump failed, and were replaced along with the radiator. The air bubble and occasional overheat/heat increase problem persisted at times much like yours and nobody could figure it out, as it passed all HG/pressure tests, etc., until just recently (August) the heater core failed and was replaced...which resulted in much hotter air in the cabin, and no air bubbles or overheating since.
Checking the old heater core, it had three plugged cores and was slowly leaking at the lower junction with the valve on it, and had obviously been leaking slowly for years and also allowing air into the system that nobody could ever figure out...so, check yours!

[Z-owned]

The coolant also started to turn sort of a tan color

Thats not a good sign and makes it sound like the HG.

[DCaff300ZX]

^^^True, but I forgot to mention that the heater core was also full of the tan gunk which in the past would show at times in the coolant flow and kept my mechanics-at-the-time scratching their heads. So if your HG's test out OK and no oil in the coolant, check what I suggest.
Also, the clutch fan HAS to be working perfectly so be SURE it has been replaced with OEM or is OK per BigT's advise.
I also had the first W-Pump that was replaced fail immediately, and also had the replaced fan clutch fail this year so it seems that even OEM parts aren't completely fail-safe so have the w-pump checked as well.