I have an 86 Turbo and it is somewhat of a project car. So I'm not claiming that it was fine before. The car reaches an idle temperature of almost 3/4 of the analog gauge. Like half an inch before the 3/4 mark. Just to make sure, it is running hot right? I've read that the needle should be at around the halfway point.

[www.az-zbum.com]

Thats the page I was guiding myself by, and the car always reaches the same idle temperature. Replaced the radiator cap, thermostat, waterpump, did a flush, fan clutch is fine. The only thing left would be the radiator, or a hose or something that is bad.

So what else could cause overheating? What do I check? Even if it is something dumb like a disconnected vacuum line or connector, and if so which ones would affect it?

The temp gauge should be at the 1/2 way point or lower, under normal conditions.

Does it get hot at idle? Does the fan clutch lock up so the fan spins at idle?

Does it overheat at speed? I'd check to make sure the radiator is not blocked. My '85 Turbo had so much pet fur that there was only a small space for the air to flow through the radiator.

If the car has A/C there's an electric fan in front of the radiator that should come on when the A/C compressor is on.


It's a big pain to remove the radiator because you have to pull off the whole front bumper! If you go that far, have the radiator professionally cleaned.

Yes, I've done all my testing while having the car on idle. Haven't really driven it much. The fan always spins even at idle. And yeah, it overheated at the beginning when it was pushed, so much that steam could be seen coming out from the front of the hood. I did note that on the az-zbum site it said that overheating at high speeds could be because of a clogged radiator, but I didnt know how much effect it could have at idle.

I know there is always that possibility of a blown head gasket in the back of my mind. I've read about the signs, oil in coolant, coolant in oil, the milky ring, smoke coming out of exhaust, and some others but those don't appear. Well, except the bubbles in radiator...I have seen those appear with the radiator cap off. But I'm not sure, does this mean it is definitely blown and would it cause overheating like that?

Also heard about cracked manifolds a couple of times, but there isnt much info on the other sites about them because apparently the problem is usually one of the others I've checked.



Edited 1 time(s). Last edit was 2009:06:23:12:04:12.

wouldn't put a lot of faith in the stock analog gage. It might be bad as might the sender. See if you can rent/borrow one of those temp guns that give you the correct temp by pointing at the radiator. Also I have noted that it takes a bit of running for a z31 engine to get full of coolant. Did you dilute the coolant to 50/50. pure glycol is not a good coolant as the viscosity is such that the water pump doesn't circulate enough coolant. you stated that you are seeing bubbles in the coolant. that isn't a good thing.

Steam coming from where exactly?

Also have you tested the thermostat. Boil it in a jug to see if it opens.

sounds like a cracked radiator.

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1985 300ZX n/a

See my Z project

I will try to get my hands on a temp gun. I couldn't really tell where the steam was coming from that day, I'm, guessing somewhere in the front, not from the heads if that was what you were referring to. But i'm not sure. The car still smokes now, but I'm pretty sure it is just some old sludge on top of the exhaust manifold burning up.

About the bubbles. Don't they appear when "burping" the system? And could they possibly be caused by air coming in not necessarily from inside the engine? Maybe a leak around the hoses/thermostat?

Also noticed some blocked off hoses around the intake manifold. The car doesn't have the AIV, but they don't seem like they were blocked too good, I will try to fix it today.

i dont know if its normal because i dont have a/c but when i turn on the blower fan, the temp needle moves along with it in sync.



Edited 1 time(s). Last edit was 2009:06:24:03:51:05.

You need to re-read Zbums site on over heating. There are alot of parts you need to test, that you probably don't have the tools for. A temp gun you don't need at all.

Inspect the radiator for any leaves/trash. Also check to see if any rocks have made a nice dent in the radiator, just something good to check for as well.

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1985 N/A Z31 5 speed 2+2-burgundy exterior and interior.

MY Z31

"RIP ol' red she didn't make it after the wreck, she died at the scene on 8-8-08"

Do a coolant system/cap pressure test.

Re: Engine Overheating. What to check? new
Posted by: red october
Date: Yesterday, 06:48AM


Do a coolant system/cap pressure test

this will save you lots of time!

If I wanted to install an aftermarket temperature gauge, would I install it where the temperature switch goes? I mean, that is where I would like to put it, but I guess I need a gauge with a sender that will fit there. Or where else would it be possible to get a reading from?

There is no reason to get an aftermarket gauge.

Well I did the coolant pressure test like you guys suggested and found it leaking around the thermostat and probably the water pump. So I did them again. I'm gonna be patient and wait the 24 hours the gasket maker says.

It was suggested somewhere else that I test the temperature with a cooking thermometer so I used a deep fry one and stuck it in the radiator fill hole. In the end it hovered around 170 which was the temperature my thermostat opens. While the car gauge read almost at the 3/4 mark. Then again, I don't know how accurate this method is and I still have a feeling it's overheating.

Still was getting bubbles in the radiator last I checked, but I never burped it by putting it on jack stands. I found the leaks and wanted to fix them first. Could the bubbles have been caused by the leaks or not burping it? Or does it 100% mean its a headgasket problem?

Lastly, I don't know if I mentioned it here but the valve ticking noise finally dissapeared. Now there is another steady clicking noise that is quieter than the lifter noise, and it does not increase with RPM's. Any ideas? Honestly, im not too sure what sound the car *should* be making so thats why I ask.

When you get it all assembled do the coolant system pressure test with the system full and all the air removed at warmed up temperature. Do the test again, if it holds pressure you don't have a bad head gasket.

I put it all back together (minus the timing belt cover for now) and thankfully the thermostat and water pump stopped leaking. Something came up and I didn't have much time to test, but I noticed bubbles in the radiator fill hole when I first started the car back up. Hopefully just because the radiator wasnt burped. Well, I had enough time to pressure test, and yeah the leaks stopped and I didn't notice any others. Still overheated the same as before.

My radiator cap says 13 lbs. I used a pressure tester from autozone with a gauge that says if I want to test 13-15 lbs (dont remember exactly what range) I should test it at the arrow which was located at 16 psi. The Motorad instructions said if the arrow dropped slowly there was a small leak. If it held constant pressure for at least 2 minutes that it was ok. Well, the needle dropped from 16 psi to like 13 or 14 in like a minute or two. However, after that it stopped dropping and stayed steady for yeah, at least two minutes and probably more. This is when I was interrupted and I had to stop testing.

Is this good or bad? I was told that this car's cooling system's capacity is 13 or 14 pounds so I'm hoping maybe thats why it dropped from 16. Also, the instructions mentioned checking the dipstick for any water or bubbles, didnt see any.

Sounds good to me. If the radiator cap is designed to hold 13 psi, and you held 13-14 for over 2 minutes, everything should be working correctly.

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1985 300ZX n/a

See my Z project

Cycle the cooling system for a few days and test again - it just needs to be burped, thats why it lowered to 16 psi.. air bubbles compress within the system.

Daily update time. First thing I did was top up the radiator with a little bit of coolant. Did the pressure test again and it held at 16 psi. Then I just ran the car without the radiator cap. Didn't see a lot of bubbles at first. Most of them which were very tiny. After running it a bit more I revved the engine and frothy white bubbles formed in the coolant around the filler neck. Weird?

I will cycle it for a few days like you said. In the meantime is there anything else I should check for?

no