Strange things afoot with the temperature gauge...

[rhodedog80]

Yes, that is an adaptation of a line from "Bill and Ted's Excellent Adventure." No. I'm not ashamed. Even if it did star Keanu Reeves.

On to the reason for the post: I have a 1996 240sx. I love the car. I have recently started doing my own maintenance on the machine as I have a mechanically sound mind and am actually handy with tools.

So far, so good. Recently, however, I have noticed that when I am driving my car, my temp gauge will spike up to "H" very quickly, but when I push the accelerator to higher RPMs, it makes the temp needle dip rapidly - and many times stay in the mid-temp range. This baffles me, and I was wondering what I might do to debug it further. I was thinking three possible things:

1. Water pump
2. Cylinder gasket leak
3. Head gasket leak

There is a slight oil leak, but no smoke visible when I accelerate or upon initial startup. I would say temp gauge malfunction, but I can smell the coolant... I have parked the car because I don't want to damage the car by driving it hot. I also don't want to chase rabbits as I disassemble the beast.

I'm just looking for a direction or ideas from experienced 240sx owners/mechanics.

[allenms240]

Sounds like there are bubbles stuck in the cooling system. Jack up the front of the car, take of the radiator cap, turn your heater on full blast, and let the car idle. The air should rise to the top, and escape through the open radiator. Do this for about 10-15 minutes.

[rhodedog80]

Hmm, yeah. I probably should have put that at number four. Guess I'm a pessimist.

So, these air bubbles would cause a car to overheat rapidly like that?

Thanks for the reply, btw.

[allenms240]

Yes, they do. I don't know the EXACT reason, but here is my theory.
So you have air bubbles in the cooling system, which are obviously ALOT hotter than the coolant itself. As the air passes by the temperature reading sensor, it records the temperature as extremely hot, then as the bubble passes, the coolant sends the sensor a cooler reading, which is the spike you see. I don't think the car is actually overheating, but its just the hot air that's giving the hot reading to see on your gauge. That's just my theory on how it all works.
However, these air bubbles DO cause a spike in temperature readings. And since the car isn't staying hot, that's what I believe it is.

These air bubbles are sometimes easy to remove, sometime rather difficult. I've been going on and off trying to remove them from my girlfriend's vert, but haven't have the time to really sit down and jack it up and flush the system. But that's worse case scenario. Give it a go and report back with results or questions.

[rhodedog80]

Will do. A flush may not be a bad idea for me anyways.

I'll let you know how it went.

[biggie]

Sounds like thermostat is getting stuck, but air pocket also possible.

[rhodedog80]

Sounds like thermostat is getting stuck, but air pocket also possible.

Yeah, I'm replacing the the tstat, regardless. We'll see.

It is HOT out today!