Strange overheating issue.

[yodabiri]

Puzzling cooling system question.

I friend of mine just got a mustang with a 200 ci engine, and he changed the waterpump because the old one had a messed up bearing and was making a loud and abonoxious noise. So he changed it and now the car sounds great, no noise, but what's interesting is that when he drives it, the temp gauge shows the car starting to overheat (the needle goes above half) and then cools off and this only happens in the first 15-20min or running the car. We tried bleeding the system, and drove the car up on ramps and revved the engine for 15 minutes or so, to see if any big bubbles would come out of the radiator, but other than one bubble, nothing. The problem still persists. We cannot figure it out to save our lives. I think we tried everything we can think of short of changing the thermostat, but that is unlikely to be the source of the problem because it worked great with the old pump, so why should it not work now?

If anybody has run into a problem like this, please let me know and I will pass it on to my friend.


Thanks in advance!

 

[BUZZ]

Sounds like the thermostat is sticking. I think I'd change it just to see if it is. Just how hot is it getting? Half way on a factory gauge doesn't sound very hot. Is it above 220*?

 

[ludwig]

That's because he has a water pump that's working now. From a cold start, T-stat stays closed until the water behind it is hot enough to open it, meaning at the top of the water jacket. Meanwhile, all the water behind - below the T-stat - is getting hotter and hotter. There is a slug of hot water past the temp sender when the T-stat opens. Then it settles down to normal function.

It gets particularly warm when the car is parked for a short time and there is no circulation. Once it is started, there is a slug of hot water past the temp sender that was heating to max in the block. This goes into the radiator and cooler water comes in behind it.

Get a 160* or 170* T-stat if you are worried.

 

[mugsy]

Yup, sounds normal to me. The water in the block is heating up first, then the t/stat opens and all that hot H2O mixes with the cooler H2O in the radiator. This will lower the temp for a little bit and then the whole system will be back up to operating temp. Without a thermometer it's tough to tell if a probelm is really going on. Trust some numbers before a postion on a H--C gauge.

tanx,
Mugsy 8)

 

[kukm66]

Anytime you replace the water pump, you should replace the thermostat as well as a precautionary measure. They do not cost much at all and will save you a lot of headaches later.

 

[yodabiri]

I thought that the thermostat is most likely the problem now. I will let my friend know. He is looking to swap a 250 in, so I dont really know if he would want to bother with the thermostat before switching engines.


Thanks for the responses

 

[wallaka]

I'll agree, it's probably not a problem, just normal operation. You'll really see it with a good aftermarket gauge. I have seen temperature fluctuate (especially on boat engines, where the coolant is colder) simply by depressing the throttle at idle. It's just an unusually sensitive stock sender or gauge.

 

[CobraSix]

Really, to me sounds like you have some air still in the system somewhere. Here is how I bleed my cooling system.

Run it for 15 minutes in your driveway with the cap off. Keep an eye on it. If at any time you see the fluid level in the radiator go down, fill it back up immediately. Keep doing that until you see it stop dropping. Then I like to turn the car off and let it sit for 5 minutes, again keeping an eye on the radiator level. AFter 5 minutes, start it back up and check the level again. once it starts coming up, put the cap on.

The problem doesn't sound like the thermostat issue, sounds like low coolant and air in the system. Thermostats are not a digital system (On/Off). They are an analog system that open gradually until full open at the designed temperature. If you see sudden spikes like you did, it's either low coolant, or if you are in a cold environment you have ice in the system due to improper coolant mix ratio.

 

[mugsy]

You might want to check to see if the 200 and 250 use the same t-stat. When I put the 250 head on my 200 I want to say the physical t-stat size looked the same (but the hose size was definetely different between the two heads).

 

[MustangSix]

If this is the stock gauge, you may also want to check the instrument voltage regulator. If it sticks, the gauge tends to go high.