Hey everyone,

I'm having a problem with my 1991 RS / 305TBI overheating. I checked the stickies and didn't really find anything that would point me in the right direction. I hope someone can help me out from a previous experience or something.

I bought the car in August of last year and drove it quite often when the temperatures outside were around 60F-70F. I put the car away for part of the winter while I did some tuning up and the smog pump removal mod.

Now, that the roads are pretty much cleared out where I am, I have been driving the car more often. I noticed the other day while I was driving along that my temperature gauge was higher than normal (220 +/- 10). This was city driving, haven't drove the car on the expressway. The radiator fan was on and so was the heat. After a few minutes of driving and the temp gauge getting pretty close to the red (260) the temp dropped pretty quickly back to around 150 where it usually is.

The previous owner installed new cooling lines and radiator so I figured maybe the 50/50 mixture was off and the bottom of the radiator was frozen because it was below freezing outside.

I went ahead and just did a complete cooling tune up.

New waterpump, thermostat, gaskets, proper pre-mixed 50/50, and radiator hoses. Bled the system after the job and started driving the car again.

Still does the same thing. Normal city driving while the temperatures were below freezing the car likes to get past 220F and near red then drop completely down to ~150.

The only thing I can think of is my air dam being missing, but I thought that was primarily for highway driving? I should also note I didn't have any problems with the car overheating when I drove it during the fall where the temperatures were way higher (without an air dam)

Bad temp gauge? Or is there something I am missing?

Thanks for any input or advice!

 

Your car will overheat no matter what speed if your air dam is missing or heavily damaged. What condition is your A/C condenser in? If the fins are flattened it will restrict air flow to the radiator.

 

Wouldn't make a ton of sense though, the temperature always drops down pretty quickly. I took all of the A/C components out of the car. The radiator looks to be in pretty good shape, not leaking or not too many bent fins. I'm starting to think maybe there's an air pocket that isn't allowing the thermostat to open all the way.

When the car got past 220 I pulled over and felt the upper rad hose, wasn't that warm.

 

Purchase a Laser Light Temp gun about $30 at Harbor Freight,these temp gauges are usually off about 20*+.Is the car Boiling over when you shut it off??If not, your not over heating.

 

That kind of rise and drop...does it do it constantly? Looks a lot like an air bubble in the cooling system? Have you burped the system since you did all the parts swaps?

 

Is the overflow tank filling up or are you losing coolant? Bad radiator cap where spring will not hold pressure allows coolant into overflow tank or out to ground. Air dam does nothing when stopped/idle so solve the problem at idle first. What is the gauge showing when the fan comes on? Should give you some indication the gauge is about right. Easy to test the therm in a pot of water too. Keep us posted.

 

Ahh,forgot about a possibly bad radiator cap. That will mess everything up.

 

It happens once while driving. Happens again after the car has been turned off and sits until it cools down.
It is full, not losing coolant. The fan is always on, previous owner has it wired that way. Thermostat was tested before putting in car, and it opened once the water boiled.
Brand new cap.


However, I DID find a solution for anyone that runs across this problem. It was a trapped air bubble somewhere near the thermostat causing the problem. I drilled two holes on the thermostat, reinstalled in the car, and burped the cooling system. It was wanting more than I lost taking the thermostat off so it finally accepted all coolant. I drove it on the highway hard and through the city, stayed at 150F.

Horray!

 

That's too cold, you want it to be around 190-200º Fahrenheit. Otherwise the oil will never get hot enough to burn off contaminants, along with constantly running rich furthering contamination. The thermostat housing should have a bleed valve on it, as drilling holes in the thermostat partially defies the point of it being there.

 

SO it was an air bubble. Needed to burp the engine more. Putting the front end up a bit would have done the trick without drilling.

Drilling holes is an acceptable practice, with high-compression, high-horse power motor. I believe he has a defective thermostat if he needs to drill holes with a basically stock motor.

Boxx is correct in that 150 is too cold. You want a 180 degree thermostat at the coldest. These cars were designed to run 195-220, especially in stock trim.

 

SO it was an air bubble. Needed to burp the engine more. Putting the front end up a bit would have done the trick without drilling.

Drilling holes is an acceptable practice, with high-compression, high-horse power motor. I believe he has a defective thermostat if he needs to drill holes with a basically stock motor.
Boxx is correct in that 150 is too cold. You want a 180 degree thermostat at the coldest. These cars were designed to run 195-220, especially in stock trim.







That's too cold, you want it to be around 190-200º Fahrenheit. Otherwise the oil will never get hot enough to burn off contaminants, along with constantly running rich furthering contamination. The thermostat housing should have a bleed valve on it, as drilling holes in the thermostat partially defies the point of it being there.

 

That or use a quality thermostat that meters flows, like the Stant SuperStat, versus the on/off nature of cheaper units. It's what I put into my high comp 357 and it made a world of difference. I was thinking either the thermostat he used is either defective, too low of a temperature rating, or upside down even.

 

It's quite possible to put in upside down. I've even seen some that were put in backwards, not upside down, just backwards, that have caused problems. I use a Stant myself, though I had a Mr. Gasket in there for three years with no issues. Just swapped it out when I was replacing the radiator a year ago.

 

#1. Stock gauge reading 150°? Its not 150°…

#2. Just because the coolant is at "150°", which is most likely closer to 180-195°, does NOT mean the oil is the same temp. The oil is hotter than the coolant temp the same as the metal is hotter than the coolant temp. Its the basis of how the cooling system works. Typically the oil temp is ~20° if not more, over the coolant temp.

I run a 160° thermostat, my oil gets up to the proper temp to burn off contaminates just fine. Even before the turbo was installed.

You know where my temp runs with that 160° thermostat? 175-180°…

The rating of the thermostat is NOT the temp the car will run, its only the temp that the thermostat OPENS.

#3. Drilling a small bypass hole has been used for decades and is a perfectly acceptable practice. Why else would they sell failsafe thermostats?. Ever had a thermostat get stuck closed? Just dont drill a GIGANTIC hole in it.

 

I agree. Going to replace the thermostat again and try your suggestion with jacking up the front. At least now I know it isn't something else!

 

Yes the oil runs hotter thanks to friction, but the faster the oil can get up to temp the better.

Yes we all know the temp rating is when it opens, but it does have an effect. My car's previous owner had an 180º in there with holes in it and it wouldn't get over 170º. After putting in the 195º SuperStat it gradually gets up to 190-200º and stays there. Of course climate had more of an effect than anything probably since it hasn't been above 30-50º where I live.

Or spend the extra bucks for a good stat instead of rigging it, either way.

 

I picked up a Failsafe 180F thermostat, seems to be accepting more coolant while burping it. Heat is working, temp is around 190F on the gauge.

Just gotta test drive it in a few minutes here.

 

Sounds like you should be good to go then.

 

Indeed! Test drove again on back roads and the expressway. Temperature was reading around 190-200 the entire time. All is well, and it doesn't take 50 miles for the car to warm up.


Thanks everyone for your input and advice!

 

Glad to hear it! Now enjoy responsibly.

 

Then the hole was too big. There is a specific size it needs to be. Doesnt matter if you get a "good" stat or not, its going to fail eventually. Is it going to fail closed or open? It can fail either way, so it doesnt really matter if you have a good one or not.

 

Well everything fails eventually, but the good ones don't have to be rigged as they meter flow which was the point being made.

 

And if it fails closed and overheats the engine? Thats the point I was making. All it takes is once.

I once bought the best tstat money could buy, and all I can say is I have had better luck with the cheap failsafe tstats.

And the "good" ones dont meter flow any differently than the "cheap" ones. Ever had a tstat fail closed in your car?

 

That's why you pay attention to the car's vitals?

Yep, they do: http://www.stant.com/index.php/engli...at-thermostat/

Nope. I always go through and replace all expendables whenever I buy a car.

 

And thats the only time you replace them? LOL

Do you also trust stock gauges?

 

you have to know that the temp gauges in these cars are "inaccurate". heres what I did with a similar problem .I bought a IR heat gun. point and shoot type. I found that my perceived overheating actually wasn't. having said that. debris can build up between the AC condenser and the rad. and your air dam MUST be in place for proper air flow

 

This tool is a Godsend,it really puts you at ease just knowing that your not running Hot


http://www.harborfreight.com/non-con...ing-60725.html

 

Also with every water pump change interval.

It depends, if they're the dummy gauge that graces the presence of most cars then no. If it's an actual thermometer then yes.

 

My factory temp gauge reads 20* hotter than the actual tempterature of the car, verified by when the temp switch activates and by thermal IR temp gun. So I don't trust factory gauges for nothing.

 

The stock gauge is woefully inaccurate.

 

I don't have a third gen, so I wouldn't know. That's why you should always double check with a used car like the chap above with his temperature gun.

 

I'll tell you what!it sure as hell beats chasing a problem around

 

Indeed, the right tool for the right job.

It's good to know though that the third gen gauges are wonky though, as I'm looking at acquiring one. Odd though as the second gen gauges are pretty accurate and can be recalibrated if needed.

 

There's a company in cali that will recalibrate third gen gauges, but it's really expensive.

 

Also good to know, was wondering about that. I'd probably just find an aftermarket solution that could be implemented and look original. Of course I'll worry about that after getting one.