Testing Thermostat Functionality

[jmackro]

OK, here's the backstory: I was driving my 2800 CS a few weeks ago when I got caught in stop-and-go traffic. After a few minutes, I noticed the temperature gauge climbing. Did a quick U-turn, started moving again, and the needle dropped back to normal. So, how to diagnose what's wrong?

I had replaced the water pump and viscous fan coupling a year or so (and very few miles) ago. The radiator is fairly new. Coolant full. My first diagnosis was that the A/C condenser wasn't allowing sufficient airflow through to the radiator. It's the original condenser, as far as I know.

Brought the car to my mechanic to get his opinion. He looked at the condenser and pronounced it OK. Suggested that the thermostat might be sticking and we agreed to have him change it (old and replacement thermostats rated 70° C). After changing it, he left the car idling at his shop and said that the temperature gauge stayed in the normal range. So, the problem should be solved, right?

I had asked him to give me the old thermostat. Today I gave it the boiling water test and it seemed to open pretty wide (see picture below). My questions are:

- Looking at the picture, with the thermostat fresh out of boiling water, is it opening fully (sure looks like it to me)?
- Do thermostats ever become intermittent? That is, stick only sometimes? My mechanic pointed out a shiny part on the thermostat shaft where he believes it was binding. But it sure seemed to open in the pan of boiling water.
- If the old thermostat was not defective, then what other issues might explain my symptoms? Yea, I know: a clogged radiator.

 

[Stevehose]

Have you bled the system?

 

[jmackro]

Thank you for that suggestion.

I did bleed it when I replaced the water pump. Still, it would be easy enough to do again. But would air in the cooling system cause these symptoms?

 

[HB Chris]

Classic symptom of partially blocked radiator happened to me in our 2800. Speed improves airflow.

 

[Krzysztof]

I had such effect in my E30 and it appeared to be viscous coupling. I was told it is very common story even if the "visco" is not old.

 

[BarnE9]

Thermostat sure does look to be wide open but since you've replaced it you can strike off the list! Have you been for a drive since it was changed ie not just idling?
If still overheating, bleeding the system cant hurt though I dont think an airlock would cause symptoms you describe. Note your newish radiator, but its only going to be as clean as the sludge in the rest of your engine allows. Maybe try a coolant flush - I did one recently and seems to have been beneficial

 

[sfdon]

A test for folks that don’t want to do the “stick the shop towel in fan blades test, while the engine is running”.
If you stand by your Windshield while the engine is warmed up and can feel the hot hitting you from 3 feet away and then turn the key off and watch the fan come to a stop in 2 turns- It ain’t your fan clutch

 

[jmackro]

Thermostat sure does look to be wide open but since you've replaced it you can strike off the list! Have you been for a drive since it was changed ie not just idling?
If still overheating, bleeding the system cant hurt though I dont think an airlock would cause symptoms you describe. Note your newish radiator, but its only going to be as clean as the sludge in the rest of your engine allows. Maybe try a coolant flush - I did one recently and seems to have been beneficial

Yes, doing a flush is a good idea. I did replace the coolant a year+ ago when I did the waterpump, but could service it again.

I have driven it since the thermostat was changed and it has stayed cool. But I haven't gotten stuck in traffic. The problem seems kind of intermittent, which is why I asked whether anyone has seen a thermostat fail intermittently.

A test for folks that don’t want to do the “stick the shop towel in fan blades test, while the engine is running”. If you stand by your Windshield while the engine is warmed up and can feel the hot hitting you from 3 feet away and then turn the key off and watch the fan come to a stop in 2 turns- It ain’t your fan clutch

My mechanic grabbed the spinning fan with his bare hands! Well, not quite. He pressed his finger against the backside of the circular hub - not the blades - to see if he could stop or slow it down. It looked kind of dangerous to me, but he pronounced the viscous coupling OK.

I like your test of watching the fan decelerate after the engine stops - will try that.

 

[HB Chris]

Just let it idle, if it gets hot it is a partially blocked radiator, use a thermal thermometer, it should be hot at input and colder at output down low. You can borrow mine.

 

[jmackro]

Just let it idle, if it gets hot it is a partially blocked radiator, use a thermal thermometer, it should be hot at input and colder at output down low. You can borrow mine.

Hmm... I don't have a thermometer, but are you going to be at The Finish Line this Saturday? If so, I'll bring my coupe.

 

[HB Chris]

I will be in Santa Barbara but home tomorrow and Thursday

 

[Honolulu]

Insufficient cooling at low speeds or when stopped (but adequate cooling when moving) suggests not enough air moving through the radiator because the fan clutch, fan itself or drive belt is not/no longer in the game. It is less likely to be a partially blocked radiator.

 

[Dick Steinkamp]

Brought the car to my mechanic to get his opinion. He looked at the condenser and pronounced it OK. Suggested that the thermostat might be sticking and we agreed to have him change it (old and replacement thermostats rated 70° C). After changing it, he left the car idling at his shop and said that the temperature gauge stayed in the normal range. So, the problem should be solved, right?

I had asked him to give me the old thermostat. Today I gave it the boiling water test and it seemed to open pretty wide.

It sounds to me like it is fixed. If he let it idle in his shop for a while that should be the same as being stuck in traffic for a while. Let it idle in your driveway for a half an hour and see if you get the same results your mechanic did.

The T-stat is 70 C. The water temp should be monitored during a test to see if the T-stat starts to open at 70C. Plunking it into boiling water only shows it will open at 100C. It does not test that it is performing correctly.

My guess is that the T-stat was defective and the mechanic fixed the problem.

 

[jmackro]

It sounds to me like it is fixed. If he let it idle in his shop for a while that should be the same as being stuck in traffic for a while. Let it idle in your driveway for a half an hour and see if you get the same results your mechanic did.

Yes, agree, with one exception: I had the A/C on when I had the recent incident of rising temperature, but I'm not sure my mechanic tried it with the A/C on. However, there's no reason I can't do the experiment that way.

One other idea: The electric fan ahead of the condenser works OK, but is original. An aftermarket e-fan would generate more CFM through the condenser and radiator and might address this issue. Thoughts?

 

[Stevehose]

I have the original condenser fan and have no change in temp when the ac is on or off. If it doesn’t need bleeding then either the radiator, water pump, or fan clutch is not keeping up.

 

[sfdon]

Let’s go through this step by step...

the correct thermostat for your car is an 80* C thermostat.
It should open at 80* C

Putting a thermostat in a pan of boiling water is not how to test it.
Place it in a pan with a thermometer and note the temp when it opens

The OEM fan is a joke. Throw it away. Install a PUSHER fan with minimum of 1500 cfm with curved blades.

 

[wilies13]

I just checked on ebay. A new thermostaT from a really good supplier like Behr (Febi Bilstein, Wahler,..) has an price of about 20....25 Euros.
In such cases I just apply risk management and replace the part. The damage in case it suddenly refuses a good function could be much, much higher.

 

[Krzysztof]

Risk Management:

Risk Definition ==> overhating the engine
Risk Mitigation & Corrective Actions ==> replace the thermostat

 

[Mike Goble]

Install a PUSHER fan

I'm a PUSHER fan...

 

[jmackro]

the correct thermostat for your car is an 80* C thermostat. It should open at 80* C. Putting a thermostat in a pan of boiling water is not how to test it.
Place it in a pan with a thermometer and note the temp when it opens

Yes, I see your point. The fact that my old one was fully open at 100 C doesn't tell me what it would do at a lower temperture.

The OEM fan is a joke. Throw it away. Install a PUSHER fan with minimum of 1500 cfm with curved blades.

Many people have mentioned the Spal 30102056 14" pusher. Can this be installed with the condenser in place, working through the grille openings? Or do the radiator and condenser need to come out to get access?

For extra credit: Is there a generic temperature sensor that will fit into the existing, threaded fitting on the lower radiator port? I am running a later radiator that I sourced from Mesa Performance. My idea is wiring this sensor into the fan relay to switch on the pusher when the A/C is off and coolant temperature high. And yes, a diode would be needed to ensure that it doesn't switch on the compressor fan clutch as well.