buick fanatic
May 26, 2001
i replaced my thermostat from a 180 to a 160.when i run the car the check engine light comes on and goes out in at random.i checked the code and code 15 comes up.will changing the thermostat cause this.
Quite possibly. I have performed this mod when i rebuilt my engine, and have since gone back to a 180 stat. I would not recommend going below that on an n/a engine. It benefits the turbo cars, which can run consideribly warmer on their own. And helps them resist detonation. My engine actually performs better with the 180 anyhow.
well i replaced the 160 with a 180 and after a day of running i got the code 15 again.i replaced the sensor a couple of months ago so i don't think it's the sensor.maybe the connector to the sensor?can you buy one of these or is it another junkyard run?thanks.

I've been having the same problem the last couple of months eventhough the (coolant)sensor is brand new. After running a resistance check on the coolant sensor it checked out o.k.

It does'nt always come on, just after a long run.
(And yes I also use a 160 degree thermo).

Clear the code and try it again. (Disconnect the battery for 10 secounds). See if it still happens.....or perhaps check th coolant sensor connector....:(
yeah i think it's the connector,because it was a little worn when i replaced the sensor.i'm going to check it out this weekend.
I'm not sure that the NA guys want to be running 160° thermostats. I found this in "The Buick Free Spirit Power Manaul" (1980):
....a street engine should always be run as hot as possible without encountering problems. Fuel economy and horsepower are improved with hotter running engines. The engine will be more efficient operating at 220 degrees than at 180 degrees...

Hotter cylinder temp equals more power. On a turbo engine, a 160° thermostat helps combat detonation. Less detonation, more boost. more power. This gain from more boost far exceeds the loss in power from the colder cylinder temps. But if boost is kept steady or the engine isn not turbocharged (and detonation is not a problem) then the lower temps hurt performance.

I know this goes against what many people say about cooler incoming air being denser and therefore supporting for fuel, more power. The trick is to keep the incoming air cool, but the cylinders hot. Keep the 195° thermostat for the cylinders. The incoming can be drastically cooler by disabling the THERMAC air cleaner (but the cool air will cause a off-idle stumble).
Another thought. :)

Running a 160° termostat on a NA would make sense if the timing was being advanced to the edge of knocking. The gains from the timing advance, like more boost, could outweigh the loss from the cooler cylinder temps.

Anybody advancing thier initial timing? How much? Going to premium fuel should come before a colder thermostat, IMO.
Perhaps it is because my engine is at the stage of being pretty well worn out....but it does'nt seem to make much differance on my N/A engine. Currently, the hottest my engine reaches is 180. (And that's after a real long run).

I don't have any economy problems... 18-20 mpg is standard on the highway. And as far as pep, well it is substantially peppier than what it use to be when I first joined this forum. (You guys sure know your stuff:D ).

I've run it at 20 degrees advance but that screws everything up. (The engine has fixed adjustments due to the ECM. The slightest alteration and the curb idle will be too high and the trainy bangs in on drive or reverse). 18 degrees is stock and it runs great. (Let the EST do its job). I've tinkered around with it so many times and the end result always is that stock specs are best.

I can probably put the thermo to the test when the engine is finally rebuilt. I will keep in mind what jdub and Rich mentioned.
OK, I did some more reading (it's cold in Chicago).

I have an artcile from am '87 Car Craft were they tried several aftermarket PROM chips in a stock '84 Firebird 305. They found little difference in performance from the chips, but did notice an actual performance gain while going to a cooler thermostat. (Some chip vendors send you a lower thermostat with the chip. It appears the thermostat did more than the chip.)

Why? Well on an ECM car, the lower temp signal tricks the computer. The artcile says the ECM believes it's under a lighter load and therefore advances the timing.

Bruce Plecan ( member) once suggested that with the lower temp, the ECM adds more fuel to the mixture. He kept the hotter thermostat, but put in an in-line resistor to fool the ECM into reading a lower temp. This way, he gets benefits of both ways. :)
I would imagine that the resistor would be attached to the signal wire that connects to the coolant sensor........pretty neat trick! ;)

Rich, your N/A build-ups are a good start....most N/A Regals don't come with the temp gauge on the dash pod either, (some Limited versions probably do I assume). This gauge and an air/fuel gauge should go hand and hand.

As in my set up, at start-up the temp gauge shows the warm up process. The A/F gauge does not register any activity. This is because the A/F gauge is attached to the O2 sensor and the ECM is operating on an 'open loop' condition. At this point there will not be sufficient voltage at the O2 sensor to register any activity on the A/F gauge.

The coolant sensor at a given temp, (X- the unknown), will signal the ECM to send sufficient voltage to the O2 sensor. Hence, the A/F gauge will then register activity....('closed loop'). The ECM now will attempt to keep the engine carburation as near to a stoichiometric condition as possible.

On my set-up, X- the unknown is 160 degrees! :eek:
Is this when the thermostat opens up? This is an everyday occurance for my Buick.

These two gauges alone lets one know that the ECM is functioning the way it was meant to.
My two cents worth

The O2 sensor is a voltage generating device. When it reaches it's working temp.,controlled by exhust temp., it sends a signal to the computer causing it to go into closed loop mode.

The temp. sensor is a resistive device. This modifies a voltage sent to it by the computer and the computer can then read the coolant temp. from the return voltage.

These two sensors ,along with others, allow the computer to calculate the required A/F mixture.

Puting a resistor in the temp. sensors cuircuit will fool the computer into thinking that the engine is colder than it really is. This will result in the computer supplying a richer mixture to the engine.

During closed loop the computer regulates the A/F based on compromises of power /fuel economy /emmissions. The closed loop system only works during cruising and light to moderate accelleration. At WOT the computer goes into open loop mode and does what ever it's going to do. Usually the manufacture sets up the WOT conditions for a rich mixture to protect the engine,and reduce warrenty claims.

Is it therefore necessary to have a richer mixture under closed loop conditions? After all if you want to go faster all you do is press more on the gas.

If you are drag racing the car a richer mixture may be benificial when loading the engine against the torque converter for take off. But only if your not already spinning the wheels! :)
:rolleyes: I'm curious....perhaps someone might know this one.

What in the world does the OEM water temperature sending unit (water temperature switch), do on these cars? (This is for the cars that only came with the hot engine idiot light).

The coolant sensor works in conjunction as part of the CCC/ECM system. I read that the coolant sensor provides a switch point signal for the hot engine light. The coolant sensor and the water temperature switch are attached to the intake and are two differant and seperate componants. I still have'nt figured out what purpose does the water temperature switch serve if any? After all, there is no temperature gauge so what is this switch doing.... does anyone know? :confused:
I'm pretty sure that the sensor is for the ECM only and the switch is for the idiot light. Cars with a temperture guage will have two sensors.

Try pulling the lead to the switch and see what happens. :)
You're right Rich,

As in my Buick manual states, 'A single contact temperature sensitive switch is located on the intake manifold. Coolant temperature above 258 degrees closes a set of contacts within the switch which then light a red signal on the instrument panel'.

Also, running a cooler thermostat on the N/A's must have an effect on the fuel mixture. Unfortunatley, I don't know if the effect is beneficial, (speed/performance), or if it's a hinderance, (poor gas milage/emissions). It's very hard to tell w/o some type of hand held engine diagnostic analyzer. :(

Also, my manual suggests a 195 degree thermostat...
Originally posted by Freddie's Buick

Also, running a cooler thermostat on the N/A's must have an effect on the fuel mixture. Unfortunatley, I don't know if the effect is beneficial, (speed/performance), or if it's a hinderance, (poor gas milage/emissions). It's very hard to tell w/o some type of hand held engine diagnostic analyzer. :(

Also, my manual suggests a 195 degree thermostat...
I have been doing some reading on Dodge sites laterly...I know
I just cann't find the right site again.
They talked about terms...160 was to cold and it would set off the check engine light. A 180 seemed to work the best...
I wanted to cut & paste because they also talk about emissions, I try from memory(remember my age) In a nut shell, the hotter the engine the lower the emissions, when you switch to a lower term the emissions will run higher. So any one that needs to have a emissions test, I would change back to a 180 at least long enought to pass.....Now if I could only remember were I read it :confused:
Jim, what you are writing about seems to make sense.....My Buick manual hints about a 'certain calibration temperature'. But it neglects to give out that information. It only mentions that the thermostat opens at 195 degrees. So it would be a good guess that the coolant sensor and the thermostat play a major role in determining good drivability on an ECM vehicle.

Originally posted by b4black

I'm not sure that the NA guys want to be running 160° thermostats........Keep the 195° thermostat for the cylinders.

Originally posted by b4black

(Some chip vendors send you a lower thermostat with the chip. It appears the thermostat did more than the chip.)

What Rich wrote hints toward this idea.

One can assume that if on the N/A cars this calibration temperature is 195 degrees, anything less will affect drivability.