what is the correct tstat & more

cougar20th

Registered User
What is the correct thermistat temp for a 89 tbird sc. im trying to get rid of the problems that just cooked the new catyltic converter. I was told that the car is staying in a default program and added to much fuel. it has a 180* in it.
 
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ive been using 180 for years it runs great in my 89 i also use colder plugs but stock is 192 or 195
 
If the thermostat is working right, then a 180 will not cause problems. I also used one for years, as have many others.

Check your ECT (engine coolant temperature) sensor. That is the sensor with primary control over open-loop and closed-loop operation. If that is bad, it will screw up the mixture. I think it hides next to the cam sensor? I forget.

It could be lots of other sensors (fun, right?), but the ECT is the first place to start.
 
Sorry but I disagree. If you are running your car in the winter I suggest you use a stock thermostat which is 195 degree T-stat. It ensures your car gets out of cold start strategy (ie fuel tables). I had a 89 and ran a 180 and diluted my oil from the excess fuel washing down the cylinders. You COULD also block off a portion of the radiator with cardboard if you don't want to swap T-stats for the winter. No problem with running a 180 T-stat in the summer ONLY.

Concerning cooking your cats......I would check your crank pulley and make sure it hasn't started wobbling. If you see any wobble the harmonic damper is broken and possibly your timing shifted allowing fuel to be burnt in the manifolds. Are you down on power or have you seen the manifolds glowing?
 
well ill be putting a 195 into it. its cheap enough to do to eliminate a possibility. theres also a miss when its running. not all the time but its there. motors got plenty of power. im going to check everything tomorrow. i just had the motor out and did heads and such about 1000 miles ago. I had to do the motor because of cloged cats and to much presure popping the head gaskets. starting to wonder if im chasing a problem from before that didnt get fixed like i thought. i know the car is throwing a TPS code. So im thinking its a combination of problems. the car is used primary as a winter car.
 
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If it's adding too much fuel, check your O2 sensors as well. I had one get clogged up and it was caused by a bad O2 sensor.
 
Hmm, interesting. I didn't notice any problems with the 180 thermostat in my 1989, and I used to live in the same area as Duffy Floyd. I still got about 26mpg highway, and I definitely didn't cook my cats. On the other hand, it was a base model (no SC), so it had a speed-density EEC instead of an MAF. Maybe that was the difference. Oh well, "your mileage may vary."
 
Duffy Floyd said:
No problem with running a 180 T-stat in the summer ONLY.

Since these engines like to run hot sometimes, why wouldn't you run a 160° thermostat in the summer??

Ira
 
running a 160 Tstat could possably not allow the coolant to stay in the radiator long enough to actually cool. EIth my stangs however I noticed with a 160 it would cause the carto run rich and take longer to warm up. Once warm it would in the summer run a lil hotter then normal. This may apply to the SC as well.

180 is generally a good compramise
 
If you have a missfire it will cause unburnt fuel and destroy the converter. Fix the miss first.

The EEC has a paramater called time_to_wait_for_closed_loop. This is a parameter that begins to count after the car is started. If the car has not achieved commanded operating temperature after this amount of time the car will go closed loop anyway. The means that the car will run closed loop (14.7:1) AF even if it has no thermostat and never hits even 100 deg. However, the car doesn't run closed loop under all conditions at all times even after it is warmed up. Load, rpms, TPS angle, and other things can trigger open loop. If ECT is not above a certain figure then the startup fuel tables will be used for open loop operation. So this means that the car will go closed loop very quickly after startup, but it may go very rich every time you step on the gas if you aren't warm enough. So cruising around with a 160 deg themostat might work fine, and WOT will be fine, but if you drive in hilly areas or around town a lot mileage will be poor because you'll be in open loop a lot due to acceleration. Open loop in part throttle cold start is 10-11:1 whereas open loop in part throttle after warm up is 12.5-13.5:1.

But that is not likely the problem in this case. A missfire will always cause raw fuel to go to the cats and generally cause them to overheat.

{edit} I ran a 160 deg thermostat successfully for years before I knew any of this but that doesn't make it right. Also any thermostat that is not in sync with your cooling fan will cause erratic engine temps. Sometimes cooler than stock, sometimes hotter than stock. Making the engine run at 190 deg or less with the use of a 180 deg thermostat and a properly calibrated chip greatly reduces cooling system pressure which has other benefits that you can imagine for yourself. ;)
 
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If you are getting TPS codes then that is probably why you are running open loop. I don't think the 180 deg. t/stat is a problem (or even 160) because our cars go to closed loop very early, like 140 deg. Check out the TPS adjustment article on the TCCOA site.
 
Joisey Jim said:
I don't think the 180 deg. t/stat is a problem (or even 160) because our cars go to closed loop very early, like 140 deg.
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No they don't. This is what I have been trying to point out. Going closed loop is not temperature dependent. It is TIME dependent. The difference is if the set temperature is not reached, then OPEN LOOP operation sources mixture from the startup tables. Open loop is used on acceleration, WOT, and some other circumstances whether the engine is up to temperature or not.

On ALL SC's, the temp switch from startup tables to normal fuel is 180 deg and only a chip can change that. However, cruising AFR will not be affected by thermostat.
 
Dave,

So if I get your drift correctly the factor that causes the car to run rich when below 180 degress is strictly a function of which fuel tables are in force then? (Cold Start vs. a "Normal Fuel Table? )Open Loop to Closed Loop transistion is related primarily to time after the engine is started (assuming all the "other" conditons to go Closed Loop are met).
 
That is correct. Closes loop operation begins after a counter expires in the EEC. I can't access or change it on the 89-90's but I can on the later ones. If you drive on the highway you'll probably see no particular difference in mileage with a 160, but if you drive in a hilly area or around town the time spent in open loop will be greatly increased and the car will run rich a large % of the time.

There is also a parameter that states temp_change_for_base_fuel. In this case if operating temp is reached, it must drop a prescribed temp after that to go back to cold start up. On some years it's 180deg switch and a 20 deg drop window. Other years it's straight 180 deg with no drop window. On some 5.0 cars it is a straight 120 deg.

It is also worth noting that the switch to turn on the knock sensor is 186 deg. ;)
 
So if I understand any of this, it means that there is really no advantage at all to running a 160° t-stat, now is there??

Ira
 
No there is not. Running a 180 can even have some drawbacks depending on driving conditions and whether or not the EEC has been programmed for it.
 
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