1987 GMC V2500 TBI 350/TH400 Thermostat Help!

[Turbo Dog]

The opening temperature of the thermostat does not control the operating temperature of the engine. It only sets the temperature at which the coolant starts flowing. So the lower the opening temperature the longer it will take for the engine to reach operating temperature. If the Robert Shaw has an opening larger than the stock size it may or may not help. The thermostat opening size is used to regulate the speed of the coolant flow. Coolant flowing too fast is just as inefficient as coolant flowing too slow. It needs a set amount of time to allow it to transfer engine heat to the radiator shell. Flowing too fast and it can't transfer enough heat and flowing too slow it retains heat from the shell.

 

[bucket]

The opening temperature of the thermostat does not control the operating temperature of the engine. It only sets the temperature at which the coolant starts flowing. So the lower the opening temperature the longer it will take for the engine to reach operating temperature. If the Robert Shaw has an opening larger than the stock size it may or may not help. The thermostat opening size is used to regulate the speed of the coolant flow. Coolant flowing too fast is just as inefficient as coolant flowing too slow. It needs a set amount of time to allow it to transfer engine heat to the radiator shell. Flowing too fast and it can't transfer enough heat and flowing too slow it retains heat from the shell.

The thermostat 100% controls the operating temperature of the engine, so long as the vehicle is moving fast enough to have sufficient airflow through the radiator. Your theory only works if the thermostat cannot support the amount of flow needed to keep the engine at the temp at which the thermostat opens. Which most thermostats can support the flow.

If you have two trucks cruising down the road, one with a 195 stat and one with a 180 stat, they are going to have different operating temperatures.

 

[BillK]

Matt,
If you go to the parts store and give them the vin or tell them what you are working on they should be able to give you the correct thermostat. As far as that goes the Chevy dealer probably will not be much more expensive and will be an oem part. I prefer using GM parts as much as possible. Has served me well over the years and they really are not that much more expensive.

 

[Matt69olds]

It came with a 195 thermostat from the factory. GM did that from an emissions concern.

I’m betting you have a lazy thermostat. I have seen you exact concern many times over the years. The first start in the morning, the temp goes way beyond it’s normal range, then the thermostat opens a d it’s fine until the next day. Replace the thermostat, all is good.

If the thermostat is really old, it might have deteriorate to the point is maintains a cooler operating temp than it normally would. In other words, the factory 195 opens far sooner than its 195 rated temp. Who knows, the truck might like the factory temp, it might get better mileage, or it might not care. Thermostats are cheap and easy to change, might be worth trying both.

 

[Bextreme04]

I don't agree at all that TBI engines require a 195 stat. That's the beauty of electronic fuel injection, the ecm sees the engine temp and adjusts the fuel accordingly. So really, the stat temp is even less important on a TBI engine.

I've had several TBI engines over the years and have ran the usual 195, 180 and 160 stats. And no thermostat on occasion too. My best running and most peppy feeling TBI engines (AKA, my favorite ones) have had 180 and 160 stats in them. And also the base timing set to 8-12 degrees too, that's very important as well. I just don't recommend the 160 stat for winter driving if you like a good heater.

Some things GM did at the time were solely for emissions reasons and the attempt to achieve maximum efficiency. IMHO, those things don't matter now and they should take a back seat to what makes the engine run and perform better. If I have the choice of 20 more hp or .25 more mpg, I'm going with the horse powers.

This is not necessarily true. Fuel injection will usually have an "open loop" and "closed loop" fueling cycles. It will stay open loop until the engine gets to a certain temperature and will not start looking at the narrowband O2 sensor and adjusting fuel trims until it enters the closed loop cycle. This is because it will run the engine richer than stoic(just like a carb'ed engine) to aid with warmup and the narrowband O2 sensors wont know what to do with that. Depending on what the closed loop minimum temperature is, you could end up never entering that mode and therefore the engine will never start adjusting the fuel trims. It will still run OK, but you might never end up getting your best fuel mileage and top engine performance.

 

[bucket]

This is not necessarily true. Fuel injection will usually have an "open loop" and "closed loop" fueling cycles. It will stay open loop until the engine gets to a certain temperature and will not start looking at the narrowband O2 sensor and adjusting fuel trims until it enters the closed loop cycle. This is because it will run the engine richer than stoic(just like a carb'ed engine) to aid with warmup and the narrowband O2 sensors wont know what to do with that. Depending on what the closed loop minimum temperature is, you could end up never entering that mode and therefore the engine will never start adjusting the fuel trims. It will still run OK, but you might never end up getting your best fuel mileage and top engine performance.

On a cold start, open loop, the ECM still uses the coolant temp sensor to determine fueling. Also, a 180 stat is not nearly cold enough to keep the ecm from going into closed loop anyway.

Edit... I didn't read very well. But my second sentence still applies.

 

[bucket]

But anyway, that is always the very first thing I do to a GM TBI 305 or 350. Install a 180 stat and bump the initial timing to 8-12* btdc. It helps power and economy both.

 

[Bextreme04]

On a cold start, open loop, the ECM still uses the coolant temp sensor to determine fueling. Also, a 180 stat is not nearly cold enough to keep the ecm from going into closed loop anyway.

Edit... I didn't read very well. But my second sentence still applies.

I don't know enough about the TBI tables to know when the switchover to closed loop happens, but on other cars I've seen colder thermostats cause that to happen. Most newer chevy tables I've seen have closed loop starting at 185 coolant temp, which you will probably hit with a 180 thermostat. The TBI being OBD1 has some benefits and some downfalls, one of them is that you can still move the distributor that much to get initial timing adjusted without it having a panic attack. OBD2(Vortec) will have a complete meltdown if the distributor is more than a few degrees off zero.

The engine will operate more efficiently and make more power at the warmer temperature though... there's a reason that all newer chevy engines came with a 195 thermostat that keeps it running around 210 degrees, and it isn't because the engine runs worse at those temps. The big issue with our trucks is most people have the air filter pulling straight out of the hot engine bay... so more engine heat means hatter intake temps and MUCH less dense air. If you have a way to pull in cold air from outside the engine bay, run the hotter thermostat. If you are worried about overheating, get a better/bigger radiator.

 

[bucket]

Everything I've posted is based off of actual experiences with GM's TBI engines used in all conditions and mpg figures recorded. Ignition timing is the big one. The factory spec of 0° makes for a lazy engine. Advancing the timing improves power and efficiency, while the slightly cooler stat enables the more advanced timing in all conditions, even towing in hot weather. The cooler stat by itself does not make much difference. But it doesn't hurt real world driveability or efficiency.

 

[bucket]

Oh, I forgot to add, I seem to recall mine going into closed loop within a minute or so of operation. It's something I was specifically paying attention to before and after swapping to a heated O2 sensor. The reason for the swap was because it was supposed to be better with the long tube headers I had installed a year or two prior to that.

Which BTW, I found that to be another common myth that was false. The ecm had zero trouble going into closed loop with the stock, non-heated O2 installed in the collector of the long tube header. The heated O2 only reduced the time needed for closed loop operation in the dead of winter.

 

[HotRodPC]

On a cold start, open loop, the ECM still uses the coolant temp sensor to determine fueling. Also, a 180 stat is not nearly cold enough to keep the ecm from going into closed loop anyway.

Edit... I didn't read very well. But my second sentence still applies.

That's one of the things I was taught wrong in Cali Smog Tech class. We were told closed loop happened about 185-190. Who knows, maybe in very early Cali only systems like the C4 (Computer controlled cat conveter) cars it was true, but as far as our trucks which should be OBD1 and CCC (Computer Command Control) systems with electronic carbs with AF Mix Solenoids and TP sensor, Closed loop should happen 120-145 degrees depending on models, and that goes for other GM vehicles too including TBI and TPI.

 

[Turbo Dog]

The thermostat 100% controls the operating temperature of the engine, so long as the vehicle is moving fast enough to have sufficient airflow through the radiator. Your theory only works if the thermostat cannot support the amount of flow needed to keep the engine at the temp at which the thermostat opens. Which most thermostats can support the flow.

If you have two trucks cruising down the road, one with a 195 stat and one with a 180 stat, they are going to have different operating temperatures.

I'll politely disagre...it is the radiator that determines the operating temperature of the engine. What I was talking about is how the thermostat helps the radiator by controlling the flow rate of the coolant. Other than that it only controls the point that the coolant begins to flow.

 

[bucket]

I'll politely disagre...it is the radiator that determines the operating temperature of the engine. What I was talking about is how the thermostat helps the radiator by controlling the flow rate of the coolant. Other than that it only controls the point that the coolant begins to flow.

There's too many variables to say that though. Road speed, radiator size, ambient air temp and thermostat temp can all change the engine's operating temperature.

 

[Turbo Dog]

There's too many variables to say that though. Road speed, radiator size, ambient air temp and thermostat temp can all change the engine's operating temperature.

You've got that right!

 

[Jwernatl]

On a cold start, open loop, the ECM still uses the coolant temp sensor to determine fueling. Also, a 180 stat is not nearly cold enough to keep the ecm from going into closed loop anyway.

.

How do you know if open or closed?