1987 GMC V2500 TBI 350/TH400 Thermostat Help!

[Matt Frediani]

Greetings,
I live in Northern California and have to keep everything OEM to pass smog. I have a nice 87 GMC V2500 with a 5.7L TBI 350 with Th400 Trans.

The problem. Every so often my temp gauge shoots up and pegs. Mostly when the truck is not warmed up properly and some times when driving around town. I suspect the thermostat. I want to change it but dont know what temp stat I need to order. I checked my OEM service manual and it does not say what operating temp stat to order. I need the correct one because I have heard its critical to the ECM and passing smog with the TBI.

The gauge seems to work fine, the truck normally runs with the temp gauge at 12 o'clock.
Can you please help me figure out what would cause this and what temp thermostat to install?

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[82sbshortbed]

I have a 180* stat in my BBC 454. Seems to like it just fine. Nice looking truck.

 

[Matt Frediani]

Im wanting to put in the correct stat that GM recommends for the TBI 350. I 80 will work but I have heard its not correct and will most likely not pass tough california smog laws. thanks though.

 

[Itali83]

All my tbi trucks were 195 thermostats. I would inspect the wiring to the sending unit as well. If the gauge literally goes from normal temp to pinned in one second, for sure it’s an electrical problem. If it slowly creeps up then it’s the stat possibly sticking. Good luck.

ben.

 

[Matt Frediani]

Yes, It slowly creeps up. does not peg quick like a bad wire or ground. I will order a 195 stat and install. Do any of you guys have a recommendation for a high quality thermostat?

 

[Curt]

Coolant systems need serviced every couple years.Hoses,thermostat,belts.May look your fan clutch over too.

 

[Curt]

Stant is a well known brand most auto part stores carry.Might be a good time for antifreeze change anyway.Most auto parts stores accept used coolant too.Might ask while your there.

 

[waterpirate]

Very nice rig! I run 180 in my 87, but I am a long way from California smog enforcement. lol
Eric

 

[QBuff02]

Take a look at the fan clutch. They fail one of two ways, it either will not disengage and will spin faster than it's supposed to and you will hear it audibly louder than it should be. the other way they fail is they won't engage as they warm up and will basically freewheel. Pop the hood and give it a spin, if the fan spins easily with almost no resistance it has failed and needs replaced. Low speed driving when the fan isn't operating correctly and there is insufficient airflow through the radiator will allow engine temp to creep up. As for the thermostat, hotter is better for emissions. I'd bet it has a 195 degree thermostat in it. If the fan clutch is in fact good, service and flush the cooling system and replace the thermostat.

 

[75gmck25]

I would recommend using a high flow 195 degree thermostat, which are better quality, and usually have a small bypass hole in the face to prevent air locks. Robert Shaw is a well known brand, and Mr. Gasket thermostats look like they might just be a repackaged Robert Shaw.

GM used 195 degree thermostats from the factory, and that is still the best choice because the engine is more efficient at that temp. It’s even more important for a TBI truck because the computer uses the temp to determine how to optimize the fuel mixture.

 

[bucket]

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.

 

[Matt Frediani]

So if I pop my hood and my truck is stone cold from sitting over night, If I can spin my engine fan ,the fan clutch is bad?
I tried to stop it with a magazine when th truck was running and it seemed it was engaging.

If Im using a 195 stat, what fan clutch do I need?

 

[Snoots]

So if I pop my hood and my truck is stone cold from sitting over night, If I can spin my engine fan ,the fan clutch is bad?
I tried to stop it with a magazine when th truck was running and it seemed it was engaging.

If Im using a 195 stat, what fan clutch do I need?

NO!
If it's stone cold it should spin easily.
If it's hot it will spin but you almost have to force it.

You 195 stat should be good.

 

[Edward Koleman]

Why not pop your old thermostat out and see which one you have? It will be stamped on it. Then you will know for certain what to get... ‍

 

[Raider L]

Yep, Robert Shaw. I've used them ever since I rebuilt my truck due to the fact it has a much larger opening and being all brass has a better sensitivity to the temp. and will open what it says it's set for. They're just a better built thermostat. I just put a new one in my truck and it's a 180 and it was running the other day and I wanted to see where it was opening at and it started opening at like 180 and a 1/2 a degree. Where most Stants have a 5/8" opening the Shaw is closer top an inch.