Coolant System Temp Fluctuation

[LEEVON]

It's been a weird week for my truck. '87 TBI 350, bone stock no modifications. The engine is newer with good compression, runs very strong, no leaks, etc. My only complaint is that it has gotten poor mileage (consistently 12mpg). It has smelled rich at idle, but I haven't investigated yet. I was cruising last week and the temperature approached 240/250 according to the dash gauge. There were no apparent leaks, no crud in the coolant or signs of head gasket problems, and coolant is circulating very well. I replaced the thermostat with a stock 195 and topped it off. Since then the temperature has been fluctuating from cold (160-ish) to hotter (210-ish) regardless of driving patterns which I verify with my infrared thermometer at the water pump outlet and gauge temp sending unit. I know that 210 isn't entirely bad, but it should be holding steady at 195, not climbing or dropping for no reason. I can diagnose/fix the cooling system, not too worried about that.

But here's where I get lost. Immediately after the thermostat replacement the truck started idling very high, at the same time my mileage got much better and the rich smell was gone. I suspected the coolant temp sensor (CTS) or maybe the idle air control valve (IAC) or a vacuum leak or faulty fan clutch was causing the coolant AND idle issues. What I found was that the cruise control unit was holding the throttle open. I have no idea what changed with that or why. I haven't used the cruise or touched anything in that area. I wonder if the old thermostat was the wrong temp, causing the computer to compensate fuel/timing and some wiseguy rigged the cruise to fix idle speed issues.

So to my question; What's the best way to diagnose the cooling system? Pressure test? Ohm check on the CTS? And what's up with the cruise unit being way too tight on the throttle shaft all of a sudden? (probably not going to get an answer on that one, but worth a shot). I want to fix the cooling system, then tackle my idle and gas mileage problem.

Thanks

[Troy 82]

I had those Temp fluctuation after a coolant change one time. I burped the air out & it solved it for me.

[lha1992]

I have a weird temp fluctuation also on my '89. It started right after putting the new radiator in but then stopped. Then after having to do the intake gaskits on it and flushing the coolant and what not its below 195deg where it should be at on the highway but then at other times it'll be right at or about 195. Just seems too cool to me for some reason. Runs cooler than it use to and seems to be burning more gas.

I replaced the coolant temp sensor back in 2010 or 2011 but maybe it went bad also? IDK getting ready to do a big service on it and see.

Prolly check your fan clutch which might be faulty on my own truck also and see about the ohms on the CTS too. I need to re-torque my TBI down after the intake gaskit replacement also. Prolly even re-torque the intake too just for vacuum leaks possibly...

[83GMCK2500]

I will try to find the thread, but I know the TBI motors are really particular about the EGR function when it comes to fuel economy. Not sure if it would affect temperature...

The GM troubleshooting guide for the TBI motors is AWESOME, you can find them on ebay. They are a huge help.

[tucsonjwt]

Just a thought, if you do need a fan clutch I recommend a genuine GM/Delco clutch. It costs way more than aftermarket, but you can get it from online GM vendors for a somewhat reasonable price. Mine works fine and I also used a GM water pump when I went through my cooling system. I don't have TBI on my 83 BB, but I put in a 180 degree thermostat and the dash gauge shows around 160 degrees steady once it is at operating temperature(never measured the actual temp with IR gun) even in 110 degree desert heat. I'm sure that is way too low for a TBI engine, but good parts throughout the cooling system do help cooling efficiency. Of course, I am sure you already checked for no white smoke out the tailpipe.