Fluctuating Coolant Temp

98 3.8L with 156K miles, odd symptoms. Car starts and runs great but with Low Coolant light on. As she warms up, the Low Coolant light turns off and then operating temp goes higher than usual when you come to a stop, a little more than halfway up on the gauge and she starts to idle a little rough. Never overheats though. Revving the engine in neutral while stopped has no effect on the temp, but moving forward will get it to drop a little. Except if you get on the hiway and cruise at 70, then temp comes back up. Not sure if this is the T-stat, cooling fan or something else. Also not sure if the rough idle ties in to this or is due to the overdue plugs/wires change. Thoughts?

Start with a coolant flush and new thermostat.

Then clean the MAF sensor and throttle body and see what its like from there.

so is it low on coolant? or the low coolant light is just on?

Sounds low on juice..

Bleed it..burp it.. see if that helps.

The coolant level is right on the low side, and I suspect that as she heats up enough coolant expands back into the recovery tank to turn the light off. After reading some of the other threads there probably is an air bubble somewhere in the system. I picked up a t stat a auto zone this weekend and plan to change that along with the coolant and plan to change both this week. Thanks for the too on the MAF, I hadn't thought of that .....I'm guessing if I do a search on the forum ill find the.cleaning process?

The coolant sensor is located in the side tank of the radiator, about 2/3 of the way up. Pull the radiator cap when the car is cold and check the level in the radiator. It sounds like you are checking the tank only.

Ahhh....thanks for that! The coolant level sensor on the Suburban is in the overflow tank. Actually there is no cap on the radiator, the tank pressurizes along with the radiator, I assumed the GP was similar design.

Update: Have not installed the T-stat yet, but temp seems to hold at 190 on the hiway and then spikes to 215 at the stoplight. A long run on the hiway produced a spike to 230-ish at the off ramp stoplight. Revving in neutral to circulate the coolant seems to have very little effect, but obnce you get moving again the temp drops like a rock back to 190 and hold steady. So if the coolant level is 1/3 down in the rad, this whole issue may be coolant related and not the t-stat at all. I was holding off adding coolant until I canged the t-stat, but I'll top her off tonite and see what happens next.

ahhh reason for the spikes is that you have the stock fan settings in the pcm. Fans aren't programmed on at all until 216F/228F. Therefore when you have 190F and stop..you've lost all airflow. Up spike the temps. My suggestion on this would be to have your pcm programmed to the temps of your thermostat.

Update: Added about a gallon of 50/50 mix last night...filled the radiator and about half of the overflow tank. Ran to work this morning and temp held at 190 regardless of moving or not, no spikes whatsoever. Looks like back to normal, but the lurking question....why was the coolant so low to start with? Guess I'll just have to watch it for leaks. Never been inside the motor on this thing at all, so hopefully not IM gaskets.

Actually idles a little better but still rough. Going to pick up a can or the CRC MAF cleaner, probably look at plugs and wires also and see if that helps. Thanks to all for help and ideas!

Not trying to scare you.. but search on L36 failure and my screen name. I'd consider a pressure test to check things out.

Yes, I saw that thread the other day when I was reading on the forum. Here's the question though, I've got a fair amount of miles on since the coolant dropped, so if that gallon went into the LIM wouldn't it be hydrolocked already? Or does it need to fill up more to get completely hydrolocked?The car isn't smoking, there's no evidence of coolant in the combustion. I would think I should be able to see a wet plug or two also?

you wont really see coolant on the spark plugs.

so wheres the coolant going? likely your engine is eating it and burning up and out the tail pipe.

it will hydro lock when it leaks bad enough to just pour into the cylinders, after you turn it off. then it wont crank over.

and do that pressure test for the coolant system, if it dont hold pressure its leaking somewhere as well.

you should do your lim and upper intake gaskets asap, its kinda maintenance around here.

A bad radiator cap could cause mysterious coolant disappearance as well.

I know I'm on borrowed time there...think I will throw in a set of plug and do a pressure test and see whats what. Is there a step by step on the IM gasket replacement?

sure would be, search for it in the how to section.

Update on the situation: Changed the plugs and cleaned the MAF and throttle body. Runs a little better, but still rough at idle. Checking the AT fluid last night and notice the coolant overflow bone dry again. Is it even worth investing in a pressure tester at this point or should I just bite the bullet and plan to do the intake manifold gaskets? No signs of any coolant leakage at all, so it must be burning it, probably what's causing the rough idle, correct?