Coolant Temp Cycles during warmup.

I just replaced the water pump on my 98 K1500 350. Now as the engine warms up, the coolant temp gage shoots up to 210, then falls down below the normal running position. Then it goes back up just short of the first high point and falls again. The system goes through about 7 cycles before it stabilzes at a temperature, which is the same spot it ran at before I changed the pump.

I'm guess the gage may be chasing the thermostat. I refilled the coolant system per the factory maintenance manual, which says to just top off the radiator, allowing a fair amount of time for it to burb. Then it says run the engine until warm, let it cool off and top off the radiator.

Well, I've been driving it for a week and it continues to cycle. I've topped of the radiator a couple more time, adding a few ounces at most each time.

Do you guys think there is still air in the system? Does this bleed procedure sound sufficient? Wouldn't a weeks worth of driving force remaining air out of the system?

Thanks in advance for any advice you can lend.

I've seen some trucks that do that. I think it's from cold water getting up to the thermostat, so it closes partially, then heats up, opens again, lets cold water back up, etc. If it's not overheating, and the heater blows hot air, I wouldn't worry about it.

Also, you had the heater on when burping it?

Yes, I had the heater on. What's so puzzling to me is that all I changed was the water pump. The thermostat is the same so I didn't expect to see a different warm up trend. So far it hasn't overheated, but seeing the temp gage go that high made me nervous. In the summer, I've pulled 5,000lb boats with the truck bed full of gear up a ten mile long heat with the outside temp at 100 F, and the temp gage didn't go that high.

Thanks for you reply.

Maybe the new water pump just pushes the water through much faster, so cold water hits the thermostat. As long as it stabilizes, it's probably fine.

The thought had crossed my mind that the new pump characteristics may be different. It's a new GMB pump from Schuck's Auto parts. I din't feel like paying through the nose at the dealer. I hope that wasn't a mistake.

Thanks,

My temp gauge does the same thing But only cycles 1 time after I changed the water pump, no problems ...

It sounds like they gave you the wrong water pump. There are two types of chevy water pumps- one for regular rotation and one for reverse rotation used with serpentine belts. If you put the wrong one in it will cycle like you described until the entire system heats up. Check the stock number of the pump you bought with the store to make sure it is correct.

Oh and by the way-Yes it happened to me-- at Checker auto parts

I just replaced the water pump on my 98 K1500 350. Now as the engine warms up, the coolant temp gage shoots up to 210, then falls down below the normal running position. Then it goes back up just short of the first high point and falls again. The system goes through about 7 cycles before it stabilzes at a temperature, which is the same spot it ran at before I changed the pump.

I'm guess the gage may be chasing the thermostat. I refilled the coolant system per the factory maintenance manual, which says to just top off the radiator, allowing a fair amount of time for it to burb. Then it says run the engine until warm, let it cool off and top off the radiator.

Well, I've been driving it for a week and it continues to cycle. I've topped of the radiator a couple more time, adding a few ounces at most each time.

Do you guys think there is still air in the system? Does this bleed procedure sound sufficient? Wouldn't a weeks worth of driving force remaining air out of the system?

Thanks in advance for any advice you can lend.


My 98 5.7 did the same thing as yours is doing. After I drove it a couple days it stabilized. I think these systems may be hard to get the air out of until it is warmed up and cooled down a few times. Ever since the water pump it will run up to 210 then drop like a rock below normal and then stabalize at normal. I don't know why it does it, But it has done it for about 100,000 miles and it has never overheated. I even had a scanner on it and it read the same as the gauge. I recently replaced the water pump again at 200,000 miles and this time I let it run and burp coolant for more than 1/2 an hour and it didn't fluctuate like it did the first time. I still goes up, drops and stabalizes though. I even replaced the thermosat three times when I did the first pump because it was acting like it was sticking. Bottom line is I think it is just air in the system.

Mbumgua,

Did you have a temp cycling problem, and did you go back to Checker and get the correct pump? If so, did your cycling temp go away?

Thunderbolt,

In the last week that I've been driving my truck, the number of temp cycles has dropped to 2 from 8, so maybe there is air in the system. Shouldn't it bleed itself out after some time?

Thanks for the feedback.

Mbumgua,

Did you have a temp cycling problem, and did you go back to Checker and get the correct pump? If so, did your cycling temp go away?

Thunderbolt,

In the last week that I've been driving my truck, the number of temp cycles has dropped to 2 from 8, so maybe there is air in the system. Shouldn't it bleed itself out after some time?

Thanks for the feedback.

Mine eventually got better and I think yours will too. For some reason it just seems difficult to get the air out of the system on these trucks. I would keep driving it and see what happens. As long is it is not overheating it isn't hurting anything. Just make sure there is coolant in the overflow tank and after you drive it awhile longer check the level in the radiator. If it doesn't quit doing it you may need to replace the temp sensor for the gauge. I don't remember which one is which though and I believe there are two or three on it.

Problem solved! I replaced the thermostat, which possibly addressed two problems. First, I placed the original and the replacement thermostats in a pot of water on the stove and noticed that the original was just a bit slower to open than the new one, once the water got to 195F. This probably accounts for the overshoot in temp during warm up. Second, if there was any air trapped in the system near the thermostat, it was removed when I opened the system.

The engine runs a bit cooler than before, and I think that is due the slightly sooner opening of the new thermostat.

One nice thing I noticed during this is that since I owned my 1979 K10, Chevy went to a o-ring on the water outlet fitting. No more waiting for sealant to cure!

Thanks for all the comments!

My 98 5.7 did the same thing as yours is doing. After I drove it a couple days it stabilized. I think these systems may be hard to get the air out of until it is warmed up and cooled down a few times. Ever since the water pump it will run up to 210 then drop like a rock below normal and then stabalize at normal. I don't know why it does it, But it has done it for about 100,000 miles and it has never overheated. I even had a scanner on it and it read the same as the gauge. I recently replaced the water pump again at 200,000 miles and this time I let it run and burp coolant for more than 1/2 an hour and it didn't fluctuate like it did the first time. I still goes up, drops and stabalizes though. I even replaced the thermosat three times when I did the first pump because it was acting like it was sticking. Bottom line is I think it is just air in the system.

That's exactly what it was. When you changed the water pump, something I did first week of January, we left the thermostat off while we filled up the radiator. When the fluids got to the top of the engine we put the thermostat on and continued to fill it up. It did cycle for a while after letting it run and I just let it do that and filled the radiator up completely a day later. But the reason it does that cycling is because the air heats up faster than the coolant and the sensor is reading the air temp.

Also the gauge sender is in the head, gets hotter faster than the thermostat in the upper front intake. See this pretty common. One fix for this is to drill a small hole in the edge of the thermostat, the small flow will allow the stat to sense the water temp quicker.