Coolant Temperature Swings

[MS-Dmax]

Thanks to all that post here, it is an amazing wealth of info.

I purchased a 2007 2500HD Classic in April 07. Everything has been great until the cooler weather came along. My truck had been showing and engine temp all year just under 210. I thought that was normal until it got colder here. Then, my temp would drop to around 170 when it was below 50 outside. During use, it would fluctuate from 170 - 210 depending on load and outside temp. I took it to the dealer and they replaced the thermostats. They told me the first one opens at 180 and the other at 185. My brother's truck (1 year older but a LBZ) stays at 185 no matter what. I know something is not right but I have no idea where to get the dealer to go looking next. I've never had a truck with 35 degrees of swing! Any help would be much appreciated.

36000 miles, mostly highway w/out load, some pulling to 10K lbs. Completely stock under hood. Serviced regularly as recommended.

[Mark Rinker]

All three of my Dmaxes run 'way cooler' on the temp gauge in cooler weather. Guess I am used to seeing the swing. Along with the swing goes lower fuel mileage.

The '02 got new thermostats about a year ago, it was doing the same thing the other day when I drove it.

Throw the winter front on below 15 degrees, that will help.

[DieselMonk]

Mine did the same thing and I got the dealer to replace the thermostats and the temps are still going up and down like a jojo. The only way to stop that is to put the winterfront on. :D

[Mark Rinker]

If its really erratic after the thermostats have been replaced and the weather (temps) moderate, check the coolant temp sensor.

[MS-Dmax]

Thanks to all that have replied. I'm leaning toward a problem with the temp sensor. I guess I wasn't very clear in my first post but I live down south here in Mississippi. The coldest this truck has seen is 26. I could understand covering the radiator to increase temps when it gets real cold but I start seeing the swings at 50. Just this morning on my way in to work, it was 50 outside and the temp swung from about 180 to 210 throughout the 50 mile drive.
The only thing that makes sense at this point is the sensor or gauge. All the other LBZ's I've talked to are steady.

Thanks again for all the replies.

[Dakster]

My LMM does that in 50 degree weather, although I only noticed it at idle. As soon as I started to drive, it jumped back up to around 210. I thought (and still do) that it is completely normal...

I live further south than you so the truck hasn't been in temps below low 40's. (that's above zero for you folks in the Northern parts of the US)

Not alot of help, but don't be surprised if the "Functions as intended" phrase is found on your dealer work order...

[DieselMonk]

Mine did that since brand new out of the show room. Thought that is "normal" hmm... maybe I get that thing warmed up and get a scanner on that and check that coolant temp sensor. :cool:

[Dakster]

Dieselmonk,

Let us know what you find out... I'm getting my nit pick laundry list ready to go to the dealer. I'll add that one if you find it is not "normal".

Thanks.

[MS-Dmax]

Since I was pretty sure the temp shouldn't swing 40 degrees in mild weather, I asked the dealer to check the sensor and gage when I took it in last week for an oil change. Also, I told them the high idle function wasn't working.

They couldn't get the scanning tool to communicate with the truck computer so I'm dropping it off tonight for them to do more extensive "research" into the problem.

This should be fun. I'll keep everyone posted as this develops.

[Kennedy]

The temp gauge is only there to give a general idea of temp. It is far from precise. You wil likely find that in warm weather your engine will run in th elow 190's and in cold weather the low to mid 180's. The winter front wil help to bring this back up. Changing parts is a waste of resources regardless of warranty or not.

[DieselMonk]

Sorry for the loooong delay, but I did finally take the winter front off, put on a scan tool and drove around for a while. Came to find out that the coolant temperature senor only swung 2C in either direction max. However my coolant temperature gauge is swinging waaaay more than that. To me: there is nuttn wrong with the sensor. My bet is on the instrument cluster. Anybody else can verify?

[MS-Dmax]

Well, they had my truck for almost a week. The communication problem was an operator error issue.

The truck temp continues to swing 30 degrees or so going up the road. For some reason, I cannot get it through their heads you have to drive the truck to see the swing!

Thanks for the info DieselMonk. I think you have come closer to finding the problem than the dealer has.

Mr. Kennedy, I want to clarify that I don't expect the gage to be precise, but at least my temp should be steady. A swing up or down a few degrees would be acceptable. Changes in normal operating temperature would be expected across the seasons, but not while driving down the road. My gage reads about 170 at idle (warmed up). Start driving and it will climb up to 210 depending on the load and going up or down a hill! The swing is over 20 degrees just going over an over pass.

It does this at 70 degrees ambient. It does it at 40 degrees ambient. It has nothing to do with using or not using a cold front.

I'm not into swapping parts to troubleshoot either but I don't think anyone would accept their fuel, battery, oil, or any gage swinging 20% while going down the road at a constant velocity. I watch all my gages carefully to insure the health of the vehicle.

I love the truck and with the exception of this minor quirk, it has performed flawlessly in the 43000 miles I put on it (1 year the end of this month).

DieselMonk, if you find anything else or confirm your thoughts, I would really appreciate the info. Does anyone know the method the gage uses? Does it get a voltage to the gage or current?

Thanks to all who read and reply!