Engine/Transmission (1994 - 1998) erratic temp gauge

[CStansbury]

Hey Guys,

I noticed my temp gauge acting funny when I was bringing the truck home Saturday. It would creep up to about 200, then drop to 140 within about 5 seconds. It did this over and over down the highway cruising at about 2000rpm. Think it might be the sending unit? Maybe salt corrosion as there is quite a bit on other exposed fasteners? I didn't see this exact issue in any old posts so I was curious if this has happened to anyone else. Thanks!

Chris

 

[EKlem]

Some temperature fluctuation in the gauge is normal. On my truck, it swings between 150 and 190. The problem has to do with the thermostat I believe. It is kind of out of the loop of the main flow of coolant and some people have found that the thermostat itself does not get good flow through it. Cummins came out with a revised thermostat and at least one person on here has tried drilling a few strategic holes in theirs. As far as I know, you sound like you are experiencing the same thing as everyone else.

 

[CStansbury]

Thanks EKlem, I had seen the posts about temp flux due to thermostat, but I just couldn't imagine it changing that quickly. I'll have to see about the revised thermostat.

 

[Joe G.]

That's normal behavior for a '95. Mine has always done it. In fact, if it does not do that I know something is wrong. It maintained a steady reading one time when I went from home in Eureka, CA to Roseburg, OR. While I was there I removed the thermostat. The rubber cover had slipped and jammed the 'stat open. I replaced it. Now it is behaving like it always did.



The cause is the size of the cooling system. It behaves a little differently in the summer than in the winter. It takes some time to heat up the coolant in the engine to the opening point for the 'stat. When it opens the coolant in the radiator has had plenty of time to cool so the coolant in the engine drops which closes the 'stat again.

 

[CStansbury]

Well, I had read about that and I just couldn't believe it until I saw it happen to me. Sorry for the redundant posts guys.



The PO had some plexi winter fronts for the grill. It doesn't really get cold enough here for that, but maybe it would help keep the engine sludge free with all the little start and stop trips here around town.

Chris

 

[Matthug]

Same here in my 94, stat opens at 190-200ish and drops all the way down to 160 everytime the stat re-opens.

 

[ttbird]

same here, if not moving something wrong. been doing it now for over eleven years. I guess you get use to it.

 

[REFLOYD]

should my 98 be doing that too? it get to 190 and just stays there doesnt move at all

 

[Joe G.]

I don't know about your '98. There have been a lot of panic complaints because of the gauge variations so Cummins might have put a different 'stat in the later engines.



However, the only ways I can think of to dampen the gauge variations is to restrict coolant flow through the 'stat or have it leak a little. I'm not sure I like those ideas. If the coolant flow is restricted I would think that the engine would tend to get hotter in a heavy use situation. If the 'stat leaked a little then it should take longer to warm up with you really need the heater. Maybe I'm wrong about that, but I trust the way mine works.

 

[RJPotts]

My '96 does it too. No problems yet. It will climb slowly to 190-200 or so and drop to 150-160 within a few seconds, then start climbing again.

-R. J.

 

[McLaughlin]

Seems to be the thermostat working. Look at the bright side, at least your thermostat seems to be working :-laf . My seems to be stuck open. Just pulled a 4K trailer from Pheonix and except for one long hill by Flagstaff, my truck never got to close to 190. Most of the time the temp gauge was setting a 140 or below. Looks like it is time to replace the darn thermostat.