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2002 Dodge Ram 1500 5.9 temperature keeps changing

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Dylan Lowery

Aug 19, 2015, 7:41 PM

Post #1 of 7 (917 views)
2002 Dodge Ram 1500 5.9 temperature keeps changing Sign In

I recently bought a 2002 dodge ram 1500 5.9 SLT and it run great. Everything in the truck run like it should but I'm not a expert at vehicles and I'm just a paranoid kinda guy, sadly.

But I've been noticing that my temperature gauge is at different temperatures when I'm driving. The truck itself has never run over halfway on the gauge I'm pretty sure its not over heating.

When I crank it up on a cold start the truck heats up at normal pace and it usually goes up to about 3/4 of the way to the half line. Its been getting up to 100 degrees here in NC but it stays in the same place on the back roads and highways whenever I'm keeping a constant speed. It will sometimes drop down to 1/4 of the whole gauge whenever it's under 85 degrees outside.

But whenever I'm sitting at a red light it creeps up to right under the halfway line on the full gauge (where I thought every other car is suppose to be naturally) which usually takes about 2 minutes tops. But When I start driving it drops back down.

And whenever I've been driving for a while and cut the truck off and start it right back up, the gauge will go up to right under the halfway mark but drop back down as I start driving.

If anyone knows what's going on I'd really appreciate the help! Thanks in advance

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Aug 20, 2015, 4:10 AM

Post #2 of 7 (882 views)
Re: 2002 Dodge Ram 1500 5.9 temperature keeps changing Sign In

What is this using for a fan(s) for the radiator? Big fan with a clutch or maybe electric or would you know if altered?

IDK - about all RWD dominant vehicles temp will stay steady once warmed up if super cold up to reasonable hot which this is. Talk 117 (real air temps recently in AZ) with full sun I forgive a vehicle. Engine is used to and can or should easily handle 100F area if all is right.

IDK - take a look. Does radiator look clean (A/C condenser is in front of it) of a lot of dirt of bugs? Some can have unseen dirt in between and need to take some parts off to look and or clean out if the problem at all.

Being a truck I can't know what fan set up you might have. Some folks might change it for extra cooling that isn't working? You need to look and tell us,


Dylan Lowery

Aug 20, 2015, 12:50 PM

Post #3 of 7 (864 views)
Re: 2002 Dodge Ram 1500 5.9 temperature keeps changing Sign In

There is a big radiator on the passenger side that has a fan coming from the pulley so I guess it's a clutch? And there is a small fan on the driver side that is electrical. I'm guessing for the transmission? And they both have bugs on the but I just got them off and I can see through both radiators so I reckon they're not clogged up

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Aug 20, 2015, 8:55 PM

Post #4 of 7 (854 views)
Re: 2002 Dodge Ram 1500 5.9 temperature keeps changing Sign In

OK - The biggest fan when warm should be pulling lots of air thru as felt over the engine with hood open - the belt driven one should be strong when fully warmed up and A/C on.
If a regular clutch driven one would not coast well when you shut engine off spun by hand would stop or observed when someone else shuts engine off as it should be tight.

Idea is to pull air hard when needed and not so much when already cool enough. There are some primary fans with a clutch that can totally freewheel.

My trouble is it's a truck so smaller radiators are used or selected as optional for transmission oil, engine oil and power steering. A/C's radiator is called a condenser up front near grille. That will catch the debris, bugs and may have the smaller coolers in front of it.

If in doubt have it checked. It's hot and will be again or just work hard so has to be right.

I'm a bit confused that engine radiator isn't the whole area possible nearest the engine on a truck? Is something blocking you view as seen from behind?

Anyway - most all of this layout temp gauge would get to one point and stay there - not so on all assorted vehicles. It's possible for this to be at it's limit but still safe and movement a warning actually.

In short (IMPOSSIBLE FOR ME) things that allow gauge to fluctuate would be...........
* Incapable cooling due to lack of fan power
* Way too strong of an antifreeze mix. Many folks will think more is better and it isn't. 50/50 or set to a -34F is a standard. Check it and if unknown when flushed last put that on the list.
* If really working a vehicle hard owner's manual should say you can adjust the antifreeze mix down to even just -20F or so as it would cool better. Forget that unless extreme use. Hate to say NC and 100F isn't hot but mountain driving, trailer hauling, stronger A/C wanted and used and so on the better cooling the better and forget efficiency for fuel. Engines cost moreMad than fuel.
A true fan clutch can be bad every 5 years or just gets weaker by the year. They can be replaced with HD ones on trucks especially - go ahead and google out a parts site and look (just for the info) on cooling system parts applicable to your truck exactly. You'll find brand choices and assorted options.

When in any doubt and you don't absolutely know get it checked out and serviced if it hasn't been. There is a schedule for changing antifreeze at least.

Own a total HD truck myself. Shyed away from much truck stuff simply for space reasons so less experience on my part on the extreme options for trailer hauling up to aftermarket options for cooling. My own has a cooler for everything and actually hate that but all works fine. Needed and wanted for extremes but also another thing to break IMO for my personal needs and would opt out if bought new - most would want anything and everything for extreme working vehicles.

I suggest you just check that services are up to date, check that antifreeze and condition of radiator plus fan clutch as best you can or get an opinion from a shop.

Heat kills engines, lack of oil change service also kills engine. It all counts. This is the hot time for most of the lower48 states even here in New England. Same as NC - annoyingly hot for a shorter while here than you but same idea.

Point is you and normally folks go all year and don't pay attention to the extremes upon us by season so we notice and good that you do,


Dylan Lowery

Aug 21, 2015, 5:07 AM

Post #5 of 7 (844 views)
Re: 2002 Dodge Ram 1500 5.9 temperature keeps changing Sign In

Okay thank you! You're very helpful. I'm going to take it to the shop Monday a d let him change the fluids and have him inspect the radiator and fan. I'll update the thread when I find out What the problem is. Thank you!

Dylan Lowery

Aug 28, 2015, 8:08 PM

Post #6 of 7 (806 views)
Re: 2002 Dodge Ram 1500 5.9 temperature keeps changing Sign In

Well I got the oil changed and all fluids checked. They were all good. I went ahead and put a new thermostat in and the gauge line seems to be at a steady point while driving. But its still only goes 1/4 of the way up so I reckon I'll change the fan or something..?

(This post was edited by Dylan Lowery on Aug 28, 2015, 8:27 PM)

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Aug 29, 2015, 1:30 AM

Post #7 of 7 (797 views)
Re: 2002 Dodge Ram 1500 5.9 temperature keeps changing Sign In

Dylan - Relax for a bit and either learn what it's supposed to behave like or get things checked as you did BUT........
Saw your private message that said it isn't leaking anything - keep the questions in here for ideas and suggestions from all.

Start from the top. You noticed it reads warm to hot on a gauge on your dash during a rather warm spell and there will be. You said NC I'm in MA so similar only meaning it's not like I don't see 100F temps and what they do. I go inside and try not to stare at gauges for the worst of it - just calm down.

I can't say exactly what is used for this truck for every detail of it's cooling system just that a traditional layout engine like this are frequently belt driven fans with a clutch for cooling the engine by pulling air thru the radiator. It's now old to a vehicle so the radiator, fan driven by clutch and antifreeze are as old or do you know? If you just got this say so. Owning a vehicle you are in charge of keeping track of when things were done and what it due to be done on a schedule. Fine, you just had an oil change, fluids checked (tons but which ones and how checked?) and new thermostat and now you say your temp gauge reads 1/4 of it's way "up" as you put it and pretty much stays there. This layout it really should choose ONE spot for warmed up and stay there Summer temps or Winter. New thermostat job is usually a great time to flush out coolant and put in new and it isn't clear that was done? Seriously - in general if you had this done by a shop it would be suggested to flush it out and many would have to take out old thermostat anyway and drain antifreeze and would put back new with any suggestion of a hot running engine by you mentioned - so need to know what it really looked like when there and what they really did or if you don't know how can we?

Let's focus on outside temps and temps you see on this truck. It is/was warmer but nothing out of control. It would if at the same time you were hauling a heavy load, hills bring out the weak areas of a cooling system but if all was well not really that hot for a vehicle. 100F even is actually cold to your engine itself. They run at closer to 200F or so normally plus or minus a little. Some engines (all electric controlled fans on many) will run warmer on the gauge and are warmer and normal. This one shouldn't really fluctuate that much or at all ONCE at it's temp of the rating of the thermostat. Got it?

When you come to a stop or shut down an engine like to get fuel the temp goes up in engine a bit, some will spike as seen on a gauge and when started quickly go back to normal for each vehicle. At that time the cooling system's condition matters and shows that it works well by coming into normal. If it couldn't that's a problem.

YOU MAY NOT HAVE A PROBLEM? It's good to pay attention so you know what to expect and when it isn't right.
Suggestions have already been made. Of all you now have an ordinary oil change (wasn't on the list) and a new thermostat and said there are no leaks and temps read -- I think you mean OK by saying 1/4 up? So what do you want any of us to do?

If the fan clutch is worrying you replace it. They are a common replacement item and there should be choices for a truck of how heavy duty you want or need.

It appears you are not doing any of this work by what you said so the questions should be to the tech/mechanic or just who or what situation did you choose? Just anyone with some fair know how or a trained tech/shop set up?
DYLAN! If you are that unfamiliar with what is normal and what service needs this truck should have on a routine. These things will be listed out in an owner's manual. The common suggestions of any machine will naturally say to use the dealer for that and parts sold by the dealer. That's not practical or possible for all so many shops and techs for vehicles are out there.

It's up to you to care for it like about anything, vehicle or not or seek help. The web and sites like this can help but is not a replacement for in person inspection with suggestions if you are in doubt about anything about it...........


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