pipefitter

As the title states, I'm a newbie, and look forward to asking some questions, and even answering some (if I can!) on this forum. My question regards the oddball temperature action on my '96 12 valve. It has 93,000 miles, and a couple of years ago, I noticed that the temp gauge never climbed off cold. I replaced the thermostat and flushed the cooling system. The t-stat is from NAPA. When the gauge now climbs to 190 it falls off to 140, climbs back, falls off and so on. Wondering if it's the t-stat, or the sending unit. Any ideas?

Robert Rausch

That's normal operation for a stock thermostat. There has been lots of discussion of this over the years. I like this because with a little deduction I can figure out relatively how hot the radiator water is by the action of the gauge needle. This comes in handy for me while towing in hot weather.

pipefitter

Thanks. Out of curiousity, I bought a t-stat from a local Cummins dealer, but haven't installed it yet. The one I replaced a couple of years ago was the original and must have been stuck open. I just wondered if the one from NAPA wasn't as good as oem.

9812vram

The book says this is 100% totally normal, especially under load - but it's not. I believe this was an attempt to "fix" a design flaw. The book says it's normal, thus MOST people are satisfied with this and don't worry about it. Not me. Rushing cold water though a hot engine is a good way to crack a head. Ask me how I know!
The 12v thermostat housing is oddly hanging off the side of the head, so for the most part, temperature change is due to saturated heat - which takes time to happen. The only flow through the housing when the thermostat is closed is a tiny bit of leakage past the thermostat (it doesn't seal 100%) and a small amount of flow through the air bleed at the top of the housing. Since the air bleed is at the top (and heat rises, think convection) the hottest coolant rises to the top and flows out the air bleed. The thermostat cannot sense this as it's "sensor" is low and to far back in the housing. Once the coolant get's hot enough the thermostat eventually picks this up from the heat saturation - but it's already too late - it begins to open and as soon as it opens at all, it creates flow which suddenly passes 180+ coolant past it which causes it to open too far to compensate. This is because a 180 thermostat for example will begin to open slightly at around 170 so that by 180, the coolant is already being regulated, flowing through the thermostat some and giving it a steady temperature reading, which allows it to act quickly and do a super job of regulating temperature. So it senses 170, opens a bit, suddenly sees 180 and says "whoa! gotta open more!", so it opens too far. In our case (poor housing design) the thermostat suddenly sees 180 degrees, so it opens half way (for argument's sake) which suddenly releases a gulp of cool rad coolant into the hot engine. This obviously drops engine temperature and your temp gauge real quick. Now the thermostat sees 140 coolant and completely closes starting the cycle over. Good way to crack a head or blow a head gasket, especially in my climate where the water entering the engine from the rad in winter can easily be 100+ degrees colder than the engine's 180F!
The way around this is to take your smallest drill bit and drill one or two holes towards the inside of the flange of your thermostat. This seems to allow just enough coolant flow through the thermostat to allow it to sense temperature on time and react on time like it was designed to. The result is a temperature that climbs to 180 and stays there. The side effect is that it takes a bit longer to warm up and cools off a bit quicker in winter. Start with one hole, then go to two holes if needed. My truck needed only one hole, but a friends truck needed two.
I've also found with thermostats that the only aftermarket ones that work properly, carry the name brand STANT.

pipefitter

That sounds reasonable. The reason I bought, (but have not yet installed), a Cummins t-stat is due to an issue with a Toyota I own. It's an '87, 22RE 4 cylinder, and in the winter, the gauge showed the engine almost overheating before it opened and the temp dropped off. On a vehicle with an aluminum head, that can cause head gasket issues. With a Toyota t-stat, it simply heats to normal operating temp and stays there. Thanks.

joem

9812v ram is correct, my friends combine with a 6bt in it has a totally different T-stat, and it does have a tiny hole in it that the automotive one doesnt- he discovered this a decade ago when his combine held a even temp but his truck didnt, Keep in mine that the iron block on the cummins doest seem to effected by the sweeping tempatures as all. As for your Toyota- I also Own a 90' Toy but mine has the 3.slow engine in it- done my share of head gaskets until I learned that you must do a 100 mile engine warm re-torque and always use the Toyota red coolant in our older rigs- the new Toyota pink is a acid baised Organic OAT coolant that will eat up brass and solder fittings.
Take care, enjoy your Cummins!

pipefitter

Thanks again. I bought that '87 new. I've only replaced one head gasket, and that was when I replaced the timing chain at 175,000 miles. Those little 22R's are tough. It has 296,000 miles on it now, and I'm afraid the constant expanding and contracting cause by the temperature spiking may have caused another one. But, I am going to try drilling a hole in the diesel's t-stat before I install it. Thanks for the response.

ronnie

drilled mine today works great now

22r's had three of them. last forever, rust or accidents did them in with still good engines

infidel

Totally normal, a waste of time and money to fix.

When these trucks were sold new the dealer was supposed to tell the new owner the following and a sheet came with the owner's manual that said:

NOTE: THE COOLING SYSTEM USED ON THE CUMMINS DIESEL ENGINE EQUIPPED VEHICLES PROVIDES THE CAPACITY AND PROTECTION OF THE HIGHER GVWR AND GCWR RATINGS AVAILABLE ON THESE VEHICLES. OWNERS SHOULD BE ADVISED OF THE NORMAL SLOWER WARM UP AND LOWER ENGINE OPERATING TEMPERATURES ASSOCIATED WITH DIESEL ENGINES.

THE COOLANT TEMPERATURE GAUGE WILL FLUCTUATE ON DIESEL ENGINE EQUIPPED VEHICLES. IT IS INFLUENCED BY VEHICLE LOAD, OUTSIDE AMBIENT TEMPERATURE AND EXTENDED IDLE TIME. THIS FUNCTION IS CONSIDERED NORMAL

GAUGE FLUCTUATION WILL BE NOTICED MOST OFTEN UNDER STEADY STATE (HIGHWAY) DRIVING.
PLEASE ENSURE THAT YOUR CUSTOMER IS AWARE OF THIS NOTE PRIOR TO RELEASING THE VEHICLE.

In 1998 after years of complaints Cummins electronically turned the coolant gauge into basically an idiot light.
The temp still fluctuates, you just can't see it.

9812vram

You're entitled to your opinion. Like I said, some folks are totally content with a 50+ degree temp fluctuation because "the manufacturer said..." while others see it as a problem and seek ways to fix it.

Are you sure Dodge turned the 24v temp gauge into an analogue idiot light? The thermostat and housing on a 24v head is very different than that of a 12v, but I believe they share the same temp sensor and they also have the same instrument panel... I think they may have just fixed the problem...

joem

I know on my friends 99' the temp swung up until the last ecu update he did for the 3 cylinder idle.

Robert Rausch

315,000 miles on the stock thermostat in my '95 and no problems yet.

pipefitter

I'm glad I'm not the only one that has observed this. I could understand and accept it if it only did it when I was towing or carrying a load, but driving along at 45 mph and seeing the gauge drop from a little over 190 to 140 freaks me out. I have bought a Cummins t-stat for the truck and will install it soon. When I replaced the oem stat with one from NAPA, is when I started to see the temp swinging. Prior to that, the truck always ran cold, and I thought the stat was stuck open. Thanks for all the replies.

Robert Rausch

Can I have your old NAPA one--the one that swings the temps?

chariotdriver

Like Infidel said, Dodge changed how the gauges worked in 98. My 98 is different than the other two but it still has swings they just seem to be dampened/delayed quite a bit, the swing isn't nearly as wide/big. For instance, the oil pressure stays at "0" after it's started for so long that the "ding-ding-ding" crap goes on for 7-8 seconds. Ended up installing a mechanical gauge which shows almost instant oil pressure while the factory gauge indicates zero for several seconds. Darn computer interfaces; I would prefer the raw data...........just another example of one person messing their pants forcing the rest of us to wear diapers. If you don't like the big swings when the t-stat opens; you have a couple options IMO: 1. drill a small hole in the t-stat flange to ensure there is always some coolant flow through the radiator, or, 2. drill out/score the jiggle pin which will do sort of the same. Your warm up times will be longer. Our diesel generator at the cabin had a jiggle pin installed in the t-stat flange (air bubble elimination???). Since we use the thing for co-generation it has a heat exchanger on the exhaust and coolant for heating, we ended up snipping the jiggle pin to make the coolant heat exchanger work better. Our fuel consumption over a 5 day period (24 hrs/day) went up 1 or 2 gallons while our waste heat recovery went through the roof