I've had 2 NAPA thermostats fail since the first of the year. Just wondering if anyone knows of any particular reason why they could be failing aside from poor quality in the first place and what others are using Cummins or aftermarket?

The right Cummins replacement thermostats have "jiggle pins" on the sides of the plate. Dont get the wrong type.

I took my engine number to cummins and just assumed I would get a 190, well I shouldnt has assumed because it called for a 180. It was nearly $50 and the 190 was nearly $100 so I went with a napa unit. The t stat in my truck now isnt 3 months old but I will replace it tomorrow.

Based on feedback across the forums I would run nothing but Cummins thermostats. Everything else seems to fail early or wildly swing.

I just put one from O'Reilly's and it works just fine.

I have the cheapo from O'Reillys for a couple of years now no problem I read the temp from the Scangage. not sure how easy or accurate the stock cage is to read from the truck dash.

I have run the 180 and the 190 cummins with the wild swings , though mine are low and not high.

Based on feedback across the forums I would run nothing but Cummins thermostats. Everything else seems to fail early or wildly swing.

There's plenty of guys with swinging temps running Cummins thermostats which would rule that variable out. This whole topic perplexes me and trying to chalk up all these similar engine temp issues as cheap garbage thermostats just doesn't add up. I'm not saying that getting a bogus thermostat isn't possible but I think there's other reasons this is happening to so many people. Here's what the service manual has to say about it: TEMPERATURE GAUGE READING IS INCONSISTENT (FLUCTUATES, CYCLES OR IS ERRATIC) 1. During cold weather operation, with the heater blower in the high position, the gauge reading may drop slightly. 1. A normal condition. No correction is necessary. 2. Temperature gauge or engine mounted gauge sensor defective or shorted. Also, corroded or loose wiring in this circuit. 2. Check operation of gauge and repair if necessary. (Refer to 8 - ELECTRICAL/ INSTRUMENT CLUSTER - DIAGNOSIS AND TESTING). 3. Gauge reading rises when vehicle is brought to a stop after heavy use (engine still running) 3. A normal condition. No correction is necessary. Gauge should return to normal range after vehicle is driven. 4. Gauge reading high after re-starting a warmed up (hot) engine. 4. A normal condition. No correction is necessary. The gauge should return to normal range after a few minutes of engine operation. 5. Coolant level low in radiator (air will build up in the cooling system causing the thermostat to open late). 5. Check and correct coolant leaks. (Refer to 7 - COOLING - DIAGNOSIS AND TESTING). 6. Cylinder head gasket leaking allowing exhaust gas to enter cooling system causing a thermostat to open late. 6. (a) Check for cylinder head gasket leaks. (Refer to 7 - COOLING - DIAGNOSIS AND TESTING). (b) Check for coolant in the engine oil. Inspect for white steam emitting from the exhaust system. Repair as necessary. 7. Water pump impeller loose on shaft. 7. Check water pump and replace as necessary. (Refer to 7 - COOLING/ ENGINE/WATER PUMP - REMOVAL). 8. Loose accessory drive belt. (water pump slipping) 8. (Refer to 7 - COOLING/ACCESSORY DRIVE/DRIVE BELTS - DIAGNOSIS AND TESTING). Check and correct as necessary. 9. Air leak on the suction side of the water pump allows air to build up in cooling system causing thermostat to open late. 9. Locate leak and repair as necessary

Based on the design of the thermostat there is going to be a swing. I know the 3rd gen specs off the top of my head so I will use those. 190° thermostat. Opens at 186°-193°, and goes full open at 207°. They drop to 180° when idling, and seems to hold 180° pretty easily, with proper idle rpms and winter front. It will rapidly drop to 180°, and then takes a long time in cold weather to get below that. So right there we are looking at a 27° swing from idle temp to thermostat opening, and 215° is normal pulling a grade in OD with the thermostat full open, so 35° swing. If your loaded that right there is a big swing, thou most of us rarely ever see the thermostat go full open. In the 9K miles I drove last year I towed about 55% of them, and only saw full open a dozen or so times. The issue with older, and newer cheaper, thermostats is the wild swing that is built in. As the thermostats age they develop wear spots from rarely going full open and the spring weakens. This is why when empty you can approach full open temp, then swing back below cracking temp. The thermostat is slow to respond, then goes full open and allows too much cold coolant in. When I changed my thermostat 2 years ago it had 52K miles on it. I would cruise at 195-198°, and see as high as 207° on the initial crack of the day, with 213° being my normal hill pulling temp. When I changed the thermostat it made a HUGE difference, I cruise at 188-193°, see about 195-197° on the first crack of the day, and 202-205° is normal hill pulling temp. I don't have any swings, aside from the first crack of the day dropping about 7-8°. So you have to consider the difference between normal swings, and faulty thermostat swings. 90% of owners of these trucks are scared to death of 210°, but they don't realize that's only 3° above a fully open thermostat, and that 225° is the max allowed temp by Cummins. There seem to be far more swinging issues on new non-OEM stats than OEM ones. If an OEM one swings it's normally worn out, which happens much quicker on these trucks due to the size of the cooling system.

There's plenty of guys with swinging temps running Cummins thermostats which would rule that variable out. This whole topic perplexes me and trying to chalk up all these similar engine temp issues as cheap garbage thermostats just doesn't add up. I'm not saying that getting a bogus thermostat isn't possible but I think there's other reasons this is happening to so many people. Here's what the service manual has to say about it: TEMPERATURE GAUGE READING IS INCONSISTENT (FLUCTUATES, CYCLES OR IS ERRATIC) 1. During cold weather operation, with the heater blower in the high position, the gauge reading may drop slightly. 1. A normal condition. No correction is necessary. 2. Temperature gauge or engine mounted gauge sensor defective or shorted. Also, corroded or loose wiring in this circuit. 2. Check operation of gauge and repair if necessary. (Refer to 8 - ELECTRICAL/ INSTRUMENT CLUSTER - DIAGNOSIS AND TESTING). 3. Gauge reading rises when vehicle is brought to a stop after heavy use (engine still running) 3. A normal condition. No correction is necessary. Gauge should return to normal range after vehicle is driven. 4. Gauge reading high after re-starting a warmed up (hot) engine. 4. A normal condition. No correction is necessary. The gauge should return to normal range after a few minutes of engine operation. 5. Coolant level low in radiator (air will build up in the cooling system causing the thermostat to open late). 5. Check and correct coolant leaks. (Refer to 7 - COOLING - DIAGNOSIS AND TESTING). 6. Cylinder head gasket leaking allowing exhaust gas to enter cooling system causing a thermostat to open late. 6. (a) Check for cylinder head gasket leaks. (Refer to 7 - COOLING - DIAGNOSIS AND TESTING). (b) Check for coolant in the engine oil. Inspect for white steam emitting from the exhaust system. Repair as necessary. 7. Water pump impeller loose on shaft. 7. Check water pump and replace as necessary. (Refer to 7 - COOLING/ ENGINE/WATER PUMP - REMOVAL). 8. Loose accessory drive belt. (water pump slipping) 8. (Refer to 7 - COOLING/ACCESSORY DRIVE/DRIVE BELTS - DIAGNOSIS AND TESTING). Check and correct as necessary. 9. Air leak on the suction side of the water pump allows air to build up in cooling system causing thermostat to open late. 9. Locate leak and repair as necessary

Short off cold weather operation and the gauge(sender maybe, I did not see it mentioned)being bad, the other items all lean towards over heat conditions and not over cooling, I have orderee another temp sensor to see if it is that. Maybe a wiring problem. Temps were in the low 90 here yesterday and I noticed my fan came on a few times that I was not expecting. Maybe normal with the heat. I dont remeber seeing anything in the FSM that addressed an overcooling situation. I also have another speedo at the house and I think Ill plug it in when I get back home and see if that makes a differance.

John, I'm not downplaying your intelligence since I like your analytical point of views. But.....you cant ignore the fact that there is plenty of people with engine temps which hardly swing at all. Like me. My engine temps lay solid at 190* unless idling in the cooler weather for long periods whereby I might see 185*. And when pulling hill or driving "faster" on the freeway I may see 195*(ish). When towing the temp will consistently maintain 195* until I stop and then it will settle back at 190* again. My truck has ALWAYS run this way, even when I had the OEM thermostat. So why with my cheap NAPA thermostat will I not get the wild swinging engine temps but another guy will? As I mentioned, I think there's other reasons this issue is as rampant as it is. I've never had an engine that would swing engine temps 30*+.

- - - Updated - - -

Short off cold weather operation and the gauge(sender maybe, I did not see it mentioned)being bad, the other items all lean towards over heat conditions and not over cooling, I have orderee another temp sensor to see if it is that. Maybe a wiring problem. Temps were in the low 90 here yesterday and I noticed my fan came on a few times that I was not expecting. Maybe normal with the heat.

I dont remeber seeing anything in the FSM that addressed an overcooling situation.

I also have another speedo at the house and I think Ill plug it in when I get back home and see if that makes a differance.

My bad guys..... This is what the FSM says about "diesel" engine coolant system. I would have edited my post but you already quoted me.

TEMPERATURE GAUGE READING

INCONSISTENT ( ERRATIC,

CYCLES OR FLUCTUATES)

1. Heavy duty cooling system,

extream cold ambient (outside)

temperature or heater blower motor

in high position.

1. None. System operating normaly.

2. Temperature gauge or gauge

sensor defective.

2. Check gauge. (Refer to 8 -

ELECTRICAL/INSTRUMENT

CLUSTER - DIAGNOSIS AND

TESTING)

3. Temporary heavy usage or load. 3. None. Normal condition.

4. Air traped in cooling system. 4. Fill cooling system (Refer to 7 -

COOLING - STANDARD

PROCEDURE).

5. Water pump

5. Replace water pump.

6. Air leak on suction side of water

pump.

6. Check for leak. (Refer to 7 -

COOLING - DIAGNOSIS AND

TESTING)

___________________________________

And to answer your question dripley.

TEMPERATURE GAUGE READS

LOW

1. Vehicle is equipped with a heavy

duty cooling system.

1. None. System operating normaly.

2. Temperature gauge not

connected

2. Connect gauge.

3. Temperature gauge connected

but not operating.

3. Check gauge. Refer (Refer to 8 -

ELECTRICAL/INSTRUMENT

CLUSTER - DIAGNOSIS AND

TESTING)

4. Coolant level low.

4. Fill cooling system. (Refer to 7 -

COOLING - STANDARD

PROCEDURE)

John, I'm not downplaying your intelligence since I like your analytical point of views. But.....you cant ignore the fact that there is plenty of people with engine temps which hardly swing at all. Like me. My engine temps lay solid at 190* unless idling in the cooler weather for long periods whereby I might see 185*. And when pulling hill or driving "faster" on the freeway I may see 195*(ish). When towing the temp will consistently maintain 195* until I stop and then it will settle back at 190* again. My truck has ALWAYS run this way, even when I had the OEM thermostat. So why with my cheap NAPA thermostat will I not get the wild swinging engine temps but another guy will? As I mentioned, I think there's other reasons this issue is as rampant as it is. I've never had an engine that would swing engine temps 30*+.

What do you use to watch the temp? Remeber the dash is ECM driven, and not always the same from year to year or flash to flash. Just like oil pressure and voltage, the coolant temp is not 100% accurate. I watch mine on a OBDII reader and it moves far quicker and further than the OEM gauge ever indicates. I really doubt there are any out there that only run one temp, it's probably a lack of attention. I have a buddy with a '10 that swore his truck ran 205° all day every day, never changed towing, empty, etc. then I called BS and he watched it more, turns out it moves a LOT. I guess I should have clarified a little more. If you watch it like a hawk it will be moving a bit. Every little hill will raise 1-2°, big hills even more, etc. But if you just glance at it every now and then it won't look like it really moves.