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I'm currently in the midst of performing a tune-up on my new-to-me 2002 Dodge Dakota SLT Club Cab 4x4 with the 4.7L V8. One of the items on my to-do list is to replace the thermostat and thermostat housing, as the previous owner snapped off one of the housing retaining bolts within the timing cover and substituted a C-clamp in its place. Definitely not ideal. I am overhauling the entire cooling system anyway due to a suspected internal leak within the radiator, so this project will kill two birds with one stone and ensure me of a clean start with the brain of the cooling system.
I am planning on primarily using this truck for towing purposes, and perhaps some light snow plowing duties as well. With this in mind, I'd like to ensure that any parts I replace are well-suited to the cooling requirements of relatively severe service. I have therefore extensively researched the idea of running a 180° thermostat for greater cooling over the OEM 195° 'stat. Ultimately, I have decided against it due to the slight decrease in fuel economy many have reported, the higher potential for sludge buildup due to the cooler temps, problems that can develop over time from running a richer-than-stock fuel-to-air ratio, and a decrease in cabin heating ability.
Although I've decided in favor of a thermostat which opens at the 195° stock temperature, my research on the temperature debate led me to another technical question which I've only seen broached once or twice. Namely, is there a special "coolant bypass block off" feature unique to the OEM thermostat (Mopar P/N 68174083AA) which gives it an advantage over aftermarket options?
I first read about this possibility in a thread over at DodgeDakota.net, where a user by the name of "Rob C" stated that all aftermarket thermostats will result in less efficient engine cooling than the OEM Mopar-branded thermostat due to their inability to block off the coolant bypass port in the engine block when fully open. In a related thread, this user then went on to explain how the OEM thermostat functions in relation to this coolant bypass port.
In a nutshell, the theory is that the 4.7L V8 engine has a coolant bypass passage in the block — located directly behind the thermostat — that enables coolant to circulate from the water pump to the cylinder heads / block and back again in a closed loop when the engine is cold, prior to the thermostat opening. According to the above threads, the OEM thermostat contains a special extension on the back (spring side) which extends several inches beyond the rear of the thermostat. When the engine reaches operating temperature and the thermostat opens, this extension serves to restrict / block this bypass port, effectively forcing more heated coolant to flow through the radiator instead as it is now the path of least resistance. The net effect is supposed to be a greater cooling effect than can be achieved by aftermarket thermostats missing this extension on the rear. EricTheCarGuy has a helpful YouTube video containing a good explanation of how this type of system works.
I dug through my copy of the Factory Service Manual, and it sure seems like Rob C is on to something. Page 7–34 (PDF page 233) had this to say on the matter:
![109826]()
![109827]()
This information verifies the existence of a coolant bypass, as well as a feature on the rear extension "post" on the OEM Mopar thermostat designed to block off 50% of this flow when the thermostat is open for more accurate coolant temperature control. FSM Page 7–3 (PDF page 203) contains a helpful diagram illustrating coolant flow throughout the cooling system. I've taken some creative license with coloring to illustrate my understanding of how this coolant bypass block-off feature works.
Cold Engine Coolant Flow (thermostat closed, bypass journal open):
![Text Line Diagram Map]()
Warm Engine Coolant Flow (thermostat open, bypass journal restricted):
![Text Line Map Diagram World]()
This brings me full circle back to the reason for authoring this thread — is this special "coolant bypass block off" feature truly unique to the OEM Mopar thermostat? If so, how much real-world impact does it actually have?
According to Rock Auto's catalog, the Mopar thermostat is 4x the price of the cheapest alternative, and I can find no visible difference in the listing photos for the aftermarket 'stats and the Mopar P/N 68174083AA.
I am eager to hear from those who have replaced OEM thermostats in their 4.7L V8's with aftermarket units regarding whether the OEM version contained a post on the rear that was missing on the aftermarket replacement. I'd also like to hear what beneficial effects you believe this coolant bypass block off feature may provide, if any.
I am planning on primarily using this truck for towing purposes, and perhaps some light snow plowing duties as well. With this in mind, I'd like to ensure that any parts I replace are well-suited to the cooling requirements of relatively severe service. I have therefore extensively researched the idea of running a 180° thermostat for greater cooling over the OEM 195° 'stat. Ultimately, I have decided against it due to the slight decrease in fuel economy many have reported, the higher potential for sludge buildup due to the cooler temps, problems that can develop over time from running a richer-than-stock fuel-to-air ratio, and a decrease in cabin heating ability.
Although I've decided in favor of a thermostat which opens at the 195° stock temperature, my research on the temperature debate led me to another technical question which I've only seen broached once or twice. Namely, is there a special "coolant bypass block off" feature unique to the OEM thermostat (Mopar P/N 68174083AA) which gives it an advantage over aftermarket options?
I first read about this possibility in a thread over at DodgeDakota.net, where a user by the name of "Rob C" stated that all aftermarket thermostats will result in less efficient engine cooling than the OEM Mopar-branded thermostat due to their inability to block off the coolant bypass port in the engine block when fully open. In a related thread, this user then went on to explain how the OEM thermostat functions in relation to this coolant bypass port.
In a nutshell, the theory is that the 4.7L V8 engine has a coolant bypass passage in the block — located directly behind the thermostat — that enables coolant to circulate from the water pump to the cylinder heads / block and back again in a closed loop when the engine is cold, prior to the thermostat opening. According to the above threads, the OEM thermostat contains a special extension on the back (spring side) which extends several inches beyond the rear of the thermostat. When the engine reaches operating temperature and the thermostat opens, this extension serves to restrict / block this bypass port, effectively forcing more heated coolant to flow through the radiator instead as it is now the path of least resistance. The net effect is supposed to be a greater cooling effect than can be achieved by aftermarket thermostats missing this extension on the rear. EricTheCarGuy has a helpful YouTube video containing a good explanation of how this type of system works.
I dug through my copy of the Factory Service Manual, and it sure seems like Rob C is on to something. Page 7–34 (PDF page 233) had this to say on the matter:
This information verifies the existence of a coolant bypass, as well as a feature on the rear extension "post" on the OEM Mopar thermostat designed to block off 50% of this flow when the thermostat is open for more accurate coolant temperature control. FSM Page 7–3 (PDF page 203) contains a helpful diagram illustrating coolant flow throughout the cooling system. I've taken some creative license with coloring to illustrate my understanding of how this coolant bypass block-off feature works.
Cold Engine Coolant Flow (thermostat closed, bypass journal open):
Warm Engine Coolant Flow (thermostat open, bypass journal restricted):
This brings me full circle back to the reason for authoring this thread — is this special "coolant bypass block off" feature truly unique to the OEM Mopar thermostat? If so, how much real-world impact does it actually have?
According to Rock Auto's catalog, the Mopar thermostat is 4x the price of the cheapest alternative, and I can find no visible difference in the listing photos for the aftermarket 'stats and the Mopar P/N 68174083AA.
I am eager to hear from those who have replaced OEM thermostats in their 4.7L V8's with aftermarket units regarding whether the OEM version contained a post on the rear that was missing on the aftermarket replacement. I'd also like to hear what beneficial effects you believe this coolant bypass block off feature may provide, if any.
