I have been keeping track of my coolant temps with an OBDII scanner. Over the summer when temperatures were in the mid 90's to about 100 degrees my temps were normally 195 on the highway, 200 around town and 205 sitting in traffic.

Now that the temperatures outside are in the 40's to 50's I am seeing 201 on the highway, 205-208 in town, and up to 212 at a stop light. After it gets up to 212 at a light it almost immediately drops to 203 once I start moving again.

The only thing that has changed since the summer is I have had the coolant system flushed and I replaced the throttle body heater plate and gasket. The engine driven fan doesn't spin freely when cold if I give it a good fling it will go around about a third of a revolution. When the engine has been running for a while it may spin for about a third of a revolution if I try to spin it by hand.

Any ideas?

Thanks,
Mike

 

So your viscus clutch is shot and I bet you didn't replace your t/stat and it is now plugged up because of the flush.

 

You will experience a slightly warmer engine temp when it is cool out in certain conditions. The electric fan does not run when the ambient air temperature is below a certain level. So, at idle or speeds below about 52 mph you will run a bit warmer when using the climate control system. This is really noticeable when temps are just below the criteria for the electric fan to function but, are still warm enough for the a/c to cycle.

 

IMHO the rad may have been flushed, but it is not 100%. If you check temps on fins from top to bottom should be within 10F. Big difference is sludged up lower rows. Cools ok at speed, but not enough when slow or parked. I ran my D1 at 3000 - 4000 rpm tody, and down the highway at 60 mph, 183-187F, with a 180 stat. 60F outside. Don't think you need to be topping 212.

 

This is NOT TRUE at all.

For starters, the engine cooling function of the aux fan is TOTALLY independent of both speed AND air conditioning switch points!

Secondly, your stated speed and air conditioning switch points are also wrong and the operation mentioned is incorrect. The speed is 50mph however even when running below 50mph the system will not turn on unless ambient air temperature is above 82 degrees. You imply that the operation of the HVAC system is somehow connected to the temperature readings he is seeing for the cooling system? The only connection is that they are both capable of turning on the aux fan - but for different reasons and operation of AC shouldn't impact coolant temp as the thermostat should simply open more and/or the clutch fan should engage more to counteract the slight reduction in cooling efficiency from the warmer AC condenser.

Third; the operation of the aux fan for engine cooling is possible at ALL speeds and at ANY ambient air temperature. As per RAVE manual the on point is 230 degrees and the off point is 221. In fact the system is even capable of turning on the aux fan after the engine is shut off to prevent heat soak.
(I believe that bit of information is incorrect however, as I've never seen any DII do this with the key off, and my DII routinely activated the aux fan at idle (no AC on) implying temps above 230, yet wouldn't remain on if I turned the engine off. The trigger temps for key-off operation are supposedly 221+ so obviously it should have stayed on!)





Back to the OP's post - this has happened to me before and it turned out to be a tstat which had clogged sensing holes as Disco Mike eluded to - however it doesn't take a flush to clog them. I inspected my new (~10k mile) tstat and one of the 4 holes was already clogged. If any of the 4 (if you're using an OE-style thermostat) sensing holes are blocked in your thermostat housing, then while your truck is at idle and the cooling system is bypassed, it will incorrectly "sense" the temperature of the coolant flowing over the spring, causing it to open later. In a bypassed state, almost all of the coolant flow is sent "around" the thermostat through the heater core.

While reading the RAVE descriptions very carefully, I think I finally understand yet another fluke in the bypass system and one which could explain both your problems AND mine.

"In cold ambient temperatures the engine temperature can be raised by up to 10°C (50°F) to compensate for the heat loss of the 10% exposure to the cold coolant returning from the radiator bottom hose. "
lol! It's not "compensating" - it's incorrectly reading the temperature of the actual engine coolant because the temperature has dropped so much by the time it went through the (colder than normal) radiator!

So what they're saying is that in a bypassed state, the sensing holes still allow enough flow through the radiator and over the thermostat to allow it to open once temps finally rise. The problem with this is that the return temperatures from the radiator will be SO COLD in the winter (since 10% of the flow is going through the radiator, it has the ability to drop temperature significantly by the time it actually emerges into the thermostat) that the thermostat will CLOSE further due to these colder temperatures from the sensing hole flow, allowing the bypassed coolant to increase in temperature. 50 degrees is obviously extreme and would only happen if you were in Siberia or something and the ice cold 10% flow out of radiator/across the tstat caused it to basically shut lol.

The funny thing is that they are totally aware of the issue when they mention that the problem will cause engine temps to rise at idle as high as 50 degrees (?!) since the tstat sees the colder return temps from sensing flow. They watched this happen in a lab... and knew there was no way around it with the bypass cooling system design!

Obviously NO amount of cooling action from the aux fan or clutch fan will have any impact on this problem, since there isn't any coolant flowing through the radiator in the first place! In fact, as the temps rise, the ECU will trigger the aux fan which will cool the radiator and the return temperatures to the thermostat even more!

Clutch fan operation is unaffected since it's based on actual physical temperature of the radiator which is obviously cold w/ the 10% flow.

As I mentioned above, once you increase engine RPM the bypass spring opens allowing full flow over tstat, which instantly detects the true coolant temperature and quickly opens - additionally, once it does open, all the coolant is passing through a VERY cold radiator so your temps immediately drop back to normal.

Unfortunately there isn't much you can do to resolve the "flaw" - except that it shouldn't be too bad (as you can see with the temps you posted, which are within operating range) if the system is working to original performance. I'm actually not so sure there's anything wrong with your tstat, as my aux fan was turning on in cold weather at idle which implies idle temperatures above 230 degrees which were much higher than yours. Note that my fan did NOT turn on when I first replaced the thermostat while it operated normally - this makes me think you're fine and were just noticing the design issue I noted due to colder return temps. If your temps jump significantly (230+ as I guess mine were..) then look into the tstat or other components.

In summary, the fact that your temps drop once your RPMs increase tells me it must be bypass-system-related, and the fact that it's worse now then in the summer narrows it down even more and tells me it must be due to colder return sensing temperatures while bypassed, causing the thermostat to close slightly more and thus causing coolant temperatures to increase.

As far as failures or replacing parts goes.. the ONLY two elements of the entire cooling system that would involve RPM-related temp increases would be the water pump or sensing-hole clogging as this would effect the return temperature of the coolant entering the thermostat housing. The waterpump would NOT explain why this is happening now with the colder outside air temperatures. This leaves us with the tstat (clogged) as the only part that could even influence your symptoms if you wanted to replace something, but it sounds like yours it working perfectly to be honest with you. Even a brand new tstat will have that increase in temps at idle (even higher in winter) - so I say leave it. You only noticed it because of the OBDII reader, the system was within spec though. I noticed the exact same thing as you however my only hint was the aux fan turning on!

If you notice a dramatic 10-20 degree increase (in idle/bypassed state) one day, I'd say one of those sensing holes finally clogged, but you'd be looking at temps around 225+

 

Whew! The points about the clogged disc metering holes are spot on. Not sure that the bypass works exactly as you point out, but the end result of blocked stat or blocked rad will cause overheat issues.

 

Thanks for the replies. The detailed description about the sensing circuit in the T-stat makes a lot of sense. I'll keep an eye on it but it sounds like a logical explanation.

Regards,
Mike

 

Here's pix and plumbing flow chart. The softer spring of the bypass part of the stat allows the disc to descend at rpm above idle, supply warming up coolant to the heater core, more comfort quicker in winter. Also used in other cars, like Mercedes and Fords. As the main stat warms up, it opens up and forces the disc back closed, at which time the metering holes become the link between hot engine coolant and the thermostat main body.

Also note that a somewhat blocked radiator can reduce the heat reaching the face of the fan clutch, preventing it from reaching full coupling.

 

at Estorilm, What I said was in fact true. Is it the op problem? I dont know and neither do you. Take for instance this example. Its 65deg out and sunny, your idling at the starbucks drive thru, you have you climate control temp set for 68deg. The solar heat gain from all that single pane un-insulated glass that makes up a large percentage of the d2 is causing your inside cabin temps to spike rapidly. The climate control system in turn cycles on the a/c compressor to compensate for the temp but, unlike during the summer time when its 90 deg out the a/c fan does NOT kick on. So, now you have the additional load of the a/c compressor paired with somewhat warm outside temps and your ultra-gauge will tick up a few deg more then what you saw in the summer time. This heat load will not open up the thermostat any more then it already is at idle due to the nature of the bypass system and the fan clutch is already engaged to its 70 percent or so coupling rate and is limited now by the slow idle speed. The only thing now that can drop temps any more would be for the electric fan to kick on and it wont because the outside air ambient temps is too cool, unless the engine temps climb to 212deg. If you dont believe me spend less time typing your op ed piece and go verify it for yourself.