Whelp after finally replacing my tstat and upper hoses last night (had been procrastinating for a while, but it's starting to get warmer) I've got some thoughts. Maybe some of you had been following what I thought was going on with our cars and the bypass system, and I'm a little more confident now that I was on the right track.

First of all, the problem is fixed - my aux fan doesn't come on after ~10-20 seconds of idling (this would happen previously even in 20-30deg ambient temps). The stock radiator and clutch fan should have MORE than enough cooling capacity with the AC off to keep temps below the aux fan trigger temp (212 on 202 off).

My hunch was that, in a bypassed state (or even semi-bypassed, ie. at idle but with tstat slightly open) the tstat wasn't getting an accurate reading of coolant returning from the engine.

The issue with the bypassed cooling system is that when it's bypassed and the tstat is closed, you're basically isolating everything in front of the engine (tstat, radiator, fans, etc) except for the heater core loop. A small % of coolant flow is allowed out of the main coolant manifold and through the tstat in all situations, but this (to me) is risky since a number of variables can impact the performance of that small amount of coolant.

SO here's your moment of zen (or mine, anyways..). When I removed the tstat from my car last night, one of the 4 "sensor" holes was blocked!

I went to my friends shop and grabbed a random used DII tstat off the shelf and one of those 4 holes was also blocked! Obviously you don't need to be good at math to realize that this is a 25% reduction in flow for temp sensing purposes (or more importantly, surface area over the tstat spring area).


I'd also like to note that I personally believe a one-shot coolant system bleed is impossible on these cars, at least as per the RAVE manual, since this doesn't handle the heater core loop. I dropped my SLS to the bump stops (ie. nose high) raised the expansion tank, filled it all till solid coolant came out of bleeder, lowered it, and filled to normal level. Upon starting the car for ~30 seconds with heat on max, I shut it off and repeated the process.. letting out a LOT of additional air.

After getting up to normal temp (it was 11PM, was at my work shop, and freezing..) I noticed the famous "water fall" sound.. but I wasn't going to wait for the system to depressurize so I could bleed it. Drive home was 100% fine, and I popped the hood before going to work this morning to rebleed it. The tank was probably 60% empty ( so there was that much more air in the system which had sorted itself out on my drive home). After raising the tank, bleeding, and topping off.. everything is PERFECT. The fan will NOT come on regardless of how long I idle, even with the AC on (condenser tends to heat soak the radiator and reduce efficiency.. yes I'm aware that AC alone would not trigger the aux fan if it's cold enough outside, but coolant temp sensor still has ability if required).

Should be noted that I drained and flushed everything (took block plugs out as well) so I probably had more air in there than most forumers would have if they were just replacing a tstat or waterpump.

In conclusion, you need 100% flow of over the tstat surface to keep efficient cooling at idle (through the sensor holes in tstat) or else temps will increase (most likely) at a proportional rate to the reduction in sensor plate surface area caused by a blockage. Once it heats up enough, even the 75% remaining area (in my case) is hot enough to "tell" the tstat to open. This is why our DIIs still "work" (and why, once you give it gas / accelerate.. the problem goes away, because the system is no longer bypassing and has an increased flow/more accurate read of coolant temperature.. opening and reducing temps almost instantly, hence why my fan turned off previously when I revved it past ~1200rpm even with faulty tstat).

Sorry for the long post, but I had a series of "eureka!" moments over the past few days while studying the system, culminating in the stuff I did last night. Hopefully this helps someone out - our cars aren't possessed and it's not a matter of "luck" but rather lack of understanding a seriously complex cooling system based on symptoms that don't seem to make sense (running too hot.. but only at idle..?) I also need to say that this is a likely contributor to head gasket failure, since your car can be seemingly functioning perfectly fine even though you're running ~25 deg too hot every time you idle (think parking lots, traffic, etc). This can be going on for YEARS and most people wouldn't notice the little hint (in my case, early activation of aux fan).

Good luck!


what it looks like (suppose it's possible that more than one of these can block on some cars) -

 

Good find, thanks for posting.
That thermostat looks like a grandaddy btw, I would get a new one if that is the one in my truck.

 

Interesting write up. Many folks on this and other forums have also concluded that the stock cooling system is junk too.

You may find it interesting to search for the inline thermostat mod and read through how others have replaced the stock thermostat...

Here is a discussion of the results of the mod over on D-Web: http://www.discoweb.org/forums/showt...thermostat+mod

 

Thanks for the comments.

I'm not however saying that the stock system is junk.. it's just over-complicated and probably not worth the risk to the vehicle just to have quicker warm-up times in the winter (although none of us are going to complain when it's 10 degrees outside!)

I'm laughing at all the misinformation in that thread by the way - take it all with a grain of salt (people saying stock LR system meters coolant temp after passing through radiator, stuff like that, which is just total BS)

I would never do the inline setup like the people above. First of all, it requires just as much work (possibly more?) under the hood as it does to replace an OEM tstat and tune-up the stock system to full performance.

Second, and this is a BIG one.. the in-line system works because it uses the heater core loop as the primary coolant flow when the tstat is closed. In this condition, with that mod.. 100% of the water pump's force is directed through your little heater core. This is BAD. The RAVE manual specifically states that at engine speeds higher than idle, pressure is relieved from the heater circuit because the bypass spring opens. It's still bypassing the radiator, but it flows through the larger hose and out of the engine's main coolant manifold instead.

Once the engine is up to temp with the inline mod, coolant can flow normally through the larger hoses and main coolant manifold.


BASICALLY the only thing that mod accomplishes is the elimination of a safety feature lol, beyond that you're using the same style thermostat as the stock system, it's just not in a funky plastic can!

 

thanks for the post, as one with a warm idle, I am always interested in learning more. Quesiton, does the thermostat have two valve, one temp activated and one pressure activated. My understanding is that at idle, the pressure valve closes and forces all of the coolant through the heater core, with a small amount that passes through the four small holes in the valve itself. At running speed, the pressure valve open due to the pressure of the water pump and the temp valve opens upon the coolant reaching a certain temp, 180 to 195. At idle, is the pressure valve closed regardless of coolant temp? Does hot coolant cause the temp valve and the pressure valve to both open? It doens't seem to be the case. It seems that at idle the pressure valve closes which makes the temp valve moot. Is that a correct assumption? Look forward to the knowledge of the board. Thanks Phil

2000 DII warm idler
2001 DII cool as a cucumber at idle

 

Glad you liked the post - and good question, as this was the concept I had trouble getting my head around at first (very confusing).

To answer your question, the valves are fully independent; yes the pressure valve is always closed at idle regardless of temperature (and has no temperature activation method).

If you can picture this.. at idle, if the temperature gets too warm (through sensing holes) the thermostat opens (allowing coolant through the bottom of the tstat) - remember the funky "tee" pipe on the top of the fan shroud? Well, the pipe feeding the top of the radiator is always TRYING to flow coolant through the radiator, but it can't till it has a clear path (this happens when the bottom of the tstat opens) - off it goes into the water pump (the third pipe on tstat.. the angled one). That's the "hot" way to flow coolant above idle.

The "cold" way is when the bypass opens. Picture bottom of tstat blocked, nothing can go through the top of radiator (so that part of the "tee" pipe on radiator shroud is isolated/stopped) - the other part of the tee feeds the top of the tstat, to the bypass valve. Above idle, that valve opens and coolant is sucked through that valve and directly into the water pump (again, through the angled portion of third tstat hose).

Third scenario which you seemed to understand fine (but I'll explain for the others) is the cold/ AND idle at the same time. Tstat is shut, so the radiator side of tee hose is stopped. It's at idle so the bypass is also shut (closing the remaining portion of shroud tee hose) - nothing can leave the main coolant manifold of the engine. However two things still happen, obviously coolant is flowing at a slow rate through the heater core - AND a small percentage of hot coolant is leaving the tee hose.. passing through the bypass spring, and letting the thermostat still sense the temperature. This is why those 4 sensing holes are so important (and it's in this scenario where our problems come into play). Both my tstat and the random one at my friends shop had a blocked sensing hole, meaning temps will continue to rise at idle till the tstat finally "feels" the hotter temps from the remaining 3 holes.

Cheers!

 

Here you go. Hopefully this doesn't do more harm than good. Obviously I'm not going to quit my day job in hopes of a career in anything artistic lol.

Only read the color for the scenario you're looking at, (ie. cold flow.. only read/look at things in blue) the text near tstat area will explain why the flow is going where it's going.

Hope it helps.

(yes... I'm bored at work today)