Has anyone heard of this phenomenon before? I mean it's an axial-flow waterpump so I'm confused as to what would cause its efficiency to decrease? It should simply be connected or not - nothing in between!

I'm glad I caught onto the hint - the aux fan had never been working since I bought the thing a few months ago. I finally replaced it, but at idle the fan is almost always on.

I went to my friends shop and hooked up a scan tool to read the coolant temperatures, and sure enough they were fairly high (it was only mid-60s out) I think around 216 degrees or something.. but at that temp the tstat should be wide-open and the aux fan will activate. It wasn't increasing temperature, but it was still too hot for idle (figure if I was in traffic on a 100 degree day, flowing the same amount of coolant with the same amount of airflow.. it would almost certainly overheat).

We revved the engine and the temps INSTANTLY tanked 10-15 degrees and the fan turned off.

Do you think the impeller is backing off from the housing just a tiny bit, and screwing up the flow/efficiency at idle? The impeller is pressed onto that shaft so there's no way its slipping obviously.

Maybe this is a good time to trash the clutch fan and go electric since I need a new waterpump apparently!


ANYWAYS like I said, I'm glad I took the hint and hooked up the scan tool - I'd advise you all to watch out for a similar problem! The vehicle is a 99 disco II with 150k on it, not certain but I'd assume this is the original water pump.

 

It is not uncommon for idle temps to be 102-103degC and I believe the faster your engine turns the faster the fan will turn. It won't turn at a faster RPM than the pulley that is attached to the crank.

 

I've seen where a water pump impeller has spun on the water pump shaft, but that would only happen at higher speeds, and would cool fine at idle. I doubt it's your problem though. My hunch is your radiator could also be becoming plugged. The increased resistance may be causing less water to flow at low speed when the pump can't produce as much pressure. At higher revs, increased pressure overcomes the resistance allowing more flow across the radiator. I imagine you've investigated this, but ensure your fan shroud fits the fan well, no holes in the shroud, etc. I personally wouldn't ditch the fan for an electric. It should cool fine as is. I'm active on the Corvette forums and they commonly have cooling problems. You see guys trying all these things like electric fans and all that ....it usually comes down to the radiator and improper shrouding (improper air flow).

Mark G

 

Interesting, as i've had the same concern on my 04 se. Heres what i found-the thermostat should start to open around 179f and be open by 204f. When at idling speeds my temp reaches upto 211f. As soon as the rpms increase a sprung diaphram inside thermostat will open and allow cooled air from rad into the engine thus lowering the operating temp on the road. this happens regardless if the tstat is open. The system is designed this way to force coolant through the heater core to shorten engine warm up times during idling. I find my operating temps when veh is in motion to be 195-205f which is ideal. its just the way the cooling system is designed to operate

 

the radiators also have small ports. D1's are worse than D2's. but a backflush on the radiator wouldn't hurt at your mileage.

 

Interesting information guys, thanks!

ol rover, that seems to make sense about the way the tstat operates, but I'm confused as to how this would work if my ambient temperatures increase. It sounds like you're saying it just won't flow that much till the rpms increase, but my temps were already fairly high - if I had been outside when it was 40 degrees warmer out (which is possible around here!) then I'm not sure I'd be able to keep it cool, since my aux fan was already switched on and the clutch fan was locked up.

It's possible that there's an issue with my radiator, but wouldn't the water pump be a more likely culprit based on how quickly/easily the temps decrease when the engine is revved past idle? It's a cross-flow style radiator so wouldn't it only be possible for a few fins to be blocked.. but not the whole thing? None of this should be an issue at idle, since the radiators are designed to be adequate in extreme conditions like heavy towing up hills, on hot days, off-roading, etc. Radiator capacity should be fine at idle.. I think the pump just isn't flowing it.

Hmmmmm. Shroud is intact.. never been messed with, and with both of those fans on the thing can throw out some serious air.

Actually now that I think about it, if it was a fan problem the temperatures wouldn't instantly decrease when the RPMs increased. That can only be flow-related, or possible radiator related.. but again since they dropped so quickly it seems the radiator was working fine, it just didn't have water flowing through it yet.

 

Bump - just replace the waterpump and tstat (since I have one sitting here) and go from there I guess? I SHOULD probably do hoses at the same time, but that looks like a PITA and they're so freaking expensive.

 

interesting thead, i have the same symptoms on my cooling system.
sorry i dont have any ideas or suggestions.

i got this truck not even a month ago and within the first week it over heated. scan gauge would read 230 at idle and when reved or driven would drop to 205.
there was air traped on the system. after i bled the air out it goes up to 215 (at idle) and it drops relatively fast if i rev or on the road.
is 215 too hot? i allready have a new thermostat but have not installed it yet. was thinking to flush the coolant out this weekend but rave says "replace the plugs gaskets" which i dont have.

 

I did all the hoses on mine, as well as thermostat and water pump and they weren't really that expensive. I also added water wetter.

I don't have an IR thermometer so I can't give specific numbers but my temp gauge stays dead center.

 

Drain and flush the cooling system and rplc the tstat-this helped my idling temps drop form 230f to 210-215f. I feel waterpump/rad/fans are ok. If you download the Rave manual it will help you understand how this stuff all relates. my aux fan comes on at 207f or even less if the a/c is on. Our cooling system is not typical