Flowkooler Water Pump and "Low" RPM

[WillyWagoneer]

Hey all, a couple guys and I were discussing Flowkooler water pumps at a local 4x4 meet last week, and the discussion of what's considered "low" rpm came up. Some guys insisted anything below 1500 rpm was low, while others drew the line at 3000 rpm. What are your thoughts?

This goes back to my post from a couple weeks ago. I'm considering a Flowkooler pump to help keep my temps down when cruising at 70 mph @ 2700 rpm. The temperature really only climbs when I exit the highway and stop at the first light, so that's why I'm considering upgrading the water pump for the low rpm flow gain. I'll also go to a new 180 degree thermostat. The radiator is brand new.

Thanks,
Willy

[WillyWagoneer]

I think I answered my own question by double checking BJ's Off-Road (https://bjsoffroad.com/flow-kooler-high ... 91-amc-v8/)
"Flow Kooler water pumps solve low-speed heating problems by doubling the water flow rate at low rpm and idle. At 3,500 rpm, the flow rate returns to normal to prevent too much water flow at high speeds."

[tgreese]

In the days of carburetors, low speed operation was considered above idle, but "low." Highway speed is say 2000 or so. Modern cars tend to cruise at lower speeds like 1500-1800, but that did not happen prior to efficient automatics with lots of gears. They drop down into a lower gear in response to any additional load. I'd say low RPM is above idle to 1200 or 1500. Certainly not 3000.

You have a fan shroud? Coming off the highway, not enough air flow is indicated. One might suspect "latent heat" from the highway run, but I don't know how realistic that is. Kinda think that's gone after a brief time at low speed. You could try leaving the hood ajar and see if that helps - supposed to keep the engine compartment cooler. Could be your water pump is not pumping - you could replace it and keep the existing one for a spare. Back in the day, it was common for Jeepers to carry a spare water pump. I assume the pump came with the Jeep.

Thermostat won't help, unless your current one is defective. What about your radiator? My experience says that's the usual problem for old cars. Looks fine, doesn't leak, but does not cool.

[WillyWagoneer]

It has the stock fan shroud, a new fan clutch, and the new radiator was installed two years ago. Granted the radiator and fan clutch were installed by the prior owner, but I have the receipts from the shop that installed them, and they do look like new parts. The water pump definitely looks like the original water pump. No clue about the thermostat; I need to take a look.

The temps do drop once I get back up to 35 - 45 mph, which may indicate an air flow issue, but since the water pump gasket needs replacing to begin with, I am planning to start with a new pump.

[Yeller]

Warning: cooling system science forth coming

Part of the cooling equation at higher rpm is water pressure in the block from the water pump exceeding the flow capacity of the thermostat, which is part of its job. That pressure drops the temp jumps up from the heat soak of then engine, just like when you shut the engine off, in extreme cases you may even see/hear early signs of boil over in the form of light percolating sound and possible steam bubbles in the over flow line. That water pressure is beyond what the pressure within the radiator from the cap, this pressure action is what causes the lower radiant hose to collapse when the system is not fully purged of air. That pressure prevents hot spots in the block that will boil causing the previously mentioned “percolating”. It’s not over heating yet but if those spots are not kept wet and not boiling, which takes pressure to happen, you can get an over heat condition before you actually overheat the engine. The flow cooler pump does a good job of keeping the block pressure up at lower rpm’s reducing that temp spike you are describing.

In essence you can have a very efficient radiator and fan and still have heat spikes when engine rpm drops. It is not always a lack of airflow. That being said I would run down the highway and immediately stop and hold a piece of paper in front of the grill to see if the fan is drawing air. Unfortunately one thing we come to accept as normal is new parts prematurely failing or not functioning at all. If you wanted to be scientific about it you could use a no contact tachometer on the fan to see if it speeds up sitting idling as the temp increases, it should.

[tgreese]

My experience at the dealership did not point to an no overheating problem with these cars. Not that the cooling was especially effective, but that new engines in proper tune with the factory parts did not overheat. Hard to say more, other than my one historical and two present V8s are fine with the factory equipment, and I did not see overheating complaints in the '70s when I was working.

So yeah, it should be ok with the factory equipment if its all in proper condition. You have to start eliminating unknowns, and the water pump seems like a good start. I would replace the thermostat with the correct (195F?) one, since it does not make much difference to cooling capacity.

[Yeller]

Tim is completely correct, but all it takes is one thin spot in the casting of a cylinder wall and you have some odd issues. Boring cylinders during rebuilds can exaggerate the issues. That’s why “most engines” don’t have any issues but there are some that do. Not that anything is wrong with one that does have issues, it just takes more effort to make them happy.

[tgreese]

... you could use a no contact tachometer on the fan to see if it speeds up sitting idling as the temp increases, it should.

I presume this is part of the viscous clutch thing; when it's warm,the clutch couples the fan more tightly to the shaft? No background here, but that seems like what it should do.

Ya know, this sounds a lot like too little advance at idle. You come off the highway, sit at idle at an intersection, and the temperature goes up.

Realize there is also HDC on these Jeeps (heavy duty cooling) which - as far as I can tell - is nothing more than a CTO that changes to manifold vacuum for the distributor when the engine temperature exceeds (something). There are other gadgets in the vacuum circuit that modulate vacuum somehow, but I think switching to manifold vacuum is the gist of it.

These Jeeps normally have a CTO that connects the distributor to ported vacuum once up to temperature. Ported vacuum is blocked mechanically by the throttle plate at idle, giving less advance at idle. Manifold vacuum must be better for cooling at idle, since it ignites the lean charge earlier for more complete combustion, sending less heat into the cylinder walls (I presume).

Not an expert, but this issue results from the lean idle mixture that these engines run at. Again, in my experience, these engines run fine with factory equipment, but I was gone from the scene by 1979. Presumably one could richen the mixture at idle (increasing HC emissions) or advance the spark at idle (increasing NOx emissions? or reducing exhaust gas temperature? Or both?), both improving cooling.

Another approach would be to set your overall advance (at the crankshaft) for highest vacuum at idle. This should be the most efficient setting for idle. However, this also may introduce too much advance overall and result in pinging. Probably better to bypass the spark CTO and run at manifold vacuum all the time.

Just a comment - my J20 is operating with manifold vacuum to the distributor all the time. For me, this is mostly because I do not have all the vacuum devices needed to hook up the factory vacuum plumbing. It seems fine in terms of operating temperature at idle, but Massachusetts is not a very demanding locale re that issue. Plus I have no smog inspection here, though I expect the smog guys would approve it if it passes the tailpipe test. They are not going to trace out all the vacuum plumbing!

[Yeller]

There’s a lot to the cooling package, Tim brought up some very common over looked ones. Lack of timing at low rpm makes many engines boil. Retarded timing is usually harder to cool than too far advanced.

[Yeller]

Tim yes it’s part of the viscus clutch. Gets warm, speeds up cools, slows down. On my J Truck you can stand and watch it speed up and slow down, no tools required.

[SJTD]

Not a single comment that you need to slow down flow. Can this idea finally have died? Makes me want to weep with joy.

[Yeller]

Not a single comment that you need to slow down flow. Can this idea finally have died? Makes me want to weep with joy.

Did until you brought it up!

I’m with you, glad it’s gone.

[Herk]

Not a single comment that you need to slow down flow. Can this idea finally have died? Makes me want to weep with joy.

The theory that "the coolant is moving too fast to pick up the heat" is hooey. If that was the case, it would be moving too fast to shed the heat in the radiator. The rate of flow, unless it's TOO SLOW, isn't a factor. It's a single pump closed loop system so the flow rate is the same throughout, as is the pressure (which is controlled by the cap). In basic thermodynamic terms the performance of a a heat exchanger is a function of surface area and the rate the cooling media moves through it. The engine is a heat exchanger and the radiator is a heat exchanger. The same coolant moves through each at the same rate. So the rate is on both sides of the equation and cancels out. The system will reach an equilibrium point based on the amount of heat in the engine and the rejection capacity of the radiator.

An engine that has a hot flash after the vehicle is slowed down is probably suffering from a lack of flow. If the radiator is new, and was installed correctly, (that is, the system was completely flushed before it was installed so a bunch of crap from the block didn't immediately plug it up), it's most likely the pump. I have removed old water pumps to find the impeller had completely rusted away. I've also seen new radiators wrecked by not getting all of the rust and scale out of the engine.

[SJTD]

I was hearing it the other way that it was moving too fast through the radiator to dump heat ignoring the fact that while it might be dumping more heat in the rad it would also be picking up more heat in the engine. And boiling.

I was considering selling pumps with small impellers and calling them the "Slow Cooler".

[devildog80]

So....I have what appears is the OEM radiator in my '84 GW w/401 & 727 transmission.
As it had sat for 19 years before revival, figured the radiator had clogged a bit through outside fins with dust and oxidation, so I purchased a coil cleaner used on home A/C units.
Soaked the cleaner full strength on outside of radiator, A/C evap coil, and secondary auto trans cooler as they are stacked in the front of the truck.
Not sure what numbers I had before doing this, but after the fact I have warmed up at idle 176* inlet on top hose and 124* outlet at bottom hose. So roughly 50* cooling when idling I think is good, or should I have better numbers?

[Yeller]

I'd be happy with that. the proof will be in adding thermal load by driving I-17 up to Flag and see what it does.

[letank]

So....I have what appears is the OEM radiator in my '84 GW w/401 & 727 transmission.
As it had sat for 19 years before revival, figured the radiator had clogged a bit through outside fins with dust and oxidation, so I purchased a coil cleaner used on home A/C units.
Soaked the cleaner full strength on outside of radiator, A/C evap coil, and secondary auto trans cooler as they are stacked in the front of the truck.
Not sure what numbers I had before doing this, but after the fact I have warmed up at idle 176* inlet on top hose and 124* outlet at bottom hose. So roughly 50* cooling when idling I think is good, or should I have better numbers?

sounds good, but what is the ambient temperature?

[devildog80]

Around 104* with 50% humidity being monsoon season here.