: Cooling fans and hard driving- temp spikes

03-23-09, 08:43 AM
I had a call from a customer who had the stud kit installed by a garage and he said his car is exhibiting symptoms similar to my '98: I've come to the conclusion that it's not HG related, but rather GM did something wrong when they designed the cooling system:

- Under WOT runs well over 100MPH, temp stays perfectly normal. Slowing down is fine.

-3 or 4 WOT runs in a row to this speed, and then slowing right down to 30 MPH or less or coming to a stop, the temp will spike. Never to the red-zone but past 3/4 on the guage. No loss of coolant, no smoke, no steam- nothing. The temp will drop down within a few miles of driving or idling for a bit back to half.

I've pinpointed this to the cooling fans. at 100 MPH you have a lot of air flow through the rad. No need for the fans to come on. You've built up a lot of heat in the engine. Now you slow down to nothing, and the fans only come on after the temp rises to 225 I believe. The fans have come on too late to get rid of the heat that was just created in the engine. So the temp will spike and drop off in a few minutes.

Does this make sense to anyone? I don't blame the HG's or the studs because you can keep it at WOT at any speed and the temp stays right on normal with no coolant loss or white smoke.

I think GM should have programmed the PCM to kick the fans on after a WOT blast immediately if the speed dropped below 50 MPH, or something similar.

03-23-09, 09:46 AM
Seems like it could be a possibility.
I would try to figure out how to run the fans 100% (hard wire?) and see if this solves your problem.

03-23-09, 12:33 PM
Jake, I think some service manual research will turn up that the fans never operate over 45 mph. They just aren't needed. After some pretty wild runs my 2002 will go up to 225 IF I come to less than 10 mph and stay there a while, but never to 3/4 gauge - never. There's a LOT of heat in the manifolds and crossunder pipe after a couple of 110 mph WOT's, and some of that heat radiates into the lower block sides, right at the lower water jacket area, so a short temp spike is entirely possible under your described conditions. If the fans come on at 225 and the engine cools, that's within the design specs. Heck, the temp cycles from 195 to 225 all day in traffic without an A/C mode turned on, so a bit of cooldown after a hard run is to be expected. Think of the wild temperatures reached by the valve train, crank, rods, and lower piston skirts, and the ONLY way to cool them is oil flow, which then goes to the cooler in the radiator tank, so at the end of a hard run that 225 degree oil is actually also heating the coolant.

03-23-09, 06:23 PM
yeah Sub you're right the engines are crammed into a tiny space, the older Olds small blocks had all kinds of room for the heat to radiate and escape. Actually my 403 boiled over after shutting the engine down right after a trailer-towing run on a hot day. That was the only time I had an issue with the older iron V8's- I needed a larger rad anyways for that engine. I used a rad from a 307 powered Delta 88.

03-24-09, 10:45 PM
try leaving the trans in 3rd gear when you come down in speed after a high seed run. This will keep the engine revs up slightly thus helping the waterpump do its job better. These pumps dont do well at low engine speeds. I think this is so the pumps dont cavitate at the 6700 rpm rev limit of the L37 N*.

My 98 wont move over the straight up position.


03-25-09, 07:43 AM
Thanks Hogg, maybe it's something about the cooling system that I'm missing. I've got a new rad in the car as well. Collapsing hose maybe? Doubtful I think. I haven't checked my fans in a while either- hopefully both are still working, not just one.

03-25-09, 10:44 AM
figure out a resistor ohm'age that'll bring up the temp to 225 sooner, as long as it's a linear resistor circuit, some electrical diagnostics w/the FSM should answer this question and a simple work around. Another thing to do would be to wire in a junction from the pedal assembly when you're at WOT (same signal as the switch that deactivates your A/C compressor if it's on) and kicks on the cooling fans, though as it's been said high speed isn't really where you want it. Or, just wire in a manual 'on' switch to the fan if you know you'll be doing harsh runs, this is the easiest/simplest...

03-25-09, 08:08 PM
Easiest way out of this one is to not sweat the small stuff. Every internal combustion engine on earth has a temperature spike upon slowdown after a hard run. Sort of the same effect as cooling down a horse after a sprint. You're trying to find/create problems where there aren't any.

03-25-09, 11:26 PM
sub said a few posts back that the oil entering the oil cooler in his radiator was (following high speed runs) heating up the coolant in the radiator which could cause a temperature spike... well would installing an external oil cooler and blocking off the oil lines entering the radiator prevent or at least lessen this problem?...

my father and i have been building up a 1974 chevelle with a 502 GMPP big block and recently we have been having coolant temperatures being a tad bit high... we decided to block off the transmission cooler area of the radiator and run the tranny fluid through its own independent cooler.. temps went from 210 215 to a rock steady 185. and this is iron block and aluminum heads.

03-25-09, 11:54 PM
Not hot oil entering the cooler in my radiator - any radiator. An overheated transmission has exactly the same effect. (The transmission temperature alarm which trips fans to fast is set at 305 degrees in a 4T80E !!!)

The lube oil cooler setup in a daily driver passenger car/sports sedan is in the radiator for a very good reason: to insure that the oil remains at about the optimum temperature for flow, lubrication and to be hot enough to vaporize acids and moisture in order to pass them off to the PCV system. An external oil cooler has no way of insuring constant temperatures unless you build in a thermostatic bypass at the filter adapter connections.

The temperature spike on cooldown after a hard run is of no consequence.

You could still run the transmission cooler in the radiator tank if you went to another row of coolant tubes or a larger radiator. I did some crazy work on my '65 Chevelle SS, but it had the old Muncie 4-speed, so no problems there.....