Getting Hot on the Freeway

[Mick Je]

Ok, my 1970 GMC has now had all the different cooling ways you can do: The stock clutch fan, Flex Fan, and now electric fans. My truck has a mild 402 .30 big block. My trouble is with the electric fans, everything is good around town, but once I start going over 55mph (spinning 2200 rpm) with A/C on or not on the fwy, the truck eventually starts getting in the 220-230 range and that's way too hot!!! But if I get stuck in LA traffic it cools and runs good! Before I got these fans I did a lot of research comparing other brands and I got the Derale Dual High Output fans (4,000 cfm) with their high output controller. In the future I would like to add a gear vender to lower the rpms. So far I changed thermostats from 180 to 160, then I took the thermostat off and currently trying water outlet restrictors. I was wondering if any of you guys have ideas that might help.

Thanks

These are the fans I have.

[geezer#99]

You need an air dam under the rad. You need to create a low pressure area behind and below the rad so that the high pressure air can flow thru the rad and down into that low pressure area.

[volksworld]

this is an incredibly stupid question but are they spinning in the right direction?

[airdale94]

I would try removing those covers on the shroud (above and below each fan). It sounds like you're not getting enough airflow at highway speed. The 4th Gen F-bodies have a dual fan setup that has flapper doors on the shroud that open up at higher speed and allow more airflow. Might check your A/C condenser also. Blow it out with air and or water.

[67ChevyRedneck]

Something is wrong with your cooling system. Your vehicle should not need a fan of any kind after about 45 mph, even on a 120 degree day.

Put the thermostat back in. If the water is just constantly circulating it will never have a chance to cool.

I would look into a leaking head gasket, plugged radiator, or other issue.

What size radiator do you have? With a 402 and A/C you should be running a factory 4 row at a minimum.

Originally Posted by airdale94

I would try removing those covers on the shroud (above and below each fan). It sounds like you're not getting enough airflow at highway speed. The 4th Gen F-bodies have a dual fan setup that has flapper doors on the shroud that open up at higher speed and allow more airflow.

I was thinking the same thing as well.

[Lee H]

I agree, at speed the air is not adequately moving through the radiator. I think that flat shroud is causing a great deal of turbulence limiting the flow through the radiator. Is there any gap between the shroud and radiator? Air, like water, follows a path of least resistance.

[nzc10]

I agree with 67chevyredneck sounds like a blocked radiator my girlfriends car used to do the same thing on the highway I removed the radiator and took it to the radiator shop who flushed it out found out it was 60% blocked no problems after that

[LUCKTRUCK]

perhaps timing or air fuel mixture?

http://www.automotiveu.com/SolvingOverheating.htm

[Mick Je]

Before I put the electric fans on
-I put on a Weiand Water Pump
-I had to re-core the radiator and its a repro bass stock big block one
-New Water hoses and coolant

And when I got the fans I took those flappers off. The fans are designed for puller applications and fits my radiator coils perfectly with a 2" gape between the fan and coils.

And it starts getting hot over time going 55 mph and over.

[davepl]

The factory cooling system for the big block is more than sufficient.

The factory fan and shroud will move a lot more air than the box you pictured (unless those are some serious $400-per fans or something).

Remove random pieces of the factory setup (like the shroud) and all bets are off.

How's your vacuum advance?

[Mick Je]

I do have a engine oil cooler and trans oil cooler in front of the condenser and radiator.

[marcus.boldt]

If it was me, I would take it out on the highway and let it start heating up, then find a safe spot to work on it, and remove the fans and the housing. Jump back on the highway and see if it's worse at 55 or if it starts cooling.
Personally I think at highway speeds, those fans are just in the way.

[Oldtruckfanatic]

Before any of that id definitely verify the fans are spinning the right direction just turn them on and hold some tissue in front of the radiator. If it pulls the tissue toward the radiator its correct. If its blows it away fix the problem. All of the above are posts are very good advice. The fan direction is just easy to check first.

[HalfCaseKustoms]

I have a stock set up on my 350 and having the same problem. Fine around town in any temp. Over heats on the highway

[airdale94]

Might check timing like LUCKTRUCK said: Also make sure your waterpump bypass/heater core(hot) is plumbed, both you and HalffCaseKustoms. I know some don't use them on both big blocks and small blocks. But I have a 283CI and I had 76CC 350 heads on it and I put a shutoff valve on the heatercore feed. No problems. Swapped to some HO 305 heads and I would overheat on the highway. Opened up the shutoff valve and overheating went away.

[67ChevyRedneck]

Also check for a vacuum leak.

I just remembered a couple months ago I needed a longer vacuum hose immediately for a trip in my mustang (was literally about to leave for the Hot Rod Power Tour) and I stole it from the C10's vacuum advance. I then forgot I did that. Truck ran kinda funny, but temp was OK at idle, but was getting up to 215-220 on interstate. Scratched my head a bit, then remembered the missing hose. Put it back on and truck was fine... so could be a vacuum leak too (or timing as already mentioned).

[truckdude239]

Lower hose colposing under load

[slomotion]

Do you know for sure your gauge is accurate?

[Acorn]

X2 or 3 on checking your timing and advance. It never hurts to check it anyway.
Start with some basic troubleshooting. And don't get such tunnel vision that you miss something simple.
When did this problem first start? Did it cool fine with stock fan? Try driving on the freeway but drive slower. Like 500 -1000 rpm slower and see if that makes a differance.
Everyone here is giving good advice. (as they always do!) It's bound to be something simple. Maybe take a brake, clear your mind and start over when your fresh.

[Mick Je]

Well just found out that my radiator core is shot, which I thought that was all good, but that should be the problem. Thanks for all the ideas!!! Getting a new brass core tomorrow!

[Tom Vogel]

I thought you already re-cored the radiator. ??
PS how about a link to see more pics of your truck?

[special-K]

This was obviously a coolant flow issue and first thoughts go to radiator. I saw it was cored and was stumped. It would have had to been something else restricting (or not giving...water pump) flow. Thoughts kept going back to question radiator work. I'm not surprised. Now just baffled at what wasn't done that was said to be done?

[67 cst swb]

I will be interested to see whether a new radiator fully fixes your problem.
All that aluminum looks pretty holding those fans, but its acting like a stop sign for the air at highway speeds. (especially if your fans are spinning backwards)

[Classic Heartbeat]

I will be interested to see whether a new radiator fully fixes your problem.
All that aluminum looks pretty holding those fans, but its acting like a stop sign for the air at highway speeds.

I think he found his problem with the plugged radiator. The shroud does not stop enough air to create a problem. Enough air still passes through the fans even if not turned on. I would be interested to know where the thermostat is located for the fan system. WES

[67 cst swb]

Originally Posted by Mick Je

Before I put the electric fans on
-I put on a Weiand Water Pump
-I had to re-core the radiator and its a repro bass stock big block one
-New Water hoses and coolant

My thought is...
He either got ripped off on the re-core job or is being told that his current radiator is shot and needs a new radiator now, when this one is actually fine.
Unless of coarse, he used a Hi-Volume water pump also and everything is just passing through the system too fast and not getting a chance to cool.

I will just be interested to see the outcome.
It will be great if it solves the problem, as I'm sure he's tired of this issue.
.