Not Your Average Overheat Question

[mosel70]

'70 C10 396, TH400 3-spd trans, mild cam, 454 heads, timing 36 total
Overheats at low speeds or at stops. If I go straight to highway at 65 it is fine. If it overheats at low speeds and then I hit the highway, it won't cool down, stays in the red. With rad cap off, I see good flow in radiator. With engine warm, in gear at a stop is 700 rpm, but in park it goes to 1800-2000 rpm.

Here's my question: With temp in the red, temps on both ends of lower AND upper rad hoses show 210-213 degrees. No 10-20 degree difference. Why?

New:
Mishimoto 3-core rad
Mishimoto twin puller e-fans, 2600 CFM; 1x 1100 cfm pusher fan
2x gauges
2x temp sensors
rad hoses
2x water pumps (clockwise looking from front bumper), FlowKooler
trans cooler inside rad
Edelbrock 750 carb, mixture screws turned 1 1/2 turns out
Edelbrock RPM Airgap intake manifold
HEI distributor
Hooker headers
Rear gear ratio 3.73

[Steeveedee]

Does it puke coolant when it overheats, or do you just see a high temp on the gauge? Could be a bad transducer (sender) or temp gauge. Nice looking under the hood, BTW.

[Overdriven]

Couple things I'm concerned with, and/or need verification about. Engine rpm when warm is 700rpm in gear but 1800-2000rpm out of gear? I've never heard of carb issues causing overheating but something is off there.

The other is you say there's 2 water pumps? Obviously I see the block mounted one but there's another?

[B. W.]

If your top & bottom hoses are reading the same temp then the radiator is not getting rid of the heat (I know, that's obvious)

Possible causes:
radiator too small
not enough air flow (fan shroud(s)
no thermostat (coolant is flowing too fast)

or any combination of the above

[HO455]

Just so I am clear. You have two fans pulling air through the radiator? And an additional fan pushing air? If so the first thing I would do is remove the pusher fan. My experience with pusher fans is they only work as a single unit covering a such of the radiator as possible. They are always a poor option to be used only if no other fan will configuration will work as the blades will deflect as much air as they push through.

[kwmech]

I have never had any luck what-so-ever with electric fans. Just about tried them all. Last go-around was with my tow truck. Gen6, 454, nothing fancy single 3'' exhaust with iron manifolds, HEI and a Q-jet. Dual electric top of the line fan assy, self contained shroud. Always tried to overheat when Idling. I put a stock fan shroud on with a 7 blade fan and a fan clutch. Never had a problem since.

And as stated before, if you have the same temp at both hoses the radiator is not doing its job

[toolboxchev]

A radiator can look all fine and dandy by opening the cap and looking inside. Color of the coolant can even look great yet when you drain it from the bottom is will be a nasty brown color.

How do I know this! Been there. Most all coolant flush products are not that good at removing stuck on corrosion inside the radiator. Look at the ingredients of Prestone Super Flush. Sodium Citrate.

Everyone says Aluminum in Radiators are great products. With the old Copper and Brass Radiators you can use products like Sodium Hydroxide, Oxalic Acid to flush your system with no harm to the internals on the block. They must be neutralized.

Back flush the entire system and pull your drain plugs on the engine block when you do.


A lean setting on the carb will make the motor run hot also. I had this and a couple other issues like a cracked manifold, and other problems too.

[dave6672]

As toolboxchev just said, I would make the carb is not too lean. I once spent s lot of time working on my truck's cooing system and the whole problem ended up being that the carb was too lean.
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[67 chevelle]

Originally Posted by HO455

Just so I am clear. You have two fans pulling air through the radiator? And an additional fan pushing air? If so the first thing I would do is remove the pusher fan. My experience with pusher fans is they only work as a single unit covering a such of the radiator as possible. They are always a poor option to be used only if no other fan will configuration will work as the blades will deflect as much air as they push through.

Agree ,, get rid of the pusher ,,, sounds like you have a "lean issue" , too much timing can also cause excess heat

[joeydurango]

Idle in park is way high... mine idles in gear (warmed up) at 600rpm, then at about 800rpm in park. Don't think this is a cause of your overheating issue, but definitely seems odd, as Overdriven mentioned above.

36 total timing seems a little high as well. Have you tried it back down at around 32?

Finally, I agree with Kwmech... I've used electric fans and, while mine did work decently, that was with a big, non-stock radiator. On my Super I'm using a six-blade, stock-size fan with a clutch, and a proper shroud, zero issues.

That is a nice looking engine bay.

[70STOVEBOLT]

You mention the idle mixture screws being turned out 1.5 turns but how is the carb calibrated? Is it the "out of the box" tune or did you calibrate it with a jet/rod kit? Sounds like it's a lean carb problem.

[demian5]

You guys have hit everything that could possibly be. Let's see what he finds.

[BossHogg69]

You may have 36* total timing, but what do you have your initial timing at? What about vacuum advance? Is it hooked up and where?

What year are the 454 heads? What head gasket? Some issues may be there if using a Mark IV block (series flow of the coolant to the heads, which 396 is) with later Gen V/VI heads (parallel flow of coolant to heads) without the proper gasket.

[Shinysideup]

It idles @ 1800-2000? Holy timing batman! That will run hot.

Thats a great looking engine bay.

[Andy4639]

The timing and idle are way off. You need to set the carb air screws to as much vacuum as possible at idle with a nice smooth idle also around 650 -800 RPM's.
The base timing should be set with vacuum advance disconnected and plugged at both ends, the carb end should have your vacuum gauge hooked to it so you can set them to max. This will require some playing with the screws. Keep both screws to the same amount of turns. You will have to set the idle screw also as you do this to keep it the same.

[tdangle]

Also check the choke make sure it's coming off of high idle. 1800 - 2000 sounds about right for the high idle

[Andy4639]

Originally Posted by tdangle

Also check the choke make sure it's coming off of high idle. 1800 - 2000 sounds about right for the high idle

Good point the choke should not be on when doing all this. Normal operating temp when setting up the carb.
Also make sure you have the advance hooked to manifold vacuum which is done by placing your finger over the inlet and it should tug real good.

[mosel70]

Really good replies, thanks to everyone.,

I only have one water pump, a FlowKooler.
https://www.summitracing.com/parts/BRA-1671
I took the front 1100 CFM pusher fan off. Now only 1300 Mishimoto twin puller fans (2600 CFM total) on. Carb mix screws 1 1/2 turns out, it came new out of the box at 2 1/2 turns out. Edelbrock said that's about right. No vacuum leaks anywhere. It starts bleeding coolant from rad to overflow tank when super hot. 19lb rad cap. Electric choke working fine.

So, my son-in-law came over and got me to stop wanting 800 RPM at idle just because it feels smoother. He said I've got to live with the slight rumble from that cam. Now it idles at about 1200 in park, and 550 in gear at a stop. Still too much of a drop and not sure why.

Motor nearly overheats at a stop but much better than deep red on the gauge needle.

I've ordered two of Derale's 2100 CFM 14" fans (4200 total)
https://www.summitracing.com/parts/DER-18214
with their 16795 PWM Fan Controller
https://www.summitracing.com/parts/der-16795
They should fit my Mishimoto fan shroud perfectly and give me a tight seal.

I'll let you know if it fixes the overheat. I don't like it when guys ask for help, get lots of answers, but never tell you what actually worked. You, too? Ha!

[geezer#99]

Originally Posted by mosel70

Really good replies, thanks to everyone.,

Now it idles at about 1200 in park, and 550 in gear at a stop. Still too much of a drop and not sure why.



a!

It does that because of a lack of enough initial timing to idle at 750 in park and 600 in gear.
To compensate at present you likely have the curb idle screwed in enough to let it idle like it does.
Two problems arise from that. Retarded timing makes it run hot. Increased primary throttle opening has it idling on the power circuit.

A test to check for too much throttle opening is to screw in either mixture screw and see if it quits.

What's your timing curve like?
How much initial?
How much mechanical?

[Andy4639]

2500 bare min on cfm fan for a big block! 2 fans @4200 means they both have to be on all the time. One fan is only 2100 CFM This isn't enough to keep the BB cool!

The turns on the carb are starting points not the rule. The idle in park you need to adjust them to the max vacuum period! No question about it. It's the only way to do it. 2 turns maybe right for your setup but until you do the adjustment trust me it needs to be checked. Check out their video on it.

Cooling. Rule of thumb!

1,250 CFM for a 4-cylinder
2,000 CFM for a 6-cylinder
2,500 CFM for a 8-cylinder
Note: Big horse power means big fans!

You Tube

[mosel70]

Thanks for last two replies today.

I just turned the mixture screws in all the way one at a time from 1 1/2 turns and it decreased RPM from 850 to 550 but did not stall engine.

I can't check timing right now because when I installed a new water pump I also installed a new harmonic balancer and adjustable timing pointer. So I need to find TDC again to get the pointer right and will then check timing again.
https://www.summitracing.com/parts/SUM-164700 (timing pointer)

One thing of note is my vacuum advance is connected to port (passenger) vacuum on my Edelbrock 750 carb.

Thanks again!

[geezer#99]

That should be your ported vac port. Should be no vacuum at a normal idle (700 rpm).
Your high idle likely shows some vacuum.
You'll need to use a piston stop to find true tdc #1 to set your pointer.
Info here on that.
http://www.crankshaftcoalition.com/w...op_dead_center
You can make a piston stop from an old sparkplug or buy one.

[Andy4639]

I just finished doing this to my 64 SS Impala. It has ran rough since I bought it but I lived with it like you until last weekend. I bought a new cap and rotor and installed. I found 1 bad wire and replaced it also. I then started messing with the carburetor and found the vacuum advance was hooked to the ported side passenger side of the carb so I swapped them. I then hooked the vacuum gauge up and fine tuned the 2 idle air screws to get max vacuum and it's running great now.
I do need to buy a new set of plug wires and plugs for it though but for now it's fine.

[mosel70]

Hmmm, manifold vice port vacuum. I see alot of people talking about that online.

This week I'll put a t-fitting on my manifold line which I have a vacuum line running to my TH400 trans and cap off the port vacuum port on the carb and I'll let you know what happens. It would be really cool if that helps my idle issues and especially overheat issues at idle. Thanks!

[HO455]

The vacuum advance line on the distributor needs to be hooked to a ported source. There are several reasons for this. It allows the advance to return to the same location everytime you let off of the throttle allowing the idle speed to be more consistent. As engine load affects vacuum and you don't want the that affecting your idle quality.
When the engine is at cruise speed on the road and you let up on the throttle manifold vacuum spikes high with a non ported source the ignition timing maxes out while the engine is experiencing a lean condition making conditions ripe for preignition detonation. With a ported source the timing is reduced when the throttle closes. This is why virtually every auto manufacturer during the carbureted era used a ported vacuum source.