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.

[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

[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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[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]

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.

[Andy4639]

Vacuum is constant at idle. No you don't want ported vacuum you want manifold. Once your up to around 3000RPM's the vacuum advance is where it is and want change any. At wide open speed you have no vacuum.
Hook it to the manifold vacuum.

This is the article I have and read. It's long but has all the info you need to make the right decision.
This was written by a former GM engineer as a response to a similar question on a Corvette board:


As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.