[many birds]

I’ve got a 69 firebird with a recently rebuilt relatively stock 1968 400 (only the cam has been changed) and auto trans. It has a 180 degree t-stat and a 15 inch fully shrouded electric fan and a 8 bolt water pump with cast impeller. During the rebuild process, I did the peening of the water pump plate. The electric fan is set to kick on at about 190. Radiator is stock and in good shape. Also, the heater core is connected and in good shape. Trans is th400 with a slightly above stock stall TC.

Problem is that the car tends to run warmer than I like. At idle, in park, it reaches about 190 And the electric fan is able maintain this. In drive, in moderate stop and go traffic, it hits 210 to 220. Once it reaches these temps, it takes a while for it to come back down even if traffic lightens up and I’m at highway speeds-the fan runs continuously. My carb is finally set to provide adequate fuel at idle, the timing is 14 initial. I have tried both ported advance and manifold vacuum advance. It runs cooler at low speeds with manifold vacuum advance. Temps reach 230 with ported at low speeds. Also, with the fan running, the under hood temp gets pretty high—suggests the fan is pulling a fair amount of heat? have verified the temps using a laser/IR temp gun pointed at the t-stat housing.

On a positive note: No evidence of detonation on plugs, yet.

Where should I start—change the electric fan back to a 19 inch flex or clutch fan? If so, will a mechanical fan cool better at idle than the electric? Or Is this a water pump issue? Or something else?

[steve25]

Once vehicle road speed gets above 25 miles per hour you do not need a fan if all is right as that's what fan clutches are for with the stock set up, and if you have a electric fan running at those times it's only blocking that 25 mph or greater amount of air from getting thru the rad and cooling things down.

Temps getting up to 230 with vacuum advance working leads me to think that your Cam is in retarded.
What is the cranking comp you read when you do a test?

[STEELCITYFIREBIRD]

Whats the ambient air temp? Is your electric fan moving alot of air?
How much vacuum advance? Did you set your idle mixture up in gear, engine well heat soaked?
External Trans Cooler?!?

Myself I'd go to the clutch fan setup and an external trans cooler.... if you don't have one.

[Schurkey]

Originally Posted by many birds

a 15 inch fully shrouded electric fan

It takes a REAL electric fan--and usually two--to equal an engine driven fan and fan clutch.

First Guess: Not enough fan(s).

Second Guess: Too much of the radiator is covered by a shroud that doesn't have movable louvers that close with the fan running at idle, but open at higher speed to increase airflow.

Of course, all the "usual suspects" are in play. Retarded timing, lean fuel mix, dragging brakes, plugged radiator cooling tubes, corroded radiator air fins, faulty thermostat, incorrect pulley ratio between crank and water pump, lack of OEM air dams (if used) and the typical list goes on and on.

[dmac]

Have your fan come on at lower temp as a first, easy step. Maybe that will be enough, and try a lower t-stat. Starting with a lower temp in traffic will give you more time before it gets hot . Maybe even go as low as 160 on the fan temp.

[dataway]

Are you running a stock water pump pulley?

Sounds to me like an rpm based water flow issue. If the fan can maintain temps in park at idle, but can't maintain temps in gear in traffic .... to me points to the water pump not moving enough water at low rpms.

What is the difference between idle in park rpm and idle in gear rpm?

[Schurkey]

GM doesn't turn on the electric fan until 210. Takes more than that to turn the second one on.

[My442]

Check your total timing (Vacuum and mechanical).

I chased a problem like this and found that weak springs in the distributor allowed to much advance,

Also, an aluminum radiator is significantly better than stock.

[70GS455]

Originally Posted by Schurkey

GM doesn't turn on the electric fan until 210. Takes more than that to turn the second one on.

That works well when the tstat is rated at 200 or so and the designed coolant operating point is 200 or more

Sent from my SM-T817V using Tapatalk

[racegto65]

Originally Posted by Schurkey

It takes a REAL electric fan--and usually two--to equal an engine driven fan and fan clutch.

First Guess: Not enough fan(s).

Second Guess: Too much of the radiator is covered by a shroud that doesn't have movable louvers that close with the fan running at idle, but open at higher speed to increase airflow.

Of course, all the "usual suspects" are in play. Retarded timing, lean fuel mix, dragging brakes, plugged radiator cooling tubes, corroded radiator air fins, faulty thermostat, incorrect pulley ratio between crank and water pump, lack of OEM air dams (if used) and the typical list goes on and on.

Agree with Schurkey. Fought this same problem with a friends '66 lemans. 2 small electric fans custom shroud and ran hot. Finally went with a Taurus fan due to space limits and helped tremendously until the car was totaled.

[many birds]

The cranking compression is 175 psi. No, unfortunately I did not “degree” the cam—it’s a Voodoo 703 installed dot to dot.

Ambient air temp has been anywhere from 85 to 98 degrees. Yes, the fan moves a lot of air—It looks like the flex a lite unit rated at 3000 cfm—it was installed by the PO. Vacuum advance is limited to 15 degrees. Idle mixture was set in park without the vacuum advance. No external trans cooler, just the radiator.

The shroud does not have moveable louvers. Timing is as previously mentioned—14 initial, 15 vac advance, and stock springs on hei distributor. Radiator is in good condition from what I can see. Not sure how check for clogging on the inside. Coolant is Dexcool orange stuff. All pulleys are stock. Upper air dams are in place. Only missing lower air dam/spoiler below the valence.

I tried setting the fan to lower temp. It didn’t help once the stop and go on uphill driving started. Lower t-stat is only going to buy a little extra time, probably not going solve the problem completely.

Idle speed in park varies between 800 to 900 depending on heat soak, etc. In drive drops to about 750 to 825 rpm. Difference is about 100 to 150 rpm.

On newer GM’s second fan kicks on at 230. But that’s on newer cars that are specifically designed to operate reliably and efficiently at these temps. They also run 195 t-stats. I’m not sure what the specs were for our older engines.

["QUICK-SILVER"]

You posted this in a different thread..

Originally Posted by many birds

I looked into the distributor and found the that it had been recurved by the previous owner. The advance springs were so puny that sometimes the rotor was saying a little stuck in the advanced position. It did have new weights and plastic bushings installed. I found some springs sitting in my random parts bucket that I got with the car (picture below). I’m guessing that the thickest/rustiest springs are the stock ones. I went with the middle ones even though they are a little mismatched. Timing now holds steady from about 1200 rpm down to 450ish. The engine starts severe stumbling below 500.

Timing holding steady from 450ish to 1200 tells us something is wrong. Should've picked up 5°~10° in that RPM spread. If timing doesn't go up at cruise the engine is going to run hot.

How far away is the fan and shroud from the radiator? Tight fit limits air flow to the fan opening. And only cools what's right in front of it.

Clay

[many birds]

Fan shroud is tight fit. Essentially sits on the radiator. I thought about that flow restriction also, I’m not sure how much difference it would make in stop and go traffic, however. Certainly might be an issue at steady speeds above 30 mph.

What rpm should should I be at when setting the initial timing? The way I did it was: set idle speed, Set initial timing. Then adjust mixture screws for best vacuum. Then hook up vacuum advance and finally set the set the idle speed again.

Should I go to a lighter advance spring? The current setup was just part of trouble shooting surging and poor idle quality.

[STEELCITYFIREBIRD]

“It didn’t help once the stop and go on uphill driving started.“

Now what would cause the extra heat under this condition ?
Converter?
Timing?
Load ??

[many birds]

Originally Posted by STEELCITYFIREBIRD

“It didn’t help once the stop and go on uphill driving started.“

Now what would cause the extra heat under this condition ?
Converter?
Timing?
Load ??

Could be all of the above. However, it’s a relatively light car with a 3.55 rear end. Didn’t sound like it was struggling. I’m going to mess with the timing again since it is the easiest and cheapest option. Converter is questionable. It looks stock, but the previous owner told me it’s was a 2000 stall—that’s sort arbitrary since stall speed depends on vehicle weight, axle ratio and engine torque output.

[mchell]

You would probably see an instant improvement with a 2 row 1 1/4 tube or greater aluminum radiator......I tinkered for far too long trying to improve my cooling, when my radiator took a dump I found that it was the source of all my cooling issues. Been using good aluminum ever since.

I know there are those that claim to run cool with with a stock radiator and 600hp engine with a/c blowing on a 98degree day in Rush hr NY city.....but I’ve had waaaaaaaaay better cooling with a good aluminum radiator..... fwiw

[ta man]

Take your rad cap off is the dexcool leaving a orange scale on the inside..if it is you need your rad cleaned. That stuff is nasty!

[Navy Horn 16]

Typically if it's hot in traffic, you have airflow issues. If it's hot at cruise, you have water flow issues.

When I went from a mild 400 to a wild 469, I had to upgrade the radiator. It was a stock unit left over from when the car was an esprit, so it was probably designed to keep a tiny motor cool. Not good enough anymore. I replaced it with a drop in Cold Case radiator, and it's been fine ever since. 160 on the highway even in Texas summers. It may hit 200 in traffic. NBD.

[Cliff R]

What pistons were used for the relatively "stock" rebuild? How far were they down in the hoes at TDC? What is the quench distance?......Cliff

[Formulas]

As stated in post #13 fan shroud is a tight fit, take a pic and post probably be obvious what's going on as you seem to have most things right

If your fan shroud sits virtually against the radiator on the back side and just has fan holes its a huge restriction

Stop and go speeds the stock fan is pulling air through the whole radiator area not just a 15 inch hole

.