1979 401 cooling issue

[AZChief]

I have a 1979 Chief with a transplanted 401, all stock. Here's the issue: Running the Chief on the highway at 50mph or faster, and the temp gauge on the cluster reads at the 3/4 mark with the underhood aftermarket gauge reading 195-200 degrees. Once I turn on the A/C, the temp immediately shoots up, the interior gauge is almost pegged, and the underhood aftermarket gauge reading around 230 degrees! Turn the A/C off, and the temp drops back down again.
Get off the highway and run at 40mph or slower, with stoplights, and the temp stays at the 3/4 mark or around 195-200 degrees, with the A/C on OR off. Go figure.
I replaced pretty much the entire cooling system, as I live in AZ and purchased the Chief not too long ago. It has a re-cored 3-core radiator, new water pump, new thermostat and waterneck, new radiator cap, new hoses, and fresh fluid with a good mix of anti-freeze. The fan is not damaged and sits in the shroud at the right depth.
Any ideas as to why it cools ok at lower speeds, but at higher speeds with the nice ram air, it gets really hot?

[440sixpack]

I would suspect the radiator. how long ago was it recored ?


It's possible your bottom hose is collapsing . especially if your radiator flow is impaired. I've never seen it on a Jeep but it can be an issue. that's why many hoses have the spring in them.

[AZChief]

The radiator was re-cored about a month ago, when everything else was replaced. The bottom hose is new and also has the spring in it, I made sure of that.

[rang-a-stang]

Maybe you are running lean and the engine is really making that much more heat that you can't cool it?

[AZChief]

That thought crossed my mind, didnt think it would make that much of a difference, but Im going to check that and the timing tomorrow.
Any other ideas from anyone, please chime in!

[FSJunkie]

It's probably lean and/or timed poorly.

It's also a 401 in Arizona. One of the many reasons I like 360's.

[gsmikie]

bad fan clutch if no fan clutch bad pitch on the fan blades if it is an aftermarket fan the blades are flattening out at higher rpm and slowing air flow

[AZChief]

FSJunkie, There's not much difference in cubes between a 360 and a 401, are you saying the 401's run hotter?

[yossarian19]

He is - I have no firsthand knowledge but have heard people say that 401's run hotter than 360's.

I don't think your problem is the fan, both because it isn't doing anything at highway speed and you don't overheat in town.

Put a light behind the radiator and look through it and the condensor from the grill. You want to see nice, clear light coming through the fins. If it's jammed with bugs or the fins are bent, fix as needed.

With the engine at idle, keep well clear of the fan blades (seriously) and snap the throttle open for a few seconds. Watch the lower radiator hose. Did it collapse from suction? If so, get a new one with a spring if possible but a new one anyway.

It's always possible the radiator has something wrong with it, rebuilt or not, but I'd be looking at air flow through the condensor & coolant through the lower hose first.

[440sixpack]

401's will run hotter than 360's just like 400 chevy's run hotter than a 350. it's the thin cylinder walls. that said you shouldn't have problems unless you're towing.


Fuel and timing will effect engine temps some but they effect EGT's mainly. if your problem is as severe as it sounds it's cooling system related.


Is it blowing water out ? just wondering about a compression leak.


I still suspect the radiator. seldom have I seen a cooling problem that a radiator didn't cure.

[tgreese]

Has the 401 been rebuilt? As mentioned, the cylinder walls are thin. The larger 401 bore combined with the existing bore spacing requires that the cylinder walls be thinner than the 304 or 360. These blocks are spec'd for 20-thou overbore maximum, as I recall. If the cylinder wall is too thin next to the cooling passages, heat from combustion travels quickly into the cooling water instead of being carried away with the exhaust gases. If you search, you'll find many instances of 401s bored say, 60-over with hot spots in the bores that made them overheat terribly.

[AZChief]

The 401 has not been rebuilt, it has roughly 120K on it, and runs just fine. Fires right up without skipping a beat! I will try checking the lower radiator hose just to be sure, although it is new as well. Im going to check on the passages through the fins of the radiator and condenser, as well as checking the timing and fuel mixture, and also the compression, as suggested.
Thank you all for the suggestions. It may be a week or so before I can get to everything, but I will touch back here afterwards to report on the findings.
Thanks again!

[Ristow]

No mention of a new fan clutch yet. Put a fan clutch on.

[wiley-moeracing]

they make a severe duty/police version for the fan clutch, use that instead of the standard fan clutch.

[AZChief]

It does not have a fan clutch on it, I assumed it didn't come with one because it really doesn't look like one was ever there. It goes from the pulley, to the spacer, to the fan. And the fan is set into the shroud just perfectly. It's a '79, we're they all coming with clutch fans already at that point?
I'm just not sure the fan is the issue, because it cools just fine (195) at lower speeds and at stops, when the cooling system is depending on the fan. It only gets really hot on the highway when I turn on the a/c. In these situations, it's being cooled by ram air, not the fan.