76 340 duster overheating at idle

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Matthew Sermini

Matthew Sermini

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Hey everyone, this is a follow up thread from about a year or more back about my 76 duster overheating at idle. I’ve been busy so haven’t gotten around to my duster until recently. I finally got a temp gun and used it today to check out different spots on the engine bay while the engine was running/ shut off.

I have a picture of my engine bay setup right now and I know I need a fan shroud minimum. I’m prepared to redo the cooling system with a radiator and shroud from Mancini but I haven’t called them yet to order anything because I want to make sure I can get a radiator that will fit/ reccomendations from you guys.

Car was original /6 w ac. Currently has a 22” 3 row aluminum radiator (I don’t remember the brand). I’ve been told a 26” is the way to go but my core support opening is 22". I had bought a rock auto 26" 2 row open style radiator years ago and i remember it not fitting with the mounting locations. It was also probably junky anyways. I’d prefer an oem look for my car or period correct so I’d prefer to not put in electric fans or another aluminum radiator and shroud if possible. I know im asking a lot but that’s what I’m going for thanks.

I also just installed a new fan clutch from summit, on my Oem 7 blade fan and that has definitely helped slow the overheating issue but it is still present.
180° or 190° thermostat. I don’t remember exactly but was also planning on getting a 180° high flow. Timing might need to get backed off but I haven’t put the gun to it since I built the car and am planning on getting the engine tuned once I know the cooling system works as it should.

Temp numbers after driving around this morning then idling at my house for about 10 minutes until the temp gauge was creeping up

Upper hose: 170.9°
Lower hose: 171.1°
Thermostat housing: 207.6°

A minute or two after shutoff
Upper hose:186.9°
Lower hose: 182.4°
Thermostat housing: 221.7°
Center of intake: 241.3°

IMG_5847.jpeg


IMG_5846.jpeg
 
j par said:
How about driving it.. does it cool off once you get it in the wind?...
Click to expand...
Yes. Never had heat issues while driving, just at idle in heavier traffic. Also I stopped at the store for like 15 minutes and had to hold the pedal to the floor to get it to fire back up. Zero leaks and no white/black smoke when starting ever.
 
Matthew Sermini said:
Hey everyone, this is a follow up thread from about a year or more back about my 76 duster overheating at idle. I’ve been busy so haven’t gotten around to my duster until recently. I finally got a temp gun and used it today to check out different spots on the engine bay while the engine was running/ shut off.

I have a picture of my engine bay setup right now and I know I need a fan shroud minimum. I’m prepared to redo the cooling system with a radiator and shroud from Mancini but I haven’t called them yet to order anything because I want to make sure I can get a radiator that will fit/ reccomendations from you guys.

Car was original /6 w ac. Currently has a 22” 3 row aluminum radiator (I don’t remember the brand). I’ve been told a 26” is the way to go but my core support opening is 22". I had bought a rock auto 26" 2 row open style radiator years ago and i remember it not fitting with the mounting locations. It was also probably junky anyways. I’d prefer an oem look for my car or period correct so I’d prefer to not put in electric fans or another aluminum radiator and shroud if possible. I know im asking a lot but that’s what I’m going for thanks.

I also just installed a new fan clutch from summit, on my Oem 7 blade fan and that has definitely helped slow the overheating issue but it is still present.
180° or 190° thermostat. I don’t remember exactly but was also planning on getting a 180° high flow. Timing might need to get backed off but I haven’t put the gun to it since I built the car and am planning on getting the engine tuned once I know the cooling system works as it should.

Temp numbers after driving around this morning then idling at my house for about 10 minutes until the temp gauge was creeping up

Upper hose: 170.9°
Lower hose: 171.1°
Thermostat housing: 207.6°

A minute or two after shutoff
Upper hose:186.9°
Lower hose: 182.4°
Thermostat housing: 221.7°
Center of intake: 241.3°

View attachment 1716278531

View attachment 1716278532
Click to expand...
Temps look good to me. I don't know why you are measuring the temperature at the middle of the intake manifold by the head, that will of course be hotter due to the exhaust passage. Put a shroud on the radiator and fix your temperature gauge.
 
canyncarvr said:
That is a heat soak issue. Different from simply 'running hot'. Resolving that problem is a good bit different.
Click to expand...
Any idea on the causes or solutions? A couple months back I drove to the store, stopped and got gas, drove a minute down the road and hit a stop light. Took off then the engine died and took a couple minutes to start back up. I figured it was from the cold gas going into the hot engine. I have an edelbrock avs carb
 
Matthew Sermini said:
Any idea on the causes or solutions? A couple months back I drove to the store, stopped and got gas, drove a minute down the road and hit a stop light. Took off then the engine died and took a couple minutes to start back up. I figured it was from the cold gas going into the hot engine. I have an edelbrock avs carb
Click to expand...

The problem is unlikely cold fuel going into a hot engine. I have proved that to myself by using a cool can. Unfortunately, using a cool can as a necessity in every day driving is way too much of a hassle.

Resolving a heat soak problem will involve keeping as much heat AWAY from the carb (and away from fuel going INTO the carb) as possible. Ceramic coated headers as an example. Running fuel lines as far away from heat as possible. Running a fuel return line regulator (like an injected system). Shielding the carb from heat (heat shields, phenolic/bakelite/wood spacers). Blocking manifold heat.

The #1 problem with fuel boiling in the carb is simply the fuel we have available to use. It's mostly garbage. Vapor pressures are high so the fuel ignites easier. That's fine when it's -20º. When it's 115º? ...not so good.

As an overall rule of thumb, DO try to NOT run a winter blended fuel on a warm/hot day. Of course...you don't know if you are pumping a winter blend or not. You can talk to your local distributor. They know...and will usually tell you when the switches (from winter to summer/from summer to winter) happen.

If you live in a warm climate, such blend changes may not take place.
 
remytherat said:
A fan shroud goes a long way…
I bet that and messing with the timing at idle will solve the issue
Click to expand...
Agreed.
I had a similar problem with mine. Even with the aftermarket radiator, my 360 would slowly overheat in heavy traffic or standing still. It was only going at 30 mph+ that it would start cooling down.

My 360 has 10.2 compression with a 750 Street Demon carb (with the phenolic fuel bowl) on top of an Edel Air-gap intake. So I had goodies to help cool the fuel down.

The low speed/idle slow overheating issue was resolved once I put a fan shroud on.
 
Last edited:
FYI:
I run a stinking ancient, factory 318 rad circa 1973>75, in the factory slanty core support, with a 400plus hp 360 which will pull itself around all day at 550rpm and with 5 degrees of advance.
The rad in your pic will be fine, AFTER, you do the following;
1) install a shroud, and
2) Restrict the bypass hose,
3) move the clutch up closer to the rad, and
4) install a hi-flow pump, or at least an anti-cav. plate on the current one, and
Probably optional for you are;
5) If you have to, install a Thermostatic clutch or a direct-drive fan and
6) reduce the size of the w/p pully closer to the crankpully or slightly smaller than

Opinions;
1) A bigger rad, in your configuration, will NOT solve your problem.
2) After the system is functioning correctly, you can try a thermostatic clutch... but not a chance would I install a standard viscous-drive clutch, on an engine with a history of running hot..
3) as you have found out, measuring the hose temperature is meaningless.
4) IMO, This is an easy fix, and you will not need a bigger rad.

What you are actually looking for is a DIFFERENCE in temperature of close to 30 degrees, (minimum 20*) between the inlet side of the pump, and the outlet side of the stat house. Your system can't do it because;
1) not all the air that the fan is pulling, is coming thru the rad. Without the shroud, the air is just sneaking up from below, or simply recirculating the hot underhood air. The pump pulls whatever is easiest.
2) The pump is recirculating hot water from the bypass hose
3) the pump may be internally bypassing water around the open-sided vanes, and or, may only have a 6-blade impeller. and
4) if your pump-pully is BIGGER than the crank pulley, then it will be turning at fewer rpm than the crank. Most guys will tell you that this is a no-no. Whereas, I maintain that, with a pump that has the anti-cavitation 8 blade impeller, AND with a restricted bypass hose, I'd leave that change for last.

Other
>I would probably reroute that fuel line back to where it belongs, NOT running above the heater hoses and header. At idle, your header-pipes may run in the 400s, whereas the intake at 200 or so. Your IR gun cannot measure air-temp, lol.
>How close is your battery negative cable to getting into the fan?
> your KD spring looks a lil wonky to me.

Happy HotRodding
 
Matthew Sermini said:
Hey everyone, this is a follow up thread from about a year or more back about my 76 duster overheating at idle. I’ve been busy so haven’t gotten around to my duster until recently. I finally got a temp gun and used it today to check out different spots on the engine bay while the engine was running/ shut off.

I have a picture of my engine bay setup right now and I know I need a fan shroud minimum. I’m prepared to redo the cooling system with a radiator and shroud from Mancini but I haven’t called them yet to order anything because I want to make sure I can get a radiator that will fit/ reccomendations from you guys.

Car was original /6 w ac. Currently has a 22” 3 row aluminum radiator (I don’t remember the brand). I’ve been told a 26” is the way to go but my core support opening is 22". I had bought a rock auto 26" 2 row open style radiator years ago and i remember it not fitting with the mounting locations. It was also probably junky anyways. I’d prefer an oem look for my car or period correct so I’d prefer to not put in electric fans or another aluminum radiator and shroud if possible. I know im asking a lot but that’s what I’m going for thanks.

I also just installed a new fan clutch from summit, on my Oem 7 blade fan and that has definitely helped slow the overheating issue but it is still present.
180° or 190° thermostat. I don’t remember exactly but was also planning on getting a 180° high flow. Timing might need to get backed off but I haven’t put the gun to it since I built the car and am planning on getting the engine tuned once I know the cooling system works as it should.

Temp numbers after driving around this morning then idling at my house for about 10 minutes until the temp gauge was creeping up

Upper hose: 170.9°
Lower hose: 171.1°
Thermostat housing: 207.6°

A minute or two after shutoff
Upper hose:186.9°
Lower hose: 182.4°
Thermostat housing: 221.7°
Center of intake: 241.3°

View attachment 1716278531

View attachment 1716278532
Click to expand...
Is it puking coolant? If not, it isn't overheating. Center of intake? That's the heat riser passage and hot exhaust flows through there.
 
When you get a shroud if you have a gap between it and the radiator seal it with some foam weather strip. Doing this made a big difference on the class 7/8 trucks and construction equipment I used to work on. Also keep an eye on how far your fan is in the shroud.
 
We took my car in this afternoon. With the vacuum advance blocked off engine file was at 0° so we bumped it up to 10° at idle. I’m gonna run it and see how the car is affected. I’m still gonna work on the shroud. I know it’s gonna help a lot
 
i'd start with a shroud.. fan has no chance at pulling much through the radiater. personally i'd find the proper v-8 copper radiator for it too..
 
Only one degree difference between the top and bottom hose. You need to fire that radiator.
 
RustyRatRod said:
Only one degree difference between the top and bottom hose. You need to fire that radiator.
Click to expand...
And quit measuring the temp of the heat riser passage. Like I asked, is if puking? If not it isn't "overheating".
 
Matthew Sermini said:
We took my car in this afternoon. With the vacuum advance blocked off engine file was at 0° so we bumped it up to 10° at idle. I’m gonna run it and see how the car is affected. I’m still gonna work on the shroud. I know it’s gonna help a lot
Click to expand...

If it was timed at TDC, if I was your engine I'd be puking my guts out at idle too. :)

Stock mopar SB engines tend to like 10-14 BTDC. When you have the timing set near TDC, there is a lot of unburnt fuel headed out into the exhaust port and manifold/headers where it finishes the combustion process. Folks that break in an engine and see glowing headers know all about this. Retarded timing.

Nice job j par with the question right out of the gate!
 
Hard to see from the pics, but looks like a BIG water pump pulley. WP pulley needs to be 10-20% smaller than the crank pulley; that speeds up water flow AND fan speed. Unless the pump pulley is smaller, you will be wasting time & resources trying to fix the problem.
If engine is stock, set initial timing to 15*.

More info here: www.stewartcomponents.com
 
Bewy said:
Hard to see from the pics, but looks like a BIG water pump pulley. WP pulley needs to be 10-20% smaller than the crank pulley; that speeds up water flow AND fan speed. Unless the pump pulley is smaller, you will be wasting time & resources trying to fix the problem.
If engine is stock, set initial timing to 15*.

More info here: www.stewartcomponents.com
Click to expand...

like ya said its hard to see but that pulley looks to be the proper 70-up small block non a/c pulley.

looks to be in the proper area anyway...

1721558623785.png
 
For the stalling,

I'd point that temp gun at the bottom of the fuel pump. Once mine hits 120 I start seeing bubbles at the carb.

I wedged a piece of cardboard in front of the fuel pump and the bubbles disappeared. Worth a try
 
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