76 Duster Overheating at idle

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Matthew Sermini

Matthew Sermini

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I have some questions I was hoping I could get help with. I have a 1976 Plymouth duster. Original slant six car now with a 340/904. I have a 22inch aluminum radiator in the car (only one that fits with original mounting points) but it seems to get hot a little too quickly at idle. I took the car out last weekend and it was getting pretty hot within 5-10 minutes of idling around at a show. The car doesn’t overheat when going down the road but I’m looking for solutions for when I’m in heavy traffic or at a car show. I don’t have a fan shroud on, and everything is within a couple years old. Car also doesn’t like to start if I go into a store for like 20 minutes but as soon as I get back on the road any vapor lock issues clear up. Newer thermostat and new fuel pump in the car. Any tips or input would be great.

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First, "WHAT" is hot? Give us a number. Secondly, lets get some pictures of your fan and how close it is to the radiator. Also, what thermostat do you have?

One more little tidbit. These overseas radiators have NO consistency whatsoever. You may have a good one and you may not. Get an infrared temp gun from harbor freight. Get some readings at the thermostat area. Post those numbers. Shoot the upper radiator hose at the inlet of the radiator and at the bottom hose at the outlet and post those numbers. That will tell us whether the radiator is doing it's job.
 
I’m not sure of the actual temp when running, I’ve been going off of the original gauges which I got fixed. It might not be completely accurate but if I had to guess it runs around 200. I’ll get a heat gun and post some numbers. The fan is a standard small block clutch fan, sits maybe 3 inches max from the radiator

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Matthew Sermini said:
Also the thermostat is from oreillys but I think I have another from classic industries laying around.
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Needs to be a high flow thermostat. I like running a 160. People will argue, but that's what's worked best for me through the years. Whatever you run, the high flow part is very important. Also make sure your pulley ratio is right. The water pump should be turning faster than the crankshaft. In other words, the water pump pulley should be smaller than the crankshaft pulley.
 
tedsweet said:
200 is not over heating
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I agree with that, but it does not leave as much "room" for error as say an engine that runs 170 or 180. During stop and go traffic in 90 degree plus heat and high humidity, 200 will soon be 230 and that's too hot.
 
so spend money trying to fix a problem that doesnt exist. 170 isnt even hot enough to get moisture out of the motor oil
 
I had that issue running 195 and then waiting at a long light it would go up to 220 quickly. I changed to a 165* thermostat, a 4 inch circular shroud and added a pusher 16" electric fan. Now it will idle in traffic well, and if it rises I switch on the fan and it's fine. It's a cammed big block, they seem to run hotter to me.
 
RustyRatRod said:
Needs to be a high flow thermostat. I like running a 160. People will argue, but that's what's worked best for me through the years. Whatever you run, the high flow part is very important. Also make sure your pulley ratio is right. The water pump should be turning faster than the crankshaft. In other words, the water pump pulley should be smaller than the crankshaft pulley.
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I will make sure to get a high flow thermostat. Also, The pulley for crankshaft and water pump look to be about the same size from the photos I have. I got the pulleys from a 318 I parted out so I assume that they are correct.
 
44070dart said:
I had that issue running 195 and then waiting at a long light it would go up to 220 quickly. I changed to a 165* thermostat, a 4 inch circular shroud and added a pusher 16" electric fan. Now it will idle in traffic well, and if it rises I switch on the fan and it's fine. It's a cammed big block, they seem to run hotter to me.
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thank you for the input! I will definitely try this
 
Matthew Sermini said:
I will make sure to get a high flow thermostat. Also, The pulley for crankshaft and water pump look to be about the same size from the photos I have. I got the pulleys from a 318 I parted out so I assume that they are correct.
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Measure them and see. Also, make sure to inspect how clean the coolant is. If there's a lot of rust and corrosion in the system, it will be brown and discolored. It may need flushing. But that temp gun is your buddy. Those things can really tell you what's goin on with your cooling system.
 
As RRR said, pump speed is very important at idle. I think the heavy duty cooling option had the water pump turning at 120% of the crank speed. Think mine has a 7.25 crank pulley & a 5.75 water pump pulley to get this ratio.

A shroud would help a lot, just make sure fan is 1/2 in, 1/2 out of the shroud. I learned that the hard way.
 
Matthew Sermini said:
I have some questions I was hoping I could get help with. I have a 1976 Plymouth duster. Original slant six car now with a 340/904. I have a 22inch aluminum radiator in the car (only one that fits with original mounting points) but it seems to get hot a little too quickly at idle. I took the car out last weekend and it was getting pretty hot within 5-10 minutes of idling around at a show. The car doesn’t overheat when going down the road but I’m looking for solutions for when I’m in heavy traffic or at a car show. I don’t have a fan shroud on, and everything is within a couple years old. Car also doesn’t like to start if I go into a store for like 20 minutes but as soon as I get back on the road any vapor lock issues clear up. Newer thermostat and new fuel pump in the car. Any tips or input would be great.

View attachment 1715937380
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You need to have the fan about a half inch away from the radiator when you don't have a shroud... and you need a fan that covers a good portion of that radiator ..probably about 17 to 19 in fan.
 
Bewy said:
Check idle timing. Retarded timing causes hot running.
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This is another great point! If the engine is worn, the timing chain might have a lot of slack in it. As it stretches, the timing retards itself, so certainly something to check. Our friend down under, @Bewy is sharp. He's always on top of things, even though he's under them. LOL
 
At idle the fan can't pull any air through the rad due to there being no shroud.
The shroud makes the fan bring air through the rad intead of beating it into submission.
192 is the stock temp stat on the later Magnum engines. IIRC every MoPar I have ever owned had a 192 stat. My 92 Dakota can idle all day in 95° temps with the AC on and not overheat. I tried a 180 and it wouldn't warm up and I got an error code for not getting warm enough.
Try a shroud before throwing money and parts at it.
 
The question about the temperature is twofold.
1. Is how hot ?
2. Did it rise suddenly? Parked after driving doesn't count. It always rises quickly when parked and shut off.

This from page 10 of the 1975 Plymouth Valiant Owners Manual.
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'76 wiring is different than '75 but the gages information is pretty much the same.
The gage should be labeled the same as the one shown here.
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Center is around 185 * F
That's below the opening point of most late model t-stats.
I'd expect the needle to be a bit to the right of center on a fully warmed up engine with a 190* t-stat.
After restarting after a short drive, the temperature should drop but will do that better at idle with a fan shroud.
 
Matthew Sermini said:
Car also doesn’t like to start if I go into a store for like 20 minutes but as soon as I get back on the road any vapor lock issues clear up.
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That's probably not vapor lock. Vapor lock is what happens in the fuel lines. The carburetors runs off the fuel in the bowls. The only heat related restart issue that can occur is when vapor in the line expands with enough pressure to force open the bowl's inlet valve. If there is liquid fuel above that expanding vapor it can cause flooding (bowl overflow).

It could be the choke or cold idle settings or too advanced initial timing. So based on the information so far there's not enough to conclude the initial is too advanced or not sufficiently advanced. Although this is probably not the cause of what you observing you should know that:
A factory 340 will be fine with something like 12 - 14* BTDC at 650 rpm. A hot rod will want a bit more initial.
A stock factory distributor will be set up for something closer to 5* BTC @ 750 rpm for emissions purposes to put more heat into the cylinder walls. All 340s were produced with a Clean Air Package so will have distributors with the long fast advance off of idle.

Matthew Sermini said:
I took the car out last weekend and it was getting pretty hot within 5-10 minutes of idling around at a show. The car doesn’t overheat when going down the road but I’m looking for solutions for when I’m in heavy traffic or at a car show.
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Based on the difference between moving and not moving, that points to a difference in air flow between moving and not moving. Fan shroud and maybe a front hood seal if not present would be the first things to try.
 
Slap Stick said:
At idle the fan can't pull any air through the rad due to there being no shroud.
The shroud makes the fan bring air through the rad intead of beating it into submission.
192 is the stock temp stat on the later Magnum engines. IIRC every MoPar I have ever owned had a 192 stat. My 92 Dakota can idle all day in 95° temps with the AC on and not overheat. I tried a 180 and it wouldn't warm up and I got an error code for not getting warm enough.
Try a shroud before throwing money and parts at it.
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I've never run a fan shroud.
 
Mike69cuda said:
As RRR said, pump speed is very important at idle. I think the heavy duty cooling option had the water pump turning at 120% of the crank speed. Think mine has a 7.25 crank pulley & a 5.75 water pump pulley to get this ratio.

A shroud would help a lot, just make sure fan is 1/2 in, 1/2 out of the shroud. I learned that the hard way.
Click to expand...

Do you know the application of the pulleys you're using? Are they single groove?
 
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