Guys,

The subject is a 74 Dart with a 318 in it.
The problem is I don't have the service manual for it, so I can't read up on something. Specifically, how is the resistor in the line for the choke heater supposed to work?

The problem I am having is that the choke seems to come off all the way, then after driving a while, the choke comes back on part way.

I checked for voltage before the little resistor bolted to the top of the motor, and there is 12 volts before the resistor. After the resistor there is NO voltage- which seems to be allowing the choke heater to cool off and allow the choke to come back on.

I guess engine heat isn't hot enough to keep the choke off on its own.

So I'd like to know, how is this thing supposed to work?

Is there something about the ambient temperature that if it is too warm, it shuts the choke heater off after a while? What about some kind of time delay? Does it stay on for a certain number of minutes or something, then turn off on its own?

What is funny is that I did not remember having this problem when the outdoor temperature was colder. Now that it has been in the mid 70s, I have the problem.

Thanks for your help.

Get the service manual (and the other two books described in this thread) as quickly as you can. There is no substitute.
What you are calling the "resistor" is, unless someone has installed a non-stock component in place of the correct one, actually a choke heater controller. It grounds via its housing where it bolts to the cylinder head, sees +12v whenever the ignition is switched on, and looks at the temperature in its close vicinity. When that temperature is below about 60°F, it does not pass +12v along to the wire leading to (and powering the heating element in) the choke thermostat housing. When the temperature reaches about 60°F, it begins sending +12v to the choke heater to kick it off faster. Once the temperature is hot enough that the choke is surely all the way off, it no longer sends +12v to the heater. The slant-6 cars got a single-stage controller. The V8 cars got a 2-stage controller with a small ceramic resistor attached. I put a 2-stage controller on my '73 slant-6 Dart and its cold start and warmup behavior is about as perfect as they get.
Fun.
See above.

It sounds as if your choke heater controller and your choke thermostat both have problems. If I were you I'd save myself a lot of aggravation trying to find original-type parts and replace the entire lot with an (adjustable!) Electric choke kit.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks Dan.
Yes, it has been interesting finding the car suddenly fast idling.

I had tested the choke heater control with the engine cold and found 12 volts at both terminals. Then with the engne warmed up, there were 12 volts at one terminal and 0 at the other. I thought I had a bad ground, but it sounds like this is how it is supposed to work.

I have the factory service manual somewhere but can't find it. That is why I decided to post. Thanks for the leads on the other two books.

I have always had slant 6 cars, so I see that the choke heater control is different on a 318 in that it has the resistor on the side that you mention. I wasn't sure what that was for.

It seems like it may be the choke coil itself not being sensitive enough to the heat coming up from the intake manifold (as opposed to the exhaust manifold as on a slant 6 car).

I may also try shielding the choke coil with some aluminum foil to keep the wind off of it and perhaps keep the heat in better. This generally worked on GM cars I had with hot air chokes.

Or I may get a new choke coil...

Do you think there would be any harm in by-passing the choke heater control and running 12 volts to the choke heater at all times that the engine is running? Would that damage the choke heater at all?

Neither necessary nor helpful. The "wind" isn't causing your problem. You've got at least one faulty component.
Burn it out. And cause fairly substantial extra current draw all the time...more current through your bulkhead connector...more current through your ammeter...dimmer(!) lights and slower(!) fan and turn signals at idle.

You ever hear of Julia Child, the famous chef? She used to let people talk at length about how they were gonna make some classic dish with less fat and fewer calories. Canola oil instead of butter! Applesauce instead of sugar! Baked instead of fried! And then she'd say "Why not just make the damn thing properly and eat less of it?".

Just sayin'.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

OK! Thanks!

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Super Six Conversion Stromberg

I have to wonder if the intake manifold exhaust crossover is plugged.

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Joshua

D'OH! Right, it's a V8. The crossover is almost certainly plugged, and the heat riser valve is likely faulty (or missing). That's an even better reason to go ahead and put in the electric choke kit, which does not rely on the intake being heated (though eventually you'll want to pull the intake, clean out the crossover, and repair the heat riser as needed).

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

OK... I was going to ask about how the intake manifold gets heated, because the heat has to come from somewhere to heat up the choke coil.
I thought the intake had a coolant passage through it, and I thought maybe the coolant was low (the coolant is not low..)

But you're saying it gets heat from the exhaust?
I just went out there to look at the exhaust manifolds... it's a little hard to tell because the driver's side exhaust manifold is right over the steering column (amazing how they fit that in there) and the power brake booster also makes it hard to see very much. The passenger side manifold doesn't seem to be the one that has the heat riser in it.

What I CAN tell you is this... I unbolted the choke coil from the intake manifold. While I had 12 volts before and after the choke heater control, the choke heater was not getting hot!

When I re-bolted it, the choke heater DID get hot. So it must ground through the mounting bolt.

I can say that the choke heater does seem to stay on if the key is "on" but the engine is off (therefore the ambient temperature around the choke heater control stays low...)

The temperature at the top of the engine must get warm enough to turn off the choke heater control, but NOT warm enough to keep the choke coil itself from putting the choke back on...

I just went out again to look and realized that the exhaust heat must come from the driver's side exhaust manifold to heat up the base of the carburetor, because how else would the EGR valve (which is on the driver's side of the carburetor) get the exhaust gases to transfer into the intake?

So if the crossover is plugged then the EGR valve must also not be working...

I see that the driver's side exhaust manifold is also shaped differently than on the passenger side. It goes above and in front of the spark plugs... they needed to do this to clear the steering column...
it's not like that the passenger side...
interesting...

There are exhaust crossover passages in both heads and manifold. Some cars don't have a thermostatic heat riser valve, but those that do generally have it in the passenger's side manifold. The exhaust, when the heat riser is closed, is forced to flow from the passenger's side head through the intake manifold to the driver's side head and exhaust manifold. Chances are excellent the passages in the manifold and heads are blocked with carbon. This requires intake manifold removal to correct. You already know some of the symptoms of it being plugged. It's amazing the difference when the manifold heat works.

Yes, there is a coolant passage in the manifold, but it's there to join the coolant outlets from the heads and hold the thermostat. The coolant only heats the manifold incidentally. Some small block trucks had coolant heated intakes, but no LA powered cars that I'm aware of.

Electricity needs a path in and a path out. Unbolting the heater cut off the return path.

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Joshua

Thanks Joshua. I'm realizing you are totally right on what is going on here. It all makes a lot of sense. This is my first 318 car, I haven't had it very long so I'm finding out a lot about it the more I poke around.

My car does not appear to have a heat riser, though it's a '74, so I would have thought it would have had one, since it is from the emissions era.

[As an aside- by the 80s, they were using coolant heated intake manifolds more than exhaust heated intakes (at least on GM cars that I had back then).]

OK... so a few minutes ago I took the EGR valve off and found that there seems to be quite a bit of oil in the crossover passage- I don't know what to think about that!

Incidentally, the car also seems to be getting poor gas mileage (like about 16 mpg even on a long highway trip. Not that I know what a 318 normally gets...) I checked it over carefully and couldn't find anything really wrong, even after putting new spark plugs in it. I'm sure everything else is OK (i.e. carb, ignition). I now suspect that if the crossover passage is plugged, the choke must come on enough that the air/fuel ratio is richened by the choke blade being slightly closed, and that is probably what is lowering my gas mileage.

Anyway I think I know where I need to start now...

Are you sure your 318 didn't USED to have a heat riser valve? After a few decades I've found that the external vacuum motor can rust & fall off, leaving the butterfly valve flapping (or stuck) in the manifold. It's sometimes not easy to see unless you (safely) lift the car & look from underneath. (always possible someone replaced a manifold somwhen.)

A partially applied choke will definitely foul plugs, exhaust, & kill mileage.

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63 Valiant Wagon
225 - 4 bbl

Chrysler used bi-metallic springs on their heat risers in this era. Do you have a Chrysler product with a vacuum operated heat riser?

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Joshua

Vacuum-operated heat risers were used on some '77-'82 Mopar V8s. Thermostatic on the others. Always in the passenger-side exhaust manifold. Most of the aftermarket replacement manifolds have no heat riser provision at all.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.