Thermal modulator question

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mgarcia1387
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Thermal modulator question

Post by mgarcia1387 »

Good evening everyone,

So long story short. The previous owner changed the r16 to a u20 but kept the same thermostat housing and disconnected/plugged all the vacuum lines.

So I got a thermostat housing with the Thermal modulator ( see pictures)

I have this thermal modulator on top of the thermostat housing. How does it come off? What is that brass piece inside it? I'm trying to make sure it works before I replace it and reconnect the vacuum lines. Am I incorrect in assuming these lines should not be plugged as they serve a function during idle as after I drive the car for 20 min and it warms up the idle gets very high. (How high you ask? I can't tell you as the tachometer dial is missing lol)

Thank you for your help.

Mike
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Gregs672000
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Re: Thermal modulator question

Post by Gregs672000 »

Welcome Mike, we're here to help. My understanding of a FUNCTIONING emissions control system on the U20 is pretty limited. However, what will make things work right has a lot to do with what is still hooked up and what is not, as well as what distributor curve you have (pre-smog or post-smog). From what I can see in your pics, I do not think you have any smog pumps hooked up, and therefore you have no "system" left to work with.

What we need to do is help you to configure your engine to pre-smog design, which includes recurving the distributor (not hard with a kit, or replace with a much better later Nissan EI dizzy, one of the best mods you can do). My replacement thermostat housing has a plug fitted to it, so I'm sure the old modulator will screw out if desired, or the whole shebang can be replaced (see vendor options and check their sites). For vacuum lines off the intake manifold, they should be plugged or sealed in what ever way you desire. There should be one vacuum line from one carb to the vacuum advance canister on the distributor, but other than that you don't need any. Regarding your idle creep, we can help figure that out with some more questions and info as it can be heat related or linkage related.

What is your mechanical skill level? Access to tools? Regardless, I would encourage you to read through some of the tech wiki for various mods or tuning etc. We can help you get it running right! Keep posting questions etc.
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notoptoy
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Re: Thermal modulator question

Post by notoptoy »

Typically the idle increasing as it warms up is due to the accelerator cable being set tightly with no slack. As it heats up the cable "shrinks" causing the idle to increase. Make sure there is a healthy amount of slack in the accelerator cable when cool and see if that helps!
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david premo
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Re: Thermal modulator question

Post by david premo »

So the function of thermal modulator is to protect the engine from overheating by advancing the timing of the ignition by switching it from carburetor vacuum to manifold vacuum. The difference is at idle carburetor vacuum is 0 and manifold vacuum is at its highest. Smog systems used during this time period were very rudimentary, they incorporated a air pump, a thermal switch, an anti backfire valve and a fan clutch.
The way it worked is the engine was leaned out as much as possible for a higher combustion temperature coupled with an air pump to add air to the exhaust manifold to keep the burn going as long as unburnt fuel exists. When the temperature would get too high due excessive idling or low speeds the thermal valve would expose the distributor vacuum can to manifold vacuum and advance the timing which would begin to cool the combustion chamber. In addition to the the vacuum switching the fan clutch had a bi-metallic that would lock the fan to pulley speed increasing the cooling of the engine. Cars of this era would typically have a “wandering” idle as the fan and thermal switch would kick in and out. The big problem with disconnecting the thermal valve, is you have literally pulled the pin on the grenadine and your waiting for it to go off.
Anyway the thermal valve has three ports 1st port is manifold vacuum M. The 2nd port is carburetor vacuum and the 3rd port is distributor advance vacuum. By manipulating the different vacuum ports it keeps the engine from overheating and damaging it, if it’s not working all bet’s are off and the engine will likely have extensive overheating damage.
mgarcia1387
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Re: Thermal modulator question

Post by mgarcia1387 »

Gregs672000 wrote: Sun May 01, 2022 8:37 pm From what I can see in your pics, I do not think you have any smog pumps hooked up, and therefore you have no "system" left to work with.
Do you mean the 3 vacuum lines? Or is there something else I am missing?

Does that piece inside the thermal modulator do anything specific?

Thanks!
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Re: Thermal modulator question

Post by mgarcia1387 »

Also I was reading this (see attached)
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Gregs672000
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Re: Thermal modulator question

Post by Gregs672000 »

Wow Dave, what a great summary and understanding of that system!

Mike, what I'm talking about is what Dave mentioned, namely the air pump. It would be quite obvious to you that yours is missing if you had ever seen one :lol: . It was a pump the size of a small alternator that was run off a belt on the crank. Most of the cars here today have long since removed that system as it was prone to failure and continued to safely run their cars in the original U20 configuration, i.e. prior to the emissions systems that started in '68. So, what we need to do is to help you learn what distributor ignition curve you have, and make sure the engine is timed at idle accordingly (17degrees pre--smog, 0 post-smog). Early distributor had an ignition curve that added about 15 degrees advance, added to that initial 17; later had all 34+ degrees built in so it's set to idle at 0. You have to look inside the distributor to know. May need to adjust the idle mixture, and we can help you get those SU chokes and cables working properly if not.
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Gregs672000
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Re: Thermal modulator question

Post by Gregs672000 »

The piece inside the thermo modulator is like a small, temperature actuated switch, and probably works on the same principle and manner that a thermostat does.
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Re: Thermal modulator question

Post by spyder »

I removed my modulator and plugged the hole with a hex screw in plug. I can't tell if you still have the emissions pump and associated plumbing. You want to check the distrubritor weights to make sure the weights are the 7.5 degree not the 17.5 degrees.

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Re: Thermal modulator question

Post by david premo »

Hi mgarcia,
To answer your question best there is typically a check ball inside the valve that defaults to ported carburetor vacuum, and when the temperature reaches the set point probably about 205 degrees F, the wax pellet in the bottom of the unit expands to lift the check ball. What occurs next is the manifold vacuum routes to the distributor vacuum advance canister and advance’s the engine timing which cools down the combustion chamber temperature. Additionally the clutch fan engages at nearly the same time to increase the cooling of the coolant in the radiator. As I in the previous response the engine at idle will continually idle up and down, while the fan clutch will engage and disengage as demand dictates. This wax pellet is nearly identical in design to how the thermostat in the cooling system works, it too has a wax pellet that is filled with metals and other materials to set its opening and closing temperature needs.
Dave
Last edited by david premo on Tue May 03, 2022 11:38 pm, edited 1 time in total.
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Gregs672000
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Re: Thermal modulator question

Post by Gregs672000 »

And since he doesn't have an air pump etc hooked up, he doesn't really need to worry about the darn thing, right? :lol:
Greg Burrows
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Re: Thermal modulator question

Post by david premo »

No he needs to worry if he still has the 17.5 cam.
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Re: Thermal modulator question

Post by Gregs672000 »

How come? Aren't there a number of cars running around with the emissions system removed but running the smog dizzy set at 0? I'm not doubting you, just want to understand. Are the SUs different? Different needles? Thanks Dave.
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Re: Thermal modulator question

Post by david premo »

Greg, SU needles are not the problem, it’s the excessive combustion chamber heat generated by the heavily retarded timing and lean mixture with no safety, ie the thermal valve and fan clutch to prevent engine melt down. Hence the reason I have said disconnecting the system without recurving the distributor is the pin pulled and waiting for the explosion. Lean mixture equals higher temperature in the combustion chamber as well as retarded timing adding to the increased temperature. Then stir in the air “smog” pump to add air into the exhaust to continue burning any remaining unburnt fuel. Which is why the factory manifold is double wall up by the cylinder head, to contain the heat.
Dave
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Gregs672000
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Re: Thermal modulator question

Post by Gregs672000 »

Ah, I did not know that. So recurve or replace the dizzy!
Greg Burrows
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