Cold Weather Modulator

[bilbo]

My truck (1983 2.3L) has what I think is called a 'cold weather modulator.' This looks to be a damper in the intake hose that pulls air across the exhaust manifold I assume to warm the air charge into the engine. Has anyone noticed any problems with this system? When I bought the vehicle this was all blocked off so it couldn't operate. After putting everything together I hooked it all back up.

The truck would start and run well when warming up, but wouldn't idle well at all when hot. It would eventually stumble and die. In the process of troubleshooting this, I remembered this was unplugged so I re-blocked it off. The truck now idles fine when hot, but has a low but for the most part stable idle when warming up. Is it possible the thermostatic valve is not working properly and pulling heated air into the carb causes enough trouble to screw up the idle?

 

[tomw]

If this has a 'vacuum motor' that moves a flap to re-direct incoming airflow over the manifold, or from 'cold air', there should be a thermostatic valve that closes when cold, and opens when hot. When closed, manifold vacuum will pull the flap, until the thermo switch gets hot, and then the flap will be relaxed by spring pressure.
On EFI, the vacuum thermo switch is mounte in the air cleaner box at the bottom. It has one vacuum line leading to the intake, and one to the motor on the 'stove'.
tom

EDIT
I may be wrong about the sensor valve. I know it has a bleed that lets the vacuum leak, so the flap opens and allows cooler intake air. Thus it may only have one vacuum line to it. Vacuum from manifold, to motor/operator, to thermo switch that opens when air is warm enough, releasing vacuum pull on the flap motor.
tom

 

[bilbo]

There is a vacuum motor, that connects to a flat disc shaped thing on the bottom part of the air cleaner housing, with another line connected to manifold vacuum. It sounds like what you described, with the disc thing being the thermostatic valve. I'm working on repairing some rusted leaking gutters, then after that I'm going to try testing that system out. We'll see what I find, if anything.

 

[RonD]

Yes, many just disconnect/block off this air pre-heater.

It should only pull air from around exhaust manifold at first startup, this helps engine warm up faster which lowers emissions
It should switch to "cold air" after 4 to 6 minutes, if it doesn't then yes that can be tough on engine running correctly
Air from around hot exhaust manifold is much too hot/thin to get good power from and engine.

"Hot air rises"
That is because it is "lighter"/"thinner" than cooler air, so hot air has less oxygen in it
Gasoline burn ratio is 14.7:1 Air:Fuel, this is a WEIGHT RATIO, not volume
So 14.7 POUNDS of air to 1 POUND of gasoline
Or 14.7 grams of air to 1 gram of gasoline

Hot air weighs less so you have to add less gasoline, which means less power
Cold air intakes(CAI) were popular back in the carburetor days because most carbs pulled air from inside the engine bay, which was hot air after warm up, so running an air tube to front grill to pull in cooler air helped gain the most power at that air temp.
All fuel injected engines used CAI since the 1980's

Point being if you suck in air from around the exhaust manifold AFTER warm up the engine will not run well and it will lack power

Most intake manifolds have coolant circulation, this helps keep air warm enough even in cold weather, pulling air from around exhaust manifold should only happen after cold start and only for a few minutes after that

 

[Mark_88]

I think I replied to this in another post and it appears you have the answer to the question.

I found the alternative to this pre heater was to use a block heater in really cold weather. I put one of those inline heaters and would plug it in for anywhere from 30 minutes to an hour and the truck would start and run fine with ready heat in the hoses to warm up the cab in seconds.

It made life much easier and actually cost much less to run than calling CAA for a boost and all the extra time waiting before I could see through the windshield on frosty mornings...

 

[bilbo]

RonD, that makes sense and seems to align with my suspicions. Just for kicks, when I get a chance I'm going to test my theory by opening the flapper with the Mityvac.

I did put a new block heater in when I rebuilt the engine. I'm well acquainted with them being in NE North Dakota. It gets fairly cold here in the winter, and a block heater is almost a necessity. I agree they make a massive difference in starting and warm-up time. I have a Taurus without one that hasn't refused to start yet but does take ages to warm up in the winter.

I always got a kick out of the 'cold air intakes' that were available. On nearly every car I've owned, there is a hose or duct that pulls air from outside the engine bay. To me, that's already a 'cold air intake,' and always thought a more truthful name would be 'filter relocation kit' as they seem to just move the filter from one end of the duct to the other.

 

[RonD]

Filter relocation kit.................. I like it

Like a good paint job, decal or LED lights, doesn't get more power or better MPG, just makes you feel better, the look

Same thing under the hood, the look

 

[tomw]

I think a bit differently. There is a bi-metal inside the switch that opens when ambient air, or intake air, is above a certain point. When it opens, it opens the vacuum to atmosphere.
It should stay closed at lower temperatures, though I do not know the exact number. Cold air is cold air, and heating 0C or 32F air to 50-ish or ?? will help fuel economy, emissions, stumble & stutter & stall problems, and cost you nothing. Once the engine & intake have been heated, there's less chance of icing, stumble, stall, and emissions should be better, as the system goes into closed-loop operation.
You really want that thing to work, especially in the C and ND areas. It gets very cold in the winter, and the other three seasons, all three weeks of them, are still not real hot, and the engine will thank you.
FoMoCo will go to lengths to reduce the parts count and thus the cost of their product, while still meeting emissions and driveability concerns. They would not have built the stove, installed heat resistant tubing, designed and added the flap, put on a motor(mover), installed vacuum lines, routed the lines, added a thermo 'switch' for no reason. It cost $$ to add that thing. They ain't dummies.
It is my opinion that in colder climates the vacuum switch, stove, flap, etc should be a helper. I would even go to the effort of routing a piece of tubing to the cab to be able to turn on the stove when the engine was cold if I could not get a replacement vacuum switch.
Here in GA, it's not so cold all the time, so my flapper function is not critical to starting, however, the EFI lima needs as much help as it can get when cold. My engine is very susceptible to stalling when I first start it up, even though it is running at or over 1k rpms. I really have to feather the throttle. It has always been that way, so I don't think anything is wrong, but is certainly is in need of all the help it can get.
tom

Why not do a test of mpg with the vacuum line plugged and then operational?

 

[bilbo]

That's what I'm going to do. Once I'm driving it again (it has no windshield at the moment) I'll first check if I can induce the erratic idle behavior by manipulating the flapper with a manual vacuum pump. If that test is positive, I'll see if I can find the temp at which the thermostatic valve opens, or if it's stuck open/closed. If I can, I will make it functional again. However, it's a possibility that I won't be able to find replacement parts if any are bad.

Another theory I dreamed up was if the diaphragm on the flapper were leaking, and the thermostatic valve was bad, it would be a vacuum leak into the manifold. Maybe when cold and choked the engine can handle this better, but when hot it's too lean?

 

[tomw]

I'll first check if I can induce the erratic idle behavior by manipulating the flapper with a manual vacuum pump.

All you'd have to do is to run a piece of vacuum line to the thermo switch in the air cleaner, to the 'vent' side, and back to the cab. When you want heat, plug the vacuum line with a golf tee. When you don't want heat, pull the tee.
I have seen the valves on Rock I think.
tom

 

[bilbo]

I've finally had a chance to look into this again. There is a page in my engine/emissions diagnosis manual on the bimetallic valve and how to test it. It checks out fine. The problem persists. It idles smooth at 850 like it's supposed to when warm, but once it's hot it idles very rough dropping down around 500 and not runnine well at all. Especially after a long stretch of highway and/or hot weather. If i let it sit for 10 minutes or so usually it will idle fine again. I replaced both fuel filters. The larger mesh one was packed full of junk, but no help with the idle. Vapor lock Maybe? Would vapor lock go away if the engine is off for a short time, like getting gas? The mixture screw cover was off when I bought the truck, so God knows what the POLICE did with it, but we did adjust it some right after the rebuild. Is there a starting point for that mixture screw? All I've been able to find is a procedure using propane that I'm not equipped to do. It's a carter YFA feedback carb.

 

[tomw]

Another theory I dreamed up was if the diaphragm on the flapper were leaking, and the thermostatic valve was bad, it would be a vacuum leak into the manifold. Maybe when cold and choked the engine can handle this better, but when hot it's too lean?

If that is the case, the cure would be a golf tee in the offending vacuum line. You could install the golf tee and observe hot idle reaction.
Have you checked the cold idle cam for proper setting? As the engine warms up, the thermostat spring relaxes, allowing gravity to move the cold idle cam to a different spot, which should make the idle return to 'normal'. If there is a separate stepped cam, it is adjusted for xxxx rpm with the engine warmed, which will generally be lower with a cold engine during the warm up where the thermostat spring controls its position & the choke flap. IOW, the idle will be too fast when set with a warm engine, but should be reasonable with a cold engine. This cam may have a 'v' or '^' marking that is the spot to adjust the idle when hot. You have to manually set the cam there, and adjust the screw to spec.
I don't know the carb, but a flaky hot idle is likely due to more air or less fuel, a lean condition. Have you checked the throttle shaft bushings for wear? If they leak, it is almost impossible to get a good stable idle. If you can wobble the shaft, it's too loose.
If you have a distributor with multiple diaphragms, it may pay to check the hot idle ignition timing. Some of the diaphragms may fail, and vacuum may be applied where it shouldn't or not applied as needed, buttering up the timing when hot. Are there temperature operated vacuum switches involved with the timing or carburetor? They may have become dysfunctional, messing with timing or hot idle mix/settings.
tom

 

[bilbo]

The CWM system checks out fine; no leaks in anything and it operates as it should. The high idle cam has been adjusted correctly, and moves out of the way as it should so the TSP can control the idle. The distributor has only one diaphragm, and it checked out when I put the engine together. The throttle shaft has a slight amount of play but it isn't terrible. Maybe it's enough to cause issues? Spraying ether around the area doesn't have any effect, though.

I know there are a few temp. controlled vacuum switches on there. I'll have to take a look at what each one does and see if they're operating correctly.

One other thing, there is a dead spot just off idle when hot. I seem to hit it every time I shift when I am letting the clutch back in and starting to increase throttle and the truck will lurch a bit.

 

[Mark_88]

One member (Angie or her husband) recently posted about finding an uncommon vacuum leak...it turned out that the brake booster seal was broken and because this runs off engine vacuum it was causing high and erratic idle...

Some earlier Rangers didn't have boosters but if yours does it might be part of your idle problem. Worth checking out...

 

[bilbo]

Mine has power brakes, but the Booster checks out fine. I'm going to borrow or buy a timing light again and see what's going on there. Ithe looks like my advance line runs from a temp controlled valvenvironment maybe that's faulty or hooked up wrong.