Allpar Forums banner

1 - 13 of 13 Posts

·
Registered
Joined
·
78 Posts
Discussion Starter #1
I have a 88 Dodge 1/2 ton 4wd pick w/ a 4 barrel carbureted 360 engine. My question for now is the position of the heat riser. This is vacuum operated. When i put vacuum to the line the valve moves in and out. I eventually need to fix the lines but for now i need to know the position of the valve.

I've read it's closed and when it warms up it opens. I've also read it's initially open, closes when started and opens when warm. If somebody could verify which sequence is right i would really appreciate the help.
 

·
Super Moderator
Joined
·
32,249 Posts
Are you talking about the flapper plate mounted in one of the exhaust manifolds? Because that's called a heat riser valve. Or is this the pre-heater valve that's in the air cleaner snorkel? If the latter, the flapper in the snorkel is closed to airflow through the snorkel on a cold start, and draws air up through the pre-heater tube that comes off the exhaust manifold. When the engine warms up, the flapper opens to draw cold air in through the regular snorkel opening.
The vacuum tubes can be connected to either port on the thermostat at the bottom of the air cleaner, there is no polarity to it.
 
  • Like
Reactions: chuzz

·
Registered
Joined
·
982 Posts
Bob, he must be talking about the air cleaner door, I can't find anything on a vacuum operated heat riser valve. I only remember seeing them on Chevrolet small blocks.

If it is the one in the air filter, they are open with no vacuum and closed with vacuum. Running they can vary from closed to open depending on the ambient temperature and amount of heat needed. These were to help the lean mixtures the emission carburetors had to run properly atomize.
 
  • Like
Reactions: Bob Lincoln

·
Super Moderator
Joined
·
21,620 Posts
Some of the 360/4bbls with stricter emissions had a vacuum operated (power) heat riser valve in the exhaust manifold. I don't know where it sits with no vacuum applied, but it does close when cold to run exhaust gases under the intake manifold floor to heat the incoming air/fuel mixture. As the engine warms and the choke opens, the valve opens because less heating is needed.
I would have to look in a service manual for specific tests.
See inset items 20, 21 and 22 here:
00000613.png
 

·
Super Moderator
Joined
·
32,249 Posts
Bob, he must be talking about the air cleaner door, I can't find anything on a vacuum operated heat riser valve. I only remember seeing them on Chevrolet small blocks.

If it is the one in the air filter, they are open with no vacuum and closed with vacuum. Running they can vary from closed to open depending on the ambient temperature and amount of heat needed. These were to help the lean mixtures the emission carburetors had to run properly atomize.
And they were still used on TBI engines. I built such a system for my 92 3.9L V-6, even though with MPFI, it's claimed that it's not needed. It does make the engine run more smoothly when cold, and gas mileage did increase almost 1/2 mpg.
 

·
Registered
Joined
·
982 Posts
True, I forgot TBI, never had a running one. The 86 convertible had a very dead 2.2L TBI in it and none of my other vehicles have ever been TBI. I can see where it could help, the warmer air is thinner and requires less fuel to get the desired A/F ratio.
 

·
Registered
Joined
·
982 Posts
Some of the 360/4bbls with stricter emissions had a vacuum operated (power) heat riser valve in the exhaust manifold. I don't know where it sits with no vacuum applied, but it does close when cold to run exhaust gases under the intake manifold floor to heat the incoming air/fuel mixture. As the engine warms and the choke opens, the valve opens because less heating is needed.
I would have to look in a service manual for specific tests.
See inset items 20, 21 and 22 here:
00000613.png
Probably California, but I saw nothing on the 1988 even light duty versions. That's what I was looking for before I said anything. If they work like the GM ones, with no vacuum they are open (fail safe mode). I wonder if they were as prone to sticking as the older thermal only valves? The old rule we went by in my shop, on the big blocks, due to the location (between 6 and 8 on the exhaust manifold) they would rattle at idle, and if it rattled you were good, the small blocks, you had to actually reach down and roll the counterweight to make sure it was free. I rebuilt many of them, it's how I learned to do a nice stick weld, doing MOPAR heat riser valves.
 

·
Super Moderator
Joined
·
21,620 Posts
I saw them here in NY on the B300 maxivans and police cars, usually along with the mini-ox cats and smog pumps.
We were Chrysler-Plymouth, so I didn't see many Dodge pickups.
 

·
Registered
Joined
·
78 Posts
Discussion Starter #9
Sorry I didn't clarify better. It's not the flapper in the air cleaner snorkel. It is the heat riser valve off the passenger side exhaust manifold. I'm trying to get the sequence for that since I read two conflicting statements (on another site). One said it's closed when cold and not running and opens as the engines heat up, the other said it's open when not cold and not running, immediately closes when started and opens as it warms. up. It a 360 4 barrel in Pennsylvania.
 

·
Registered
Joined
·
982 Posts
I could find no description of how or when it operates, but here is the vacuum system diagram for it, the system is referred to as the Early Fuel Evaporation system or EFE. The nearest I can find is the solenoid valve closes it by vacuum until the engine reaches a set temperature, the charge temperature sensor is looking for air temp in the manifold of 60° F at which point the solenoid switches to the EGR amplifier and the Power Heat Valve opens. Hope this helps, BTW it is only on the under 8500 GVW models.
360 4V under 8500 GVW.jpg
 

·
Registered
Joined
·
4,787 Posts
. . . . I'm trying to get the sequence for that since I read two conflicting statements (on another site). One said it's closed when cold and not running and opens as the engines heat up, the other said it's open when not cold and not running, immediately closes when started and opens as it warms. up. . . . .
The confusion is created because the operating concept for opening and closing is different between a thermally activated heat riser valve and a vacuum operated heat riser valve. On a thermally activate heat riser valve, as the surrounding area cools the bimetallic spring closes the valve. This is similar to a thermostatically control choke plate on the carburetor. With a cold engine the heat riser valve is closed. With a closed heat riser valve, part of the exhaust gas is forced through passageways cast into the intake manifold from one side to the opposite exhaust manifold. This warms in the intake air passages and allows for better driveability during cold engine start. As the exhaust gas warms the valve area, the bimetallic spring expands and opens the valve and allows exhaust gas to escape directly into the manifold and not pass into the intake manifold runner area.

With a vacuum controlled heat riser valve and with the likelihood of a malfunction due to lack of vacuum or improper vacuum control, the default at rest and with a cold engine is for the valve to be open. Once a cold engine is started a thermal control vacuum switch valve opens and allows vacuum to close the valve. When the thermal control vacuum switch reaches a certain temperature, vacuum is shut off from the valve and the valve returns to its default position, open.
 

·
Premium Member
2017 Charger Pursuit AWD
Joined
·
619 Posts
I saw them here in NY on the B300 maxivans and police cars, usually along with the mini-ox cats and smog pumps.
We were Chrysler-Plymouth, so I didn't see many Dodge pickups.
M-body squads, all of them, used the regular heat valve in the passenger exhaust manifold.

The trucks had two separate CCEV switches (not solenoids) for engine vacuum. One ran the air pump diverter valve and canister purge (Quadrajet equipped trucks and squad cars had that), and the other went to the vacuum controlled flapper. One was located in the intake, the other in the radiator top tank. EGR was routed through the vacuum amplifier and a timer, but I used to plumb it into the CCEV on my old squads to clean things up a bit.

One big difference between the old TQ and the QJet: the design of the choke!
The TQ used the old divorced choke in the manifold. This was one of the worst designs - so many guys complain about Lean Burn, but this old choke was truly junk. The QJet had the choke right on the side of the carb (squads), and you could drill out the rivets to EASILY adjust the choke's tension. It was so simple, and so effective, I used to remove the stupid flapper and just ran with the choke and heated air flap in the snorkel. Even in -30°F temps, that old girl ('85 Fury squad) would start right up, and ran beautiful.
 

·
Registered
Joined
·
78 Posts
Discussion Starter #13
I appreciate the responses. Special thanks to AllanC. Your post cleared everything up. I'm not getting vacuum and with an estate going on and two houses to sell i just wanted to make sure it's in the open position for now and will get aroud to fixing it in the fall. Just like you said i read two different sequences. One said it starts out closed (no vacuum) and when the temperature comes up it applies (vacuum) and opens. At first that made sense since that it matched the even older spring and counterweight, closed when cold and opens when heated.

The second one i read said it starts out open when not running (no vacuum) , closes when started (vacuum) and opens when heated (no vacuum). At first that seemed overally complicated but your explanation makes perfect sense. If the vacuum is interrupted you would want the valve to fail in an open position since that's where it is the majority of the time. Much thanks. At least know for now the way it''s sitting it's in the open position which is good enough for the summer.
 
1 - 13 of 13 Posts
Top