Under the hood mysteries: Distributor Vaccum Control Valve.

What is it? what does it do? is it an emissions control device?

This is what a Distributor Vacuum Control Valve or Ported Vacuum Switch looks like



This little device may not appear to be all that important in fact most people believe it to be a direct part of the evil emissions system, but it has a big job to do when your engine gets too hot. The threaded end looks a bit like an engine temperature sensor or a thermostat, and that's because it is.

It's normally mounted on the intake manifold or engine block where it can sense coolant temperature. On the 351 Cleveland or 302 there is a port for it right on the water pump housing around the alternator.



The three ports at the other end are for vacuum connections. One goes to full manifold vacuum, another to carburetor vacuum or ported vacuum, and the third goes to the vacuum advance on the distributor.



At normal engine temperatures, the distributor gets ported vacuum, which is 0 psi at idle. During extended idling on a hot day in bumper to bumper traffic, the engine temperature can start to rise pretty fast, especially with air conditioning on. That's when this little device goes to work.

When engine temperatures reach about 225 degrees, this switch changes the distributor vacuum advance source from ported to full manifold vacuum, which is high at idle. This advances the timing and engine rpm increases.

The result is increased radiator fan speed and coolant flow through the engine and radiator, which tends to bring the temperature down.

Once the sensor determines the coolant temperatures have dropped to a safe level, it reverts back to ported vacuum and the engine timing returns to normal, returning the idle rpm to its normal setting.

Chances are most people have never even noticed when this device is protecting their engine, they just notice that the temperature gauge drops a bit, or the HOT light on the instrument panel extinguishes.

This Valve is a compensation to the emission system. It addresses a problem that can occur with lower timing at idle and engine overheating. since all 71-73 cars were designed to use ported manifold vacuum as an emissions reduction at idle if you have an A/C car with heavy load on accessories and you want to maintain stock functions of the engine, its a good idea to restore this valve if it is missing from your vehicle.

Personally i think its pretty trick having the ability to change the type of vacuum the distributor sees at idle based on Engine temperature, even if the trade off is reduced idle emissions, and emissions control is evil.

now at a car show when somebody points to one of these valve and doesn't know what it is or takes out a cross and shoots garlic at it, you can tell them what you learned here .

Thank you. I can now speak fluent Ported Vacuum Switch!

Good info thanks

Great write-up as usual! Thanks!

Learn something new every day. Thanks for the info!

Got to see mine work the other day. About a week ago the car started running like the timing chain had jumped a cog. We tracked it down to a faulty distributor vacuum module. We replaced the module. While driving around town in hot weather, I pull into a parking lot and heard a click from the engine. Then the RPM jumps to 1200 so I am thinking, Now What? Had I not read your article, I would still be under the hood, ???

mike

Very informative! I took the one off my 73, 250, 6 cyl, way back in the 70's trying to improve the idle. The 6 cyl version only has two vacuum ports. I also removed another vacuum device, had a light blue plastic piece on it, that was attached to the bolt on the valve cover. It had three vacuum ports and a ground wire. These are in a parts drawer in the garage, maybe I should clean them up and reinstall!

The part you removed was a spark controller, another cool little gizmo.

Thanks! I have been wondering about this for years!

What I was looking for, good informations.

(thank you Timachone )

Very good article on the vacuum switch... I have an electric ported vacuum switch on my 71 j code car. Very strange devise for sure. I was told that it was part of the California emission controls but, I am not sure if that is correct.

any A/C car would have it as standard equipment. it was extra protection needed because of the extra load from the Compressor on the motor.

the only relation to emissions is maintaining ported vacuum when the engine is cold or normal temperature, only going to full manifold vacuum when the engine is overheating.

from a performance aspect full manifold vacuum advances timing at idle which gives more performance more initial timing over factory set.

with ported vacuum is 0 at idle, which keeps the timing retarded at factory initial this increases exhaust temperature and decreases emissions when sitting in traffic, stopped, or in park with engine running. the second the throttle is pushed the vacuum shifts to full manifold vacuum the distributor vacuum kicks back on and away you go..

there is a delay caused by ported vacuum when you stomp on it compared to full manifold vacuum which is where the performance is lost.

with ported. vac is 0, timing is set to initial, you stomp on it, vac goes up increasing the distributor advance and car goes.

with full manifold vac. vac is 17" HG, timing is Initial + vacuum advance, you stomp on it, the distributor is already advanced and advances more giving more timing and more power, thus power output is higher.

you can tune ported to be very much like full manifold vacuum if wanted.

ported really isn't a bad thing especially when dealing with open chamber heads on V2 and 1972-1973 V4 motors. open chamber heads do not like advanced timing before 3000rpms they tend to cause detonation/ping.

the best performance would be a 1970-71 V4 with closed chamber heads, then you would see a big jump in power going to full manifold vac at idle.

Very good article on the vacuum switch... I have an electric ported vacuum switch on my 71 j code car. Very strange devise for sure. I was told that it was part of the California emission controls but, I am not sure if that is correct.

Click to expand...

I remember when you were looking for something that had to do with that switch. Was it the wire harnes that connects it? Mine did not have the electric one or for that matter the other one either. Someone removed it.

I do have some overheating issues when ideling in traffic. I wonder if I should look for one and put it back on. Would I have to do anything else other than re connect the vacuum hoses or would the timing need to be reset?

Definitely a good educational thread about this mysterious device. I never knew what exactly it did. Really sounds like it was there for a good reason.

if you are overheating at idle you need to first look at your cooling system.

Radiator clogged, too small.

Fan problems

shroud.

plug heat range too hot.

problems with headers/exhaust.

timing issue.

fueling issue, too lean, ping detonation.

water pump issues.

thermo.

the DVCV is only used in emergency situations and should never come on except for last resort.

72HCODE said:

What is it? what does it do? is it an emissions control device?

This is what a Distributor Vacuum Control Valve or Ported Vacuum Switch looks like

This little device may not appear to be all that important in fact most people believe it to be a direct part of the evil emissions system, but it has a big job to do when your engine gets too hot. The threaded end looks a bit like an engine temperature sensor or a thermostat, and that's because it is.

It's normally mounted on the intake manifold or engine block where it can sense coolant temperature. On the 351 Cleveland or 302 there is a port for it right on the water pump housing around the alternator.

The three ports at the other end are for vacuum connections. One goes to full manifold vacuum, another to carburetor vacuum or ported vacuum, and the third goes to the vacuum advance on the distributor.

At normal engine temperatures, the distributor gets ported vacuum, which is 0 psi at idle. During extended idling on a hot day in bumper to bumper traffic, the engine temperature can start to rise pretty fast, especially with air conditioning on. That's when this little device goes to work.

When engine temperatures reach about 225 degrees, this switch changes the distributor vacuum advance source from ported to full manifold vacuum, which is high at idle. This advances the timing and engine rpm increases.

The result is increased radiator fan speed and coolant flow through the engine and radiator, which tends to bring the temperature down.

Once the sensor determines the coolant temperatures have dropped to a safe level, it reverts back to ported vacuum and the engine timing returns to normal, returning the idle rpm to its normal setting.

Chances are most people have never even noticed when this device is protecting their engine, they just notice that the temperature gauge drops a bit, or the HOT light on the instrument panel extinguishes.

This Valve is a compensation to the emission system. It addresses a problem that can occur with lower timing at idle and engine overheating. since all 71-73 cars were designed to use ported manifold vacuum as an emissions reduction at idle if you have an A/C car with heavy load on accessories and you want to maintain stock functions of the engine, its a good idea to restore this valve if it is missing from your vehicle.

Personally i think its pretty trick having the ability to change the type of vacuum the distributor sees at idle based on Engine temperature, even if the trade off is reduced idle emissions, and emissions control is evil.

now at a car show when somebody points to one of these valve and doesn't know what it is or takes out a cross and shoots garlic at it, you can tell them what you learned here .

Click to expand...

mine has the electrical connections at the top. original on my 1971 grande w 351c 4v.

smartypants said:

mine has the electrical connections at the top. original on my 1971 grande w 351c 4v.

Click to expand...

Did the car come with A/C from the factory?

My '71 Mach 1 "H" code 2v has that device. However, early after I got the car, there was an issue with engine temp, and when this switch opened, the advance made the damned thing ping even under carefull acceleration, so I by-passed it. I remedied the temp issue but left the switch in-op, which isn't necessary any more, as temp stays within limits. I understand it's purpose, but haven't needed the cooling effect it is designed for, knock on wood.

Wasn't the purpose of the 3 port switch to activate the retard side of the dist. dual diaphragm (canister) and retard the timing for emissions at colder engine temps? Something to that effect anyways.
The base timing on the M code was 6 degrees initial, so I'm guessing the retard might set it back to 2 or 3 degrees.
On my M code, I recurved the dist. to an initial of 14-16 degrees (now set at 14*). There is no longer a need for the 3 port switch as the dist. is now a single diaphragm. I actually removed the switch and use that spot for the idiot light temp sensor which allowed me to use the proper temp sensor location for the mechanical gauge.

Oh, yes, I didn't mention that I had removed the original distributor and installed a single advance, non-California distributor soon after buying the car. I believe the switches function was to deny any vacuum to the distributor at startup thereby reducing emissions and speeding engine warm up, when the switch then allowed vacuum to the advance once the engine achieved temp. The system works well enough, but the pinging deal if too hot wasn't sitting well with me. There are times when going thru the desert towing my boats, when pulling a hill that you can see where the engine would build more heat, but you have to keep going and get over the hill to keep the airflow thru the radiator, and were the switch in place, the engine could start pinging under load and you sure don't want THAT. Anyway, that's why I rendered it in-op.

Spike Morelli said:

Oh, yes, I didn't mention that I had removed the original distributor and installed a single advance, non-California distributor soon after buying the car. I believe the switches function was to deny any vacuum to the distributor at startup thereby reducing emissions and speeding engine warm up, when the switch then allowed vacuum to the advance once the engine achieved temp. The system works well enough, but the pinging deal if too hot wasn't sitting well with me. There are times when going thru the desert towing my boats, when pulling a hill that you can see where the engine would build more heat, but you have to keep going and get over the hill to keep the airflow thru the radiator, and were the switch in place, the engine could start pinging under load and you sure don't want THAT. Anyway, that's why I rendered it in-op.

Click to expand...

Ah yes, a far better explanation Spike. I was "close!"