Thermal Vacuum Switches

[Elizabeth73]

As many of you know I have done a small engine overhaul. We put everything on the new intake that was on the old intake (2bbl switch to 4bbl). Now it idles rough and we suspect that there is a vacuum leak on this part or it is faulty.

I have already read the thread Under the Hood Mysteries and saw that it increases rpm's when the engine runs hot.

My question is: Can a faulty switch cause the car to idle rough? Can I just take this out and plug the port in the intake?

 

[72HCODE]

it could, but it is easy to test.

just hook up a tube to the switch and blow or suck air through it.

put your thumb over the top port then hook up a tube to the lower or middle port, and blow or suck air.

if you blow/suck air from the bottom port and hear a leak in the center port then you know you have a leak on the switch or the switch is defective.

the ports could of been mixed up during service as well or you have a broken hose somewhere in the system.

additionally a hose could of popped of somewhere else, or cracked and is leaking or the problem can be somewhere else.

another easy test, is just cap off the lines and see if there is a change. you could even cap off your distributor vacuum advance which would tell you if that is leaking.

now you put a new intake on the engine and that alone can cause vacuum leaks.

if an engine was machined and plained then the new intake might have to be milled to allow it to sit correctly.

if you used the turkey pan that goes under the intake with a new intake that can cause problems. you could have a bad seal on the corners of the intake.

another test is using carb cleaner spray around the intake with the engine running and listen for the rpms to increase that points to a vacuum leak and where it is coming from.

 

[Don C]

Yes, it can be removed and the hole plugged. However, you may not be able to pass emissions inspection, depending on your DMV regulations.

Those are used with the double vacuum advance, which should also be replaced with the single unit.

And, yes it can cause the engine idle to be rough.

Because you also have a new intake manifold I would also make sure that it's gaskets and the carburetor gasket aren't leaking. It's not easy to get the manifold on square and level.

 

[Elizabeth73]

No turkey pan used for this build. Spray test done on intake earlier, no change for increased rpm's.

To be honest, I am not even sure we have the lines hooked up right on the thermal switch. I think I will just plug it and see where that gets me.

I will have a couple of plugs in the intake which makes me think I am missing something. What sorts of things should be plugged into the new 4v intake?

 

[Don C]

Power brakes, vacuum modulator (automatic transmission only), vacuum for air conditioning (if so equipped) controls.

 

[72HCODE]

the Distributor Vacuum Control Valve is not an emissions device, and it is not part of just the dual vacuum advance system.

the Distributor Vacuum Control Valve is a kind of band aid because of ported vacuum caused by emissions. you can run without it and have no effect over passing tail pipe emissions. this is because our cars were designed to run ported advance.

the Distributor Vacuum Control Valve is used on either single or dual vacuum advanced distributors with or without EGR when the car was equipped with A/C from the factory.

cars before 1969 used full manifold vacuum verse the later cars that used ported.

the difference between full and ported occurs at idle. ported is 0"HG at idle and full manifold is 15-25"HG

what does that mean?

it means at idle when using full manifold vacuum the vacuum advance was fully engaged which advanced timing. Advanced timing besides more power also cooled the exhaust gases leaving the car, the engine also increased RPMS which in turn increased the cooling Fan speed, this increased emissions so in the 1960s when you sat in heavy traffic you basically gave yourself cancer by breathing.

then came Ported vacuum. ported is 0 at idle, so that meant retarded engine timing or not moving from initial timing usually between 4 and 8 degrees. this increased exhaust gas temp and burned off some emmisions, which made the air cleaner.

but at idle you had a hotter running engine.

So you have a engine with some emissions changes, running retarded timing, with A/C and sitting in heavy traffic and it is 95 degrees outside.... you needed a Fail safe.

in comes the Distributor Vacuum Control Valve:

what it did was allow the vaccum to switch between ported and full when the engine reached over 210-220 degrees.

So before the radiator went into melt down and the cap did a 'thar she blows' interpretation of '3 mile island' the Distributor Vacuum Control Valve kicked on,,,, you switched from ported to full manifold vaccum, this cooled the engine by reducing exhaust temps, advancing timing and increasing Idle and cooling fan RPMs.

when the engine cooled back down the Distributor Vacuum Control Valve turned off and you went back to ported.

due to the configuration of the mechanical advance it wasn't a good thing to run on full manifold vacuum unless you re-curved the mechanical advance. Non-A/C cars usually just had a plug in the water pump housing where the Distributor Vacuum Control Valve went.

but the Distributor Vacuum Control Valve was used on either single or dual vacuum advances again as a last ditch effort to cool the engine before it grenaded itself.


the configuration of what to put on the new intake depends on how you carb is configured.

first do not plug all vaccum accessories onto the same tree.

why?

if you put all the vaccum accessories on one side of the intake 2 things happen, first you are fighting for vaccum and you will change the intake air fuel mixture inside the intake in one area.

what that means is you could cause the vacuum booster for the brakes to fight other accessories for available vacuum capacity.

that can result in the booster not working correct at certain times when the engine has low vaccum like under hard acceleration and a sudden heavy brake.

so you want to try and balance accessories that need vaccum front and rear or use additional ports on the carb.

most carbs have a specific port on the back for either a PCV or brake booster. so use that port for the brakes.

use the rear port to run the A/C climate control system on the intake. on the front use that port to run the PCV system.

this will balance out the vaccum usage in the intake manifold.

additionally each vacuum system has a special reed valve that acts like a battery capacity for vacuum.

the booster has the valve at the top of the unit and it should be checked for operation a blown reed can cause extra strain the vacuum capacity and act like a vacuum leak. the booster is designed to have 2 good heavy brake assists before it runs out of vacuum the reed valve holds this extra vacuum capacity inside the booster and it compensates for the engine outside of cruise when the load is going up and down on the engine.

so the engine is making vacuum and the booster has capacity to store some of this vacuum which helps balance the vacuum usage out.

if you have a A/C car then you also have a vacuum canister on the passenger side of the engine bay near the charcoal canister for the vapor return system. this canister acts as a vacuum storage battery for the climate control system.

it stores vacuum when engine vacuum is high just like the brake booster.

it also has a reed valve inside that should be inspected and tested as that can also act as a vacuum leak.

additional Vacuum systems on a mustang would be ram air intake which would run off the pcv port or a separate port sometimes on the front base of the carb.

on a cougar you would have the headlight vacuum motors

since you are hunting for a possible vacuum leak, what you should do is disconnect and cap all vacuum lines, using a vacuum gauge and your senses see if the engine runs better that would rule out a problem with the intake work you have done, if the engine runs bad with all vacuum lines capped then you know you have a top end leak somewhere.

if the engine runs correctly then slowly plug in one vacuum system at a time until the problem occurs then you will know what is causing a possible vacuum leak.

if you have a automatic trans make sure to check the transmission modulator or modulator lines for possible leaks.

a bad modulator will also suck trans fluid into the intake so if trans fluid keeps disappearing and the engine runs poor you want to look at the modulator.

sometimes the hose falls off the modulator and you get a mystery vacuum leak.

just more things to check.

 

[timachone]

Wow, the first time I understand the whole thing and how it functions!

Great - very well written, thank you, 72Hcode ::thumb::

It leaves only a few questions to me:

My 351C 4V from 1973 with a 4-speed manual was not an original A/C-car but was equipped with that EGR-system which required a two-pipe-DVCV.



Now all that EGR-system has gone long time before and I have rebuilt my engine more in direction to an 1971 351C 4V with '70 4V-heads, '70 4V-carb and a '71-style ram air system on an old school Offenhauser intake. My dizzy is an original '70 single point with 2 vacuum outlets.

Now I am on that point to decide whether I plug in a DVCV for working, a dummy-DVCV with plugged pipes or a complete dummy-plug.

Does a functional DVCV give me any advantage like described or is it neglectable?

I like the idea of a functional and original looking system but I am also aware of additional failure possibilitys I would create.

I am more into the "do-it-with-a-DVCV"-thing at the moment

If with a DVCV I would connect it like this:





I think of it as an additional System which could give me more cooling when I am in a long line of other cars in front of a mustang meeting in a hot summer for example

What do you think?

Tim

 

[Elizabeth73]

72Hcode, thank you so much for all of that information! Ill work on the car later tomorrow and make a small check list based on your points.

 

[imusa76]

here is how my 302 is setup.

I hope this also helps.

 

[72HCODE]

EGRs are a whole 'nother wall of text.

basically any EGR car late 73 and beyond they had to change to dual diaphragm distributor but also a dual transmission modulator

when EGRs came in for 73 they were a new emissions device and they discovered when the EGR would kick off as you drove the there was a positive burst of pressure which caused the trans to mis-shift and screwed up the ignition timing from the vacuum advance.

so the early to mid 73s usually had a single diaphragm, then ford got complaints and through testing found out they needed yet another band-aid for the emissions kludge , thus for late 73 dual diaphragm appeared. On an EGR equipped car you actually want them cause they reduce some of the weirdness that the whole EGR system caused.

EGR stinks in general as it just chokes the car with its own exhaust in an effort to burn off some emissions gases before leaving the tailpipe.

you can live without the DVCV

its a pretty cool little trick that ford used to make the car work better when it was overheating.

but just understand the 2 hose version is the temperature sensor for when the EGR valve was suppose to open. so it is different from the 3 hose DVCV.

the way the EGR worked with exhaust gas recycle meant the engine had to be HOT.. so it had to be off when the car was cold and warming up or the emissions were even worse out the tail pipe. Ford retarded the timing even more for 73 it was like 4-6 BTC, that got the engine warmed up fast since the exhaust manifolds were known to go Cherry hot at that point the temp sensor kicked the EGR off and the engine started to burn its own exhaust gases....

the result is the engine gets full of carbon and the carb usually gets clogged with some junk, but the EGR would basically clog itself up and stop working and then you got all kinds of stalling, backfire issues and hard starts.

but basically if your cooling system is in good shape then there is no benefit to a DVCV, it would be a similar effect to turning on the heater with the fan full blast on a hot day for additional cooling. or you could manually switch to full manifold vaccum if your hose is long enough.

it is a nice thing to have on your car but not a necessary evil

 

[timachone]

72HCODE,

wow - that's a lot of good information! Thank you very much - now I have a lot more knowledge about this systems and a better understanding of the good old technology.

Fantastic - I love this community!

Thank you very much,

Tim

 

[72HCODE]

Sometimes emmisions aren't evil just misunderstood. There was positive benefit to some of the systems that people ripped off, like the vapor collection system or the pcv that helped keep the fuel system and engine top end moisture and fuel varnish free and also helped the engine oil stay cleaner longer.

Things like the thermographer or smog pump was crap. Later egr systems were better but still robbed power then of course the whole first generation of catalytic converter system were terrible. Took another 20-25 years for them to perfect them.

People do forget living in the city in the 1950-1980s was super dirty from all the cars and stagnant air, acid rain was pretty real and buildings used to get eaten from the acid that formed in the air. You get a whiff from time to time driving your mustang around but imagine how clogged the air was back then with millions of pre emmisions cars on the road.

Fun

 

[timachone]

Yeah, you are right!

You can even smell the difference today - behind a driving Mustang from the good old days. The burned gases smell different than from modern cars. They are unfiltered. There are no catalytic converters and electronic assistants in the way - every time has it's up- and downside...

Now we have a little less air pollution at least, that is an important point. Although not every method and sanction leads to success...

Tim

 

[Bill73Ragtop]

A number of years ago I had a intake manifold leak on my 351C that drove me crazy trying to figure out where it was leaking from. I spent hours and hours trying to figure out exactly where it was coming from. Never did pin down the location so one day I pulled the intake, cleaned everything up and re-installed it. That solved the problem.

You have some great advice on what to check, but sometimes its the nature of the beast to go back to square one. Here's hoping you find the source of it without having to pull everything apart.

 

[Elizabeth73]

Well I found out a website I got information from was wrong. My plugs should not be gapped at 42, but at 35 according to a ford book.. I am hoping fixing this today will help solve my problem. I was tuning in the mixture screws in the carb for a little bit yesterday and it runs better, id say it just about there, but you can hear that something is not right.

The search continues. Ill knock possibilities out one by one. I need to replace my spark plug wires, they are original equipment. Do you think there is a possibility they could be at fault (remember, uneven idle)?

 

[timachone]

Of course the could!

Bad spark plug wires could always be a problem-causer in this direction... And they should be separated from each other right! Or you could have bad misfiring...

Depending on the firing order of a 351C:

Since plug 6 and 5 fire sequentially they shouldn't run right next to each other all the way to the spark plug. You could have a crossfire problem between 6 and 5 depending on the condition of the plug wires.

1 and 3 will be also better separated:



Tim

 

[72HCODE]

oh yes,, .042 gap can be a big problem.

first gap back to .035, even if you have an electronic ignition and a high powered coil stick with .035 (the trade off is the spark plug wears faster so after 20-30,000 miles you need a new set,,, woopy do i say)

with our cars the ignition system usually on the weak side no matter what, some cars can tolerate even a .050 gap... most do not.

that can cause unburned gas in the mufflers, backfires, loss power and vaccum.

so go back,,, gap to .35 take your time to get the gaps nice and even with the plug gapper.

get all the wires nice and tight use dielectric grease as needed.

neating up the wires can help small misfires or a certain rpm stumble due to cross fire.

next on the carb readjust the idle bleeds to 1.5 turns from lightly seated.

then test from there.

uneven idle can be caused from a couple of things. a vacuum leak, valves need tightening or you need head work, it can also be caused by leaning out the idle air bleeds too much and weak spark.. plus more things, like improperly adjusted secondaries or improperly adjusted primary where the transition slot is exposed too far by other tuning problems. something minor can mask as something major. never hurts to do a compression test sometimes if there is a question in the engine.

 

[Elizabeth73]

Thanks for the fast responses guys. I will re gap the plugs. I am hoping that has been the trouble. Ill be replacing the wires come pay day. I really don't want to waste time trying to find vacuum leaks if it is something like this.

Weird thing is the car drives great when it isnt at a stop.

 

[72HCODE]

if you have a PCV you want to inspect it as well, that can cause vacuum leaks at idle if something is wrong.

usually a vacuum leak will disappear at higher rpms because the engine is running fast enough to overcome any issues a small leak can cause.

but under 1000 rpms the problem will come back. change one thing at a time and then retest the car. next i would plug all vacuum lines to the manifold, and even disconnect and plug the brake booster for testing.

it is not uncommon for a brake booster to be the source of a leak, or a vacuum advance or trans modulator for automatics.

i found out my leak was from my booster when i put a hand vac on the vacuum input and found it would not hold vacuum.