78 C3 Vacuum readings
01-12-2018, 06:43 PM
#21
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St. Jude Donor '22
You are asking the right questions and going the right direction toward evaluating your engine's capability to make vacuum. But, to make a proper assessment, you need to disconnect ALL vacuum systems from your engine, FIRST!
What you want to know is how much vacuum your engine is CAPABLE of making, rather than what is it doing 'as it sits'. The various vacuum systems in your car could have leakages within them which will lower that 'potential' vacuum level. Also, timing adjustments can have a pronounced effect on an engine's vacuum level.
You first need to know what vacuum your engine COULD produce; then re-connect all the vacuum systems to determine how much leakage is present (by how much the vacuum level drops because of those reconnections). Then, if you need to diagnose and fix any vacuum system leaks, you can do so.
What you want to know is how much vacuum your engine is CAPABLE of making, rather than what is it doing 'as it sits'. The various vacuum systems in your car could have leakages within them which will lower that 'potential' vacuum level. Also, timing adjustments can have a pronounced effect on an engine's vacuum level.
You first need to know what vacuum your engine COULD produce; then re-connect all the vacuum systems to determine how much leakage is present (by how much the vacuum level drops because of those reconnections). Then, if you need to diagnose and fix any vacuum system leaks, you can do so.
16" is about what I get with a mild cam in my 350 (it was 17" with the std cam). If you run your engine then turn it off and can still raise and lower the headlights after a couple of minutes with the engine off your vac system is not leaking or not enough to affect the vac measurement JMPO.
17-20 inches is for a stock cammed engine. The camshaft used will affect idle vacuum readings.
as stated above, take a reading with all else disconnected (including PCV). A healthy engine will have a solid reading.
If you have an aftermarket cam I would expect lower numbers than 17-20 but contact the manufacturer and find out what the idle vacuum should be with their cam.
as stated above, take a reading with all else disconnected (including PCV). A healthy engine will have a solid reading.
If you have an aftermarket cam I would expect lower numbers than 17-20 but contact the manufacturer and find out what the idle vacuum should be with their cam.
01-12-2018, 06:46 PM
#22
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Thread Starter
and another one placed in line to provide vacuum for the Cruise Control, with a check valve separating it from the other vacuum lines. Which will need another canister???
I do not know. I am trying to learn what I need and need to do.
Are the numbers I posted what you were looking for??
Last edited by Douglas Brown; 01-12-2018 at 06:57 PM.
01-12-2018, 10:19 PM
#23
Team Owner
You left the vacuum line going to the brake booster connected. Why? Were you going to need the brakes while the car was idling in your driveway? Maybe you have a vacuum leak in the brake booster.
You ask questions, but you can't follow directions. Pull the brake booster vacuum line out of the booster and seal off the end of the check valve. That wasn't so hard, was it?
Take the vacuum reading at idle (engine fully warmed up) and record it. Then, while still at idle, loosen the distributor lock down and very gradually rotate the distributor (both directions--just a few degrees) to see if the vacuum increases. Find the point at maximum vacuum. THAT will be the reading that gives you the maximum vacuum potential for your engine. Lock the dizzy back down and start hooking your vacuum systems back--ONE AT A TIME--and measure the vacuum after each connection. If you see the vacuum level drop, you have some leakage in the system you just hooked up. Use this strategy to identify the system[s] which have leakage. You can stay connected to one system so that you can "chase" that leak until you find its specific source.
Once your get all the leaks found and fixed you will be good to go. I suspect that the headlamp relays are a likely source of leakage. The brake booster (if original) may also be a leakage source.
Wish you success on your search.
You ask questions, but you can't follow directions. Pull the brake booster vacuum line out of the booster and seal off the end of the check valve. That wasn't so hard, was it?
Take the vacuum reading at idle (engine fully warmed up) and record it. Then, while still at idle, loosen the distributor lock down and very gradually rotate the distributor (both directions--just a few degrees) to see if the vacuum increases. Find the point at maximum vacuum. THAT will be the reading that gives you the maximum vacuum potential for your engine. Lock the dizzy back down and start hooking your vacuum systems back--ONE AT A TIME--and measure the vacuum after each connection. If you see the vacuum level drop, you have some leakage in the system you just hooked up. Use this strategy to identify the system[s] which have leakage. You can stay connected to one system so that you can "chase" that leak until you find its specific source.
Once your get all the leaks found and fixed you will be good to go. I suspect that the headlamp relays are a likely source of leakage. The brake booster (if original) may also be a leakage source.
Wish you success on your search.
01-13-2018, 10:25 AM
#24
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Thread Starter
You left the vacuum line going to the brake booster connected. Why? Were you going to need the brakes while the car was idling in your driveway? Maybe you have a vacuum leak in the brake booster.
You ask questions, but you can't follow directions. Pull the brake booster vacuum line out of the booster and seal off the end of the check valve. That wasn't so hard, was it?
Take the vacuum reading at idle (engine fully warmed up) and record it. Then, while still at idle, loosen the distributor lock down and very gradually rotate the distributor (both directions--just a few degrees) to see if the vacuum increases. Find the point at maximum vacuum. THAT will be the reading that gives you the maximum vacuum potential for your engine. Lock the dizzy back down and start hooking your vacuum systems back--ONE AT A TIME--and measure the vacuum after each connection. If you see the vacuum level drop, you have some leakage in the system you just hooked up. Use this strategy to identify the system[s] which have leakage. You can stay connected to one system so that you can "chase" that leak until you find its specific source.
Once your get all the leaks found and fixed you will be good to go. I suspect that the headlamp relays are a likely source of leakage. The brake booster (if original) may also be a leakage source.
Wish you success on your search.
You ask questions, but you can't follow directions. Pull the brake booster vacuum line out of the booster and seal off the end of the check valve. That wasn't so hard, was it?
Take the vacuum reading at idle (engine fully warmed up) and record it. Then, while still at idle, loosen the distributor lock down and very gradually rotate the distributor (both directions--just a few degrees) to see if the vacuum increases. Find the point at maximum vacuum. THAT will be the reading that gives you the maximum vacuum potential for your engine. Lock the dizzy back down and start hooking your vacuum systems back--ONE AT A TIME--and measure the vacuum after each connection. If you see the vacuum level drop, you have some leakage in the system you just hooked up. Use this strategy to identify the system[s] which have leakage. You can stay connected to one system so that you can "chase" that leak until you find its specific source.
Once your get all the leaks found and fixed you will be good to go. I suspect that the headlamp relays are a likely source of leakage. The brake booster (if original) may also be a leakage source.
Wish you success on your search.
Yes as you said and I have read, I disconnected all vac, move distb. back and forth. look for highest Vac. set distrib. hook up each line looking for change. There was no change. That is what caused me to look at the elevation question; and at the main jets for lean at this elevation of 4,400 ft. my Vac is 11-12 "hg. FOR this elevation 12.75 - 17.75 "hg, (at sea level 17-22) As I hooked up each line it did not change 11-12"hg The only change was went I increased or lowered rpm and fuel mixture. THIS what I have.
NOW back to the question I had in the first place....HOW much vacuum is required to operate the Vac System in this car??
AND does what I have meet that requirement??[ I have been told 14"hg just for the Cruise Control. Some one else said If you can get the lights to go up and down after a few minuets of turning off the ignition your good to go no leaks. OK]
BECAUSE if it dose not meet the needed requirements then I need additional vac OR some way to increase what I have.
I am under the impression that vacuum is constant. TO me that means 11-12 "hg is all I can reach and hold as the system uses it. It will not increase unless I install a pump that can get say 15- 20"hg constant. If as I was asked;" to test the Vacuum upon rapid deceleration and record same" I did that and posted it. It did increased. could this be saved until needed.? would those small ast. tanks sold do that?.
Last edited by Douglas Brown; 01-13-2018 at 10:30 AM.
01-13-2018, 10:44 AM
#25
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Thread Starter
Hey look guys I am not trying to be a hard head with this vacuum thing.
I have learned some things about it I did not even think of, like elevation difference, -1" for every 1,000 ft. means the most I can get is
12.75 - 17.75 "hg. everyone assumed 17-22 "hg (sea level)
Stock cam, a mild cam reduces that a little. 11-12 "hg is all I can get. Carb metered jets are figured at 2% per every 1,500 ft. of elevation. Most would not know that.
everything in the car seems to work with the Vacuum I have, but I question that because of what has been said about needing 14"hg to operate Cruise.
I have learned some things about it I did not even think of, like elevation difference, -1" for every 1,000 ft. means the most I can get is
12.75 - 17.75 "hg. everyone assumed 17-22 "hg (sea level)
Stock cam, a mild cam reduces that a little. 11-12 "hg is all I can get. Carb metered jets are figured at 2% per every 1,500 ft. of elevation. Most would not know that.
everything in the car seems to work with the Vacuum I have, but I question that because of what has been said about needing 14"hg to operate Cruise.
Last edited by Douglas Brown; 01-29-2018 at 05:38 PM.
01-13-2018, 11:44 AM
#26
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St. Jude Donor '22
goodness.
to be clear.
you measured 15 in at 1500rpm?
if so, plenty to run cruise.
you sure like to make things difficult.
you bounce from 4 thousand feet alt to sea level.
easy enough to add a hand held vacuum pump to cruise diaphram and see it it can hold 1500 rpm at 15 in.
that is all that is happening in the vac side of the system.
did you run the electrical tests?
engage button works?
on button works?
hold circuit?
if yes, the transducer is bad.
to be clear.
you measured 15 in at 1500rpm?
if so, plenty to run cruise.
you sure like to make things difficult.
you bounce from 4 thousand feet alt to sea level.
easy enough to add a hand held vacuum pump to cruise diaphram and see it it can hold 1500 rpm at 15 in.
that is all that is happening in the vac side of the system.
did you run the electrical tests?
engage button works?
on button works?
hold circuit?
if yes, the transducer is bad.
01-13-2018, 12:51 PM
#27
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Thread Starter
[QUOTE=calwldlife;1596369685]goodness.
to be clear.
you measured 15 in at 1500rpm?
if so, plenty to run cruise.
I was just talking to the wife about that, 2K rpm 12-19 'hg, 3K rpm 14-20' hg, 4K rpm
11-21"hg. so during operation/running there would be plenty of Vacuum. Thank ou to L-46 for asking me to run the test.
you sure like to make things difficult.
you bounce from 4 thousand feet alt to sea level.
not meant to, but it is what it is.
easy enough to add a hand held vacuum pump to cruise diaphragm and see it can hold 1500 rpm at 15 in.
Yes, it is and I should have bought one before now, but life can get in the way
that is all that is happening in the vac side of the system.
did you run the electrical tests?
Started retracing wires I installed, difficult to get under dash and not get stuck or see what I am doing
engage button works?
Not sure, it is the kind with no markings, just button. I have been pushing it in all the way, which I just found out was the way to terminate the system, I should have been barely pushed it in to activate, and push it in about 1/2 way a hold ?? This area may be operator training.
on button works?
Do not know yet
hold circuit?
Same
if yes, the transducer is bad.
OK, I will keep that in mind as I test it today.
to be clear.
you measured 15 in at 1500rpm?
if so, plenty to run cruise.
I was just talking to the wife about that, 2K rpm 12-19 'hg, 3K rpm 14-20' hg, 4K rpm
11-21"hg. so during operation/running there would be plenty of Vacuum. Thank ou to L-46 for asking me to run the test.
you sure like to make things difficult.
you bounce from 4 thousand feet alt to sea level.
not meant to, but it is what it is.
easy enough to add a hand held vacuum pump to cruise diaphragm and see it can hold 1500 rpm at 15 in.
Yes, it is and I should have bought one before now, but life can get in the way
that is all that is happening in the vac side of the system.
did you run the electrical tests?
Started retracing wires I installed, difficult to get under dash and not get stuck or see what I am doing
engage button works?
Not sure, it is the kind with no markings, just button. I have been pushing it in all the way, which I just found out was the way to terminate the system, I should have been barely pushed it in to activate, and push it in about 1/2 way a hold ?? This area may be operator training.
on button works?
Do not know yet
hold circuit?
Same
if yes, the transducer is bad.
OK, I will keep that in mind as I test it today.
Last edited by Douglas Brown; 01-13-2018 at 12:54 PM.
01-13-2018, 01:46 PM
#28
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St. Jude Donor '22
a test light will do.
connections are on the transducer.
4 connections if i remember.
connect test light, 12vdc or meter, to the
on circuit, the button circuit, resume, etc.
only need to go inside car to use buttons, levers.
find the wiring diagram so you know which wires on ducer
are what.
all wires are under a plastic box type cover.
get a vac pump with gauge to verify
the inches to move/hold 1500rpms.
until you rule out something you can't go on.
pain in the butt to test the wires
connections are on the transducer.
4 connections if i remember.
connect test light, 12vdc or meter, to the
on circuit, the button circuit, resume, etc.
only need to go inside car to use buttons, levers.
find the wiring diagram so you know which wires on ducer
are what.
all wires are under a plastic box type cover.
get a vac pump with gauge to verify
the inches to move/hold 1500rpms.
until you rule out something you can't go on.
pain in the butt to test the wires
The following users liked this post:
Douglas Brown (01-13-2018)
01-13-2018, 03:27 PM
#29
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Thread Starter
a test light will do.
connections are on the transducer.
4 connections if i remember.
connect test light, 12vdc or meter, to the
on circuit, the button circuit, resume, etc.
only need to go inside car to use buttons, levers.
[I[B]]find the wiring diagram so you know which wires on ducer
are what.
all wires are under a plastic box type cover.
get a vac pump with gauge to verify
the inches to move/hold 1500rpms.
until you rule out something you can't go on.
pain in the butt to test the wires
connections are on the transducer.
4 connections if i remember.
connect test light, 12vdc or meter, to the
on circuit, the button circuit, resume, etc.
only need to go inside car to use buttons, levers.
[I[B]]find the wiring diagram so you know which wires on ducer
are what.
all wires are under a plastic box type cover.
get a vac pump with gauge to verify
the inches to move/hold 1500rpms.
until you rule out something you can't go on.
pain in the butt to test the wires
[B][I]
as to CC button no response when holding it in....So it is either electrical or Transducer. Will will see about hand held Vac tester
Last edited by Douglas Brown; 01-13-2018 at 03:30 PM.
01-13-2018, 03:39 PM
#30
Pro
Thread Starter
a test light will do.
connections are on the transducer.
4 connections if i remember.
connect test light, 12vdc or meter, to the
on circuit, the button circuit, resume, etc.
only need to go inside car to use buttons, levers.
Is there power to switch when key on??
Like the brake peddle stop light switch, key on power to 1/2 the switch
IF I check at the transducer will I have power?? at one of the connection or do I have to push in the button. ??
find the wiring diagram so you know which wires on ducer
are what.
all wires are under a plastic box type cover.
There are two wires at the ducer. hold and terminate??
I get NO power to switch at peddle.
get a vac pump with gauge to verify
the inches to move/hold 1500rpms.
until you rule out something you can't go on.
pain in the butt to test the wires
connections are on the transducer.
4 connections if i remember.
connect test light, 12vdc or meter, to the
on circuit, the button circuit, resume, etc.
only need to go inside car to use buttons, levers.
Is there power to switch when key on??
Like the brake peddle stop light switch, key on power to 1/2 the switch
IF I check at the transducer will I have power?? at one of the connection or do I have to push in the button. ??
find the wiring diagram so you know which wires on ducer
are what.
all wires are under a plastic box type cover.
There are two wires at the ducer. hold and terminate??
I get NO power to switch at peddle.
get a vac pump with gauge to verify
the inches to move/hold 1500rpms.
until you rule out something you can't go on.
pain in the butt to test the wires
01-13-2018, 08:54 PM
#31
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Location: Southern Cal Ca
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St. Jude Donor '22
ok then.
i can't stress enough to get a good guide to
the electrical side of the cruise system.
the cruise must be in "on" position to energize the
electrical.
factory service manual even has the car in the air so wheels will turn
when testing speed.
Went thru this a while ago and basically my transducer is **** up.
best of luck
i can't stress enough to get a good guide to
the electrical side of the cruise system.
the cruise must be in "on" position to energize the
electrical.
factory service manual even has the car in the air so wheels will turn
when testing speed.
Went thru this a while ago and basically my transducer is **** up.
best of luck
01-13-2018, 09:19 PM
#32
Pro
Thread Starter
ok then.
i can't stress enough to get a good guide to
the electrical side of the cruise system.
the cruise must be in "on" position to energize the
electrical.
factory service manual even has the car in the air so wheels will turn
when testing speed.
Went thru this a while ago and basically my transducer is **** up.
best of luck
i can't stress enough to get a good guide to
the electrical side of the cruise system.
the cruise must be in "on" position to energize the
electrical.
factory service manual even has the car in the air so wheels will turn
when testing speed.
Went thru this a while ago and basically my transducer is **** up.
best of luck
DB
01-15-2018, 11:11 AM
#33
Pro
Thread Starter
Spent day upside down on back under dash.
Results of find: wire colors do not correspond to wire diagram.
too many wires, does not match diagram.
Removed vac hose and wires, they were taped together.
wire is of old style single strand hard & brittle.
will have to rewire harness.....with correct colors.
Lesson learned: take nothing for granted double check before install.
It appeared I had missed wired it to begin with. Due to not having a cruise in the car, nothing to go on, at the time.
01/15/2017
Began rebuilding harness and install same.
01/17/17
After making sure I have the wire connections in the correct place I spent two days trying to re-attach the vac line to the vac release switch mounted above the brake switch. I ended up dropping the steering column, applying some white lithium grease to the end of the Vac. tube, so I could get my fat hand and short finger could reach the connection.
01/18/17
Will finish buttoning up the lower dash area and take it out to see if it works.
The circuit should be as follows: Fuse box (ignition term.),to Brake light switch connection, to Cruise Control connection, when button is pushed to engage, electric current should be at Transducer waiting to hold. To disengage press brake peddle, which disconnects vac. and electric.
If it dose not work then look at Transducer.???
Results of find: wire colors do not correspond to wire diagram.
too many wires, does not match diagram.
Removed vac hose and wires, they were taped together.
wire is of old style single strand hard & brittle.
will have to rewire harness.....with correct colors.
Lesson learned: take nothing for granted double check before install.
It appeared I had missed wired it to begin with. Due to not having a cruise in the car, nothing to go on, at the time.
01/15/2017
Began rebuilding harness and install same.
01/17/17
After making sure I have the wire connections in the correct place I spent two days trying to re-attach the vac line to the vac release switch mounted above the brake switch. I ended up dropping the steering column, applying some white lithium grease to the end of the Vac. tube, so I could get my fat hand and short finger could reach the connection.
01/18/17
Will finish buttoning up the lower dash area and take it out to see if it works.
The circuit should be as follows: Fuse box (ignition term.),to Brake light switch connection, to Cruise Control connection, when button is pushed to engage, electric current should be at Transducer waiting to hold. To disengage press brake peddle, which disconnects vac. and electric.
If it dose not work then look at Transducer.???
Last edited by Douglas Brown; 01-18-2018 at 12:27 PM. Reason: additional info
01-19-2018, 12:37 PM
#36
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Thread Starter
Yes, wires.....And trusting those who are to know better, will not take advantage of some 's lack of knowledge. And My assuming they would not do that only because I would not treat them that way.
So much for golden rule.
Thanks for the help and support.
DB
Last edited by Douglas Brown; 01-19-2018 at 12:38 PM.
01-29-2018, 05:42 PM
#38
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Thread Starter
What happened to the transducer??? how did it act??
I ask because I have had two incidents where the RPM increased on it's own via cruise control and it is not engaging each time I go to use it. The increase in rpm was when coming to a stop, each time the Cruise was not engaged at the time????
01-29-2018, 07:48 PM
#40
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St. Jude Donor '22
my transducer is bad,
windings? whatever, it don't work.
you mentioned a brake pedal switch.that disengages the cruise.
been too many years since i trouble shot my cruise.
seems there is an adjustment for speed matching.
tires in the air in gear running 45mph? and some adjustment on the transducer,
good luck
windings? whatever, it don't work.
you mentioned a brake pedal switch.that disengages the cruise.
been too many years since i trouble shot my cruise.
seems there is an adjustment for speed matching.
tires in the air in gear running 45mph? and some adjustment on the transducer,
good luck