Can someone tell me about the Thermo Vacuum sensor?
07-15-2004 | 02:44 PM
#17
Instructor
Joined: Mar 2003
Posts: 171
Likes: 0
From: Berkeley, California
resurrected!
well, this answers my question (post from yesterday), and explains why my car was running rich. woo-hoo! now hopefully i fixed enough real vacuum leaks that it will not run too lean, and i'll pass emissions.
still curious about the boost question, though--what happens when there is excess pressure in the line going to the fuel exhaust breather valve? maybe it's a check valve, so nothing!
cheers!
A
well, this answers my question (post from yesterday), and explains why my car was running rich. woo-hoo! now hopefully i fixed enough real vacuum leaks that it will not run too lean, and i'll pass emissions.
still curious about the boost question, though--what happens when there is excess pressure in the line going to the fuel exhaust breather valve? maybe it's a check valve, so nothing!
cheers!
A
07-15-2004 | 02:53 PM
#18
Race Director
Joined: Jul 2001
Posts: 14,075
Likes: 3
From: Santa Barbara, CA
Just so there's no mis-understanding, there's three separate lines.
1. There's a vacuum line going from underneath the throttle-body to one side of the "wabbit ears" above.
2. Then another vacuum line going out from the ears to the canister purge valve.
3. Then a third line going from the evaporative canister to the rubber J-boot in front of the turbo.
The 3rd line is the simplest because it's always under vacuum. Due to the turbo sucking in air from the J-boot and the restriction of the AFM, there's always vacuum here to suck up gas fumes from the evaporative canister and the crankcase vent.
The 1st & 2nd lines get their vacuum from that tiny little hole at the bottom of the throttle-body. That hole is situated such that there's only vacuum when the throttle-plate is open. You don't want fumes coming in at idle because due to the low volume of air involved, it could affect your mixtures enough to cause rough idling and emissions failure.
Even under boost, there's still vacuum in this line because it's at right-angles to the flow. But the line itself doesn't have flow, just vacuum or no vacuum. That's because it dead-ends into the diaphragm of the purge valve.
1. There's a vacuum line going from underneath the throttle-body to one side of the "wabbit ears" above.
2. Then another vacuum line going out from the ears to the canister purge valve.
3. Then a third line going from the evaporative canister to the rubber J-boot in front of the turbo.
The 3rd line is the simplest because it's always under vacuum. Due to the turbo sucking in air from the J-boot and the restriction of the AFM, there's always vacuum here to suck up gas fumes from the evaporative canister and the crankcase vent.
The 1st & 2nd lines get their vacuum from that tiny little hole at the bottom of the throttle-body. That hole is situated such that there's only vacuum when the throttle-plate is open. You don't want fumes coming in at idle because due to the low volume of air involved, it could affect your mixtures enough to cause rough idling and emissions failure.
Even under boost, there's still vacuum in this line because it's at right-angles to the flow. But the line itself doesn't have flow, just vacuum or no vacuum. That's because it dead-ends into the diaphragm of the purge valve.
07-15-2004 | 03:59 PM
#19
Instructor
Joined: Mar 2003
Posts: 171
Likes: 0
From: Berkeley, California
oh. hmm. great post Danno, that clears everything up. i thought the line seemed small for flow.
i noticed just what you were saying: i measured the pressure from the throttle body port (line (1) above), and it would pull a few in Hg, but never went above atmosphere (although this was with the car sitting, so i guess it wouldn't go above atmosphere anyways due to no load?)
but that's a real bummer, because it means that in bypassing the 'thermo valve' i really wasn't doing anything wrong, and unless the car was tested cold, the results shouldn't change much (e.g. the breather valve is always connected to the throttle body vacuum). i'll get a new switch anyways ($30).
so the rich results from my latest test might be valid. hmm... i'm starting to think O2 sensor. not least because if i put in a new one and splice into the wires, i can measure the voltage and use it as a diagnostic tool. can i really just get the Bosch 13xxx one for $40 and solder it in?
anyone know what the waveform on the O2 sensor should look like? i can borrow a portable oscilloscope from the lab and check it. i know 0.45V means perfect mixture.
cheers!
Andrew
i noticed just what you were saying: i measured the pressure from the throttle body port (line (1) above), and it would pull a few in Hg, but never went above atmosphere (although this was with the car sitting, so i guess it wouldn't go above atmosphere anyways due to no load?)
but that's a real bummer, because it means that in bypassing the 'thermo valve' i really wasn't doing anything wrong, and unless the car was tested cold, the results shouldn't change much (e.g. the breather valve is always connected to the throttle body vacuum). i'll get a new switch anyways ($30).
so the rich results from my latest test might be valid. hmm... i'm starting to think O2 sensor. not least because if i put in a new one and splice into the wires, i can measure the voltage and use it as a diagnostic tool. can i really just get the Bosch 13xxx one for $40 and solder it in?
anyone know what the waveform on the O2 sensor should look like? i can borrow a portable oscilloscope from the lab and check it. i know 0.45V means perfect mixture.
cheers!
Andrew
07-15-2004 | 06:54 PM
#20
Three Wheelin'
Joined: Jan 2003
Posts: 1,480
Likes: 3
From: Richmond Virginia
This "switch" is designed to open when coolant temperature reaches 58 degrees C. Since there is no compensation for the slightly richer mixture when the cannister is purged, 58 degrees was chosen to insure the engine would not stumble or hesitate during the purge.
The vacuum signal comes from a port in the throttle body. The port is upstream of the throttle plate, and vacuum is only present just as the throttle plate starts to open. This vacuum is very small, about 5" Hg or so, but it is enough to open the first control valve.
When the first control valve opens, hi vacuum opens the actual purge valve, which uses hi vacuum from the intake to purge the charcoal cannister.
By-passing the switch with "normal" or high vacuum will cause the system to purge continuously which is the equivelant of a rather large vacuum leak.
You can disconnect the vacuum lines to the switch, but be sure to blank off the nipple at the throttle body.
The vacuum signal comes from a port in the throttle body. The port is upstream of the throttle plate, and vacuum is only present just as the throttle plate starts to open. This vacuum is very small, about 5" Hg or so, but it is enough to open the first control valve.
When the first control valve opens, hi vacuum opens the actual purge valve, which uses hi vacuum from the intake to purge the charcoal cannister.
By-passing the switch with "normal" or high vacuum will cause the system to purge continuously which is the equivelant of a rather large vacuum leak.
You can disconnect the vacuum lines to the switch, but be sure to blank off the nipple at the throttle body.
07-15-2004 | 08:04 PM
#21
Instructor
Joined: Mar 2003
Posts: 171
Likes: 0
From: Berkeley, California
i see--thanks!
i was only bypassing the switch--linking the line from the throttle plate to the control valve, so it should have only been the normal low vacuum. the only part i was bypassing was the switch, so it should behave normally, except when the coolant is below 58C it will be purging when normally it wouldn't. i didn't hook continuous vacuum up to the switch, so it should only purge when the signal comes from the throttle.
so there is a purge valve in addition to the fuel exhaust breather (control) valve? which one is it? where is it?
for the purposes of emissions, what if i capped the line from the throttle plate, and then disconnected and capped the line from the canister to the intake boot. will i get huge fumes from the line from the canister?
cheers!
-A
i was only bypassing the switch--linking the line from the throttle plate to the control valve, so it should have only been the normal low vacuum. the only part i was bypassing was the switch, so it should behave normally, except when the coolant is below 58C it will be purging when normally it wouldn't. i didn't hook continuous vacuum up to the switch, so it should only purge when the signal comes from the throttle.
so there is a purge valve in addition to the fuel exhaust breather (control) valve? which one is it? where is it?
for the purposes of emissions, what if i capped the line from the throttle plate, and then disconnected and capped the line from the canister to the intake boot. will i get huge fumes from the line from the canister?
cheers!
-A