Welcome to the Corvette Forums at the Corvette Action Center!

Question: Vacuum leaks

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
Hi guys just wondering how I will know when I've found all of my vacuum leaks? Will the idle become smooth? Or does everything else have to be right for that to happen?
The reason I ask is I've had to make a lot of changes to things from how the car was when got it.
here's what I've done so far

fitted o2 sensor, connect vacuum to vacuum sensor, corrected routing of emissions hoses. At this point the idle went up from about 700 to 1000 so dropped it back down. Fitted choke plate.
replaced headlight actuator seals, relays and vacuum lines. New spark plugs, cap, rotor, and coil. Corrected mix solenoid travel on carb, and adjusted position of idle air bleed screw (was bottomed out, causing dwell to be at 60 the whole time). Replaced emissions hoses. Replaced rocker cover gaskets. Fixed exhaust leak at manifold.

Dwell is still below 30 (but I'll probably need to adjust this once I've found all remaining vacuum leaks). Idle is up to 800.
I believe there is another leak at the headlights. I turned them on yesterday after having the car parked for 3hrs and the left light came all the way up, the right about half way up. But after parking the car with the lights up, a hissing can be heard from the front and the lights can't be lowered without restarting the engine.
 

jj_long81

Well-known member
Joined
May 1, 2009
Messages
121
Location
florida
Corvette
L81 origional
hi antz sound like you doing what i just got done doing. do you have a mighty vac or similar vac gauge/pump. did you spray wd 40 around the carb base gasket and the intake manifold where it meet the heads with the engine running? the dwell can be set. make sure you confirm all the leaks are fixed first.
 

Vettehead Mikey

Well-known member
Joined
Aug 26, 2003
Messages
3,486
Location
Not that far from Ottawa
Corvette
1973 Coupe
There's obviously still a major leak somewhere in the headlight/hvac system. The only real effective way of fully fixing it is to test each component while isolated from all others. This is where most owners make a mistake by assuming that the component that doesn't work as expected is the one that is faulty.

As for the uneven idle, eliminate the headlight system from the list of potential causes by disconnecting the system at the intake manifold and capping the port.
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
There's obviously still a major leak somewhere in the headlight/hvac system. The only real effective way of fully fixing it is to test each component while isolated from all others. This is where most owners make a mistake by assuming that the component that doesn't work as expected is the one that is faulty.

As for the uneven idle, eliminate the headlight system from the list of potential causes by disconnecting the system at the intake manifold and capping the port.

Thanks, I think the leak must be on a relay, seal, or one of the green hoses. Since it doesn't seem to leak when the headlights are down. Only way to know know is testing them. If the distill rough after that I'll cap the port and see what happens.
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
hi antz sound like you doing what i just got done doing. do you have a mighty vac or similar vac gauge/pump. did you spray wd 40 around the carb base gasket and the intake manifold where it meet the heads with the engine running? the dwell can be set. make sure you confirm all the leaks are fixed first.

Yes do have a gauge/pump. I have sprayed wd40 around the engine, didn't notice anything. Although a mechanic I was talking too said he prefers to use ether. Said it causes the engine to rev higher, and with less of a delay. So might give that a try just to be sure.
 

jj_long81

Well-known member
Joined
May 1, 2009
Messages
121
Location
florida
Corvette
L81 origional
okay then. like mikey said remove the headlight circuit from the manifold and cap it to eliminate the entire headlight system from the engine. just cap that tee where you remove it. then do a system performance test. page 6e1-11 of gm manual. if you dont have this i will send it to you in a photo or pdf. from there follow the flow charts and you should be good to go. this test should be done before you move on to any other carb calibrations or emmisions test flow charts.
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
Ok it's been a while (just busy with other things. Have fixed all leaks on the headlight circuits, (for now anyway). Car still has a rough idle even with the port for headlights plugged. Tried spraying ether as the mechanic suggested, couldn't find anything. (although i did notice if it was sprayed directly into the carb in would lower rpm, not raise it; not sure if this means anything). haven't tried the system performance check yet, (just had a coolant hose start leaking, so need to fix that first) although I'm not sure if how this is meant to tell me if i have found all of my vacuum leaks. I did notice that part of it (depending on path) requires the dwell setting, which may not work too well as my dwell was just a guess to get the dwell to vary as it should rather than a fixed 60deg (I'm certain the screw was set wrong before I touched it).
Where to from here? Should I just carry on with the system performance check and see where it leads me? or is there something else i should look at first?
I do have a rebuild kit for the carb and all the required tools (assuming the thexton 380 can be used instead of the thexton 362; seems to be the same but one socket for each instead of a combo design), but wanted to put this off until it was needed.
 

jj_long81

Well-known member
Joined
May 1, 2009
Messages
121
Location
florida
Corvette
L81 origional
hello. do you have the service manual. its clear on the course of action you should take. along the flow chart starting at a performance check will lead you to your problem. after the performance check move on to air management system check. plug the headlight circuit and do a performance check according to manual. it will reveal things your computer wont recognize.
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
hello. do you have the service manual. its clear on the course of action you should take. along the flow chart starting at a performance check will lead you to your problem. after the performance check move on to air management system check. plug the headlight circuit and do a performance check according to manual. it will reveal things your computer wont recognize.
Thanks.

Yes I do have the service manual. Once I've got the heater hoses replaced I'll do the system performance check and see where it leads me. No point in the AIR management check though; it was removed by a previous owner (At least one of these had no idea about the 81 at all, and one i would say wasn't fit to work on any car, which is probably where half the problems started)
 

jj_long81

Well-known member
Joined
May 1, 2009
Messages
121
Location
florida
Corvette
L81 origional
without the air management system in place, and if the computer is still functioning there is a problem. my system works great. its a good system that is what makes an l81 an l81. sorry for any confusion. the performance check will not help you after the engine has been altered. replacing that equipment is almost impossible and really gets expensive. your computer could probably be unpluged and it would run just the same. but, if you will never have to pass an emissions test you could put any motor you want.
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
[QUOTE_long81;1172758]without the air management system in place, and if the computer is still functioning there is a problem. my system works great. its a good system that is what makes an l81 an l81. sorry for any confusion. the performance check will not help you after the engine has been altered. replacing that equipment is almost impossible and really gets expensive. your computer could probably be unpluged and it would run just the same. but, if you will never have to pass an emissions test you could put any motor you want.[/QUOTE]

Ok so the changes I have from stock are: long tube headers with dual 2.5" exhaust with no cat. AIR EFE and EGR have been removed. For now I assume no changes (other than messing up the settings) were made to the carb.
The computer does seem to function fine.
if I understand you correctly then the performance check would only work if no changes were made?
If that is the case, how should I proceed from here?
 

jj_long81

Well-known member
Joined
May 1, 2009
Messages
121
Location
florida
Corvette
L81 origional
with all that stuff removed how is you computer not giving you a check engine light? do you have an o2 sensor in the new header? its probably not working at all or the check engine light has been removed. most guys who have gone that far replace the carb with a non ecm carb or they modify the existing ecm carb to operate without the mixture control selinoid and they also replace the cam and distriborater. the guy to ask about the switch is gonna be cliff ruggles. hes the rodchester carb guy and hes really helpful. google him.
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
with all that stuff removed how is you computer not giving you a check engine light? do you have an o2 sensor in the new header? its probably not working at all or the check engine light has been removed. most guys who have gone that far replace the carb with a non ecm carb or they modify the existing ecm carb to operate without the mixture control selinoid and they also replace the cam and distriborater. the guy to ask about the switch is gonna be cliff ruggles. hes the rodchester carb guy and hes really helpful. google him.

No check engine light as the ecm doesn't monitor any of it. There is an o2 sensor in the header. Check engine light does work. Comes on with ignition on but engine not running. When I first got the car it was giving me codes as the o2 sensor wasn't connected. As well as a few other things. but now only before i start the engine. I'll google cliff and see what he says
 

Wajulia

Well-known member
Joined
Nov 24, 2010
Messages
131
Location
Monrovia, Md
Corvette
1981 White
I agree that the ECM does not monitor any of the smog stuff, it does turn on the EGR valve and I think it controls the air diverted, but has no feedback from any of the smog stuff.

It seems like you have done a good job in sealing all the vacuum leaks, which is time well spent, but it still idles rough. Have you checked the compression of each cylinder to assure that all the cylinders are healthy? I realize that can take all day on that car, but it will provide a lot of piece of mind.

If the compression checks out, I would rebuild the carb
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
OK haven't done the compression check yet. (but will do soon).
I have notice something though, in the system performance check it says that the pink wire to the M/C solenoid should be on the right when viewed from the harness end. ( i take this to be the terminal at the front of the car). Mine seems to be at the rear. Strangely enough it is the same on my original and replacement harness, but i have seen a photo of one with it at the front. This could have quite a large effect in thing wouldn't it? since the solenoids movements would be messed up by it being energized with the wrong polarity.
Another thought would it be worth removing the EGR blanking plate to check that there's no chance of it leaking? (EGR was removed before I got the car and I'm a little skeptical about the workmanship done)
 

Peer81

Well-known member
Joined
May 21, 2003
Messages
2,497
Location
Netherlands
Corvette
'81 Black
I think polarity isn't a problem for the solenoid, either it passes the 12v or it doesn't. As you did a great job on the vacuumleaks we can role that one out. But what is your vacuumgauge saying at idle 600rpm? How much HG vacuum at the intake manifold? Maybe you have a big cam in it bigger than the ECM can take?
Other then that try to get the dwell to walk around 30 degrees, you'll need to turn the idle mixture screws far out! If yours is at 60 degrees right now I think you're lucky it's running at all. Normally the closer you get to the 30 the better it runs but never compare it to an injection engine of course. :)

Greetings Peter
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
I think polarity isn't a problem for the solenoid, either it passes the 12v or it doesn't. As you did a great job on the vacuumleaks we can role that one out. But what is your vacuumgauge saying at idle 600rpm? How much HG vacuum at the intake manifold? Maybe you have a big cam in it bigger than the ECM can take?
Other then that try to get the dwell to walk around 30 degrees, you'll need to turn the idle mixture screws far out! If yours is at 60 degrees right now I think you're lucky it's running at all. Normally the closer you get to the 30 the better it runs but never compare it to an injection engine of course. :)

Greetings Peter

Hi peter.
I'll hopefully be able to get vacuum readings tomorrow, but shouldn't I be doing this at 700rpm since I have to 4 speed?
A bigger cam in there, wouldn't that be nice :L
I have already adjusted the dwell away from 60, its now around the low 20s. Idle didn't get any smoother, but did go from 700rpm to 800rpm. It also stopped it cutting out when I put my foot down
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
Ok, got the vacuum reading done: was fluctuating around 12-14, but would occasionally drop as low as 11. (I have no idea what this means)

Also got the compression check done, I can give exact results if needed, but doesn't seem to be any problems. All cylinders around 130-140PSI.
 

Peer81

Well-known member
Joined
May 21, 2003
Messages
2,497
Location
Netherlands
Corvette
'81 Black
Are the readings with no vacuum leaks what so ever? If so, normally with a stock cam vacuum should be between 18-20 Hg vacuum (high vacuum, small cam). With a bigger cam vacuum can drop to around 15 Hg without any problem for the ECM. But below that the ECM with think you're moving the throttle open so it will "inject" some extra fuel. This will give a low dwell number. In other words if you have no vacuum leaks you have a much bigger cam in it maybe a little to big. Sensei has written a nice topic about it and how to fool the ECM when driving a bigger cam but I can't find it. It should be somewhere here in the L81 section.
When you find the topic I would also like the read it :)

Greetings Peter
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
Checked Vacuum again today. Have a video of it here.
https://www.dropbox.com/s/u2yh3rpcwy5ldp0/20151205_162105.mp4?dl=0
very similar results. (I assume the ticking that can be heard is the m/c solenoid, since it only started once i adjusted the idle air bleed screw to get the dwell away from 60)
I did notice something odd though, when I removed the bung sealing the vac port for the headlights to connect the vac gauge, the RPMs went up to 900 and was smooth before I connected the gauge, but didn't happen again after. I would have thought creating a leak would have the opposite affect. Which makes me wonder if it's running rich, but not enough to set a code.

I would be very surprised if the cam has been changed, as it doesn't seem to perform like I would expect if it has, and doesn't look like the water pump has ever been removed, but stranger things have happened.

If everyone is out of ideas i'll ask Cliff Ruggles.
 

Corvette Forums

Not a member of the Corvette Action Center?  Join now!  It's free!

Help support the Corvette Action Center!

Supporting Vendors

Dealers:

MacMulkin Chevrolet - The Second Largest Corvette Dealer in the Country!

Parts/Accessories:

Vetteskins

Advertise with the Corvette Action Center!

Double Your Chances!

Partners

Top Bottom