Checking for vacuum leaks on L98
Safety Car
Joined: Apr 2011
Posts: 3,660
Likes: 22
Mnay like the "low-blow propane torch method where a torch thats just cracked open can be moved along the vac lines and the gas if sucked inside will cause a spike or change in engine sound/rpm. Do NOT cover the thing in flamable gas...just crack it open and follow the vac lines & fittings. Safer than ether.
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
You can do the Scotty Kilmer cigar smoke method:
Another way is to remove the vacuum lines from the plenum, throttle body and intake manifold and plug the sources one by one. The engine should run better when you plug the problematic vacuum port. Actually, what you will see is that the engine will run dramatically worse when you pull off a good one.
You might have an EGR valve that's stuck in the open position. That one's kinda hard to tell.
Another common problem is a bad brake booster. Pull off the rubber hose at the booster and put your thumb over the end of the hose. Same type of test for the other vacuum sources.
Another way is to remove the vacuum lines from the plenum, throttle body and intake manifold and plug the sources one by one. The engine should run better when you plug the problematic vacuum port. Actually, what you will see is that the engine will run dramatically worse when you pull off a good one.
You might have an EGR valve that's stuck in the open position. That one's kinda hard to tell.
Another common problem is a bad brake booster. Pull off the rubber hose at the booster and put your thumb over the end of the hose. Same type of test for the other vacuum sources.
Thread Starter
Instructor
Joined: Mar 2013
Posts: 100
Likes: 2
From: Ypsilanti Michigan
Well I did the propane torch method, and couldn't detect any rpm spikes. So I don't think I have a vac leak. I have this strange problem I've been chasing for a while where my rpm's bounce around a small bit before stalling. This is after the car's warm and the idle settles to 550 rpm. When I'm at a light just idling in neutral (manual trans) the rpms will bounce slowly from 400 to 600 rpm and either stall OR after (I give a tiny bit of gas after it hits 4) the engine "bounces" up high enough it'll stick at 800 before settling back down to 550.
List of things I've replaced (not as a direct result of idling issue, just overall);
IAC
TPS
A.I.R. Pump
Alternator
Edit: Should I just clean the throttle body?
List of things I've replaced (not as a direct result of idling issue, just overall);
IAC
TPS
A.I.R. Pump
Alternator
Edit: Should I just clean the throttle body?
Le Mans Master
Joined: May 2004
Posts: 5,866
Likes: 60
From: Hellinois
Finalist 2020 C4 of the Year - Modified
I did the cigar smoke thing once, I did find the leak but I had one small problem. I don't smoke so by the time I got the stogie all fired up and blew enough smoke in the engine to find the leak, I was sick as a dog. 
If you aren't a smoker I suggest finding someone who is.
If you aren't a smoker I suggest finding someone who is.
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Verdad Gallardo
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
It sounds like the IAC is not properly adjusting base idle or the ECM is seeing a different idle reading that actually exists:
-Did you properly set both the IAC & TPS after replacement?
-Did this problem appear just after replacing either of these elements?
Wouldn't hurt to clean the TB; be sure to use TB Cleaner only.
Are there any trouble codes stored?
-Did you properly set both the IAC & TPS after replacement?
-Did this problem appear just after replacing either of these elements?
Wouldn't hurt to clean the TB; be sure to use TB Cleaner only.
Are there any trouble codes stored?
Burning Brakes
Joined: Mar 2013
Posts: 1,131
Likes: 5
From: Illinois/Wisconsin Line
Get a 2 dollar can of carb cleaner and spray it anywhere/everywhere on any vacuum line or engine part that may cause a vacuum leak. If ur RPM GOES UP U'VE FOUND A LEAK. 85's just like mine had leaked everywhere. Just check the heater cutoff vacuum tiny hard plastic line and any of those that they put everywhere on that yr. I bet u find a few that are cracked. Check the Vacuum booster down by the dr's side headlamp all that crap down there something will be leaking, then trace that supply line all the way upto the motor. It will be leaking somewhere
U should really check them all quickly by hand by if ur lazy just give em a shot with spray
U should really check them all quickly by hand by if ur lazy just give em a shot with spray
Le Mans Master
Joined: May 2004
Posts: 5,866
Likes: 60
From: Hellinois
Finalist 2020 C4 of the Year - Modified
Get a 2 dollar can of carb cleaner and spray it anywhere/everywhere on any vacuum line or engine part that may cause a vacuum leak. If ur RPM GOES UP U'VE FOUND A LEAK. 85's just like mine had leaked everywhere. Just check the heater cutoff vacuum tiny hard plastic line and any of those that they put everywhere on that yr. I bet u find a few that are cracked. Check the Vacuum booster down by the dr's side headlamp all that crap down there something will be leaking, then trace that supply line all the way upto the motor. It will be leaking somewhere
U should really check them all quickly by hand by if ur lazy just give em a shot with spray
U should really check them all quickly by hand by if ur lazy just give em a shot with spray
That area down by the drivers side headlight is a rats nest of vacuum hoses and they get REAL nasty. The hoses there just rot away and they will leave your hands as black as can be but it is a breeding ground for vacuum leaks. Use your fingers to feel the bottom side of all vacuum hoses, I have found a lot of big holes on various cars doing that.
Burning Brakes
Joined: Mar 2013
Posts: 1,131
Likes: 5
From: Illinois/Wisconsin Line
Right They sure are. Was just playing around yesterday and those what looks like original 1985 vacuum hoses I just touched and my fingers turned black. It seems like those hose over time just turn to crap
Team Owner
Joined: Dec 1999
Posts: 34,873
Likes: 487
From: Lacey WA RVN 68-69
NCM Sinkhole Donor
That area down by the drivers side headlight is a rats nest of vacuum hoses and they get REAL nasty. The hoses there just rot away and they will leave your hands as black as can be but it is a breeding ground for vacuum leaks. Use your fingers to feel the bottom side of all vacuum hoses, I have found a lot of big holes on various cars doing that.I found one hose with a crack in it and most of the plastic lines were brittle so I wound up replacing as much as I could. The little muti-port plastic fittings can easily be replaced with T-fittings; just use extra hose to get the path of the lines to fit easily.
There are also vac lines under the plenum behind the t-stat housing for the EGR. Check those too.
Burning Brakes
Joined: Mar 2013
Posts: 1,131
Likes: 5
From: Illinois/Wisconsin Line
Anyone happen to know exactly where on the Firewall is the Vacuum feed line for the A/C heater climate control. Ive got to find it for mine. It lost vacuum and comes out through every vent.
Safety Car
Joined: Apr 2011
Posts: 3,660
Likes: 22
Well I did the propane torch method, and couldn't detect any rpm spikes. So I don't think I have a vac leak. I have this strange problem I've been chasing for a while where my rpm's bounce around a small bit before stalling. This is after the car's warm and the idle settles to 550 rpm. When I'm at a light just idling in neutral (manual trans) the rpms will bounce slowly from 400 to 600 rpm and either stall OR after (I give a tiny bit of gas after it hits 4) the engine "bounces" up high enough it'll stick at 800 before settling back down to 550.
List of things I've replaced (not as a direct result of idling issue, just overall);
IAC
TPS
A.I.R. Pump
Alternator
Edit: Should I just clean the throttle body?
List of things I've replaced (not as a direct result of idling issue, just overall);
IAC
TPS
A.I.R. Pump
Alternator
Edit: Should I just clean the throttle body?
Call Jon.
Le Mans Master
Joined: May 2004
Posts: 5,866
Likes: 60
From: Hellinois
Finalist 2020 C4 of the Year - Modified
All I can say is that it comes through the firewall somewhere around the blower motor. That's where mine leads in but I have never traced it down.
Safety Car
Joined: Apr 2011
Posts: 3,660
Likes: 22
Cudamax......
did you find the check valve behind the dist? THAT is where the line starts to hide toward the firewall and where MOST cabin vac issues begin...the broken ends on the check valve.
explore the cabin vac problem with a new check or patch in some new line. Inside the cabin the line changes to a hard plastic thats a LOT more durable and flexable than the hard line under hood.Inside rarely has a problem.
I think Ghast needs Jon...
did you find the check valve behind the dist? THAT is where the line starts to hide toward the firewall and where MOST cabin vac issues begin...the broken ends on the check valve.
explore the cabin vac problem with a new check or patch in some new line. Inside the cabin the line changes to a hard plastic thats a LOT more durable and flexable than the hard line under hood.Inside rarely has a problem.
I think Ghast needs Jon...
Last edited by leesvet; May 4, 2013 at 07:25 PM.
Thread Starter
Instructor
Joined: Mar 2013
Posts: 100
Likes: 2
From: Ypsilanti Michigan
Edit: JrRifleCoach, it's legal in my state.
Last edited by Ghast; May 4, 2013 at 08:02 PM.
Thread Starter
Instructor
Joined: Mar 2013
Posts: 100
Likes: 2
From: Ypsilanti Michigan
https://www.dropbox.com/s/w2x6z0z1ep...505_141119.mp4
there's a link to a video of my problem, this happens maybe 3-4 times during my commute. It's not a huge deal, but I would like to fix it.
there's a link to a video of my problem, this happens maybe 3-4 times during my commute. It's not a huge deal, but I would like to fix it.