Vacuum Leak Detection -tips

Ringmaster · 12-03-22, 05:40 PM

Folks,

Can anyone offer a tip on how best to isolate a vacuum leak, please?

I may have a leak on my K-Jet system, somewhere between the fuel metering head and the ISV, I think.

I hear spraying carb cleaner around the area is good. Is there anything more precise?

Many thanks in advance....

K Simmonds · 12-03-22, 07:36 PM

The carb cleaner trick works okay at giving you an idea of the general area, the rev's pick up as it get's sucked in to wherever the leak is. It can actually be quite tricky with naturally aspirated engine's to pin point a leak, especially a small hidden one as it's not as easy to block everything off and pressurise the system as one would with a turbo engine.

K-jet's in particular hate air leaks, if it has an oil breather union on the underside of the metering head boot that would be the first non obvious thing to check, the oil vapours cause the boot go very soft around said union.

What engine code are you working on?

Ringmaster · 12-03-22, 11:26 PM

It's a KU engine....

K Simmonds · 13-03-22, 07:33 AM

Okay, so a 2.2 five ten valve in an C3 chassis... I'll did out the manual and find the vacuum diagram and see where I'd check first, assuming you've already looked at the obvious connections?

Though my first thoughts as I said above it could well be where the oil breather system hooks up to the inlet, after 30 odd year this will be a weak spot.

Ringmaster · 13-03-22, 02:03 PM

Thank you....

I just tried the carb cleaner trick and it had a slight effect around the general area of the ISV hoses and the right/rear side of the rubber elbow boot. It didn't get a reaction each time I tried it though.

The problem started last year with frequent stalling with what was a dirty ISV. My mechanic cleaned out using ultrasonic cleaner etc. Stalling problem all but gone now, but idle speed is now around 900/1000 rpm and hunts a bit when cold and warm. I suspect my mechanic may have tweaked something.

Engine fires up OK from cold, but after I pull away, when I take my foot off throttle for the first time, the engine usually stalls. Start up again and she doesn't stall anymore although threatens to do so sometimes when slowing down quickly. Got a used ISV and have the same issue. I was starting to suspect the Warm Up Regulator.

K Simmonds · 13-03-22, 04:09 PM

Hmm, that's muddied the water's somewhat.

If it wasn't for the fact you have another ISV that does the same thing I'd be pretty dam confident your mechanic failed to get it working correctly and tweaked the idle speed screw to compensate for it. I've seen this before and have even resorted to doing it myself as a temporary measure on my own car, which happed to be K-jet 16V MK2 Golf.

The idle should be 800 +/- 50 RPM yes?

A small air leak will normally cause the RPM to be erratic at idle and on the low side if anything, a large one with cause it run like crap under all conditions and constantly stall at idle.

Normally it won't want to start from cold at all if the warm up reg is playing up, that is not a hard and fast rule though. They can do some weird stuff but it's not all that common.

You could have two questionable ISV's and tweaked idle screw or a really tiny air leak and a tweaked idle screw if it's ticking over cleanly all be it high.

Ringmaster · 13-03-22, 05:25 PM

The original ISV is currently in use. All this worsened after the cleaning. I too suspect my mechanic tweaked the idle adjustment in some way. I am 99% sure he isn't very savvy where K Jet is concerned.

I wouldn't be surprised if there is a minor vacuum leak somewhere. If there wasn't one at the start of this saga there could well be now, with all the removing and refitting of the ISV for cleaning. The rubber is 35 years old.

I will have to put some time aside one day and go over the whole system. At least I can use the car. Thank you for your input.

BackintheFold · 13-03-22, 09:00 PM

Check that there is no leak around the idle screw too. The rubber perishes, and the screw can fall out!

Sent from my iPhone using Tapatalk

K Simmonds · 14-03-22, 09:45 AM

Quote:

Originally Posted by BackintheFold

Check that there is no leak around the idle screw too. The rubber perishes, and the screw can fall out!

This is a good point... And the fact the rev's occasionally responded to carb cleaner in that rough area might indicate a micro leak there, trying spray it directly at the screw while wobbling it a bit will answer this.

And don't too scared of touching it, if you turn it just remember exactly where you started from, take a photos if you can and whatever you do don't loose it!

Ringmaster, no problem trying to help, please keep us updated, your best bet is to do as you suggest and systematically go over the whole system.

I'm on break at the moment, speak this afternoon/evening Stuart.

. Evening will be better for me to be honest as I'll be in chill out mode then.

Ringmaster · 28-03-22, 05:51 PM

Greetings,

On Sunday I squirted carb cleaner into where the fuel line enters the warm up valve and tightened the jubilee clip that holds the rubber bellows to the intake manifold. It was the only end that I could tighten...

The engine started well and off I went for a spin and the it did not stall when I slowed down!

Idling remained rough though. At one point the engine seemed sluggish and then returned to normal after what I would call a 'clearing its throat' moment.

Today I just fired her up in the garage. Started well. Sprayed Carb Cleaner around the ISV valve. No effect. Sprayed the Idle screw - no effect. Sprayed around where the bellows meets the intake manifold and that had some effect. The idling seemed better at first but became rough again. The engine seemed to be vibrating a bit too. Then it stalled.

It fired up again but idling remains rough and engine feels unbalanced. Idle speed is around 800-850 and adjusting the idle screw had little effect. It seems worse than yesterday. It leaves me with the following thoughts....

1. Seems like a definite vacuum leak with the bellows. I have sourced another.
2. The engine's roughness/vibration makes me wonder if cylinders are firing right. Plugs are new.
3. I'm no expert, so the engine may not be vibrating and the problem may just be the idling.
4. Maybe fuel filter and points/rotor arm (original) need looking at?

12-03-22, 07:36 PM	#2
K Simmonds Senior Member Classic Audi Club Member Join Date: Dec 2009 Location: Mid Wales Posts: 1,057	The carb cleaner trick works okay at giving you an idea of the general area, the rev's pick up as it get's sucked in to wherever the leak is. It can actually be quite tricky with naturally aspirated engine's to pin point a leak, especially a small hidden one as it's not as easy to block everything off and pressurise the system as one would with a turbo engine. K-jet's in particular hate air leaks, if it has an oil breather union on the underside of the metering head boot that would be the first non obvious thing to check, the oil vapours cause the boot go very soft around said union. What engine code are you working on? __________________ Still have my RS2 tuck up safe, also a 1992 C4 2.8 Avant quattro and a B3 quattro Saloon G60 conversion in progress.

13-03-22, 07:33 AM	#4
K Simmonds Senior Member Classic Audi Club Member Join Date: Dec 2009 Location: Mid Wales Posts: 1,057	Okay, so a 2.2 five ten valve in an C3 chassis... I'll did out the manual and find the vacuum diagram and see where I'd check first, assuming you've already looked at the obvious connections? Though my first thoughts as I said above it could well be where the oil breather system hooks up to the inlet, after 30 odd year this will be a weak spot. __________________ Still have my RS2 tuck up safe, also a 1992 C4 2.8 Avant quattro and a B3 quattro Saloon G60 conversion in progress.

13-03-22, 04:09 PM	#6
K Simmonds Senior Member Classic Audi Club Member Join Date: Dec 2009 Location: Mid Wales Posts: 1,057	Hmm, that's muddied the water's somewhat. If it wasn't for the fact you have another ISV that does the same thing I'd be pretty dam confident your mechanic failed to get it working correctly and tweaked the idle speed screw to compensate for it. I've seen this before and have even resorted to doing it myself as a temporary measure on my own car, which happed to be K-jet 16V MK2 Golf. The idle should be 800 +/- 50 RPM yes? A small air leak will normally cause the RPM to be erratic at idle and on the low side if anything, a large one with cause it run like crap under all conditions and constantly stall at idle. Normally it won't want to start from cold at all if the warm up reg is playing up, that is not a hard and fast rule though. They can do some weird stuff but it's not all that common. You could have two questionable ISV's and tweaked idle screw or a really tiny air leak and a tweaked idle screw if it's ticking over cleanly all be it high. __________________ Still have my RS2 tuck up safe, also a 1992 C4 2.8 Avant quattro and a B3 quattro Saloon G60 conversion in progress. Last edited by K Simmonds; 13-03-22 at 04:15 PM. Reason: More info.

13-03-22, 09:00 PM	#8
BackintheFold Senior Member Classic Audi Club Member Join Date: Feb 2019 Location: Near an MB ur Quattro Posts: 1,171	Check that there is no leak around the idle screw too. The rubber perishes, and the screw can fall out! Sent from my iPhone using Tapatalk __________________ 2006 A3 (daily drive) 1987 MB ur

12-03-22, 11:26 PM	#3
Ringmaster Senior Member Classic Audi Club Member Join Date: Apr 2008 Posts: 1,326	It's a KU engine....

13-03-22, 02:03 PM	#5
Ringmaster Senior Member Classic Audi Club Member Join Date: Apr 2008 Posts: 1,326	Thank you.... I just tried the carb cleaner trick and it had a slight effect around the general area of the ISV hoses and the right/rear side of the rubber elbow boot. It didn't get a reaction each time I tried it though. The problem started last year with frequent stalling with what was a dirty ISV. My mechanic cleaned out using ultrasonic cleaner etc. Stalling problem all but gone now, but idle speed is now around 900/1000 rpm and hunts a bit when cold and warm. I suspect my mechanic may have tweaked something. Engine fires up OK from cold, but after I pull away, when I take my foot off throttle for the first time, the engine usually stalls. Start up again and she doesn't stall anymore although threatens to do so sometimes when slowing down quickly. Got a used ISV and have the same issue. I was starting to suspect the Warm Up Regulator.

13-03-22, 05:25 PM	#7
Ringmaster Senior Member Classic Audi Club Member Join Date: Apr 2008 Posts: 1,326	The original ISV is currently in use. All this worsened after the cleaning. I too suspect my mechanic tweaked the idle adjustment in some way. I am 99% sure he isn't very savvy where K Jet is concerned. I wouldn't be surprised if there is a minor vacuum leak somewhere. If there wasn't one at the start of this saga there could well be now, with all the removing and refitting of the ISV for cleaning. The rubber is 35 years old. I will have to put some time aside one day and go over the whole system. At least I can use the car. Thank you for your input.

28-03-22, 05:51 PM	#10
Ringmaster Senior Member Classic Audi Club Member Join Date: Apr 2008 Posts: 1,326	Greetings, On Sunday I squirted carb cleaner into where the fuel line enters the warm up valve and tightened the jubilee clip that holds the rubber bellows to the intake manifold. It was the only end that I could tighten... The engine started well and off I went for a spin and the it did not stall when I slowed down! Idling remained rough though. At one point the engine seemed sluggish and then returned to normal after what I would call a 'clearing its throat' moment. Today I just fired her up in the garage. Started well. Sprayed Carb Cleaner around the ISV valve. No effect. Sprayed the Idle screw - no effect. Sprayed around where the bellows meets the intake manifold and that had some effect. The idling seemed better at first but became rough again. The engine seemed to be vibrating a bit too. Then it stalled. It fired up again but idling remains rough and engine feels unbalanced. Idle speed is around 800-850 and adjusting the idle screw had little effect. It seems worse than yesterday. It leaves me with the following thoughts.... 1. Seems like a definite vacuum leak with the bellows. I have sourced another. 2. The engine's roughness/vibration makes me wonder if cylinders are firing right. Plugs are new. 3. I'm no expert, so the engine may not be vibrating and the problem may just be the idling. 4. Maybe fuel filter and points/rotor arm (original) need looking at?