Vacuum Leak Detection -tips

[Obli]

Did you manage to track the issue down?

I've been trying to get my 90 20v running well after five+ years of being sat and not started. When I last drove it, it had running issues. I've got a thread about it in the Reader Restos forum.

The engine had an erratic idle, and would often threaten to stall. Once warm, the engine would become lumpy and rough. That switched in suddenly, as if, well, by the flick of a switch.

We found two splits in the inlet duct (from airbox to throttle body). Only noticed the second split during removal. Maybe I even added it! I'd advise taking it off for a closer look if you're comfortable with that. I sealed up the splits with a heat and waterproof adhesive sealant. Have a look at my thread for details. No idea if that will last as it's under constant heat and vibration but so far so good, and the product spec seems suitable.

Yesterday, I also found a hole in a small hose running from the back of the inlet manifold to the charcoal canister area. The hole was in a green plastic cap on a Y piece connector. I found that by blowing through the pipe and I could hear air gushing out. I don't know if that is an ok practice but it worked!

Also cleaned the ISV, intake and throttle body during the process, replacing air filter, plugs, oil and filter as way overdue.

Rough running when warm (I think) was due to the lambda probe. My understanding readings are used by the ECU when the coolant reaches a certain temp (80 deg C), which made sense for the sudden switch to rough running when almost up to temp. We replaced the lambda using a good condition used one we had to hand.

We also changed the 2 pin G62 coolant temp sensor at the back of the engine. It was old, cheap and easy to replace and they are known to affect engine running. ECU uses the readings from this sensor and perhaps it was giving duff readings.

I'm not a mechanic so can't be 100% sure of fixes as we've been making a few changes in a short time frame, renewing a couple of very old parts, probably original, by process of elimination / probability of cause thanks to great help here, rather than confirming the part had failed by proper testing! I've noticed the engine running improving bit by bit, day by day.

After all this, it'll probably run like a dog next time it starts!

I don't know if my advice is 100% or applicable to your engine, mind. Disclaimer!

[Ringmaster]

There definitely is a vacuum leak involving the rubber bellows.

Changing that or cleaning the original and going over it with something like Aquaseal rubber sealant, may be the answer.

Otherwise I may have an additional problem.... that's my worry.

My KU engine is simpler than yours, ie. no ECU... I have a spare ISV so will try swapping that and see what happens...

No idea what I'm doing where K Jetronic is concerned , so this is a learning curve...

Thanks for your input though....

[Obli]

I'd have a go at cleaning then patching up. I decided to do that to see if it cured the problem. Will see how my solution lasts...

I used this stuff. Not cheap but very useful, I've used it to reseal an inner door seal leak and a floor leak on my MK4 Golf.

Black, resistant to shrinking, water, mould, applies to almost any material, when cured can handle -40 to 120 deg C, can even cure underwater, doesn't attack synthetic materials. I allowed 24 hours to cure. Somewhat rubbery when cured.

https://www.screwfix.com/p/geocel-th...ails_container

Datasheet:
https://sherwinwilliams.widen.net/co...KS_PRO_TDS.pdf

I've no idea what I'm doing either, working it all out too. No harm in trying

[K Simmonds]

It does sound like you have an air leak and the rubber bellows/boots are very prone to cracking, Obli's sealant suggestion is a pretty good idea as any second hand rubber parts will likely be a little perished a unless they're NOS.

A 30+ year old rotor arm probably wants replacing, don't think the JS engine has point's, I'm fairly sure it has a hall sender in the dissy.

If the problem is only at tick over and the car is driving okay then it's firing fine.

How old is the fuel filter? These should really be replaced every couple of years, especially on cars with metal fuel tanks.

[Ringmaster]

I shall persevere with the Vacuum Leak theme.

Would you dare use the sealant on the inner surface of the bellows?

Fuel filter is due for a change.. must be 4 years old but only over 5k miles..
I've often wondered why the Haynes Manual doesn't suggest a service interval for the fuel filter on post 1985 cars.

[K Simmonds]

I wouldn't use any sealant inside the bellows, it 'probably' wouldn't harm the engine if it got sucked in but that's not an experiment I'd like to conduct on a motor I care about.

If it's only done 5K miles then the fuel filter should be okay still, though at four years for the sake of a tenner I'd change it and be sure it's fine.

[Obli]

Any sealant that slightly squidged through the splits to the inside I wiped off and made sure it was flush internally. It's messy stuff to apply. I found my finger worked best to smooth out the bead. Tried to keep the inside as clean as possible. I must say, it's quite an experimental fix but my instinct is it's going to hold out a while. I'm with Kit, I wouldn't want to test any going into the engine and I wouldn't want to apply real force to the duct. If you use the same stuff I did, it needs 24hr to cure a 3mm layer at 24 deg C. I gave it about 14 as I was I impatient and don't think I applied 3mm of it. I didn't want to cake it on.

[K Simmonds]

Originally Posted by Ringmaster

I've often wondered why the Haynes Manual doesn't suggest a service interval for the fuel filter on post 1985 cars.

Yeah, that used to bother me as well, now I've worked in the trade for years I know that nothings really altered, an engine still works in much the same way and fuel filters change's really should be conducted with much the same frequency as in the 1980's or at least half as often.

I think the lack of service intervals is partly due to the introduction of plastic tanks (one of the few things on a car I approve of plastic being used for) along with sealed system's, as well manufactures fitting much larger filters and stating it is designed to last for the life of the car, which is kind of BS.

Most modern cars are total rubbish compared German stuff made in the 1980s/90s, the ethos back then from the likes of Audi, BMW and Mercedes was that a luxury car was built to last for the life of the owner or at least 25 years. Now their really only designed to last ten years, everything you can imagine has been costed down to a minimum, hence constant snapped springs etc, I see it everyday. And don't get me started on plastic being used for anything in the engine bay where it's subjected 1000's of heat cycles so is bound to go brittle and break. As you may have guessed plastic component's are one of my pet hates.

Anyway excuse the rant, I got a bit side tracked there.

Obli gave you good advice when it comes to using that sealant, I've just read my answer back and realise it's perhaps a little sarcastic, but yeah, you don't really want to risk an engine sucking in any sort of solid, even if it's relatively soft.

Ignition faults are almost invariably worse under load, while idle issues are almost always due air/fuel ratio problems on cars of this age where there aren't a multitude of senders that could be interfering.

At least the fuel filter is easy to change on a C3 or C4 for that matter, the ones on B4 Avant quattro's are extremely hard to renew without cutting through a pinch bolt and the access for that is appalling, I have the one my RS2 to do which I can't say I'm looking forward to.

[BackintheFold]

Just a note to remind us all to avoid spraying carb cleaner near any source of ignition, like near the distributor/spark plugs/leads/coil.

[Ringmaster]

Thanks for the advice, Chaps..

Just have to find the time and apply myself to this saga.. I'll keep you posted...

They say kicking things sometimes has the desired effect.. I wonder if doing a Will Smith on it will work?