Could it be the Cold Start Valve? - Page 6

84CoupeGT · 16-09-23, 11:18 PM

I'm no expert but after rebuilding the fuel system on my Coupe, adjusting the CO leaner than around 2.0 caused it to run unevenly. Haynes manual specifies 1.0-2.0.

Ringmaster · 27-09-23, 12:37 PM

Quote:

Originally Posted by tractor Dave

Sorry I can't help then. You just need the right garage/person to look at it. Old school garage who understand old school cars. Not somewhere who expect to plug it into a computer!

I know what you mean.. none left around here..

My mechanic (who restores mainly old British marques) can't find anything wrong with the brakes, they are working perfectly. Which is what I thought would happen.

He said the brake booster I had may be the wrong one as it looks shorter that the existing part, but it has the same part no. as the current part, so I don't know what to make of that

He also wondered if refurb kits were available for it.

Anyway, he will drive it for a few more days and return it to me if nothing presents itself.
As long as it passes its soon to be MOT test I will get another year to worry about it...

I'll keep you posted.. looks like I may need to start a new thread!

Sandro · 29-09-23, 07:47 PM

Never heard of brake booster locking up brakes to be honest, but You never know.
Usually it's opposite, then it looses nitrogen charge pressure in "bomb", brakes become "heavier". If only the relieve valve stuck in it and it applied all of the pressure to the brake master cylinder. But even then it does not make much sense.
If brake booster would ever cause to lock the brakes, then brake pedal should go down and You would notice it, since it is directly connected.
Does it have ABS?

Regards the engine - have You done vacuum test on the vacuum system? Are You 100% sure there are no vacuum leaks?

Ringmaster · 29-09-23, 08:59 PM

I've heard the booster is unlikely to fail before, and I've never heard of anyone replacing one in all my years... It does not have ABS... Maybe the master cylinder is on the way out?

No vacuum test done. I did find a couple of vacuum leaks via a leaking rubber bellows and some hoses, but nothing else I can detect. I will have another look at some point.

One day all the mysteries of this world will be solved!!!

Cheers mate.

Sandro · 29-09-23, 10:12 PM

Quote:

Originally Posted by Ringmaster

I've heard the booster is unlikely to fail before, and I've never heard of anyone replacing one in all my years... It does not have ABS... Maybe the master cylinder is on the way out?

No vacuum test done. I did find a couple of vacuum leaks via a leaking rubber bellows and some hoses, but nothing else I can detect. I will have another look at some point.

One day all the mysteries of this world will be solved!!!

Cheers mate.

My first point of concern would be the part in between brake master cylinder and brake pedal rod. The one where normally You see brake servo (connected with 2 pipes - 1 from the brake accumulator and 1 "outlet" pipe to reservoir). And full change of brake fluid would be recommended. Time consuming, but worth it.

Vacuum test is very helpful. You can get vacuum tester on eBay relatively cheap. Block off throttle body inlet, connect to one of the pipes of Your choice and You will know straight away if You have vacuum issues.

maxustaxus · 03-10-23, 12:03 PM

Quote:

Originally Posted by Ringmaster

I've heard the booster is unlikely to fail before, and I've never heard of anyone replacing one in all my years... It does not have ABS... Maybe the master cylinder is on the way out?

No vacuum test done. I did find a couple of vacuum leaks via a leaking rubber bellows and some hoses, but nothing else I can detect. I will have another look at some point.

One day all the mysteries of this world will be solved!!!

Cheers mate.

Sorry to learn you are having these problems...I know how frustrating it can be.

For what it is worth, my C2 had/has a brake binding issue this year. I tried what I could myself (changed discs, brakes, checked callipers which were perfectly free when disconnected from the system). So I asked my garage chap to have a look, he thinks sediment/gunk had settled somewhere within the master cylinder or its output pipe just through age and lack of use. He suspects that this causes a trap/blockage whereby fluid struggles to return to the master which causes a slight one way pressure. Essentially, there is not as much force available to return fluid as there is to apply it, so if the system is restricted at or near the master, there are obvious results. He has cleaned this all out, and the car has been fine so far!

I am afraid that "so far" means the two miles I drove back from the garage...and I have not had a chance to drive it since. So I don't really know how well this has worked yet.

I had a similar problem on a Saab 9000. It was well known when they first came out. The material which had been used in the manufacture of either the hose or master would break down a bit resulting on an effective one way valve restriction causing slight pressure on the slave and fake symptoms of clutch failure (because the pressure was fighting the effect of the pressure plate).

This may have nothing to do with your problem, but thought it worth giving a heads up on my brake related tale of woe.

Hanuman · 03-10-23, 12:26 PM

Quote:

Originally Posted by Sandro

Never heard of brake booster locking up brakes to be honest, but You never know.
Usually it's opposite, then it looses nitrogen charge pressure in "bomb", brakes become "heavier". If only the relieve valve stuck in it and it applied all of the pressure to the brake master cylinder. But even then it does not make much sense.
If brake booster would ever cause to lock the brakes, then brake pedal should go down and You would notice it, since it is directly connected.
Does it have ABS?

Regards the engine - have You done vacuum test on the vacuum system? Are You 100% sure there are no vacuum leaks?

Not locking the brakes, but a failure in the hydraulic system- bad sensor was one, and a failing master cylinder, had resulted in locked steering and no brakes on the quattro. I don’t recall why that was possible (circa 1995), but it was interesting.

With all the tales of slowly binding brakes due master cylinder failures, I’m simply guessing that several alternate failure modes are possible. With cylinder failures in the clutch resulting in unexpected problems, I decide a while back to replace both master and slave at the same time, and that issue never recurred. Debris from a failed seal contaminated the lines, which wasn’t entirely obvious until the lines were flushed again a lot, and both cylinders changed.
Brake fluid is an often skipped item in the service history, and I put it down to that.

Ringmaster · 03-10-23, 09:26 PM

Thank you for your views, Chaps...

I wouldn't be surprised if the master cylinder is the culprit. If the issue arises again and I'm sure it will) I will change it. The brake fluid has been changed every 2 years religiously and it has been bled a number of times over the last 3 months, but the car has been used less frequently over the last two years, so deterioration somewhere within the system is probable.

MOT is on Friday so all my appendages are crossed

. If it gets through that I will have another year's breathing space to deal with the brakes and the surging idle..

Ringmaster · 27-10-23, 06:22 PM

Folks..

My C3 somehow got through its MOT!

Took the plunge and fit a new brake Master Cylinder. When the old one was removed it exposed a mass of what looked like aluminium oxide powder between it and the brake booster. Brakes feeling fine now and I await to see if the seizing problem returns.

Just need to fettle with the idling rate now.

16-09-23, 11:18 PM	#51
84CoupeGT Grown up member Join Date: Nov 2020 Posts: 309	I'm no expert but after rebuilding the fuel system on my Coupe, adjusting the CO leaner than around 2.0 caused it to run unevenly. Haynes manual specifies 1.0-2.0.

29-09-23, 07:47 PM	#53
Sandro Grown up member Join Date: Feb 2011 Location: Chelmsford Posts: 651	Never heard of brake booster locking up brakes to be honest, but You never know. Usually it's opposite, then it looses nitrogen charge pressure in "bomb", brakes become "heavier". If only the relieve valve stuck in it and it applied all of the pressure to the brake master cylinder. But even then it does not make much sense. If brake booster would ever cause to lock the brakes, then brake pedal should go down and You would notice it, since it is directly connected. Does it have ABS? Regards the engine - have You done vacuum test on the vacuum system? Are You 100% sure there are no vacuum leaks?

29-09-23, 08:59 PM	#54
Ringmaster Senior Member Classic Audi Club Member Join Date: Apr 2008 Posts: 1,333	I've heard the booster is unlikely to fail before, and I've never heard of anyone replacing one in all my years... It does not have ABS... Maybe the master cylinder is on the way out? No vacuum test done. I did find a couple of vacuum leaks via a leaking rubber bellows and some hoses, but nothing else I can detect. I will have another look at some point. One day all the mysteries of this world will be solved!!! Cheers mate.

03-10-23, 09:26 PM	#58
Ringmaster Senior Member Classic Audi Club Member Join Date: Apr 2008 Posts: 1,333	Thank you for your views, Chaps... I wouldn't be surprised if the master cylinder is the culprit. If the issue arises again and I'm sure it will) I will change it. The brake fluid has been changed every 2 years religiously and it has been bled a number of times over the last 3 months, but the car has been used less frequently over the last two years, so deterioration somewhere within the system is probable. MOT is on Friday so all my appendages are crossed . If it gets through that I will have another year's breathing space to deal with the brakes and the surging idle..

27-10-23, 06:22 PM	#59
Ringmaster Senior Member Classic Audi Club Member Join Date: Apr 2008 Posts: 1,333	Folks.. My C3 somehow got through its MOT! Took the plunge and fit a new brake Master Cylinder. When the old one was removed it exposed a mass of what looked like aluminium oxide powder between it and the brake booster. Brakes feeling fine now and I await to see if the seizing problem returns. Just need to fettle with the idling rate now.