Could it be the Cold Start Valve? - Page 4

Ringmaster · 23-08-23, 07:12 PM

Quote:

Originally Posted by ovalking

I got 60ohms (ambient temp).

I don't think it's your problem, but you need to monitor the cold start valve supply voltage to check its operation. It only comes on for 3 or 4 secs at most. This is detailed in the Haynes.

Edit: because forum can't display ohms symbol!

Thank you..

Ringmaster · 31-08-23, 11:16 AM

Fuel pump replaced... issue remains...

BackintheFold · 01-09-23, 09:01 AM

BTW.. I have discovered another trick... If I just turn the ignition on and off (without starting the engine) twice and then turn the engine on, the car starts up and drives well. Sometimes the engine surges from 800 to 1100 rpm when idling, but otherwise OK..

It's definitely pointing to a problem pump or return valve...

I don't think this points to a "dumb" component, as I can't see why the start up procedure would do anything to the fuel pump and non return valve.
It says to me its an electrical/sensor issue.

I regrert I can't be more help with what one!

Ringmaster · 02-09-23, 10:49 AM

Thanks anyway.. more food for thought.

I was considering the thermotime switch when I noticed the problem is less severe when engine was warm. I have now noticed a misfire... alas the saga continues...

BackintheFold · 02-09-23, 12:21 PM

Disconnect sensors one at a time.
When one is unplugged that makes no difference to the problem-that's the duff sensor.
Used to do that all the time for spark plugs/leads.

Ringmaster · 09-09-23, 12:59 PM

Is it a Eureka moment!? I hope so... But there is a twist...

Had the emissions checked at my local MOT centre and it was running very lean. CO showed as 0.4. They set it to 3.3 and engine seemed a little smoother. However I needed to see how it did from a cold start.

As I left the centre all my brakes seized up!!! Same problem I had 2 months ago! My mechanic found a brake fluid leak back then but no obvious cause. He drained the brake fluid and all has been well since then. Managed to limp home without setting fire to the brakes! I heard clunking noises from behind the dash and the rear, just like the last time.

Left it for 3 days as I wanted to see how it did from a cold start and it fired up well today. No stalling!!! And the brakes had freed themselves. But this is what I've noticed...

Engine revs tend to surge to 1100 rpm. Engine has an occasional faint blip.
If I rev the engine the revs drop to 1100 for a while before falling to 800 rpm.
I heard a click from the engine area when this was happening.

When I push the brake pedal the engine revs, sometimes up to 1100 rpm.

Is there any way the engine could cause all four wheels to lock up?
Or, can the brake system be affecting the engine in any way?
Some sort of vacuum pressure issue?

BackintheFold · 11-09-23, 03:48 PM

Could your brake servo be buggered?
It has a vacuum line.

84CoupeGT · 11-09-23, 06:48 PM

There is a non-return valve between the servo and the vacuum hose which goes to the intake manifold, I wonder if that is failing or loose? Or as Backinthefold says, you may have an issue with the servo itself.

Ringmaster · 11-09-23, 09:16 PM

Thanks chaps. I was just thinking that...

I'm having trouble spotting the non return valve on 7zap.

A faulty valve may explain why the brakes are failing intermittently.

tractor Dave · 12-09-23, 08:42 AM

Not up on C3 model but have you got a traditional servo or a hydraulic brake booster and bomb?

01-09-23, 09:01 AM	#33
BackintheFold Senior Member Classic Audi Club Member Join Date: Feb 2019 Location: Near an MB ur Quattro Posts: 1,190	BTW.. I have discovered another trick... If I just turn the ignition on and off (without starting the engine) twice and then turn the engine on, the car starts up and drives well. Sometimes the engine surges from 800 to 1100 rpm when idling, but otherwise OK.. It's definitely pointing to a problem pump or return valve... I don't think this points to a "dumb" component, as I can't see why the start up procedure would do anything to the fuel pump and non return valve. It says to me its an electrical/sensor issue. I regrert I can't be more help with what one! __________________ 2006 A3 (daily drive) 1987 MB ur

02-09-23, 12:21 PM	#35
BackintheFold Senior Member Classic Audi Club Member Join Date: Feb 2019 Location: Near an MB ur Quattro Posts: 1,190	Disconnect sensors one at a time. When one is unplugged that makes no difference to the problem-that's the duff sensor. Used to do that all the time for spark plugs/leads. __________________ 2006 A3 (daily drive) 1987 MB ur

11-09-23, 03:48 PM	#37
BackintheFold Senior Member Classic Audi Club Member Join Date: Feb 2019 Location: Near an MB ur Quattro Posts: 1,190	Could your brake servo be buggered? It has a vacuum line. __________________ 2006 A3 (daily drive) 1987 MB ur

31-08-23, 11:16 AM	#32
Ringmaster Senior Member Classic Audi Club Member Join Date: Apr 2008 Posts: 1,335	Fuel pump replaced... issue remains...

02-09-23, 10:49 AM	#34
Ringmaster Senior Member Classic Audi Club Member Join Date: Apr 2008 Posts: 1,335	Thanks anyway.. more food for thought. I was considering the thermotime switch when I noticed the problem is less severe when engine was warm. I have now noticed a misfire... alas the saga continues...

09-09-23, 12:59 PM	#36
Ringmaster Senior Member Classic Audi Club Member Join Date: Apr 2008 Posts: 1,335	Is it a Eureka moment!? I hope so... But there is a twist... Had the emissions checked at my local MOT centre and it was running very lean. CO showed as 0.4. They set it to 3.3 and engine seemed a little smoother. However I needed to see how it did from a cold start. As I left the centre all my brakes seized up!!! Same problem I had 2 months ago! My mechanic found a brake fluid leak back then but no obvious cause. He drained the brake fluid and all has been well since then. Managed to limp home without setting fire to the brakes! I heard clunking noises from behind the dash and the rear, just like the last time. Left it for 3 days as I wanted to see how it did from a cold start and it fired up well today. No stalling!!! And the brakes had freed themselves. But this is what I've noticed... Engine revs tend to surge to 1100 rpm. Engine has an occasional faint blip. If I rev the engine the revs drop to 1100 for a while before falling to 800 rpm. I heard a click from the engine area when this was happening. When I push the brake pedal the engine revs, sometimes up to 1100 rpm. Is there any way the engine could cause all four wheels to lock up? Or, can the brake system be affecting the engine in any way? Some sort of vacuum pressure issue?

11-09-23, 06:48 PM	#38
84CoupeGT Grown up member Join Date: Nov 2020 Posts: 309	There is a non-return valve between the servo and the vacuum hose which goes to the intake manifold, I wonder if that is failing or loose? Or as Backinthefold says, you may have an issue with the servo itself.

11-09-23, 09:16 PM	#39
Ringmaster Senior Member Classic Audi Club Member Join Date: Apr 2008 Posts: 1,335	Thanks chaps. I was just thinking that... I'm having trouble spotting the non return valve on 7zap. A faulty valve may explain why the brakes are failing intermittently.

12-09-23, 08:42 AM	#40
tractor Dave Senior Member Classic Audi Club Member Join Date: Apr 2018 Location: Wales Posts: 1,174	Not up on C3 model but have you got a traditional servo or a hydraulic brake booster and bomb?