Odd pump/grumbling noise from engine bay when coming to a stop
 

Hi,


Have had this symptom for a while, no real issues with the car other than this but im curious what may be causing it.

When coming to a stop, can be a gentle or harsh stop, doesnt really matter, i can hear something that sounds a bit like a pump, splutter and then stop. A bit like an engine stalling if it were manual and you stamped on the brakes, except its not the engine obviously. Quite difficult to describe, its not particuarlly loud, coming from front/possibly left hand side of the engine bay. It definately sounds like something that is struggling to keep running as car slows to a stop and the minute the wheels stop this thing chugs and dies.

Things I have checked/changed...

Air con, had this regassed the other day, no issues reported. Old gas was still fine and pump seems fine.
Car serviced recently, oil/filters changed etc, didnt fix the problem.
Tryed running aircon in econ mode for a while, this possibly reduced it a little but i might just have been imagining it.
Very slight leak on radiator, top it up every month or so.

Could it be the secondary air injection pump?


Thanks

Not sure if this applies to your car, but some D3s had an electric vacuum pump in the braking system. If yours has it, it might be a vacuum leak from that causing the issue, especially if you notice it when braking to a standstill.

Might be worthwhile updating your signature line to include the specifics of the car, for future reference.

Hi,

Sorry not sure whats up with my signature, i thought i had one. will update asap.

My A8 is a 3.0Tdi QS 2007.

How can i tell if it has the extra vacum pump that you mention? Is it something you can see if you lift the bonnet?


Thanks

Just checked and the electric vacuum pump in the braking system only applies to the petrol engines (BBJ, ASN, BVJ and BSM).

Now this or may not be relevant, but might be worth checking for a perished pipe. The 3.0tdi also has a vacuum system on the EGR, but no idea how it operates. The attached image might help.

Thanks,

I will have a look into this. Ive noticed zero issues really apart from the odd noise so not in a major hurry, its curiosity more than anything.

Will feedback if/when i know more.

Cheers

This will likely be your air suspension compressor. It turns off when the car comes to a stop. It also pumps out a jet of air through the outlet to purge the air dryer.

As the compressor ages and it’s piston ring gets worn, it will become noisier.

Mike

Turns out after all it was my Crank Shaft Pulley. Apparently there is a rubber seal or vibration dampener in there somewhere? It had worn down and the pulley had slid out a bit.

It all came to a head when the power steering suddely stopped working and battery not charging indicator came on. Another 100m and the car started overheating so i stopped right away and got it towed to a garage.

Got the belt and pulley changed however the garage said the tensioner bolts looked like they might strip the thread if they tried to change and was warned it may be a big job to get them out if the thread does strip. They phoned me and let me make the call so i relectently decided to leave the tensioner as is as i didnt want to add any more onto the repair bill.

I hope i havent made a bad call, repair bill was already £1400 for pulley and belt plus rear discs & pads and one rear caliper.

Hmmm so this noise has now returned despite all the work above.

Its coming from the left hand side of the engine bay, sounds like its just behind the glove box but could be further forward, difficult to tell as only occurs when in motion.

What is on the left hand side of the engine bay? Where is the AC compressor located?

Thanks

The attached image should help. The AC compressor is at the bottom right of the serpentine belt route.

Thanks for this, does that make it a possible candidate then for odd noise coming from front left engine bay area?

I have tried driving around with AC on econ mode at times and its a little difficult to tell due to the noise not being too frequent but it does seem to abate when set like this however its not cut and dried, i.e. if i switch econ on when this noise is occuring it doesnt seem to kill it instantly. does the AC continue to run on perhaps for a little while when turning econ on?

On the D3 Aircon is permanently drive, not electric clutch like in older cars. It has variable valve which regulates cooling so it makes sense that it's still working for a little while after you press Econ button until the built up pressure dissipates

Yeah interesting, that makes sense.

How big a job am i looking at to replace the AC pump?

So i just had the air suspension compressor replaced yesterday as it was taking a good while to raise the car in the morning.

Noise is still there lol! Driving my nuts.

That's a real pain when you've gone to the hassle of replacing parts. Rereading the entire thread, did you ever pursue replacing the A/C compressor (not that I'm implying that that was the cause)? Just trying to eliminate possible options.

Is it possible to try and record the noise? I know it's not great, especially when the noise you want to record isn't that loud and the engine tends to drown most of it out. There's also the issue that unless it's a very unusual sound, most people won't pick up on it.

It's a bit of a long shot and hard to say without experiencing symptoms in person. Could it be climate control fan touching the casing?

I had kind of similar experience on Q7 after replacing climate control fan. Starting up, fan running at slow speed no noise; not sure if there was noise when driving and speed pushing more air through fan and making it spin faster and touch casing but me just not hearing because of other noises when driving. Or maybe it was braking forces shifting fan wheel just a bit so that it touches casing when braking but I could hear noise coming to standstill and it would stop after couple of seconds of being stationary. Taking it apart and reassembling making sure everything was seated correctly made it go away.

Sorry just noticed replies,

So I havent pursued the A/C compressor change yet, its one of the few things i havnt touched however the AC works fine whether the noise is happening or not. For example when i start the car first off, the noise isnt there as it only tends to show when the car is moving at low speed. But i can sit stationary and AC works perfectly.

Re the climate fans possibly touching a casing, it could be that but the noise has a very definate start and stop to it that doesnt fit in my mind with a fan gently nudging up against a case.

Just to reitterate, the times i can hear it is when the car is moving at slow speed. It stops abruptly when i bring the car to a stop every time. (this makes it hard to locate as i cant open bonnet to find when stationary)
It then starts sometimes when i start moving, maybe 5-10mph+ but not always, maybe 25-50% of the time when pulling away.
I cant tell if its happening at high speeds.
Its usually worse when the car is cold but not always.
It happens no matter if i have the A/C on Eco setting.
It happens no matter if i turn the entire climate control on/off.
Its coming from the front left hand side of engine bay from my drivers perspective.
It sounds like a pump that is trying to pump something but the reservoir is dry, that kind of groany grumbling noise you get when a pump isnt pumping liquid anymore.

I will try and get a recording of it...

Thanks

Something to consider if you have access to one. Do you know anyone who has a car lift that could help you for half an hour or so. If so, raise it enough to get the wheels off the ground, have someone 'drive' it at the 5-10mph speed you mention whilst you listen in and around the engine bay, as you are familiar with the sound.

Go through the various examples of when it's likely to happen, and see if you can pinpoint it any better. You should stand a better chance, as you would be eliminating wind and tyre noise from your surroundings. You also have the opportunity of being able to move around the car both left and right, and from above and below.

Checked the PAS fluid reservoir?