Technical sudden metallic chirp noise causing slight judder at low revs - could it be aircon pump idling issue?

[trekkingnut]

I was driving the Doblo yesterday (1.9 120bhp) in pouring rain and as I got into a bit of slow traffic where revs were down to almost an idle, there were three metallic chirp noises (dnk, dnk dnk sound) about 2 seconds apart upon which the vehicle slightly juddered with each `dnk`. I thought the engine was going to cut out and so revved the engine which seemed to stop it and then wondered if the aircon compressor pulley may be the cause as aircon was off so I turned it on and didn`t hear it again or feel a judder the rest of the journey home.

My logic is assuming idling bearings on compressor can lock the pulley if aircon not running - some reading this will think I am an idiot for saying it and I hold my hands up as have no idea what it was or if the aircon compressor idles in that way or not either so any advice or anyone who had similar experience would be good to hear from. All been ok otherwise and been reliable but nearly failed mot on emissions which I think was down to not blasting the guts out of it before MOT and soot collects in back box which then gets measured during fast rev smoke testing

 

[bugsymike]

I was driving the Doblo yesterday (1.9 120bhp) in pouring rain and as I got into a bit of slow traffic where revs were down to almost an idle, there were three metallic chirp noises (dnk, dnk dnk sound) about 2 seconds apart upon which the vehicle slightly juddered with each `dnk`. I thought the engine was going to cut out and so revved the engine which seemed to stop it and then wondered if the aircon compressor pulley may be the cause as aircon was off so I turned it on and didn`t hear it again or feel a judder the rest of the journey home.

My logic is assuming idling bearings on compressor can lock the pulley if aircon not running - some reading this will think I am an idiot for saying it and I hold my hands up as have no idea what it was or if the aircon compressor idles in that way or not either so any advice or anyone who had similar experience would be good to hear from. All been ok otherwise and been reliable but nearly failed mot on emissions which I think was down to not blasting the guts out of it before MOT and soot collects in back box which then gets measured during fast rev smoke testing

In fairness I did have a customers car that used to do that from the aircon, but she used to fill it with mud driving to her horses down farm lanes, it turned out on that vehicle the electro magnetic pulley was only possible to buy with the aircon pump complete, so given the cost she part exed it as it was.
I did find squirting engine oil into the mechanism quietened it down a bit.

 

[portland_bill]

Teh aircon clutch can make such a noise as it engages, but as it was off, we can discount that. It could be the aircon trying to seize, but I would also look at the alternator pulley, and the belt tensioner.
The alternator might have a freewheeling pulley. These do give up, usually seizing rather than refusing to engage. When they do this, the alternator casing can crack. Pulleys are available as aftermarket, but need to measure diameter and any boss to space it out.
Check the casing for cracks.
Belt tensioners can chirp when they are trying to seize.

 

[trekkingnut]

In fairness I did have a customers car that used to do that from the aircon, but she used to fill it with mud driving to her horses down farm lanes, it turned out on that vehicle the electro magnetic pulley was only possible to buy with the aircon pump complete, so given the cost she part exed it as it was.
I did find squirting engine oil into the mechanism quietened it down a bit.

Thanks for reply - very interesting to hear that - so it would do it with aircon off and cause a slight jolt with each chirp and my guess is sort of correct?

 

[trekkingnut]

Teh aircon clutch can make such a noise as it engages, but as it was off, we can discount that. It could be the aircon trying to seize, but I would also look at the alternator pulley, and the belt tensioner.
The alternator might have a freewheeling pulley. These do give up, usually seizing rather than refusing to engage. When they do this, the alternator casing can crack. Pulleys are available as aftermarket, but need to measure diameter and any boss to space it out.
Check the casing for cracks.
Belt tensioners can chirp when they are trying to seize.

Thanks for reply. I had cambelt replaced earlier this year (with new water pump and tensioners - gates kit) and a new serpentine belt so serpentine belt tensioner is the one you are talking about I assume? - the alternator had no overrun on it which the mechanic mentioned to me but not sure what that even meant or even if it was correct or not! It would explain the jolt along with the chirp if it was seizing/sticking. Is there a risk of cambelt jumping/engine damage if it seizes solid?

 

[bugsymike]

Thanks for reply - very interesting to hear that - so it would do it with aircon off and cause a slight jolt with each chirp and my guess is sort of correct?

In my customers case it was making the noise with aircon off.

 

[trekkingnut]

In my customers case it was making the noise with aircon off.

That is what mine was doing but did your customer feel a slight jolt at the same time? - I thought it was going to stall the engine as the dnk noise made car jolt slightly but higher revs seemed to stop it and then I turned aircon on and no issues. Could it be a rev issue if all ok with aircon going as that in itself makes idling revs higher? - just a thought

 

[portland_bill]

Thanks for reply. I had cambelt replaced earlier this year (with new water pump and tensioners - gates kit) and a new serpentine belt so serpentine belt tensioner is the one you are talking about I assume? - the alternator had no overrun on it which the mechanic mentioned to me but not sure what that even meant or even if it was correct or not! It would explain the jolt along with the chirp if it was seizing/sticking. Is there a risk of cambelt jumping/engine damage if it seizes solid?

I was thinking about the serpentine belt. Not really 'serpentine' unless it has a complicated route, but same as auxiliary belt.
Tensioner and any idler if fitted can cause issues if trying to seize.
Ideally need to watch under the bonnet with engine running, if you can replicate the conditions, and see if anything identifies itself. A seizing tensioner will usually 'kick', but an idler cannot move, so may just momentarily seize.
Engine can be run without the auxiliary belt, even if it drives the water pump, for short periods. Battery will tolerate this for a short while, as will engine if done from cold, and stopped before any overheat. If problem disappears, aux belt is place to look. If problem persists, the cambelt, tensioner, water pump are suspects.
Some alternators have an overrun pulley, like the freewheel on a bicycle. If your mechanic has said not got one, that wont be the problem then.

 

[bugsymike]

That is what mine was doing but did your customer feel a slight jolt at the same time? - I thought it was going to stall the engine as the dnk noise made car jolt slightly but higher revs seemed to stop it and then I turned aircon on and no issues. Could it be a rev issue if all ok with aircon going as that in itself makes idling revs higher? - just a thought

I don't recall any jolt mentioned, it was more a rattle at idle, her car didn't get to idle very often as she didn't "take prisoners" when charging through the Country lanes, her accelerator pedal was on or off. As you say revs are often increased by the ECU to compensate for the extra load when aircon turned on, especially with smaller engines and hers was a 1 litre 3 cylinder model.
Another point if engine was cold and as you say pouring rain, the belt may have lost traction idling almost especially any extra load like moving the steering, I have experienced that after pressure washing an engine in the past.
All you can do is keep an eye on it and see if it happens again, there are many items around there which could affect it as @portland_bill mentions.

 

[trekkingnut]

I was thinking about the serpentine belt. Not really 'serpentine' unless it has a complicated route, but same as auxiliary belt.
Tensioner and any idler if fitted can cause issues if trying to seize.
Ideally need to watch under the bonnet with engine running, if you can replicate the conditions, and see if anything identifies itself. A seizing tensioner will usually 'kick', but an idler cannot move, so may just momentarily seize.
Engine can be run without the auxiliary belt, even if it drives the water pump, for short periods. Battery will tolerate this for a short while, as will engine if done from cold, and stopped before any overheat. If problem disappears, aux belt is place to look. If problem persists, the cambelt, tensioner, water pump are suspects.
Some alternators have an overrun pulley, like the freewheel on a bicycle. If your mechanic has said not got one, that wont be the problem then.

Thanks for detailed reply - belt looked ok from underneath using a torch so guess it needs to be removed and pulleys all checked