I just had the cam/timing belt and water pump changed by my local garage. It seems to be running very well afterwards but I noticed a noise like hissing air after I turned off the engine. It lasts for just a few seconds but is quite loud. I have looked under the bonnet and reproduced the hissing every time after shut down but cannot locate where it is coming from.
Has anyone here noticed the same thing and can anyone advise where it might possibly be coming from? As the car is new to me, I guess it could just be that I’ve not noticed this before and it is normal. But I certainly did not notice it before this work was done.
Thanks in anticipation. ![]()
Snakehips
Do you have a cone type air filter? Mine (with a K&N induction kit) behaves exactly as you describe.
If not, maybe the garage didn’t reassemble the induction parts correctly causing a leak?
no, not normal.
someone else might have a clue on here.
if you cant find the source of the hiss, get it checked out just in case
Sounds normal to me. There is a depression in the intake manifold while the engine is at idle. The the engine off and that vacuum needs filling with air from somewhere and as a general rule it sucks air round the closed throttle butterfly and through the closed idle air control valve causing the hiss. If you open the throttle once the engine is turned off does the noise stop?
Thank you for your responses above. No, my car does not have a cone type air filter. It is a bog standard Mk 2 Jasper Conran.
The noise sounds like air venting from somewhere rather than being sucked into a vacuum. It stops too quickly (in just a few seconds) for me to check if opening the throttle stops it. I have tried and it stops before I can touch the throttle linkage.
Thinking about this, it occurs to me that it could possibly be a leak somewhere in the cooling system, perhaps caused by the water pump replacement. The garage told me they did not receive the little O-ring that fits on the water pipe at the pump when the kit arrived with them, so they fitted one they believe was the correct size from their O-ring stock.
I just checked the water level and it does seem to have fallen to well below the LOW mark. That may, of course, be nothing more than water lost to replacing some trapped air in the system when they topped up with new water and anti-freeze. It’s possible, I suppose, that it is a radiator leak venting pressure (it was fine before this work was done)or they have not fitted hoses back on properly, though I see no sign of water sitting under the car.
I do hope this is not something serious or expensive as I was about to give my garage the go ahead to replace the clutch. The last thing I need is yet another expensive job needing urgent attention. The bills so far are fast approaching the value of the car!
Hi Roadster Robbie
Just a quick update that might help others if they notice the same problem. It seems your answer above, Robbie, was spot on. I took the car back to the garage that did the cambelt renewal job. They couldn’t make head nor tail of where the noise was coming from. They agreed that it was not there before they worked on the cambelt replacement so asked me to leave the car with them for a while.
The next day, they called to say that they’d had the car up on the hoist and still could not locate where the noise was coming from, though they suspected it could be somewhere in the induction system. The garage owner asked me to leave it overnight while he called in a friend who worked at a Mazda dealership.
The good news came today. The problem is fixed. Apparently, the Mazda friend said it was air venting past the throttle butterfly and he said it was not really a problem, even though it had not been doing this before. Nonetheless, he played around with the computer system and sorted it by adjusting throttle settings. Noise now gone.
So, Roadster Robbie, your response above was right on the mark. You obviously know what you are talking about where the MX5 is concerned. Well done to you - and my sincere thanks.
Kind regards
Snakehips ![]()