Hissing noise during running & after switch off

[wampit]

Hello all,

Really hope you can help or point me in the right direction.

My 68 S2 FHC has developed a hissing noise that I can't seem to track down (even used the hosepipe to ear trick).

It seems to have happened after I did the following work - may / may not be coincidental:
- Valve clearances checked (all ok)
- Timing chain tensioned
- Refitted cam covers
- Cleaned / refitted breather housing with new gaskets
- Removed / replaced thermostat
- New oil pressure sender unit
- New radiator cap (flat type)
- New expansion cap (7lb)

Sound seems to be coming from the right side of the engine bay. It also hisses for about 5 seconds after you switch off the engine. The sound is very like the compression hiss you hear when you turn the engine by hand off the crank with spark plugs in.

I've done following:
1. Removed vacuum hose from rear of inlet manifold to rule out braking system leak.
2. Removed air filter assembly to see if sound changed
3. Checked spark plugs tight
4. Sprayed wd40 on carb gaskets / thermostat gaskets
5. Tried taking radiator cap off and oil filler cap
6. Removed the drive belt to rule out water pump / alternator
7. Searched google for two hours!

Anyone had anything similar?

Thank you
Adam

[abowie]

I'd be suspecting something related to the brake vacuum circuit, given that it persists for a short time after you turn the car off and everything else is therefore stationary.

[wampit]

I'd be suspecting something related to the brake vacuum circuit, given that it persists for a short time after you turn the car off and everything else is therefore stationary.

Hi Andrew... I'd agree but I disconnected the vacuum hose from the inlet manifold and retried. Hiss was still there.

[cactusman]

The vacuum hose that runs from the manifold to the reservoir will be back at atmospheric pressure once the engine is off. So long as the non return valve in the reservoir is working the reservoir and pipe from there to the Kelsey Hayes booster will still be under vacuum. I would suspect either a leak in this pipe, the reservoir or most likely the Kelsey Hayes unit as it is nearest you when driving. If, for instance the brake pedal is not returned to fully off by the spring, then you may be holding the valve in the bellows slightly open and there will be a hiss of air that will persist for a short time once the engine is off.

[wampit]

The vacuum hose that runs from the manifold to the reservoir will be back at atmospheric pressure once the engine is off. So long as the non return valve in the reservoir is working the reservoir and pipe from there to the Kelsey Hayes booster will still be under vacuum. I would suspect either a leak in this pipe, the reservoir or most likely the Kelsey Hayes unit as it is nearest you when driving. If, for instance the brake pedal is not returned to fully off by the spring, then you may be holding the valve in the bellows slightly open and there will be a hiss of air that will persist for a short time once the engine is off.

Hey Julian... that's what I initially thought but as per my diagnosis above I removed the vacuum pipe off the manifold and tried again... this hiss was still there and after engine switch off. Thoughts turn to cooling system but the hiss really sounds like air being let out... so frustrating :-)

[JagWaugh]

Does the noise start immediately when you start the engine, or is there a delay?
Does when the noise starts depend on engine temperature?
If it starts with a delay when cold, but immediately when warm/hot> suspect cooling.

Have you got any aftermarket electronic gizmos installed?

Does it happen if you leave the oil filler plug off? (Breather)

Can you post a recording?
Does the car run fine otherwise?

[ralphr1780]

Adam, you said you replaced the radiator and expansion tank caps. Did you get your ear close enough around these?

[christopher storey]

Adam : I had a very strange problem of a similar nature on my S2. It turned out to be a pinhole adjacent to a helicoiled spark plug thread. I used one of the "plastic" metal fillers and a very small diameter aluminium rod ( perhaps 1mm diameter ) to plug the hole and it has never recurred. Have a listen with your ear tube next to all the plugs

PS also I endorse what Ralph has suggested

[wampit]

Thanks for your great suggestions Ralph, Christopher and JagWaugh :-)

But I have to confess... I've been a right doughnut!

Late last night I did remove the brake vacuum hose from the inlet manifold and tried it... I would swear blind it was making the hissing but it couldn't have been.

I've just tried the same thing now to shoot the video and guess what... no noise when disconnected. It's the vacuum reservoir pressure valve letting air out as its not shutting off. Simples.

Really sorry for wasting everyone's time. However, I have uploaded the video as it may help someone else out in the future. https://youtu.be/fD3JTbLHFk8

Sorry again! Adam