2000 Jaguar S Type, 3.0 V6, Automatic, 180,000+ miles

This car runs great, but it has an annoying humming sound.

There is a "breather tube" or "oil separator tube" at the center, rear of the engine. It is a metal tub that extends vertically from behind the engine and is about 5/8" diameter. It is topped with a rubber fitting that reduces down to 3/8" diameter and makes a 90 degree bend. That is connected to a hard plastic tube that runs under the risers for the intake manifold and connects to the intake manifold just downstream from the throttle body. That 5/8 metal breather tube vibrates. It makes a loud humming sound. You can touch the rubber fitting that connects to it and feel the very noticeable vibration. If the hard plastic pipe is disconnected from the manifold, the vibration stops.

The vibration is intermittent. Sometimes I can drive around town for half an hour with no problems. Sometimes it starts as soon as I start the car. It is most evident at idle and appears to be dependent on the manifold vacuum pressure. It stops during even modest acceleration, and returns at constant speeds.

I used rubber hoses to go from the 5/8 breather tube directly to the intake manifold, and the vibration still occurs.

Do you have any idea what could cause this? I have worked on all kinds of cars for over 50 years, but this one has me stumped.

 

Is the hose collapsing and causing it or has a small hole?

Ah - it might be worth checking what's happening to the vacuum in case it's fluctuating and this is a result instead of a cause of the fluctuation.

 

No, there isn't any problem with the hose. One of my first troubleshooting attempts was to replace the hose entirely. That had no effect.

I never suspected a vacuum problem because the engine runs so smoothly. I will check that and let you know.

Thanks for the suggestion.

 

This just keeps getting stranger and stranger. I followed JagV8's suggestion and used a vacuum gauge to check the engine. I disconnected the breather tube and checked the vacuum directly at the intake. It was a solid 17 in. Hg at idle. Also, the noise disappeared because the breather hose was disconnected. If I remember correctly, a steady vacuum between 15 and 22 in Hg at idle is a good thing.

Next, I used a T-fitting to connect the vacuum gauge in line with the breather hose reconnected. The noise returned, and the vacuum dropped to a steady 13 in Hg at idle. This indicated a problem somewhere in the breather tube assembly. I replaced all of the breather hose parts with a makeshift hose. (Note: When I did this before, I use one fitting from the original breather hose assembly.) The noise stopped and the vacuum returned to a steady 17 in Hg at idle. Problem solved - right. Not so fast.

When I turned the engine off, I could hear a short blip of that annoying noise. I heard it again when I turned the engine on. I took it for a drive, and the noise was fairly constant at about 35 to 40 mph. It was not nearly as loud, but it was there.

I rigged the vacuum gauge so that I could see it from the driver's seat and started the engine. As the needle passed 13 in Hg it blipped the sound. The same thing happened when I turned the engine off. So, I took it for a drive. At a slow acceleration, the vacuum was between 9 and 13 in Hg, and the noise was present. Also, at a steady speed of 35 to 40 mph the vacuum was between 9 and 13 in Hg and the noise was present. The noise is only present when the vacuum is between 9 and 13 in Hg.

The very small leaks around the fittings in the breather tube were not cause of the noise. They simply created a situation where the vacuum was between 9 and 13 in Hg at idle and at most driving speeds around town, so the noise was present most of the time. While accelerating, the vacuum would move out of this range, and the noise would stop. After replacing the breather hose, the proper conditions for creating the noise are only present during slow acceleration and at a constant speed of 35 to 40 mph.

So what could cause a humming or trumpeting noise whenever the intake manifold vacuum is between 9 and 13 in Hg? I have no idea!

Any suggestions would be greatly appreciated.

 

Let me start by saying I don't actually know (and don't have that engine). But some learning and good data so far

There's a hose which commonly perishes, under the intake manifold.
Now, I know of it and think of it in terms of what it does to fuel trims (OBD LTFTs). Might it be to blame / would it maybe also cause this noise?

Instead of vacuum I tend to measure LTFTs. They're sensitive and anyway I have OBD tools rather than vacuum gauges. But the two must correspond in the sense that the PCM adjusts fuelling based on various factors and vacuum must be affected or something's horribly wrong. So while I'm tempted to say grab an OBD tool with live data and check LTFTs I suspect you can figure this out fine by checking vacuum. Except if it is the hose you'll need the manifold off. Plenty of threads about the pesky hose.... if it's relevant.

 

In 40+ years of working on cars, I think this is the strangest problem I have ever encountered, but JagV8's comments set me to thinking, and I may have solved the mystery.

The car makes a strange humming or trumpeting sound in the crankcase breather tube that runs from the rear of the engine, under the intake manifold and connects to the intake manifold just downstream from the throttle body. If the breather tube is disconnected, the noise stops.

The noise is present only after the engine reaches normal operating temperature. Also, the noise only occurs at certain vacuum pressures.

The really interesting thing is that the vacuum pressure that triggers the sound varies according to the length of the breather tube. I first measured this effect with the original breather tube in place. The noise occurred at about 13 in. Hg. This corresponded to engine idle and about 45 mph.

At that point, I noticed that several of the rubber fittings on the breather tube needed to be replaced. So, I removed the original and fitted a temporary breather tube while I was waiting or the new parts to arrive. That temporary tube was much longer because it ran around the driver's side of the intake manifold rather than under it. (Note: That's the left side of the engine - I'm a "Yank" after all.) This shifted the vacuum where the noise occurred to about 10 in. Hg., which was about 40 mph.

I decided to try an experiment and replaced the temporary breather tube with one that was much longer. The point where the noise occurred shifted again to about 30 mph. I did not measure the vacuum this time.

After the new parts arrived, I replaced the original breather tube with all new rubber parts, so I knew that there were no leaks in that section of the system. The noise once again returned at engine idle and at 45 mph.

The one constant was that the noise only occurred after the engine warmed up. This made me think about which part of the intake system would change significantly once the engine warms up. The most obvious answer was the EGR valve. I decided that my next step was to remove and inspect the EGR valve. Once I removed it, the culprit seemed obvious. The gasket was broken in half, so I replaced the gasket. I then discovered that the lower mounting bolt hole had been stripped.

I think that once the EGR valve opens it draws a small amount of air under its base. This may be causing a vibration in the air column in the crankcase vent system which manifests as a trumpeting sound in the breather tube. I will not know for sure until I remove the intake manifold and repair the stripped threads in the mounting hole.

I will keep you all informed.

 

Good luck Jim.

You may just want to purchase the 90 degree rubber elbow that is located under the lower intake manifold and replace it as long as you have the plenum (upper intake manifold) off. It is a very well documented failure on the pre face lift MY's.

If you need help in the location of the elbow follow the link that is found in the first paragraph. Thank you Gus.
https://www.jaguarforums.com/forum/s...ics-faq-52720/

 

Thanks Rick,

I already have that part ready to go.

 

If it doesn't turn out to be the EGR, though it sounds like it will, maybe EVAP.

 

I finally found the time to pull my intake manifold. Sure enough, the problem was the rubber elbow below the manifold. It had been replaced in the past, but whoever did the job did not know much about mechanics.


First, the bolts holding the intake on were so tight that I had to use a 3/8 breaker bar to loosen them. They are supposed to be only about 88 inch-lbs. That's about as much torque as a stiff door knob. Since the manifold was over torqued, they had trouble getting everything to seal. Their solution was to cover everything with RTV gasket sealer. What a mess! It took a full day just to clean up the mess. The secret to making this seal is new gaskets and even pressure.


Next, they replaced the rubber elbow (approx. 5/8 inch ID) with a short section of 3/4 inch heater hose. Of course, it did not fit. It was forced into place which kinked the hose at the bend, and the over size ID did not seal around the fittings. Eventually, one end of the hose slipped of causing a gap. The air passing through this gap caused the edge of the hose to flutter which caused the noise. I replaced the heater hose with a new rubber elbow and all is well.


Thank you for all of your suggestions. They really helped guide me to look in the right place.


Jim89

 

Good news Jim, thanks for the follow up.

 

Ouch! But good to hear it's fixed

 

What a crap repair someone left you with!!

No wonder you had never seen it before. It should not have happened!
Glad you reported back but I hope we don't ever see that one again!
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