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Loud Humming sound at 600-800Rpm, disappears at 1,000 Rpm and more
My ‘91 XJS LeMansV12 is almost perfect and is in very good condition. She received all major service and replaced all hoses, etc.
I did have to rebuild the GM-400 Automatic and she is now ready for years of spirited driving.
When I received the car back from the tranny shop she warms up gently, all gages are good, and than she starts “Humming” loudly at around 600-800 Rpm somewhere in front of the engine, not “in” the engine? The engine is very smooth and quite.'
It is NOT the idler cam tensioner because that sound is higher pitch and this was checked.
We are now checking the oil line that we suspect has a valve inside that lets oil through and at a certain pressure/Rpm might vibrate enough to cause this humming sound?
The sound is obvious when you open the hood/bonnet and resonates quite loudly when you are behind the wheel in the car.
We are locating an oil line to replace the current line and see if this solves the problem.
My shop is a Jaguar expert and has not ran into this issue before and has trouble locating the sound.
- Does anyone on the forum ever ran into this issue before?
- Is there any reason why taking the gearbox out and reinstalling it have anything to do with causing this sound at lower Rpm?
Any hose or pipe with fluid running through it can create an odd noise under the right circumstances. Moaning, knocking, humming. Vibration/rattling.
I've experienced this sort of thing a few times over the years on various cars and have been lucky enough to solve the problem by simply slightly re-positioning the offending pipe or hose....which has been enough to alter the flow characteristics and eliminate the noise. The same might work for you. In your case the transmission cooler hoses and pipes would be a likely culprit.
That being said I'll mention that a moaning sound on engine start is rather common on V12s and, according to Jaguar TSBs, harmless. It's caused by air being evacuated out of the oiling system. However, this sound typically lasts only a few moments following a cold start so it doesn't really match your situation.
- Is there any reason why taking the gearbox out and reinstalling it have anything to do with causing this sound at lower Rpm?
The only thing I could think of is that the transmission mount is a bit overly complex, perhaps it wasn't reassembled correctly, or the rubber bits were not replaced, and that's creating a vibration that would transfer forward to the engine mounts or something.
Interesting thought.
Except that she start moaning more loudly when idling and getting warm, not during driving only.
When still running with a cold engine no sounds at all. Than slowly starts creeping up when getting warmer and warmer.
When discussing the replies with the shop he said none were the problem.
After spending considerable time locating the sound he zero’d in on the Oil cooler line.
He says there is a valve in this line that is probably sticking or not opening correctly or something and causing a vibration sound?!
We are trying to locate a used line, a new one is very pricy, and before removing it testing other options one last time.
The shop having worked on all older Jaguars for decennia’s says he has never ran into this issue before.
The possible issues mentioned he is familiar with.
The sound was located with this oil cooler line and that is where we are going to try to get rid of the sound problem.
When discussing the replies with the shop he said none were the problem.
After spending considerable time locating the sound he zero’d in on the Oil cooler line.
He says there is a valve in this line that is probably sticking or not opening correctly or something and causing a vibration sound?!
We are trying to locate a used line, a new one is very pricy, and before removing it testing other options one last time.
The shop having worked on all older Jaguars for decennia’s says he has never ran into this issue before.
The possible issues mentioned he is familiar with.
The sound was located with this oil cooler line and that is where we are going to try to get rid of the sound problem.
/ The Car is running fantastic otherwise.
Well done with teh narrow down.
Your market would have the Full Flow oil cooler system I reckon.
That valve is inside the oil filter housing, and rarely gives issues.
BUT
Oil filters can. NOT meaning an Elcheapo filter at all, just sometimes the oil filter can cause noises you describe.
Before spending $$ on pipes and hoses, swap the oil filter for another brand.
One model Toyota LandCruiser V8 down here did similar with aftermarket oil filters, and replacing that with a Genuine filter stopped that noise, NO idea why, we just used OE oil filters on any V8 Toyota that darkened our door.
Don't wanna be picky with your checks, however... Did they changed the alternator's drive belt? Rear/small alternator's bearing gives sucha noise and you won't spot it unitl it gives nice blue flame... Noise will also disappear with higher revs. Use automotive stetoscope on your alternator body to exclude it...
Before we replace the lines I’ll have them replace the filter.
If the online is free flowing as you say perhaps the filter is the problem. He did say we will look at any suggestions before we spend the $$$ on lines and labour.
Mechanic seems to think there is a valve in the line however.
Definitely not the alternator belt.
We spend a considerable amount of time locating the vibration-buzzing sound and located it with the Oil cooler line.
The sound appears when the engine gets its normal operating temperature. Around 500-900Rpm. And goes away over 1,000Rpm
Idling or driving does not matter.
We have found the buzzing sound at 700-800rpm, when the oil is warm, to be a faulty oil cooler thermostat.
This thermostat is NLA.
The shop believes they found one in France.
When the engine gets warm, the oil pressure gage showed very little pressure.
This is a faulty oil pressure sensor which is fairly common and will be replaced.
We thought the oil pressure may be part of the buzzing/humming issue but wasn’t.
When the parts have actually arrived, and they are good, replacing the thermostat won’t be easy.
Having an almost prefect looking and running LeMansV12 will make me happy.
Knowing all mechanicals are good makes it even better.
03-21-2021, 09:32 AM
Loud Humming sound at 600-800Rpm, disappears at 1,000 Rpm and more
