jcorenman

I think it is the oil pump.

Here's my reasoning: Noises caused by rotating parts happen at the speed of rotation. We know the idle speed (675 for auto's) and can confirm that by timing how long it takes the belt to go around in the video (7 laps in approx 5.4 sec's by my stopwatch, 0.77 sec per lap). The belt is 216 teeth (IIRC) and the crank sprocket is 24, so 9 crank rev's per laps, which works out to 701 RPM. Close enough.

I put a scope on the audio, the sound is repeating at a frequency of around 7.6 "squeaks" (or whatever it is) per second -- scope showed 38 cycles in a 5-second period.

OK, at 675 RPM the crank is rotating 11.2 revs per second (too fast for the noise), the power steering and air-pump (if fitted) are turning at about crank speed, alternator is going much faster. The air-con compressor turns slower, at around 81% of crank speed (9.1 rev/sec), closer (and worth verifying by puling the belt) but still too fast.

The cams turn at half crank speed or about 5.6 rev/sec, too slow. The water pump and tensioner idler both turn slightly faster than the crank.

And finally: The oil pump turns slower than the crank by the ratio of 35 teeth to 24, or 7.7 revs/sec-- almost a perfect match to the sound. I can't see anything else turning at a similar rate.

So how about it, oil pump experts? Is this plausible?? What could be the cause? Early 32v's (thru 87) had the aluminum OP sprockets, right? Could a worn sprocket be noisy? I thought they just got worn and ate belts...

Garth S

Kal,
Any chance you still have an air pump? Sounds somewhat like a dying one ...

Regardless, grab a stethoscope ( or long hose, wood dowel, long screwdriver), and make ear contact with every bearing in the front end .... that should pin down the source.

khalloudy

Jim!

even as an engineer, i was more than impressed with your detailed analysis. Going through every rotating component in the front of the engine, with a calculated run!

Ok, so here are some of my observations:

- Car idles at 675rpm in P/N, with Auto engaged, it will idle at 625 per spec
- Oil pressure never hits 5 bar at idle it is about 4.8 bar or so on the gauge. When warm it runs at 1.5/2bar
- Even full throttle/6600 rpm, the oil pressure never hits above 4.8 bar
- never got the low oil pressure warning

So looking at the above, the oil pump might be a culprit...

Garth,

my car has its original air pump which is still fitted. I think the AP has about 180K on it... so again might be a problem.

Note:

When the engine was fully rebuilt by Exotic Motorwerks a lot of parts were replaced including the rollers, tentionners, idlers. Air Pump and Oil Pump were not as they looked OK i guess. I think Dan is an excellent mechanic so i am confident of the items he refreshed. Having said that, WP failure is something out of anyone's hand is it is.

Here are my steps on monday/tuesday at the dealership

- Check the Oil Pump
- Check the Air Pump
- Inspect all the bearings on the front end using the stethoscope
- Remove one belt at a time and check what happens seeing what happens
- Check Belt Tensions across ALL belts (Timing, Alternator, etc... per SPEC)

I hope it is something simple like the Air Pump or Oil Pump which can be done easily... I hope :-D

jcorenman

The air pump would be simple, the oil pump is not-- it is under the lower timing-belt cover and takes a few hours work to get to. But also I think oil-pump problems are quite rare. And your oil pressures seem OK, slightly low but could be the gauge, or the temperatures, or the weight of the oil.

In any event I hope I am wrong! An air-pump failure would be much more likely, and much more easily checked-- it only takes a few minutes to remove the belts with the car on the lift.

khalloudy

Just a quick thought, was the belt minor movement acceptable or was it an indicator of some fish fish?

M. Requin

Khaled, my reasoning was based on "HOWEVER the belt was moving back and forth by about a few millimeters? but still within the roller width," but with Jim's freq analysis I'll guess not the oil pump itself but the sprocket that drives it. In any case I'll stick with taking off the covers and seeing what's going on. Good luck, keep us posted!

Lizard928

Just to note, the cams do not turn at half the crank speed.....

I cannot see the oil pump causing this noise either. There is nothing close to the oil pump for it to really rub on. And your belt movement is normal.

But a quick removal of the front covers will ensure this.

S4ordie

It sounds like the impeller scraping on the block. No firsy hand experience with this on a 928 but I've heard that sound before on another engine design. The only thing that makes me think that it is not the impellor on the block is that this has been going on for some time. On my other engine the impellor ate into the the back of the waterpump but not for too long before it just seized. Regardless, I would not start that engine again until I found the problem and corrected it.

mark kibort

why do you say this? Last time I looked, i had to turn the crank 2 times to get one revolution of the cams. what are we missing here?

mark kibort

I was listening to the frequency , and it is similar to one I had a while ago. yes, it was a water pump and it went away when the car warmed up. (Only happened when cold.). does the sound go away when warm or change?

all you really need to do is confirm with a stethascope. put it right on the metal portion above the pump that pokes out . if there is a water pump problem, it will show itself there. then, if not, you can search around for a stronger sound position.

It would be helpful to ref the engine with the video to hear how the frequency and sound changes.

probably the water pump. are you positive its not coming from the cam covers ?

khalloudy

Mark,

i do not have a stethoscope so i tried my best today to look for the sound origination point. The loudest i could get was from the right side of the engine. I put my finger at the WP and that area and there was no vibration or anything that indicated an issue... The sound is definitely more to the right and it is either in the cams area or in the air pump? The noise is constant warm or cold at the same pitch and frequency.

P.S. with respect to the cams, when the heads were done like 20K miles ago, we replaced the cam chains with thicker racing chains and the plastic parts of the tensioners were replaced as well.... i will try to get a video today of the engine reving with the sound... maybe it would help.....

mark kibort

you are going to need somethng to focus the sound from the metal to your ear. a long screwdriver will work too. you put the plastic handle pressed against your ear and then the metal part is the sound detector. If it is coming from the right side, (looking at the engine) that is water pump, alternator, steering pump-land. Sometimes, things in the back of the motor, resonate and sound like they are coming from the front. I have a bad flywheel "click" that sounds like rod bearings, but I can feel it on the belhousing where the high spot on the flywheel/timing ring is hitting the cover and makes a racket.

also, a trick is to spray brake cleaner all over that area down near the alternator, to see if it changes the sound. sometimes belt noise can make some weird noises. just a couple of well placed shots of brake cleaner will show something. it leaves no residue so its safe to spray around the front of the engine while running.

mk

Mrmerlin

after listening to the video the simple thing to do is remove on e belt at a time and see what one make the noise go away. From the sounds of it you air pump is internally going bad and either removing the belt or replacing the pump will cure the noise
( note when washing the engine the air pump bearings can be damaged by water).
In the meantime dont keep running the engine as the pump could be shedding internal parts into the air induction system that connects it to the heads.
As far as the oil pump making noise this seems very unlikely, but simply pressing on the front timing cover might give a clue .
My guess is a failed air pump

SharkSkin

Agree on this. If you want to minimize running of the engine, take them ALL off. If the noise goes away, turn the accessory pulleys by hand and/or put the belts back one at a time to narrow it down. If the noise doesn't go away with all the belts off -- well, you would have needed to take them off anyway for the next steps. Before digging too deep pull off the distributor caps and make sure nothing is rubbing inside them.

Imo000

Get a spray bottle, fill it with water and spray it onto the acessory belts. If the noise goes away, you know it's not the WP or anything related to it. If it doesn't, time to dig in.