924Board.org

[Horizonblue]

Not many 924 owners thinks much about the oil pan and I wouldn't either, if I had the original engine in the car.

But some may have noticed that I have replaced the 2.0 engine with a VW 2.4 straight six. The only modification I have done to the engine, is to rebuild the oil pan and pickup tube. The original pan was way too deep.

Here comes the problem:

When the engine(and oil) heats up, the oil pump begins to make a noise, like a set of straight cut gears running(or an oil pump that is sucking air). Only at 1200 rpm. More rpm, it goes away, idle and it is almost gone, you can barely hear it.

I don't think there is anything wrong with the pump, it gives good pressure. 6 kp/cm2 cold, 4 kp/cm2 warm. 10W40 Castrol oil.

I suspect there is something wrong with my design of the oil pan/pickup tube that can create turbulence/air bubbles.

When I had the engine running outside the car(with the original oil pan) I didn't run it for so long time that this noise would appear (takes 4 minutes of running)

The pickup tube is leak tested, so unless a crack has developed, it should be leak free. One thing I have done, that might not be the best for the flow, is that I have flattened the tube a bit in four places, so the rod bolts didn't come too close.

Does anyone have an input on what could be wrong, a design fault in the pan/pickup tube?


New pan to the left


Modified pickup tube




Oil Pan


Pickup placement


Pickup tube, flattened four places

_________________
Euro 924, 1976

"If you can't fix it, don't break it"

/P.G. Andersen

[larchie]

Since no one else has commented, the following comments might be suggestive. First off, I don't see how the shape of the pan would be relevant.

I don't know enough about fluid dynamics to definitively answer your question, but it is certain that the flow rate is changed and some turbulence is introduced by flattening the pickup tube. An oval or squarish tube would have more resistance to flow than a circular one, and the reduced cross-section of oval or squarish tubing would also reduce flow with increased velocity and decreased pressure at that point in the tube. The physics of a constriction in a tube involves the formulae for a Venturi tube. The velocity of the flow of oil is greater at the restriction but the pressure in the restriction is lower than the pressure in the wider portion of the tube.

From your pictures, however, I wouldn't think that the pick-up tube is crimped enough to inhibit flow significantly at any RPM so as to cause the pump-noise you describe. Also when the oil heats up it's viscosity is less of course, and so the flow rate should increase with the reduced fluid-friction. So, if at the same RPM, the pump sounds fine when the engine is cold but the pump is noisy when the engine is warmer, it seems to me the noise would not be caused by lack of flow, since the oil would be less viscous at the higher temperature. So the behavior you describe is puzzling.

If however the pipe is not completely filled as oil moves through it and there are sufficient air bubbles moving though the pickup tube at any given RPM, then the increased temperature would cause these bubbles to expand more than the accompanying oil, and so the rate of flow of the oil through the pump would be expected to decrease with increased temperature, all other things being equal. So you might get the sounds you describe.

The only way I can imagine getting significant amounts of air in the oil would be if the tube and screen are not submerged deeply enough and turbulence of oil in the pan allows air to be sucked in or, less likely, there is some point from the sump strainer to the oil pump where air is somehow being introduced.

[Fifty50Plus]

I looked at these pics for a long time yesterday and the only comment I thought about posting was that the pickup screen might be too close to the pan bottom. Probably not an issue.
Based on what Larchie said though, there is a way to see if that might be the problem. Try adding two or more quarts of additional oil to thoroughly overfill the pick up area. If, in fact, it was sucking air as the oil heated up this might help to diagnose your issue.
_________________
1979 924 NA race car H-Prod SCCA
1982 924 NA race car - Sold
1981 924 Turbo sold
1982 924 Turbo sold
1972 911 E race car - traded for Cayenne Diesel
1975 914 1.8 Building for H-Prod SCCA

[agfisher]

How does the inside of the pickup pipe look? Clean welds? If there are some rough welds they might restrict to flow or cause noise.
_________________
1977 924 N/A Martini Edition

[Horizonblue]

Thanks for the responce guys.

I have tried to add more oil, but the sound still appears. I have also tried parking the car in my driveway, with the nose pointing uphill, so there is more oil around the pickup, but the sound is still there.

I don't know how the welds look inside the pickup tube, I tac welded the tube completly, before fully welding it, so I could not check much inside. But off course, it would have been an idea to check the inside with a probe camera, but I don't know anyone who has one. I'm a professional welder, so it was not my concern, when I made it.

One thing about the design of the oil pan, that maybe isn't the greatest ide, is that it has a flat bottom. When oil is splashed against the flat surface, it takes more time to get to the sump and pickup, than if the pan was rounded/ had a V shaped bottom. But there is no way I can do that. There is no more space above the front cross member. So the design of the oil pan cannot be changed much, there is not space for that. Design changes must be done inside. But this hasn't anything to do with the sound.

Another thing that is a bit weird, is that when the noise appears, the timing belt starts to vibrate. Maybe it isn't the pump itself that makes the sound. Could it be the coming from the timing belt? But then, why is it only there when the engine is hot and running at 1200 rpm?

I had the cranck shaft V-belt pulley off, to gain access, when I changed the timing belt. I don't think much could go wrong when I did that, but I'd better investigate to see exactly how the timing belt pulley is mounted on the crankshaft.

Could be fun to run the engine without cover and V belt pulley, to see what happens downthere, when the noise appears.
_________________
Euro 924, 1976

"If you can't fix it, don't break it"

/P.G. Andersen

[morghen]

1. how long was the original pick up tube? are the lengths comparable or totally different?

2. did you pressure test the pick up tube? maybe you have some micro cracks/openings and the sound comes from the air being sucked in trough those. (my bet is on this one)

3. is the oil pump mecalically OK? have you checked play/tolerances/whatever needs to be checked on that specific pump?
_________________
Supercharger and EFI kits
https://www.the924.com

[Horizonblue]

1. the tube is a bit shorter(not much) that the original, here are pics of that one





2. I did not pressure test the pickup tube, since I couldn't find a way to block the strainer end easily. Instead I blocked the flange end and filled it up with diesel fuel. No leaks.

3. I haven't checked the pump at all, it has not been off the engine. I didn't do anything, since the engine seems to be in good condition.

I'm leaning more and more towards that the problem isn't air in the oil. When the sound appears, it's like someone flipped a switch. I have to be carefull on the throttle to find the spot where it comes. I don't think that the oil flow changes that much, just by adding 100-200 rpm and then suddenly it's gone again, when I rev over the spot.
_________________
Euro 924, 1976

"If you can't fix it, don't break it"

/P.G. Andersen

[Fifty50Plus]

Okay, if it is a mechanical vibration, look for harmonics at 1/2 and double the frequency (RPM). You might be able to source the sound using a long screwdriver or piece of metal resting against your head (near your ear) and the other end touching various parts of the engine. This method helps you "hear" where the noise is coming from.
_________________
1979 924 NA race car H-Prod SCCA
1982 924 NA race car - Sold
1981 924 Turbo sold
1982 924 Turbo sold
1972 911 E race car - traded for Cayenne Diesel
1975 914 1.8 Building for H-Prod SCCA

[larchie]

Fifty50Plus's suggestion makes a lot of sense. "Buzz" vibrations at specific frequencies are not unusual after installing timing belts.

So it looks like you've found the problem to be the harmonic vibration of the timing belt at a specific temperature and especially RPM. When the natural frequency of the timing belt matches the natural frequencies of the pulleys and connected shafts resonance occurs. So changing the belt tension and/or belt type will change the amplitude of the vibration. So will many other factors such a statically and/or dynamically testing the pulleys, the exact position the idler pulley, pulley misalignment, etc. Belt type also can be a factor. Hopefully balance shafts haven't been removed.

I wouldn't think you would have to check very much by the "ear on big screwdriver" technique to see the origin of the vibration since you already see the vibration in the timing belt.

So it might be worth trying to slightly change belt tension to see if the harmonic vibration changes in amplitude or occurs at a different RPM. If so, it's likely you've found the problem.

[morghen]

You can check if there is air in the oil easly...old the engine at that RPM and, turn it off and then inspect the oil filler port for oil with bubbles.

Harmonics of timing belt also sounds like a potential cause...i do remember i had the timing belt a bit too tight once and it made such a sound...but only unti it got worn a bit.
_________________
Supercharger and EFI kits
https://www.the924.com

[Horizonblue]

Yesterday I removed both timing belt covers and checked the tension, maybe it is a bit too tight, I could hardly twist it 90° as the manual says.

I have actually tried listening with an "auto-stetoscope" on one of the first occasions, when I heard the sound. I was a little confused at that time, cause it was like I could hear two sounds(with the stetoscope). I placed the probe on the pump and the sound in question was very clear. But I could also hear the sound of a gear set, running in oil. You all know how that sounds. The latter must have been the real oil pump sound, which only could be heard with the stetoscope.

With that in mind, there is a chance that the noise is not from inside the pump (air/turbulence in the pump gear).

I will start the car today, with the timing belt covers off and investigate further.
_________________
Euro 924, 1976

"If you can't fix it, don't break it"

/P.G. Andersen

[Horizonblue]

Ok Guys, call off the search, I have found the problem. (Larchie you were spot on)

The timing belt tension was too high.

When I came home from work today, I immediately started the car and let it warm up, and the sound appeared, as always at approx. 1200 rpm.

I took the timing belt covers off yesterday, and I could clearly see (with the warm engine running) that the timing belt was vibrating, exactly when the sound came. I held the smooth end of a spanner against the vibrating belt, and the noise was gone

Ok then. I loosened the waterpump and released the tension a bit. The waterpump is used as tensioner on this engine. Started the engine again (it was still hot) and the noise was gone.

I installed the covers, washed my hands, jumped into the car and took it for a spin around town. Man, what a relief that this sound is gone.

All speculations can be brought to an end.

I was seriously considering a dry sump, or at least modifying the oilpan inside as well as the pickup tube. That will not be necessary, at least not now.
_________________
Euro 924, 1976

"If you can't fix it, don't break it"

/P.G. Andersen

[morghen]

Congratulations !
Now to get the power to ~140-150 HP and then to enjoy a great 924.
_________________
Supercharger and EFI kits
https://www.the924.com

[Horizonblue]

Thanks Morghen

Yes, during the winter I will start to build an EFI intake manifold. But first I will re-jet the carburettor so it runs a bit richer. It will take some time before I have all the hardware for an EFI system. Until then, I will have to drive around with the carb.

If I can arrange it soon, I will take a new video, so I have one without the timing belt sound.
_________________
Euro 924, 1976

"If you can't fix it, don't break it"

/P.G. Andersen

[Fifty50Plus]

Congratulations!
Sometimes these investigations are like "How many engineers does it take to screw in a light bulb?' At the end you hear an answer that makes you smile.
It's good news when the solution doesn't cost lots of money and we all learn a bit more.
Nice job on the 6 cylinder install!
_________________
1979 924 NA race car H-Prod SCCA
1982 924 NA race car - Sold
1981 924 Turbo sold
1982 924 Turbo sold
1972 911 E race car - traded for Cayenne Diesel
1975 914 1.8 Building for H-Prod SCCA