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E34 (1989 - 1995)

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Old 10-17-2012, 06:55 AM
robertobaggio20 robertobaggio20 is offline
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Weird humming/moaning sound from engine solved after 1+ year

Hi Everyone,

Soapbox alert.

I am pleased and somewhat chagrined to report that I've finally solved a very irritating and annoying problem with my car that I've had for and have tried to fix via various methods, for more than 1 year.

I've had a weird humming sound that comes from the engine and is only audible while sitting in the car. Its more like a moan....similar to the sound a bad power steering pump makes, or a power steering system might make when it has air trapped in it. Or so I once thought.

I wish I could describe the sound better, but that's honestly the best I can do.

This sound would appear roughly 50% of the time while driving. It was not easily reproduceable to friends and mechanics. It tended to occur at the lower ranges of speed and was absent at cruise, but occurred particularly during gear changes. Sometimes, while at idle, I would flip the throttle from 600 to 1300rpm rapidly, and it would appear on the way up before dying down.

More than one mechanic was enlisted to solve this problem, and all of them misdiagnosed it, despite them being highly experienced. One person, in fact, was a transmission expert who kindly kept the car over one day to isolate the sound and advise me if it was related to my transmission (which had other problems and I was considering a swop or a rebuild). He had never heard this before and was genuinely puzzled. After checking it out for one day, he said that it appeared to come from the crank area but definitely not from the transmission.

Along the way, I threw all kinds of parts at the problems and spent way more that I needed to finally solve this problem.

The source was definitely nailed down just yesterday : the a/c tensioner and its pulley. Yes, I can't believe I spent that much time and money, and that the source of the problem was so straightforward and ultimately cheap compared to what I had spent, but I'm just glad that it is over and I'm very glad that the problem was not with the engine's internals or something freaky buried deep in the transmission.

These were the parts that I changed, that I ultimately don't mind doing due to its long term benefits, the performance upgrades some of them created, or because some of the components were found upon examination when removed to be somewhat defective anyway :

a. E39 pulley tensioner upgrade to the E34 m50 engine.
b. 240amps upgraded, rewired and rebuilt bosch alternator.
c. Thermoplastic water pump pulley
d. Composite impeller water pump.
e. Clutch fan delete (although this was not strictly related to the effort to trace the sound, but was done at the same time, my thanks to ethirty)
f. Full set of new OEM ignition coils. (Yes, I once put in low mileage coils into my car and found that the sound disappeared while driving. I though my old coils were not firing strongly or properly, and thus causing uneven motion in the engine. In actual fact, I had also tried out a belt spray on my engine along with the ignition coils. The belt spray was responsible for stopping this sound. I did not make this connection until 6 months later.)

These are the parts and elements that were a total waste of money:

1. Power steering pump, power steering reservoir, power steering high pressure hose (all new) and a power steering box (used), and of course alot of p/s oil.


If you have any suspicious, and unusual sounds coming from your engine, and ESPECIALLY if its difficult to localise with the hood open, I would strongly recommend doing the following :

Check all the pulleys and tensioners. Remove the belts, then hold the tensioners and pulleys one by one and shake it back and forth, up and down, etc. to see if there's any suspicious movement either back and forth or laterally. Then replace the belts and see if they are tight. Pull up on a belt and see if the other portion of the belt pushes back on its tensioner. If it does and it moves, then that tensioner is suspect.

If you find some pulleys/tensioners that are suspect, remove that belt and start/drive the car without that belt for a few minutes and see if the sound is there. However, be extremely careful about removing the belt that also drives the water pump. If you wish to try that, make sure the engine is cold, remove the belt, and drive the car for under 10 minutes, before replacing the belt. Any longer and the engine will overheat.

If the belts are not tight, the belts may be slack and may require changing.

Start the engine and watch the belts. See if they move normally over anything and see if there is any lateral movement across pulleys etc while running.

Stop the engine, hold the fan clutch and move it upward to see if there is any free play. If there is, either the fan clutch or (more likely) the water pump's (wp's) bearings are suspect.

All of the above, when busted, can create sounds, moans, hums, etc coming from the front of the car that are difficult to localise with the hood off. I'm beginning to think in fact that some of these sounds are normal, but are usually too faint to be heard at all. They only get louder when the component (s) begins to fail. So do not think that hearing any and all sound from the front has to be a problem.

How did I finally find the problem? I started looking carefully and checking carefully myself. The a/c tensioner pulley had some free play and lateral movement when grabbed and shaken, and the tensioner's pushing force did not seem sufficiently great. It looked like it could be the problem. Ultimately, I tested my theory by simply driving without the a/c belt. The humming sound was suddenly missing. I replaced it and it was back again. I sprayed belt conditioner on the a/c belts and pulley, and the sound went missing. Hosed it down thoroughly with water, and it was back. Changes\d the a/c belt to a brand new one, and the sound was still there (although reduced very slightly). It had to be the tensioner and the tensioner pulley. They have now been changed. Problem solved.

There was also alot of dirt and caked dust in the engine's front, which was easier to see once the fan clutch and fan's shroud had been removed. An hours battle with carb cleaner, degreaser, a thick brush, a toothbrush and high pressure water took care of most of it. The caked dirt and crap did not contribute to the sound, but needless to say, please keep your engine's front portion as clean as you can, dirt can only cause problems.

Lessons learned from this saga :

1. Please do not ask my why I did not ask you guys about this. I have no answer for that. It would have undoubtedly narrowed the ballpark for me. However, if you have a weird problem, don't think twice, just post it here!

2. Failure to fix a problem after it was misdiagnosed and money spent on an unnecessary repair, should increase people's motivation to find the real culprit. Professionals, who have made a profit from an unneeded repair, generally do not seem to function this way. If they can't figure it out, they'd rather not look at it, even though their diagnosis has cost you money and time unnecessarily.

3. Nobody will love your car the way you do. So educate and involve yourself in your car's repairs in a non-threatening but affirmative manner wherever possible.

4. Approach all annoying car problems with a beginner's mind. The zen approach is absolutely necessary. Ultimately, what solved the problem for me was a simple grade-schoolish step by step checking of this and that. I encountered the problem. I am neither a mechanic, nor an automotive or mechanical engineer, nor do I possess formal training in that area. However, I am damn stubborn and will not quit, and that helps.

5. Too many mechanics do not take this zen approach when they encounter a tough problem, which is a great mistake and ultimately reflects a lack of confidence in their skills, in my opinion. A seasoned mechanic, no matter how experienced, old and successful he is, needs to keep alive the curious heart of a detective...a whodunit sense. You will eventually find a model that you're familiar with coming to you with a problem that baffles even you. If you let your ego and frustration get in the way of solving this problem, you will cloud your head and not do the simple and obvious things that are required. Getting your ego or sense of frustration involved will also screw up your internal instincts, which are frequently far more important than formal education and experience. It really should not be an amateur that thinks to himself "lets get back to the bare basics and see" and then begins to devise experiments and methods to isolate or rule out components.

6. I was ultimately prepared to purchase belts of different lengths to individually isolate the p/s pump, or the alternator, or both, while keeping the crank connected to the water pump while driving. All this may seem extreme, but the willingness to create and conduct experiments is important.

7. For some stupid bloody reason, the toughest, most vexing car problems, most frequently, have the simplest solutions. I used to think that this was a coincidental irony but have now come to understand that this may be more of a rule rather than an exception. And this, as I'm sure all of us can agree, does not just apply to cars but to everything in life.

8. Yelling loudly at god helps to fix cars . It gets god's attention , and the shouting and venting clears your head, allowing you to think clearly, helping you spot the solution.

My apologies for the soapbox venting but it has been a long long loong night before the dawn.


Last edited by robertobaggio20; 10-17-2012 at 08:50 AM.
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Old 10-17-2012, 12:34 PM
Monsignor's Avatar
Monsignor Monsignor is offline
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Location: Pineapple Under the Sea
Join Date: Oct 2009
Posts: 3,285
Mein Auto: Sig
9 months to diagnose and solve.
3 months to write up explanation.

If only everybody was in depth, and i mean that honestly.

"fixed an oil leak"
"I had an oil leak and it was the gasket."

Steps taken to figure out and diagnose are the most crucial part of any fix, with an in depth analysis of what each step solved, caused, or led to.

EDIT: Please do write up or a how-to on the fan clutch delete as this would prove use for myself.

1995 525iA 250k mi
2003 350Z 90k mi FOR SALE
2014 F-150 9k mi

Instagram: @Titan_E34
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Last edited by Monsignor; 10-17-2012 at 12:39 PM.
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