300SD intermittant squeal engine pass. side
 

I have a 1985 Mercedes 300SD with 92,000 miles. It was my Dad's car and drove only in the summer. Everything seems to work i.e. car sits for a long time but vacuum door locks etc all work fine. The car is slow on acceleration but shifts smoothly. An intermittent squeal comes from near the motor mounted air cleaner but isn't the air cleaner. If you are driving the car and the rpm is around 3000 and you slightly let up on the throttle, a one to two second squeal occurs noticeable inside the car. I've tried to find it under the hood - my 11 year old can hear it too but the clackety clack makes it hard to diagnose under the hood! Thinking of EGR, turbo etc? Anyone else having any issue like this? It's not the belts - the noise is higher than that and I've retightened all with no effect.

Could be the alternator bearings. Try duplicating the noise under the same condition as before, but put all of the electrical consumables on, such as the heater fan to Max, Rear defroster, Wipers, Lights (on high beam), lighter, etc., See if this makes a difference to the squeal, especially if it now occurs at a different RPM.

If yes than its likely your Alternator bearings, if not, could also be the water-pump.

Phil

Hello Harold and welcome to the forum.

I agree with Phil's recommendation regarding the alternator. However, if the alternator is the culprit, it does not care if the vehicle is in motion. So, you can do all checking while the vehicle is in the driveway.

If you cannot duplicate the noise in the driveway, or affect its characteristics when adding additional electrical load, then you have effectively ruled it out.

Please attempt to find the source of the noise with the vehicle stationary. If this is not possible, because the noise has vanished, report back and we will attempt think it through again.

easy check would be to remove the alternator belt, start the motor and see if it goes away. don't run the car long since you'll be running off the battery completely though. if it goes away, its probably the alt bearings...

California model will have the dreaded air recirc valve and the trap oxidizer -- check for a loose intake hose between the turbo and the air filter housing. When the throttle is closed, the air recirc valve will dump boost into the intake hose, and make a squealing/swooshing noise if the hose is loose. This also means you are sucking in dirty air, so you need to check pronto.

You will also get the noise if there is a loose blowby line there, too.

If you have the trap oxidizer still in place, you need to go to a dealer and discuss replacing it with a catalyst.

Peter

300SD squeal
 

Hi guys, thanks for the replies as this is my first time trying something like this.
I don't think it's the alternator as the squeal/whistle is only for a second and is always the same pitch. My brief experience with previous alternator failures is that the pitch goes up and down with engine rpm. This squeal stays the exact same pitch (or tone). It actually is close to the pitch of a brake squeal but of course isn't the brakes. I've also jammed the brakes when driving the car when the noise starts but it doesn't affect it. I also can't get it to squeal at low rpms, only at 3000 rpm and letting off on the throttle. I have heard the noise repeating 3 or 4 times when coming up a steep hill and thought for a while it was the radiator cap so changed it with the same correct Mercedes temp spec but no luck.
I have been able to make the noise when revving the engine in the driveway and letting it drop in revs but of course a diesel doesn't decelerate that quickly when there isn't a pull from the car so it's hard to get it to do it every time. My 11 year old points to the top part of the engine and when trying it myself I can tell that the noise is also near the EGR. I changed the 1/2" hose leading under the EGR through the U shape in the air duct to the air cleaner but it didn't change anything and the original hose was in pretty good shape considering the age.
The EGR has a black dust near the vent hole but I'm not sure how to test it to make sure it's sealing correctly. I'm told that the 1985 300SD followed the California exhaust emissions so the EGR etc is set up different that the 1984 standard US (non California) model. Not sure if this is true but web sites seem to indicate this.
It seems that pressurized air is not being needed when you lift off the throttle and it is being expelled like a steam whistle. Does this still point to the EGR or something else? If you still think alternator, I'll pull the belts and try to repeat but it typically won't sound unless the car is up to full temperature.

Is it a constant pitch squeel? Or does it start higher and get lower before it stops?

My 300CD does this as well. I've decided its the turbo winding down.

The pitch is always the same. Same peep each time - sometimes the squeal/whistle will last one second but at times I've counted up to 5 seconds when going up a steep hill. It's always the same pitch and I've heard it at 30 mph and 60 mph always the same pitch. I've never heard the noise when decelerating below 30 mph or at a red light. I drove it 2500 miles in 3 days, air conditioning off and on etc (still heard it) and heard it quite often in the mountains. The only time I can get it to start is to get the engine fully up to temperature and up to 3000 rpm and then lift off the throttle. Most times it will squeal/whistle for a brief second. When going up a steep hill for a half mile or so, it will start to whistle/squeal for longer time periods even when I haven't lifted off the throttle. You can hear it in the cab of the car but have a hard time hearing it under the hood due to the diesel, but you can get it to squeal when revvving the engine and letting go. I've changed all the belts and retightened them again. No impact - same noise. Changed radiator cap - same noise. Someone said to check the overboost sensing switch hose from the back of the intake manifold to the drivers side firewall mounted switch. I pulled the line but I can blow backwards (mouth method) through the hose and into the intake manifold so the line doesn't appear plugged. It's actuallly fairly clear looking with only a minor amount of soot near the overboost switch. Still stumped.
Harold
1985 300SD 5 cylinder turbo diesel 92000 miles

I have the same car and the same problem
 

After accelerating up to cruising speed, when I let up off the throttle, I get a pressure-relieving type of sound, accompanied by a high-pitched whistle. The pitch of the noise is constant, but the amplitude tapers off. It definitely does not sound like a mechanical squeal, like a dry bearing or a belt squealing, and the constant pitch rules out noise from the turbo spooling down. Imagine opening a compressed air line very quickly, then slowly closing it over a 2-second period while it vents through a dog whistle.

I cannot get my car to make the noise by blipping the throttle in the driveway. The engine has to be under load. Since the noise occurs when you would suspect that the turbo is producing near max. boost and then the engine load suddenly drops, I suspect either the wastegate or the switchover valve that passes boost to the alda. I have had higher priority repairs to make until now, but I plan to work on this tonight.

Now, how about some input from the Wise Ones on the forum:

I can try capping off the pressure line at the switchover valve and see if that eliminates the noise. But if I do that, the alda will not be enriching the mixture will it?

I could bypass the switchover valve altogether by connecting the pressure line from the intake manifold directly to the alda. But then don’t I run the risk of overboosting the engine? This approach removes one variable but adds another, which confuses the situation.

If I unplug the vacuum line to the air recirculation valve, will it still operate? Is my understanding correct in that the recirc. valve dumps intake manifold air back into turbo intake?

On my car the air filter housing outlet is about an inch short of fully seating in the U-shaped air tube attached to the intake side of the turbo. I have the two parts duct taped together. I will remove the tape at lunchtime today and take the car for a drive and see if anything changes.

Harold: Did you ever resolve the problem?

effect of loose air filter to U-tube connection
 

I removed the duct tape from the connection between the air filter housing outlet and the plastic U-shaped tube and then drove the car around. The noise was noticeably louder. I replaced the duct tape with an extra thick layer and the noise was muffled quite a bit. Verdict is that the noise is the air recircuation valve.

I have noticed in several threads that some people unplug the air recirculation valve vacuum hose. Is this advisable?