300D engine airflow problem - PeachParts Mercedes-Benz Forum

#1 10-28-2002, 08:01 PM

I have an '85 300D that runs fine until about 45 minutes at highway speed, then it developes a rumbling sound from underneath that sounds like air to me. The sound is most prominant when the car hits a bump and the car bounces up a bit (I know, weird?) Also, it is more obvious when accelerator is released. There is a chirping or whining sound that accompanies it that also sounds like air. After the car is stopped (toll booth,) the problem goes away for another 45 minutes. The rear end checked good by a mech. I think it is turbo/exhaust related.

Right around the time I started to notice this, the car developed a miss at startup. I found a leaky injector crossover line and replaced it with no change. I have read some posts and will check some more fuel supply related items (ALDA etc.) Also, I noticed the whine of the turbo that used to be present when I backed off the throttle a little does not exist anymore. I thought that the wastegate might be stuck shut, but from what I read tonight, that does not seem to likely now.

Are these symptoms concurrent with a fuel delivery problem or an overpressure situation? The noise sure sounds like air "popping" in the exhaust. The baffalling part is that it goes away until you drive at highway speed for 45 minutes and I can't affort to pay a mechanic to ride all the way to work with me. Any help will be greatly apprieciated.

VR,
Clay

psfred · #2 10-28-2002, 08:42 PM

Check the center bearing and rubber mount on the driveshaft. You may be getting some vibration from a bad bearing as it gets warm.

That's the only thing I can think of

You might check the turbo too, although anything seriously wrong with it will cause either excessive oil consumption of poor performance.

Peter

#3 10-29-2002, 05:18 PM

Thanks Peter, I will check the drive shaft, but after another comute today, I am almost certain it is air and not metal. If it were the drive shaft, why would it go away after stopping? Can it cool off enough just from a toll booth stop? I am in Tampa/Orlando and it is still 85+ here. Is it possible to have a clogged up exhaust that creates back pressure only after 40 minutes at highway speed and then relieves itself when the engine comes back to idle? Thanks for your help.

wheelguru · #4 10-29-2002, 05:27 PM

Search for a fairly recent post about the rear CV joints. Apparently they are filled with a really thick oil, not grease. If you have a leak, enough can get out that after a while, centrifugal force makes the oil migrate out of the joint and into the boot. Stopping even for a short time lets it run back down into the joint for quiet operation again. When I got my parts car I drove it for a while, one or both joints were on the way out - i got a noise similar to yours that only occured after a few hiway miles. eventually it was present all the time, although I was lucky enough to not have one totally give out.

MVK · #5 10-29-2002, 07:03 PM

I would start with all vacuum lines, make sure none of them have coome loose from the rubber connectors, also check the EGR valve line. Check the rubber pipes on and around the turbo. One of the feed line etc. migh be leaky. Make sure the Air filter is in good shape. and mounted firmly.
The air recirculation valve can be inactivated too.
I cant think of anything else. and I know I am too general in these answrs but, these things are nice to check anyway.
Try inactivating the EGR valve and see if it makes a difference.

MVk

MVK · #6 10-29-2002, 07:05 PM

Also check the rear sway bar links.
And make sure there is no big hole in the bottom on the foor. LOL Just a thought.

MVK

psfred · #7 10-29-2002, 07:50 PM

Check for damaged or plugged exhaust, and do you have a trap oxidizer on that model? California delivery cars had one, and if it is plugged, it will cause exhaust to escape from the trap joints. Whistle or roar, depending. Ditto for a muffler full of rust and soot causing a leak, although that is usually exhaust noise.

The other thing it could be is the visco clutch locking up -- my Volvo whines like a jet engine, you can definitely hear it at highway speeds, usually only happens when I am running hard in heavy traffic on hot days (90F and up).

In that case, it is normal -- the easy check is to pop the hood after stopping and see how much air is getting blown by the fan -- should be MUCH more than usual. Fan will stop instantly when the engine is shut off, too.

Peter

#8 10-30-2002, 03:59 AM

So a leaky turbo will perform fine but still consume oil? I havn't checked mine yet - but the performance is a little down and I am still using oil.

#9 10-31-2002, 07:58 PM

Thanks psfred, wheel guru and mvk. I finally found the evidence (with your guidence) from the leaky cv joint and my new mechanic is replacing the axle tomorrow morning. My air theory was shot to hec, guess that is why I am not a mechanic. I suppose the missing at start-up is unrelated. The guy I took it to said I most likely have a weakening fuel pump due to todays diesel fuel not having a nickel additive anymore. He said that the nickel acted as a lubricant and without it some teflon seals become brittle and hence the lack of pressure at start up. Can anyone confirm this? From what I read on this site, new glow plugs are in order but this guy steered me away from them. Thanks again all for your time.

VR,
Clay

psfred · #10 10-31-2002, 09:04 PM

Missing on startup is more likely to be bad glow plugs or valves in need of an adjustment.

There is no nickel additive in diesel fuel, nor has there ever been. Sulfur has been removed, and the sulfur compounds provided some lubrication that may be missing from "new" diesel fuels.

Peter