1983 300D Engine Knock - PeachParts Mercedes-Benz Forum

jcoop · #1 01-23-2016, 08:23 AM

Hello everyone, I have been reading the Peach Parts forum for as long as I've had a W123 but now, it's finally time for me to pose a question. I just recently (1-20-16) bought a 1983 Mercedes 300D with the knowledge of an engine knock. The knocking by itself resembles a rod knock; raspy in nature but the timing isn't right at all. In fact, it sounds like someone is just randomly tapping the block with a small hammer. Here's a video I made of the car's knock: https://www.youtube.com/watch?v=fPeSE04EN9I

As you can see, the car doesn't run bad at all but the knock is very concerning. I've had 3 of these cars and this is the first one to make such a blatant noise. So, here is my dilemma. I have a 1984 300D (also depicted in the video) that runs like a swiss watch (sadly i hit a deer with it *sigh*) BUT that means pulling the engines from both cars and doing an entire swap. OR I can pull the engine out of the 1983 and have it re-worked.

If the car was yours, what would you do? Maybe, just maybe, that horrendous knocking is something else hitting or rattling on the engine as it's running? Please leave some replies as i'm sort of baffled.

vstech · #2 01-23-2016, 08:30 AM

it's certainly not a rod knock. I recommend pulling the injectors and inspecting inside the prechambers for loose parts.

if none found, swap the injectors from the good running vehicle to the knocking one and see if the noise stops.

vstech · #3 01-23-2016, 08:33 AM

it's also possible that the vacuum pump is faulty. the noise sounds like upper valve cover or injector noise.

Air&Road · #4 01-23-2016, 08:38 AM

Yes, the roller in the vacuum pump can make a heck of a knock and if it comes loose can sometimes completely wreck the engine. If it is the vac pump roller, however, you will be able to tell that it is coming from the vac pump when listening to it. It also is not a big job to remove and inspect the vac pump.

VS offers some really good advice. If after going through his scenario and checking the vac pump, you find that the knock is not injectors or vac pump, I would recommend that you put the known good engine out of the wrecked car into the driver.

My $0.02,

jcoop · #5 01-23-2016, 08:41 AM

Quote:

Originally Posted by vstech

it's certainly not a rod knock. I recommend pulling the injectors and inspecting inside the prechambers for loose parts.

if none found, swap the injectors from the good running vehicle to the knocking one and see if the noise stops.

Awesome, when things warm up (and this snow melts!) I'll pull the injectors and post my findings. I'll also take a look at the vacuum pump as i have an extra one.
Is there anything specific I should look for when I have the vac. pump off?

Air&Road · #6 01-23-2016, 10:14 AM

What you are looking at is the roller. There is a roller on an arm that follows a cam on the end of the injection pump sprocket. The roller should be tight and roll smoothly. If you can rattle that roller it is loose. If it is loose enough to knock, the problem with the roller will be obvious. If the rest of the pump is in good shape, I think you can get a new roller and pin and replace it. I don't know that for sure because I have always replaced the pump.

You will know what I am talking about after you get the pump off and look.

Hope this helps.

funola · #7 01-23-2016, 10:52 AM

Just heard the audio. Try to localize the noise starting at each injector boss on the cyl head with a stethoscope (long screw driver). If source is not at the cyl head, move onto vacuum pump, alternator, ac compressor, power steering pump, air cleaner mount, motor mount etc. anything that can vibrate. The noise is sufficiently loud you should be able to localize where it originates.

Diesel911 · #8 01-23-2016, 06:27 PM

A broken air filter bracket or the little rubber insulators will also cause a lot of noise.

jcoop · #9 01-23-2016, 07:32 PM

Alright, got home from work, cleared out the garage and got the frozen Benz inside to thaw. Going to pull the injectors and glow plugs, remove the air filter housing and then see where I stand. [ATTACH][ATTACH]

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Air&Road · #10 01-23-2016, 07:48 PM

Quote:

Originally Posted by Diesel911

A broken air filter bracket or the little rubber insulators will also cause a lot of noise.

Yep, I've heard that one before, but I forgot about it. OP, make a quick check by shaking the air cleaner.

barry12345 · #11 01-23-2016, 07:58 PM

I would not pull the injectors yet. More attempts to localize the noise first. The noise as far as I can determine from the video. Is not tinny enough or hard enough but sounds a little erratic to be a major engine problem. I also do not expect it is really anything serious.

Timing chain components like the tensioner should be kept in mind if nothing simpler shows up. Try to get a real listen to that vacuum pump as others have suggested.

vstech · #12 01-23-2016, 08:11 PM

looking at the video i dont see the air cleaner moving with the noise, but it's possible.

funola · #13 01-23-2016, 09:30 PM

Quote:

Originally Posted by jcoop

Alright, got home from work, cleared out the garage and got the frozen Benz inside to thaw. Going to pull the injectors and glow plugs, remove the air filter housing and then see where I stand. [ATTACH][ATTACH]Attachment 134590[/ATTACH][/ATTACH]

You gonna pull the injectors then listen with a stethoscope?

jcoop · #14 01-24-2016, 12:26 AM

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Before taking it apart, I ran the car to try and localize the knock. Found it to be coming from either #3 or #4 cylinder towards the firewall. The air cleaner bracket looks to be relatively new and is not the culprit. Nor is the alternator (sometimes they can make knocking noises?). I still have not checked the vacuum pump but am fairly sure that isn't it. The sound is coming from the top of the engine. The cam lobes are in GREAT shape and nothing is physically loose in the valve train. Interestingly, the last owner had replaced the timing chain but never said if the noise started after that or not.

As it turns out, I don't have a socket large enough to fit the injectors so they'll have to wait until tomorrow. I'm quite curious to see if i find anything in the pre-chambers.

vstech · #15 01-24-2016, 12:47 AM

27mm 12 point deep FULL DEPTH socket works, 1-1/16 is close enough.