Engine Knock - PeachParts Mercedes-Benz Forum

yellowdragon · #1 03-31-2006, 11:56 PM

I have a knock in my engine. I have a 1980 450 SL! The knock is coming from the right rear of the engine. I removed the valve cover to see if there is anything abnormal, it looks good so far. I'm planning on checking the valve settings tomorrow.

Does anybody have any other suggestions?

BENZ-LGB · #2 04-01-2006, 12:52 AM

Quote:

Originally Posted by yellowdragon

I have a knock in my engine. I have a 1980 450 SL! The knock is coming from the right rear of the engine. I removed the valve cover to see if there is anything abnormal, it looks good so far. I'm planning on checking the valve settings tomorrow.

Does anybody have any other suggestions?

Carbon deposits?

Strife · #3 04-01-2006, 02:41 PM

This car does not have adjustable valves, AFAIK. Maybe the cam or cam followers are worn. Have the oil tube fittings (above the cam towers) ever been changed?

t walgamuth · #4 04-01-2006, 08:21 PM

describe the situations in which you experience the knock and if it is sensitive to throttle input car speed, etc.

tom w

86560SEL · #5 04-01-2006, 10:47 PM

Not to hi-jack this thread, but as mentioned in another topic (with no response), I also have an engine knock. It only can be heard between 1600-1900 rpms under acceleration. It seems to be coming from the right side of the engine. My car has 265K and I think it is a good chance that it is worn camshaft causing it. Also too, the lifters are making a little "clacking" noise on that side as well, but I can tell that the knocking sound is distinctively different than the lifters.

When I first bought my car, the mufflers were full of holes and the car was rather loud and I could not hear the knocking at all. It was only after I installed new mufflers that I noticed the knock. I have not heard it get any louder since I have owned it (since December 05'). However, it is still a big concern of mine. I really like my car, but I would like to know that my engine is not worn out.

If it is indeed the cam causing the knock, will it damage my engine to continue driving like this? Will it eventually quit running, or.....? The car is old and I did not pay much for it, so I really do not want to spend a lot of $$$$ on it. I have heard that camshaft replacement is very costly on these cars, so I do not think my car is worth the trouble. If I can get about 25000 more miles out of my car, I will be happy. It still does not leak, nor burn one drop of oil, which is a plus.

What exactly does a carbon knock sound like? Does it occur at all RPMS? What is the best "cure" for carbon knock if that is what it is, but knowing my luck, I doubt its that simple. I have the worst luck with cars.

Yellowdragon - he is right- it could be worn camshaft/camshafts.

Thanks in advance.

Quote:

Originally Posted by t walgamuth

describe the situations in which you experience the knock and if it is sensitive to throttle input car speed, etc.

tom w

Tomguy · #6 04-01-2006, 10:59 PM

Worn cams will definetly make the engine loud - but usually the front of the cams wear before the rears.

86560SEL: Driving with bad cams isnt good in that you'll have worse fuel consumption and less power, and it'll be loud and probably misfire as they wear even more and inhibit proper combustion - I don't know what might happen if the exhaust lobe is worn but the intake isn't, but I don't think catastrophic engine failure is a result. If you DO have a bad cam, start looking around on eBay for them. Hell, I'd settle for a cam from any m116 or m117 with hydraulic studs if I were you. An old 4.5 cam, or a 420 cam may even give you more power?

What type of knock is it - octane knock or mechanical-sounding?

86560SEL · #7 04-01-2006, 11:17 PM

Thanks Tom. It sounds more like a mechanical knock, rather than the "spark knock" sound. You mentioned low fuel mileage/low power..... Well, that is what is even more puzzling - my car gets excellent gas mileage. I get about 20 MPG in town, which I have heard is exceptional for this car. Also, while not a powerhouse, power is very adequate. It is my understanding that this is not a very high HP engine, especially for a nearly 4000 lb car, so I am guessing that the power of my car is probably not much less than it was when new?

HOWEVER, I do have what sounds like a splutter at idle (from the exhaust), but I cannot feel any roughness from inside the car, or even the engine. I installed new plugs, thinking that was the problem, but it did not. It may need new plug wires. They look original, but I doubt it on a 21 year old car?

My father glanced inside the oil spout on the valve cover when we were changing the oil and while I did not notice, he said - "that lobe on that cam really looks worn". I am guessing that is the problem then huh? I do not mind the knock, I just do not want to know that my engine will soon go out, or wear out even more quickly like this.

Thanks again!

Quote:

Originally Posted by Tomguy

Worn cams will definetly make the engine loud - but usually the front of the cams wear before the rears.

86560SEL: Driving with bad cams isnt good in that you'll have worse fuel consumption and less power, and it'll be loud and probably misfire as they wear even more and inhibit proper combustions - I don't know what might happen if the exhaust lobe is worn but the intake isn't, but I don't think catastrophic engine failures is a result. If you DO have a bad cam, start looking around on eBay for them. Hell, I'd settle for a cam from any m116 or m117 with hydraulic studs if I were you. An old 4.5 cam, or a 420 cam may even give you more power?

What type of knock is it - octane knock or mechanical-sounding?

t walgamuth · #8 04-02-2006, 09:06 AM

if you have a very worn cam lobe you prob have more than one. it cant run right if it has that. if several cylinders are running with less than a full charge of air/fuel then it might be getting very good mileage, but you wont have the power it should have. and it wont actually be running smoothly. if you are used to a four cylinder, for example an eight running on six with two weak may not seem to bad but compared to a good running eight you will readily see the difference.

i would be getting the valve covers off and investigating the cams soon.

tom w

AF300E · #9 04-02-2006, 09:15 AM

Hmm, knocks are bad mmkay?

The carbon knock is duller than a mechanical knock. When I was in the workshop we had a C36 come in with a knock in it and carbon was the cause. I can distinctly remember the noise it made.
Lifter knock is more like a light rattle and usually goes away as the revs rise because the oil pressure comes up. It won't stop if the cam is worn though but you can very easily inspect the cams by removing the rocker covers.

Then the big one.
I had a knocking in my M103 (6 cylinder) and it sounded exactly like lifters.
However, it turned out to be a spun big end bearing!! You can diagnose piston related know by pulling off the plug leads one by one.

yellowdragon · #10 04-15-2006, 11:09 PM

It's been some time since I've been back here.

My knock is still there. The knock can be heard when the car is first started. When idling I can hear it. When I increase the RPM's to 3K I can't hear it. It also seems like the knock may have decreased a little after putting a few miles on the car.

Again, the car is a 1980 450SL with 170K, hasn't been driven in 8 years. I'v put about 400 miles on it, with the results above.

Any ideas? I've noticed a couple servcies offered my MB service. One to clean the valves the other cleans the fuel injection. Are these services something I should look into having done?

86560SEL · #11 04-16-2006, 12:22 AM

Hello and welcome back. I would not worry too much about it then - hopefully it is just where it set?

I am not too worried about mine now, it has not gotten any worse and as mentioned, power is adequate and it is actually smooth, save for the slight splutter from the exhaust at idle, but this is just my work car that I drive, say, 200 miles per week, if that much.

Seems like there was one interval when mine got worse. I started it once after leaving a market and it started and stalled. I restarted and it idled up to about 2500 rpms and seemed to be a little louder, but it may have been my imagination. One thing that really gets me is why this cars RPMS "races" automatically up to 2500 RPMS on startup, then goes down to about 800 RPMS within a couple of seconds. I know a lot of cars do this, but it seems harsh for those instantly high RPMS before oil is in top of the engine. 2500 RPMS on startup seems extreme - does anyone elses car go that high, or is this normal for a Mercedes?

It reminds me - there is a elderly man at work that gets in his car ( a NEW Buick), starts it and instantly races the engine to at least 4000 RPMS for about 3 seconds then releases the accelerator. Why he does that is beyond me. I cringe everytime I hear that.

Good luck with your car - keep us posted...... Happy Easter!