cj_sharky

Hi everyone and greetings from Romania!

I am the (happy?) owner of an w211 2003/177kkms E240 M112.913 2.6L gasoline engine with a stag 300 LPG system.

Please help me in investigating the strange noise that my motor is doing when below normal operating temperature.
It makes this noise from the cold start till the coolant reaches 85-90C. Then it becomes very silent.
If the engine gets cold after going downhill for a long time it starts making that noise again until it warms up.

The noise is only reproducible when engine is loaded. When revved out of gear it sounds just fine.

You can clearly hear the sound by watching these short videos.
Cold noise:
Over 85C - no noise:
I am explaining in Romanian the noise. Actually there is none, just a cat purr, as any normal car should sound like.
This is full throttle, again, no annoying sound.

Diagnosis i made:
- No error codes (misfire, lean/rich, timing etc), the car is passing the emissions test with flying colors.
- engine is running perfectly fine, has a great pull, no hesitations, smooth idle.
- Post cat oxygen sensors have been changed with bosch parts, same part number as the original ones that i took out but mercedes branded. Originals were shot, heating element failed on both. Since post cat sensors are new, they should report any issue (broken, laziness) with the pre-cat sensors. Pre cat sensors values appear fine on a OBD scanner.
- No vacuum leaks as long term fuel trims are always under 5%, usually around 1.6-2% maximum.
- Oil leaks from the top covers are solved, hoses replaced and tied with steel ties so they don't break loose.
- MAP sensor seems to be good, reads the same reasonable values as the brand new LPG map sensor.
- Sound is exactly the same on LPG and gasoline (any type, 95, 98) so it is not fueling related.
- Spark plugs are the original ones i think, they were gapped due to age to 1.2mm, i have re-gapped them to 0.8mm.
No diferece whatsoever. No excessive fouling on the plugs, they work fine. Same for the plug wires. They are conducting and they don't have any sign of misfire/arching inside.
- MAF was only checked with the tester, but values seem ok compared to other engines and response is quite quick.
Metered Air is usually proportional to the revs.
- thermostat is a bit old and it opens at around 77-80C instead of 87. Engine heats up to 90C only when pushed a bit, or when car is halted. And this lengthens the warm-up period in this 0-10C cold weather.
I know this is a problem and i will get it fix, but the cold engine noise issue would remain. And this is what really bugs me.
- i would rule out any mechanical noise as no noise when revved out of gear. Also there is no noise heard from outside, but from inside the car it sounds like you would have a racing exhaust muffler.
- it's also clear the there is no muffler issue as i've never seen a muffler sound change with engine temperature.
Also there is no muffler noise when revved out of gear.
Same for the secondary air pump.

- I will try running the engine without the MAF to see the effects.
- I will also try logging the sensor data when cold and then when hot. I suspect some timing advance differences, but i know that if they are, there is nothing i can do about them.
- I will also put the strobe light on the car, to see if there is significant timing advance change => weak chain/tensioner.
However i don't see how a weak chain/tensioner can make such combustion noise and suddenly fix itself once the water reacher 85C.


Any ideas?
Is this the normal engine noise? Doesn't sound too Mercedes like to me. Heck, even my old '92 opel/vauxhall sounded a lot better.

Thanks!
Serban

beejAMG

humming noise from the first video? belt, bearing or pulley

cj_sharky

Thanks for your answer but it think it is definitely not.
I can rev it out of gear as much as i like, no noise. It does this only when geared and engine has some work to do. Even when starting off from 0kmph to 5kmph, so gearbox noise is excluded.
Besides that, i ran a few meters with no belt, noise is the same and also noise goes away at operating temperature.
Also all of them were changed around 5kkms ago. (Pulley, idler, tensioner, belt) Parts are cheap and i did all the labor. It does not worth risking remaining stranded if one of them should fail.

During the weekend i hope to be able to inspect the EGR. Haven't touch that yet.

cj_sharky

Diagnosed the EGR, seems to be fine.
If i apply vacuum to the valve engine almost stalls and the hose/diaphragm is holding the vacuum.
Also there is no vacuum breaching the control valve.
This means that the system is fine.

Also drove a total of 50kms , all driving stiles with egr vacuum hose blocked. EGR was disabled withi this method.
Sound remains the same, so it is definitely not egr related.
No error codes either. When engine is warm up and after a few pulls it becomes quiet and smooth.

cj_sharky

Sound remains the same, growls quite annoying when cold, stops when it reaches 90C and revved a bit.
Went for a long trip and observed as while i was legally driving, 50kmph in city, 90kmph outside temperature dropped to 70C as it was -8C outside. Noise was back while engine was under load. After a few pulls it stops, but as soon as the temperature drops the noise starts again.
Definitely a bat thermostat i think, but i don't think the noise is ~stat related.

S70Houston

Sorry, I cannot make out a lot of noise on your videos based on sound quality of my speakers.


Have you checked your motor and or transmission mounts? I had this issue lately myself and the car was running quite a bit rougher when cold (rubber gets soft when it gets warm) and after replacing both motor mounts and the transmission mount, all noises/vibrations went away and the car drives like new again.

cmriv

I'M trying to makeout what you are saying....
the first thing you need to do is tell us whether it is in the area of the transmission or engine bay. and how sure are you where it's coming from????

if it only occurs when you are mobile then i can't believe for a second it's engine related. IF it was engine related you would be able to replicate the noise in park or at idle...

cmriv

maybe your chain is stretched.... timing or oil pump, either or....

cmriv

and is the noise a rattle? whistle? ping? knock? your videos do not serve much justice....

cj_sharky

i'm sorry, maybe i did not give enough details.

As i said before the engine was warm so sound was gone, but as soon as the temperature dropped a bit due gentle driving noise was back, even though engine was still running.
I know that the thermostat is responsible for the 10-15C temperature drop, or maybe the cold weather, however i don't think that the engine should garble in the first place!
The sound is reproducible when in gear and pressing the accelerator pedal i.e. engine is loaded. If i will rev it out of gear no sound whatsoever.

It s definitely not spark plugs, wires, mufflers or something mechanical as i cannot see how each of these can miraculously fix themselves after warmup and being under load for a while.
It think it is something controlled by ecu as a response to some faulty system/sensor which i am not able to find.

Let me describe the noise:
Let's say you are on a long straight. You accelerate gently from 1500 rpm upwards. You will hear some kind of knock, loud muffler sound from the engine.
Same if you would maintain constant speed up on an incline.
Should you hear any noise from the engine or not?
I am hearing it and it is annoying.
It disappears as soon as the engine is pushed a bit for 1-2 minutes, even if you would go back and drive on the same road using exactly the same driving style, that is before the engine manages to cool down a bit.

It is the same sound as in the first video.
One would say that this is normal, but it is not.
Even though the car si driven very gently, the sound is annoying as soon as the engine does some work accelerating the car.
AS you can see in the second video, i am accelerating the car and voila, no sound!

It is like someone installs a sport muffler on my car as soon as the engine is cold.

The sound can be heard from inside the car. Will try to put someone else move the car in 2nd gear with hand brake on to see if i can hear any noise from outside.

Here is a cold start. I cannot hear any rattle.

Don't think it is the chain/pump either. Why isn't there any noise when revved out of gear? Any why only when a bit colder than normal?
Why going full load automatically cures it in a minute?

cj_sharky

WE can also rule out injector malfunction, as sound is the same on LPG or on petrol, emission tests are perfect, oxygen sensor data is good, second set of oxygen sensors (after cat) are brand new so they can detect anything wrong with the first pair of sensors.
Not the case.
MAF seem to display reasonable values, MAP the same, i have verified it against a vacuum gauge.
LTFT are under 2% almost all the time, so no vacuum leaks.
Car runs fine, but the sound is kind of annoying while cruising in town on a relaxed drive.

Thought that it might be some kind of an exhaust collector pipe that cracks open when cold and fills up when hot due to expansion, but then it should sound bad even when revved out of gear.
Or maybe the CAT air pump system that has some kind of check valve?

Frankly to tell you, i don't know where the sound is coming from. I can only hear it from inside the car. If i roll the windows down no extra sound is coming from outside.

cmriv

what kind of shape is your engine/driveline suspension components in? are they all good and have play?

cj_sharky

Don't think it is drive line related.
I have some play in the universal joint, when i am moving it with my hands.
However the sound's frequency is engine rpm related, not vehicule speed related.
Therefore it is coming from somewhere before the transmission output shaft.
Since the noise disappears when warm and after being pushed a bit, it should not be anything mechanical. Note that the sound only is manifesting under load, if i revv it out of gear, no noise.

So question for all M112 users:
How does the engine sound when cold? Does it garble like in video 1, or it is quiet under 2000rpm and starts purring over 3000?

I think that there is a clear sound difference between video 1 and video 2, and believe me, the oly difference is that in the first one the engine is cold, and in the second one the engine is warn and after a fast hill climb.

Another thing about the comparison.
All videos were filmed with my mobile phone. AS you can see there is kind of a huge difference between the sound level between video 1, which is very gentle driving in 2nd gear and low speed vs video 3 in which i am going full throttle and high rpms. Video one should be dead quiet (and it is with warmend up engine) and video 3 should be as it is.

I know that mobile phones have auto gain control algorithms, however, i can assure you that the engine is a lot noisier when cold.

S70Houston

I shall ask again, .........

Motor mounts and Transmission mount in good shape?

cj_sharky

Hi, sorry for not responding.
I think they are in a good shape, as they do not look shot, cracked, dripping oil.
No vibrations are being experienced from the engine when driven or at idle.
But since i don't have another Merc with M112 engine for cross-reference i cannot tell for sure.

As soon as the weather will spare me from the cold, i will try disabling the knock sensors so the ECU will switch to a defensive timing advance.

I see that you have the E320, which is a bit similar. How is the sound from your engine when driving away from a cold start?

S70Houston

I had terrible engine vibration at cold start as already pointed out above, once the vehicle warmed up and the rubber got a bit softer, it was considerably better, but not perfect.

Ever since changing the motor and transmission mounts, no more noise and vehicle is running perfect, also at cold start.

FYI -- My mounts were not leaking either at the time of replacement.

cj_sharky

Can you describe the vibrations?
Were you experiencing the vibrations right when starting the engine?
Or while driving?
Frankly to tell you, i cannot sense any vibration Maybe a little bit at idle, but that's it.
When revved just a little bit, if one would be deaf, he will not know whether the engine is running or not.

I will try pushing the car in 6th gear while engine is stopped to see if there is any excessive play or noise coming from the engine mounts.
Have to wait for spring to come, it is too cold to work anything on the car.
While looking at some youtube videos with broken engine mounts i can tell that my engine is definitely not jumping that high when pushed. It wiggles a bit, but that's all.

S70Houston

Noise is difficult to explain. I experienced it right when I started the engine, less while driving and when the car was warm, no vibration when the car was in neutral.

cj_sharky

Sorry for your engine mount damage then. But hey, you solved the problem!
I am definitely not experiencing the same symptoms.
Will revert as soon as i will make some progress on this.

Oh, btw, i also tried pulling the plug out of each coil.
The effect is definitely noticeable, as expected. With a ignition malfunction there is no way that emission test would have passed, or i could outrun a new audi 150hp on the freeway in a friendly and perfectly safe ad-hoc contest.

cj_sharky

RAn again the same experiments with a delphi tester.
Revved and pushed the engine until the knocking stopped.

There were only 4 misfires recorded during, although the engine knocked at start all the time.
No knock recorded by the sensors.
MAx value recorded by knock sensor was 2V, minimum 0.2V. IT usually stays low when idle, and it varies while driving, but it sits between 0.3 and 1V. I could not find a corelation between the engine load, rpm and this value. Response is also quite slow, so probably the value is averaged over a few seconds.

There were some knock retards recorded for a brief period, like 1s, but value was reset quickly.It might be cause by the LPG as i was running on it.
Anyway there was no corelation between the noise and the recorded values.

When driving uphill using cruise control timing advance was spot on, a fix value, no variations.
Will have to measure it with a strobe light, to see if there are any variations.

Again, after the car was pushed hard, the noise was gone.
Ofcourse, no codes were recorded, except the EGR code stored from when i canceled the EGR vacuum command to see if it would make any difference,.

S70Houston

Ever thought about taking the car to an authorized repair shop or the dealership, have the certified mechanics diagnose and fix the issue?

I think this is what I would do in order to have this taken care of before things possibly could get worse.

cj_sharky

This is one of those "i've changed everything with new parts and it is still not working well" problems. I am afraid it will cost a lot and fixing this problem is not a guarantee.
I believe wrongly diagnosed problems and costly repairs are common everywhere, so you know what i am talking about.

I am calling this knock but it is not actually a mechanical knock so i don't think it is a big issue.

I also though about your engine mount theory. I've idled the warm engine more than 10 minutes in the parking lot. Coolant temperature was 92C, as shown in the AC secret menu. Engine was already warm as i took a short trip around town.
So engine mounts were definitely not cold. You know how tight and well insulated the w211 engine bay is.
Engine "knocked" but after being pushed it stopped knocking, although temperature dropped to 80C due to bad thermostat and speed induced cooling.
Will look into the engine mount theory more thoroughly as soon as the cold goes away.

All discussions on piston slap, rod/bearing knock are not relevant, i think.
Most of them were doing it when hot, not cold and when revved out of gear.
I cannot hear any knocking sound with the hood open, neither at idle or revved.
Engine has to be loaded.

cmriv

a crystal clear video would suffice everyone that is trying to help. There's only so much we can do via messages and words. My advice find a good MB mechanic and go from there....

cj_sharky

Can you please explain what other details i should film?
There is only one sound being heard in the foreground of the first video and it is the engine noise.
AS you can see in the second video the noise is gone although i am acceleration at the same RPM
"https://youtu.be/upnYZh2_UiA?t=1m20s"

cmriv

drive vehicle on a lift and see if you can re-produce noise.