[Windinmyhair]

Yes, sounds like a sheet metal type of vibration to me too.
Engine mounts OK?

 

[Bohemian TopDown]

Engine supports are okay, will lift the car and check those metal sheets around the exhaust. Stay tuning.

 

[SamuraiSpirit]

It sounds like transmission gears or supercharger gears to me.

I once had a krr sound when an O2 sensor was off. Also check fuel consumption and exhaust system in general.

 

[efair]

I have never heard a sound like that - but I'm not a mechanic. I know nothing about M271 engines and I don't have a wiring diagram or any literature about it... but I would use process of elimination to rule out the variable valve timing and the compressor bypass valve. You can probably unplug the camshaft magnets on the front of the engine long enough to rule them out - it might well set a code and turn on the check engine light, but it will rule them out. If the rattle stops, plug them back in one at a time and re-test to see which one is causing it.

I would likewise try to unplug the bypass valve just long enough to rule it out.

The sound does sound like it could be related to the compressor, have you checked all the ductwork?

 

[Rorywquin]

Do you really think I should have the system removed? Am I destroying the engine? Is it that bad?

Yes
Yes
Yes

If MB wanted a LPG version, they would have made 1

I had a SAAB 95 (purchased used from the original owner, instead of hiring a car for 9 months) that had done 115,000 miles on LPG and it ran like a dream.

 

[jbanks15]

i thought we talking SLK here?

 

[Alex 3]

I'm trying to understand this. If you drive a car 20 miles/day, five days/week, 52 weeks/year, that's 52x5x20=5200 miles or ~260 gallons of fuel, at current prices at least $800/year USD. That's much more than you'll spend on oil, tires, brake pads, and all the usual wear items per year.

Just changed oil and filter. $165 CDN parts only. Serpentine belt, idler and tensioner $360 CDN parts only. Rear transmission mount $80.
Tires $1500 installed in 2021. Brakes for theses cars parts about $3200.
Trans service $375. Rear diff service $250.

 

[Gkc]

Have you had your catalytic converter checked out? When my SLK320 had a rattling noise, that’s what was causing it.

 

[Avel Du]

Have you had your catalytic converter checked out? When my SLK320 had a rattling noise, that’s what was causing it.

And if it is that, do not ignore. The guts eventually become a ball and can block the exhaust.
Might be able to hear it with some gentle(ish) tapping. Place a cloth between cat and your mallet/spanner.

 

[Easycool]

Do you really think I should have the system removed? Am I destroying the engine? Is it that bad?

Yes
Yes
Yes

If MB wanted a LPG version, they would have made 1

I fully agree. I worked for a company that converted semi's. You need to change the heads and add specific programming. While LPG is very clean-burning with no emissions1st, this mod needs to be done right. Otherwise, you may be facing an engine replacement.

 

[gary_mandolfo]

Too many opinions with no facts. I would have a pilot in the seat, raise the car on a lift, raise the RPMs and listen to where the noise is. Have a short hose handy to zero in.
Nice picture of the LPG valves, really clean.

 

[Chris P.]

Should work now...

Just wondering... are we talking about a manual transmission here? If so, it sounds to me that whenever the clutch engages or shortly before you hear that rattle noise.
I spent half of my life living in Germany near the Danish border. Plenty of cars of all makes and models running on LPG (less today than in the past since diesel engines became very popular and diesel is cheaper than gasoline).

 

[Murray223]

I use to be a LPG mechanic and yes there can be head problems as it burns a lot hotter valve seat inserts can pull into the head also valves can burn if they are not changed to stellite valves, but i always found that you don't need to do that until problems occur. In Australia gas has basically phased out due to the tax the govt has put on it making it not viable pricewise

 

[Ad B]

Many opinions of good people who doesn't have experience with this cheap LPG stuff...
My Chevy C10 v8 1969 short stepside ran some 60K km with LPG, with which I was warned for the valves and seatings.
Without problems so far.
My Benzie (SLK280) is running on LPG so good, You won't notice with what fuell you're driving.
A friend (Johan) of mine is running LPG with his SLK350 R171. more as some 560.000 km now...
With his second engine...Still going strong...
👍 😎

 

[Jennyslk]

Could it be a resonance in the timing chain at that particular rpm?

 

[Dave2302]

It sounds like some loose steel part, shaking... But how is it possible that it shakes exactly at 2500rpm? I mean it stops at 2499 and 2501

Get your Camshaft VVT checked, I'm not an expert on these 200 Engines but that sounds to me like the RPM that it would engage at, certainly is on many other Engines that I see a lot of

The disadvantages of LPG are

• It causes suffocation, in case of leakage as it heavier than air.
• It is hazardous as it inflammable gas.
• It is consumed more as it has low energy density.
• It does not provide power to the vehicle in mountains or rough terrains.

I used LPG for years some cars / vans but mostly boats, I also used to do conversions when I had the large business in England.

1) If installed correctly and inspected regularly, (think annual service) it doesn't leak, and if say the tank is in a boot area, then all potential leak points, (i.e. the Valves and fittings), are encased in a sealed aluminium cover box that has a vent tube which comes out under the vehicle, and thus vents to atmo, meaning no build up of explosive vapours. Boat installs of course have that plus also a "Bilge Blower" (extract fan), which is modified to run at all times with Ignition On.

2) Tank full of petrol is inflammable and far more potential for leaks / burst

3) It does consume a tad more, but since it's only emissions are Carbon Dioxide and Water vapour it is less harmful than petrol emissions.

4) Provided it is a proper dedicated install we have never had a noticeable decrease in power, i.e. needs mapping (or distributor in the old days) adjustment to add Ignition Advance when running LPG, then it will run properly. The more recent LPG Injection Systems are way better than the old Vapouriser / Gas Ring set ups of the 70's and early 80's, but that said we had very good success with V6, V8, V12 motors running Impco Vaporisers and Gas "Carbs" .

5) Whilst on the technicalities, and re the so called techs comments about cylinder heads, All engines since the advent of un leaded fuel have hardened Valve Seats, therefore no harm is done any more than running un leaded.

With regards to Valve recession / cylinder heads as I said above all depends what Valve Seats are fitted, and if oler engines without hardned seats can always add Valve Saver kit !!

 

[Windinmyhair]

It would seem that there are a number of outdated misconceptions with regard to use of LPG in modern engines. Those who use it/have used it successfully in their motors have certainly enlightened me. Thank you. Every day is a learning day.

 

[Squishband]

It would seem that there are a number of outdated misconceptions with regard to use of LPG in modern engines. Those who use it/have used it successfully in their motors have certainly enlightened me. Thank you. Every day is a learning day.

+ 1

 

[nemiro]

LPG is actually good stuff. I had it on an old Ford pickup with a 351W, and after 200k miles, the inside of the engine was clean as a whistle. Spark plugs always looked brand new, without a trace of carbon anywhere.

LPG does make slightly less power, but you will be hard pressed to find the difference. The only issue I ever had with it was that it would sometimes be hard to start in cold weather. That probably had as much to do with the quality of the mixer I had than anything else. Other than that, I never had any issues with it, and never felt unsafe with it.

 

[nemiro]

Oh, and side benefit of LPG - extremely high octane equivalency.