What would cause a "diesel" sounding engine - PeachParts Mercedes-Benz Forum

ps2cho · #1 10-27-2009, 10:48 PM

I'd say my '88 300TE has progressively got worse in regards to its normal engine sound over the last year. It sounds like a diesel, but really scaled down. It's not loud at all, but when you open the hood you can hear its just not too happy.

What could cause it to be doing this?

I know my tensioner is fine so its not that. Could I have a bad lifter at 97k miles? Could that cause a small rough idle?

There is also a fast "ticking" at the back of the engine, that I hoped would disappear after I did my rebuild, but it did not. Could it all be associated?

Thanks,

dhjenkins · #2 10-28-2009, 08:58 AM

Quote:

Originally Posted by ps2cho

I'd say my '88 300TE has progressively got worse in regards to its normal engine sound over the last year. It sounds like a diesel, but really scaled down. It's not loud at all, but when you open the hood you can hear its just not too happy.

What could cause it to be doing this?

I know my tensioner is fine so its not that. Could I have a bad lifter at 97k miles? Could that cause a small rough idle?

There is also a fast "ticking" at the back of the engine, that I hoped would disappear after I did my rebuild, but it did not. Could it all be associated?

Thanks,

Ticking is usually a lifter. "Dieseling" could be timing related - have you "checked your teeth"?

Caseyjones955 · #3 10-28-2009, 10:37 AM

Is the tick consistent with RPM and constant? Does it appear only when the engine is warm? Only under load?

Ticking and tapping is often valve train, usually lifters but a sound likened to a diesel might be less constant/consistent and lean more toward a timing/vacuum/fuel delivery issue.

One of the sweetest purchases I made for my tool box was a fairly basic mechanics stethascope for under $20 (not the crappy $7 one from Harbor Freight). A long screwdriver works to if you place the tip at the suspected noise source and the handle firmly on any bony prominence adjacent to your ear, It tends to relay a great deal of sound and may help you isolate the tick much more effectivly. I thought I had a bad lifter on another mark inline 6 and with this trick I found that it was just a bad injector.

scottmcphee · #4 10-28-2009, 09:26 PM

Anything that causes a gasser to detonate makes it sound like a diesel, the way things should sound by the way, but I digress.
Trying running higher octane fuel to see if it diminishes.

Piston slap and worn crank bearings can also make diesel noises. Lotsa clacking in the engine.

Valve issues aren't really "diesel" sounds, because its the same set of issues for gassers. These are more top end clickety sounds.

My diesel was developing a clickety / ticking sound, using my stethoscope zeroed in on cyl#2, top end. The sound went away after a thousand miles on a new brand of oil and filter change. Shell Rotella 15w40 if you must know. I had shop grade 15w40 diesel oil in previously, and I think my Mercedes was just telling me to stop being cheap. So I bucked up the extra 25% money on good stuff... Ticky went away.

tinypanzer · #5 10-28-2009, 11:11 PM

Close inspection of the plugs may reveal what's happening.

I'm not one who thinks you are actually detonating. Reading your post carefully says that you can hear the sound with the hood open, meaning you're not driving, you're not under load, and your at idle. If your motor was detonating at idle it'd be toast by now.

So, I'm betting on a lifter or other valve train issue. Might wanna pull the valve cover and see if your cam lobes and rockers are in good shape, as well as your oiler tube.

But I do agree that a stethoscope will tell you more.

t walgamuth · #6 10-29-2009, 05:56 AM

The hydraulic lash adjusters are very oil sensitive. My 84 500 sec would hang an adjuster and make noise if I ran 15w40 oil in it. Put in mobil 10w30 and it would quiet right down.

What oil you running?

ps2cho · #7 10-29-2009, 04:03 PM

You can only hear it at idle. Once I step on the gas a little it clears it and sounds normal.

My valvetrain is very clean and looks great. I didn't notice any cam wear when I rebuilt head either. Oiler tube was cleaned and flushed out fully.

I don't really have any closer-up pics of valvetrain.

I am running Delvac diesel oil right now, but its always made the sound regardless of oil.

When I rebuilt I also followed the specs on setting timing and I was pretty much dead on, would there be any other reason why it would be off?

I will go buy a quality stethoscope this weekend and figure out where its coming from.

---

Just a thought....Does correct fuel pressure mean I am getting the correct quantity of fuel? I've noticed some weird noises from my fuel pump recently...A lot of "static" type noises from it.

tinypanzer · #8 10-29-2009, 06:19 PM

I vote for a sticky lash adjuster. But without hearing it, who knows.

It might be one of those things you can cure by seafoaming the engine oil, then changing it out after a few miles.

dhjenkins · #9 10-30-2009, 10:13 AM

Quote:

Originally Posted by ps2cho

Just a thought....Does correct fuel pressure mean I am getting the correct quantity of fuel? I've noticed some weird noises from my fuel pump recently...A lot of "static" type noises from it.

Not necessarily. A restriction in your fuel return could give you normal fuel pressure readings, while not delivering the correct quantity.

Blow some air at a low pressure through your fuel return line at the motor and see if there's any resistance. It also wouldn't hurt to measure the current on the pump while it's making those static noises to see if it's failing or partially dead-heading.

I had a stuck check valve in the fuel tank of my 380SEL, which was blocking any fuel return. The car would not start or run until that was fixed.