1974 280 SE Detonation

[carl888]

Curious if anyone is having detonation issues with their D-Jet M110 engine?  I have the car currently set up at 2 degrees BTDC (Factory recommendation) and under partial throttle at 2800 rpm it pings slightly in 3rd and 4th.  At 5,000 rpm it's very noticeable (For a fraction of a second before I back off in horror).  I'll check the distributor advance first to make sure that's fine before I back the timing off.  I am running 98 RON and the car being 1974, has the high compression 9.0:1 engine.

The W126, being 8.0:1 compression, with the ADR27 compliant timing, the suggested setting is 2 degrees ATDC (Nasty!!).  I find I can safely run the timing at 7 degrees BTDC on 95 RON with a corresponding increase in torque and a massive 20 reduction in fuel consumption compared to the factory setting!

So is it the D vs. K jet, the advance curve or the plain 1.0 point difference in compression that makes the difference I wonder?

Regards,

Carl.

[TJ 450]

The detonation sounds a lot like I was experiencing with my 6.9 (K-Jet). It does it at the stock setting of TDC. I have been meaning to retard it slightly on mine, but really, these cars shouldn't have detonation issues when they are tuned to the standard specs and are running on the proper fuel.

Could it be some bad fuel, or has it done it over a few tanks?

Other than that, I don't really have any further helpful information, as I still haven't made any progress with getting mine up to scratch.

Tim

[s class]

My 280SE is a 1980 k-jet model, and for the first few years of my ownership, symptoms just as you describe were an on-going problem.  It was most pronounced at high speed on the freeway - say 4000 to 5000rpm in top gear - anything more than gentle throttle pressure produced severe pinging.  It was most noticeable if the engine temp was a bit up, say 90 degC. 

I accidentially found the problem when I was attending to a leaking fuel hose.  The main discharge hose from the tank to the damper had always 'sweated', and I hadn't worried too much until the leak got worse.  When I finally replaced the hose, the pinging problems stopped completely and permanently.  What I surmise is that the pipe was sweating with the car standing (no suction from the pump), but when the engine was running, I believe the suction of the pump was allowing air to be drawn in aerating the fuel. 

I know that you have replaced all these hoses, and of course D-jet and K-jet are different, but perhaps you are drawing air into the fuel elsewhere?

[carl888]

Hi Tim + s class,

Thanks for the tips. The car is not drawing in false air, the gas analyser tells me so   It's also over three tanks of fuel.  I checked the advance curve again yesterday and confirmed it's fine.  So I backed the timing off 2 1/2 degrees to 2 1/2 ATDC and so far seems fine, though being in Melbourne, the storm clouds brewing yesterday meant a somewhat inconclusive road test! 

The M110 D-Jet WSM does quote to the static timing to be 0, + or - 2 degrees, so I guess I'm in the ball park, it just seems to wrong to have anything timed at a setting ATDC though!

You have to be very careful with fuel, I've had the odd bad tank, misfires, pinging in the past, the worst being a tank of fuel contaminated with kerosene, the result of which, was this:



Regards,

Carl.

[TJ 450]

That's not a happy piston.

Tim

[koan]

Is it possible there's something amiss with the A/C vacuum retard set up and now have 10 degrees more advance than you think?

Is the sad looking piston after one tank of bad fuel or was it on the way out and the kero'd fuel just finished it off?

koan

[carl888]

Okie, road tested the car, pinging gone at all revs.  However, it was cool today, and humid, perfect conditions really.  So I'll reserve judgement until the next 40 degree day.

Koan..........I checked the timing with the a/c vacuum switch both engaged and disengaged, no problems there.

With that poor piston, the other cylinders were saved because there was nothing left of the valve seats, no compression!  That one seemed particularly hardy in the seat department.  Poor ring lands though!  There was one other cylinder that had no compression too, but that was because of this:



Had the engine not been spinning at 8,000 rpm at the time under full load, I suspect I may have caught it in time!

Regards,

Carl.