Improving Mercedes OM603 engine efficiency

[320touring]

Hi All,

Back after a long time away, I now have a Mercedes s124 with the 3.0 6cyl 12v OM603 engine.

It's normally aspirated, so no turbo to benefit from.

The car is heavy at 3500lbs, and has 113bhp coupled to a 4speed auto.

I am also running it on Veg oil (SVO/filtered WVO) At up to 90% oil. The car has been run on this for years by previous owners, and seems happy enough.

The engine definitely is not in its first flush of youth (316k miles), and I've ordered

Oil/oil filter
Air filter
Main fuel filter
Pre fuel filter
Thermostat

To give it a service.

My initial plan is to carry out the service, and check the fluid levels in the autobox and diff, but I was wondering if there were any other elements that should be checked on the engine (pump/fuel system/inlet/valvetrain etc)?

Additionally, I think some more power would assist re economy - it feels as if you are ALWAYS accelerating!

Does anyone know what can be done to boost performance/restore to at least original?

[Ecky]

A rebuild wouldn't be a bad idea. I'm not all that familiar with diesels but after those miles I doubt the rings are as tight as they were when new. This is, of course, quite a bit of money to put into the car. A turbo would probably help with both power and economy, but again money.

On the cheap? Probably wait for someone else to chime in who owns a small diesel.

[cRiPpLe_rOoStEr]

Originally Posted by Ecky

On the cheap?

There is no cheap when it comes to Mercedes-Benz...

[iikhod]

Nice merc! Now that is what i call a roomy stw.

I would get the injectors and fuel pump/fuel pumps timing checked and maybe adjusted. It might bring some power back even.
You mentioned on the "introductions" thread you are getting 23mpg, sounds way too much even for an automatic.
Also, check for possible brake drag.

I once had a 124 myself, sedan with 2litre diesel+4speed manual. Sluggish but reliable, one of the best cars i had

[cRiPpLe_rOoStEr]

Originally Posted by iikhod

I once had a 124 myself, sedan with 2litre diesel+4speed manual. Sluggish but reliable, one of the best cars i had

OM601 or OM615? I don't remember to have ever seen any vehicle fitted with the OM601 and a 4-speed manual.

[iikhod]

Originally Posted by cRiPpLe_rOoStEr

OM601 or OM615? I don't remember to have ever seen any vehicle fitted with the OM601 and a 4-speed manual.

601 i think, might be wrong though. I'm relying on the internet about that.2 litre diesel and 4 speed manual were actually pretty common in finland.

[320touring]

Originally Posted by iikhod

Nice merc! Now that is what i call a roomy stw.

I would get the injectors and fuel pump/fuel pumps timing checked and maybe adjusted. It might bring some power back even.
You mentioned on the "introductions" thread you are getting 23mpg, sounds way too much even for an automatic.
Also, check for possible brake drag.

I once had a 124 myself, sedan with 2litre diesel+4speed manual. Sluggish but reliable, one of the best cars i had

Thanks, I will see if I can find info re checking the injection timing/injectors

Brake drag had also crossed my mind. No obviously warm wheels when I check.

I got started on the servicing today, hopefully that will bring some more MPGs..especially now it gets up to operating temp!

According to the EPA site, the w126 saloon (300SDL) with this engine (normally aspirated) is the closest I can find to my car (all the w124 models are saloons or turbo diesels) and it gets

21 urban, 24 highway 22 combined

Originally Posted by Ecky

A rebuild wouldn't be a bad idea. I'm not all that familiar with diesels but after those miles I doubt the rings are as tight as they were when new. This is, of course, quite a bit of money to put into the car. A turbo would probably help with both power and economy, but again money.

On the cheap? Probably wait for someone else to chime in who owns a small diesel.

I think a rebuild is quite the way off - she'll need to earn her stripes first!

[320touring]

had some of a chance to tinker with this today..

First up, the thermostat.

It's held on by 2 10mm bolts:

Top one seen here



Bottom one accessed via 3/8 ratchet and extension


It was patently obvious what was up with the old stat:


The new one was fitted (took 2 attempts to get it to seat properly) and new coolant was added and bled through.

I found removing the hose next to where the thermostat mounts means you can put the stat in its holder, and slot into the block. This means the stat stays in place and seals properly

Result?


A nice solid 90, and working heaters

The fuel level sender is also on the list of things to do..

A test drive revealed no problems so I returned to look at replacing the stop solenoid.

Initial inspection said it looked like too large a job to do in the time and light available, so it waits for another day.

Next it was "what do these switches do?"





The answer is nothing obvious, bar the green one lighting up.. anyone any ideas?

Then it was "hunt the exhaust blows"


That'll be the centre one...



And

This will be the rear one..



Gauges at the end of the 40 miles run home.



Sat nice and solid there regardless of speed.

You can see the oil gauge is showing that the oil is sufficiently warm to have thinned properly, which means the engine is at full operating temp.

I swear it feels nicer to drive - almost as if the engine is working more effectively (and the veg is warmer!)

Hopefully this will bear out in an economy increase.. almost 1/2 a tank down so should find out soon.

[cRiPpLe_rOoStEr]

Temperature control is extremely critical on a Diesel, most noticeably an IDI running on veg oils

BTW did it ever had a glowplug bypass? An uncle of mine did it in a Suzuki Vitara repowered with a Volkswagen 1.6D engine he used to own, having to press a button to control the glowplugs at every cold start instead of switching it to operate automatically.

[320touring]

Originally Posted by cRiPpLe_rOoStEr

Temperature control is extremely critical on a Diesel, most noticeably an IDI running on veg oils

BTW did it ever had a glowplug bypass? An uncle of mine did it in a Suzuki Vitara repowered with a Volkswagen 1.6D engine he used to own, having to press a button to control the glowplugs at every cold start instead of switching it to operate automatically.

It's still on the original glowplugs circuit - seems to start happily enough given a couple of pre-heat cycles on cold days