1971 Mercedes W108 280SE 3.5 V8 conversion to OM606 diesel.

pryantcc · 21-03-2014 02:05PM #1

I bought this car some months ago with a view to turning it into my daily driver. The plan is to fit a modern(ish) Mercedes OM606 diesel engine.

Structurally, the car isn't in bad shape although almost every panel has some rust issues ranging from very minor to disintegrating!

You can see the front suspension is just about absent in the pic below. The passenger door is not closed, the alignment is actually pretty good! The paint is pretty fresh, but bubbling through from underneath, so will need doing again.

Here's a close-up trying to show the bubbling above and below the trim strip. This is the worst patch for size, but I have issues on almost every panel. Around the rear screen is the worst I can see externally.

This is the inside of the passenger door. In their wisdom, someone blocked up the factory drain holes with filler and drilled new ones which weren't at the lowest point of the door!

The whole underside was sprayed with Schutz prior to sale. I've scraped off a couple of the rusty patches here, no surprises, I could see they were there before I bought it. It's mostly just surface rust which I'll blast off and re-paint.

The front screen has moisture damage also, so I'm a bit afraid of what I might find in terms of metal when I remove the windscreen rubbers!

johnty56 · 21-03-2014 05:38PM

Very nice... would it be a shame to put a diesel in it? I have one of these and half the fun is in the set up.. I can't remember what the ratio is on the rear axle but how does it compare for a diesel conversion? Let me know if you are going ahead.. I might be interested in the engine!

pryantcc · 21-03-2014 05:53PM

johnty56 wrote: »

Very nice... would it be a shame to put a diesel in it? I have one of these and half the fun is in the set up.. I can't remember what the ratio is on the rear axle but how does it compare for a diesel conversion? Let me know if you are going ahead.. I might be interested in the engine!

Thanks for your interest. Definitely going ahead! I want to drive the car every day and I can't afford to do that on petrol at 16MPG!

I'm a bit reluctant to sell the original engine and 'box as the car would never be "matching numbers" again, but that reluctance may wear off as the budget stretches!

The rear ratio is indeed too short (3.46:1) for my conversion (the car will be slow), but I plan to put in a very tall ratio (.82:1) gearbox to compensate. I might increase the wheel size too if I need to, but the gearbox gets me back to the same kind of engine-to-wheel ratio as the original diesel donor car, so I'll see what its like once driving and decide then since the wheel change is relatively easy.

johnty56 · 21-03-2014 06:19PM

Whats the back axle like? Has the compensator been replaced with a spring?

I know what you mean about matching numbers, its probably better to hold onto it if you can and have space etc.. but if you are selling it let me know!

The car itself looks pretty solid.. the trim line on that model seems to be an issue with most of them along with some of the other Merc hotspots but that one looks good!

dgt · 21-03-2014 08:39PM

This looks like an interesting venture! Where will you source the engine from?

I remember viewing a W126 which had a 307 engine inserted (another Merc) the owner said it all bolted up perfect

gctest50 · 21-03-2014 09:17PM

johnty56 wrote: »

Very nice... would it be a shame to put a diesel in it? .............

depends

The 1998-model OM606 3L straight-6 has now covered over 400,000 km (just under 250,000 miles) so you could say it’s well run in.
........ is mated to a 5-speed automatic transmission from an E55 AMG. ...........the car also runs Toyota Aristo axles

https://www.youtube.com/watch?v=YlwTxw-027o

pryantcc · 21-03-2014 09:38PM

dgt wrote: »

This looks like an interesting venture! Where will you source the engine from?

I've got one in a '99 E300 Turbodiesel. Problem is that it's a bit difficult to get the auto gearbox which comes with it working properly. You can get a controller to run it but it can be clunky at best. I'll be bolting on a manual transmission, there are plenty which fit, and it will help me with my final drive ratio.

pryantcc · 21-03-2014 10:14PM

johnty56 wrote: »

Whats the back axle like? Has the compensator been replaced with a spring?

Yep, mine has a spring. It all seems pretty solid and robust! Should be well able for the standard OM606 setup.

quietsailor · 21-03-2014 10:47PM

Would you consider convertijg to gas? It'd keep original numbers and you'd get better value economy wise

dgt · 21-03-2014 11:49PM

pryantcc wrote: »

I've got one in a '99 E300 Turbodiesel. Problem is that it's a bit difficult to get the auto gearbox which comes with it working properly. You can get a controller to run it but it can be clunky at best. I'll be bolting on a manual transmission, there are plenty which fit, and it will help me with my final drive ratio.

If it's of any use, I have a M117 autobox sitting on a pallet if you're stuck for one. Having said that, the manual would appeal to me more!

Just to add, those engines take well to compounding and 4-500 bhp is a very achievable figure if that's of any interest...

BorneTobyWilde · 22-03-2014 12:05AM

https://www.youtube.com/watch?v=_Pi0DrTOrJ8

lomb · 22-03-2014 02:23AM

What about a gas conversion? Too much labour in installing a diesel and its never going to be great in a car like that. I drove a 1980 450 sel 4.5 recently, a very nice car I thought but it needed work and Im not the man to do it or pay someone to do it (had invoices from South Dublin Merc specialist for 5k to change the distributor and fuel injectors and the problem remained, engine ran rough after car was gunned, I did a google and it looks like tank rust blocking the fuel pickup, one invoice was for 650 euros to change 18 circlips on hoses what that's around 2 euros in,parts and 648 euros in labour for a few hours work)
I think the owner gave up on it due to the unreasonable upkeep cost.

pryantcc · 22-03-2014 08:09PM

quietsailor wrote: »

Would you consider convertijg to gas? It'd keep original numbers and you'd get better value economy wise

I've thought about this alright, but I'd still be running a 40 year old engine and really want to put some miles on this thing. I like the idea that spares for the diesel are readily available, as well as the spectacular reliability reputation.

dgt wrote: »

If it's of any use, I have a M117 autobox sitting on a pallet if you're stuck for one. Having said that, the manual would appeal to me more!

Just to add, those engines take well to compounding and 4-500 bhp is a very achievable figure if that's of any interest...

Thanks, but I'm pretty fixed on the manual conversion for now. I've no plans to do anything in terms of power output. I'm aiming for economy and am more of a "Driving Miss Daisy" kind of guy. I'm very impressed by what these engines can do in standard form though, with just a heftier dose of fuel and a couple of turbos stuck on. It's incredible!

quietsailor · 22-03-2014 08:43PM

pryantcc wrote: »

I've thought about this alright, but I'd still be running a 40 year old engine and really want to put some miles on this thing. I like the idea that spares for the diesel are readily available, as well as the spectacular reliability reputation.

It's a merc - 40yrs old is nicely run in

, yeah i know what you mean, sometimes it's worth just pumping some big money into a critical part and having that peace of mind it won't let you down.

If your keeping the old engine and 'box have a hunt around the internetz and there should be storage procedures for long term storage. That way you'd have a perfect original engine if you ever want to sell her

Edit, here's a few pages form a quick google
Motorbike engine long term storage
MB long term storage

Capri · 23-03-2014 11:10AM

16mpg /??miles per week / = €?? ,

Cost of engine/conversion/bits/time-labour = €??? , (not to mention the thought of returning it to 'matching numbers' in the future )

Is it REALLY worth your time and money doing such a conversion

Maybe I'm just getting older and wiser but all this 'DAYSEL,DAYSEL,DAYSEL' talk that goes on here REALLY gets to me - diesels need more servicing and parts and oil changes. Slower and dirtier as well - yet the country seems fixated on DAYSEL as the saviour of motoring. Compare a 300SD and 350SE V8 over 3 years, initial price paid, fuel AND parts costs etc and see what wins

In South Africa they replace diesel engines with petrol ones - now THAT'S a better idea

Capri · 23-03-2014 11:13AM

lomb wrote: »

What about a gas conversion? Too much labour in installing a diesel and its never going to be great in a car like that. I drove a 1980 450 sel 4.5 recently, a very nice car I thought but it needed work and Im not the man to do it or pay someone to do it (had invoices from South Dublin Merc specialist for 5k to change the distributor and fuel injectors and the problem remained, engine ran rough after car was gunned, I did a google and it looks like tank rust blocking the fuel pickup, one invoice was for 650 euros to change 18 circlips on hoses what that's around 2 euros in,parts and 648 euros in labour for a few hours work)
I think the owner gave up on it due to the unreasonable upkeep cost.

invoices from South Dublin Merc specialist for 5k , not sure these 'specialists' know what they're about - join a Merc forum and DIY is the best option

pryantcc · 23-03-2014 08:39PM

Capri wrote: »

16mpg /??miles per week / = €?? ,

My commute is 478 miles per week, at €1.50/L for both fuel types, that's nearly €100Euro per week saved. The diesel does about double the distance to the same volume of fuel. However, if you add vegetable oil into the equation, it gets even cheaper.

Capri wrote: »

Cost of engine/conversion/bits/time-labour = €??? , (not to mention the thought of returning it to 'matching numbers' in the future )

About €700 for the bits, doing the work myself, but it will certainly take a lot of time. I plan to be clever about any modifications, making them reversible where possible.

Capri wrote: »

Is it REALLY worth your time and money doing such a conversion

It's very well worth it, over a year, the conversion saves me about €5K in fuel alone, and that's without diluting it with chip fat.

Capri wrote: »

Maybe I'm just getting older and wiser but all this 'DAYSEL,DAYSEL,DAYSEL' talk that goes on here REALLY gets to me - diesels need more servicing and parts and oil changes. Slower and dirtier as well - yet the country seems fixated on DAYSEL as the saviour of motoring. Compare a 300SD and 350SE V8 over 3 years, initial price paid, fuel AND parts costs etc and see what wins

How does a diesel need more parts? In my case, I'm eliminating spark plugs, HT leads, distributor, points, coils, rotor arm and electronic fuel pump. I'm replacing them with glow plugs.
I'll have about 170HP and 330Nm of torque at just 1600RPM. That's plenty for my requirements!

pryantcc · 23-03-2014 08:43PM

I've been working on the front suspension. The whole lot was removed, blasted and primed where required and new bushes/bearings, etc fitted.

This was the rustiest bit, some corrosion worth stopping.

Springs and brake backing plates were scruffy looking too. Turned out both backing plates had stress fractures, so I'll weld them up before re-fitting.

I only blasted the spring turrets on the subframe as they were the only rusty parts.

MercMad · 25-03-2014 03:40PM

Once you can return to original I see no harm. The motor should bolt in without too much trouble. You'll probably have to make up the mounts and I think the manual gearstick will most likely end up in a different spot.

I've seen a few W108's with the 3.0d engine (from a W123) and it looks like it was always there. You're looking for more power and usability obviously. Its a good project. I was half thinking of doing it with my 300SE W126 (1986).

If you ever decide to change the rear axle let me know. I fitted a 3.27 from a 280SE 4.5 into my W111 Coupe, having rebuilt it first, and it took 550RPM off it at cruising speed.

When you have the engine out would you give me some measurements between the chassis legs please, towards the bulkhead ? I have to build an engine for a customers 220SEb, these are based on the Fintail chassis prior to '68, and are narrower. I have been told the V8 doesn't fit, but I have a V8 so its either that or a hot M130 2.8.

pryantcc · 25-03-2014 03:47PM

MercMad wrote: »

When you have the engine out would you give me some measurements between the chassis legs please, towards the bulkhead ? I have to build an engine for a customers 220SEb, these are based on the Fintail chassis prior to '68, and are narrower. I have been told the V8 doesn't fit, but I have a V8 so its either that or a hot M130 2.8.

Yep, can do! I might struggle to remember, but I'll certainly try, and I'm sure there will be some updates here over the time the engines go in and out and in and out and in and out again, so you'll have the chance to remind me!

74merc · 03-04-2014 10:15AM

I'd be interested to know what you are going to do with the electronics. A 99 car will have DAS3 ignition surely? You would need the ignition switch and key before the ECU will allow the car to start.
I often dream about doing this sort of conversion, but had been thinking of using an engine from an earlier car with the ordinary ignition key, such as the pre facelift 202 or 124.

pryantcc · 03-04-2014 11:28AM

74merc wrote: »

I'd be interested to know what you are going to do with the electronics. A 99 car will have DAS3 ignition surely? You would need the ignition switch and key before the ECU will allow the car to start.
I often dream about doing this sort of conversion, but had been thinking of using an engine from an earlier car with the ordinary ignition key, such as the pre facelift 202 or 124.

This was a big worry for me too, but a friend of a friend runs a ECU garage type place and he sorted it out for me in no time. He has a gizmo which fools the ECU into thinking that it always has the correct key. The donor car, with this adaptation can be hotwired while the key is nowhere near it. I've yet to finish completely stripping the wiring loom out, but so far, so good in that respect!

74merc · 03-04-2014 11:57AM

Very good, that opens up a lot of options engine wise. Are you going for a turbo diesel?
As an aside, I was chatting to the owner of a 280SE 3.5 in Amsterdam a few years ago. There are quite a few of the cars there and most people have them converted to LPG. It would probably be a fairly low tech single point mixer system though, effectively a gas ring from a cooker in the inlet manifold.

pryantcc · 03-04-2014 12:07PM

74merc wrote: »

Very good, that opens up a lot of options engine wise. Are you going for a turbo diesel?

Yes, I have an OM606 turbodiesel donor engine. In the process of stripping the donor car at the moment. The auto gearbox from the donor is a no-go though due to electronics. The gearbox computer relies on a speed input from the ABS computer. Even if I moved the whole ABS setup across, the speed will be wrong because of the different differential ratio, so the gearbox would live in limp mode. It is possible to get a standalone controller for the gearbox, but the performance of it is lumpy and jumpy, OK for racing or something, but not nice for daily driving.
So, I'm going manual. My gearbox is on its way from the UK as I type. A 6 speed Merc item from a modern C class.

pryantcc · 16-04-2014 09:59AM

Work is progressing slowly! I'm in the process of going through the wiring on the donor car to make sure that once the engine is removed, everything will still work. So far, things are going well, but its a very slow and painstaking process, particularly since I'm paranoid!

Here's the donor car, a 199 300E Turbodiesel:

And the power plant. It's had some boy racer treatment, but doesn't seem to have suffered too much abuse, it still runs very well, lovely and clean.

pryantcc · 22-04-2014 11:29AM

Great success yesterday, Engine starts and runs without any of the other computers or instrument panel attached! I don't know if I have full power though, nor how to test it as the gearbox will be stuck in limp mode without its controller.
Any ideas?

gn3dr · 23-04-2014 11:37AM

That's great news and inspiring for me to get going with mine, I've got a bit side tracked with 944's recently though.
Are you writing down all the wiring changes, colours etc? Would be a huge help to others if you wete willing to share.

On the full power thing even if the box is in limp it will surely hold a gear. Tie the back of the car to a tree and see if it will pull hard. I'd pick a failry big tree though :-)

pryantcc · 23-04-2014 03:24PM

gn3dr wrote: »

Are you writing down all the wiring changes, colours etc? Would be a huge help to others if you wete willing to share.

I do plan to put together a bit of a "How-to" guide. The only downside is that I don't know the ins and outs of what the guy did to my ECU, so my solution may only work for ECUs which have been doctored the same way as mine. It will be great if it helps people though. I think the electronics part puts a lot of people off having a go with these great engines.

pryantcc · 24-04-2014 11:12AM

I'm very excited, I've finished the electrical strip down, wohoo!
I can make the engine run now with only 4 12V connections and 2 fuel pipes. It's such a relief as the electronics are the part I was most worried about for the whole job. I just have 2 permanent 12V feeds, 2 which are switched by the ignition key and 1 for the starter solenoid. Its very satisfying.
The wiring all still looks a bit of a mess but I can tidy it up once I see where to put everything in the project car.

This spaghetti is mainly the old instrument cluster connections (I hope to find a tacho and glow plug indicator light in there) and the computer diagnostic port connection.

On the other side of the donor engine bay is another box of tricks, power for the ECU comes from here and some other stuff.

The interior isn't looking great. It turns out I didn't need anything from in here and could have left the dash intact, but I needed to get in there to trace some wires in order to learn this! I'll know for the next one ;-)

So, this is it with all the required electronics sitting on top of the engine. Attention now moves to the mechanical strip down!

gn3dr · 24-04-2014 05:28PM

pryantcc wrote: »

I'm very excited, I've finished the electrical strip down, wohoo!
I can make the engine run now with only 4 12V connections and 2 fuel pipes. It's such a relief as the electronics are the part I was most worried about for the whole job. I just have 2 permanent 12V feeds, 2 which are switched by the ignition key and 1 for the starter solenoid. Its very satisfying.
The wiring all still looks a bit of a mess but I can tidy it up once I see where to put everything in the project car.

This spaghetti is mainly the old instrument cluster connections (I hope to find a tacho and glow plug indicator light in there) and the computer diagnostic port connection.

On the other side of the donor engine bay is another box of tricks, power for the ECU comes from here and some other stuff.

The interior isn't looking great. It turns out I didn't need anything from in here and could have left the dash intact, but I needed to get in there to trace some wires in order to learn this! I'll know for the next one ;-)

So, this is it with all the required electronics sitting on top of the engine. Attention now moves to the mechanical strip down!]

Might as well do the egr delete when you are in the wiring.
Great progress being made
http://www.superturbodiesel.com/std/Thread-W210-OM606-962-EGR-Delete

mannurse · 26-04-2014 06:00PM

You beat me to that car. Seen it advertised on DD in Tallaght? Although it's great to see these cars coming up frequently at affordable money. The om606 is a fantastic engine. Well done on making it work with a bypass on the ecu. Any issues with the other systems that are integrated to the ecu? Have you thought about the sensor on the bellhousing picking up a signal from the flexplate needed for the ecu? If you go manual you will need to weld some tabs on the flywheel to give this tach signal.
Did you consider using a mechanically controlled IP from an om603? That's the route I took with my w126 with 2.24 diff ratio, the car is still quick. This pump can be turned up by any diy mechanic. My w116 has an om603 in it but am swapping in an om606 using the om603 IP. Not going to turn it up though as the rear diff is 3.69 and is quick enough.

good luck with project.

1971 Mercedes W108 280SE 3.5 V8 conversion to OM606 diesel.

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