Unleaded fuel usage in older Mercedes-Benz Vehicles

[AMG]

Alastair kindly provided this information and requested it be stickied, due to the number of queries on this subject.

So, for those who want to know - the information is below:

Mercedes engines since 1957 have had sodium filled exhaust valves,hardened valve seats and valve rotators .
All these things are for preventing valve head and seat burning .
You can run unleaded fuel in a Mercedes without a problem.
There is no need or requirement for fuel additives.

Alastair


As an adjunct, I would also recommend those who have higher compression engines e.g. 3.5 m116 consider using 98RON fuel as a minimum, due to the higher compression ratios of these engines, or adjust ignition timing accordingly.

[Hendrik]

As an adjunct, I would also recommend those who have higher compression engines e.g. 3.5 m116 consider using 98RON fuel as a minimum, due to the higher compression ratios of these engines, or adjust ignition timing accordingly.

95 will do in a pinch, for those who have short arms and deep pockets. These motors are not exactly high compression racing motors, close but not quite.

[IH8]

Same deal for my cars engine with twin Zeniths?

I've run 98 Octane in it the entire time I've owned it. I've also run the additive's.
(i didn't have to speak to a mechanic for about the first 16months of ownership, he told me not to worry about the additives (he's a mercedes specialist btw))
My car's pinging now anyway. Time for some serious work.

[ADow]

I've run 98 Octane in it the entire time I've owned it. I've also run the additive's.
(i didn't have to speak to a mechanic for about the first 16months of ownership, he told me not to worry about the additives (he's a mercedes specialist btw))
My car's pinging now anyway. Time for some serious work.

Pinging can be related to ignition timing that is too advanced, but they also ping if there is carbon buildup on the piston crown. This is caused by too much slow city driving, and in some cases the mistaken belief that these cars need to be babied because they are old. The engines are massively strong and designed to be revved, and as long as the basics of the engine are sound they benefit from regular brisk driving. Check the timing and then give it an Italian tune-up. You will probably see a big cloud of brownish black gunk emerge from the exhaust the first time it is seriously fanged after a few weeks of city driving.

[IH8]

I give it a good revving every now and then. I am a 'hoon' after all, and the gearing is pretty short so the high RPM's come pretty quickly.
Pinging is intermittent. Maybe I need to have actual 98 octane fuel, rather than the crap we're told is 98 octane.

[Mercmad]

I find this often with my 6.3. I have to buy from the same place all the time as they have Real Hi octane . I am damned sure that servo's (run by the big two) are mixing fuel to top off tanks. Once upon a time the customs and excise were charged with inspecting all fuel imports and would go to random servo's to check pump volume AND fuel Octane levels. This is no longer done ...to save money .
Plenty of times I have had the car pinging and rattling ,only to have it disappear as soon as fresh fuel was added.
As has been seen on various TV programs,Servo's flatly refuse to drawn on the subject of fuel quality and who do you talk to when looking for someone from a fuel supplier?
Thanks to the quality controls being curtailed we are now at the mercy of the profit takers.

[carl888]

As an adjunct, I would also recommend those who have higher compression engines e.g. 3.5 m116 consider using 98RON fuel as a minimum, due to the higher compression ratios of these engines, or adjust ignition timing accordingly.

95 will do in a pinch, for those who have short arms and deep pockets. These motors are not exactly high compression racing motors, close but not quite.

I don't know about that, most pre-adr27 Mercedes-Benz engines stipulate 100 RON as per the owners manual.

Regards,

Carl.

[disley]

As an adjunct, I would also recommend those who have higher compression engines e.g. 3.5 m116 consider using 98RON fuel as a minimum, due to the higher compression ratios of these engines, or adjust ignition timing accordingly.

95 will do in a pinch, for those who have short arms and deep pockets. These motors are not exactly high compression racing motors, close but not quite.

I don't know about that, most pre-adr27 Mercedes-Benz engines stipulate 100 RON as per the owners manual.

Regards,

Carl.

Does 100ron exist?
While we're on the subject of additives, I read that adding a half litre of diesel or kero to a tank of gas increases the octane, is this true?
Not that I'd risk it, my little car runs great on 98. :dance:

[CraigB]

My 69 W113 cleary states 98RON in the owners manual

[Ivanerrol]

Does 100ron exist?

Yes.
Singapore, New Zuland, Japan and a few other countries

That's why some owners risk buying imported Japanese sports cars - some cars such as the original Subaru twin turbos are made to run on 100 octane

[ghogg744]

Mercmad
I use 95ron fuel dome fon mostly from BP. Getting 18L/100K. I put some fuel doc in today to give the system a good clean. Some mechs i worked with swear by it. As the car had been sitting for a while and when I test drove pinged because the deal put E10 in her. Not sure what the compression is? Who do you use to tune and where do you buy your fuel?

[ghogg744]

OK if I get what people are writing I should be using 98Ron in my 1970280SE W108 M130 6cyl injected time machine.

Thanks all great stuff.

If anyone is look for some work to do I need help with a left hand window regulator.

[Tony From West Oz]

Does 100ron exist?
While we're on the subject of additives, I read that adding a half litre of diesel or kero to a tank of gas increases the octane, is this true?
Not that I'd risk it, my little car runs great on 98. :dance:

Diesel and kerosine have a very low Octane, but a high Cetane
Petrol have a very low Cetane, but a high Octane.
Thus, adding kerosine or diesel to the fuel in a petrol engine will not increase the Octane.

For mechanical fuel injection, it has been suggested that a little diesel added to the fuel will reduce issues with the mechanical fuel injection due to the improved lubrication it provides. I have no knowledge if this is fact or fiction, but it seems to make sense to me.

Regards,
Tony

[AMG]

A lot of guys who run MFIS swear by the addition of a litre of ATF added to a full tank. Apparently it's known to improve the mileage of the M100 by a half decent amount. Also used to remove the varnish buildup in older fuel systems. Any additives should be used with a degree of knowledge about what is actually being put in - and things like fuel filters, pumps and injectors can suffer if the additive is not used with care.
In situations where the fuel system is not in good condition, it would pay to keep some spare filters and pumps handy, just in case things get dislodged etc and start to cause running issues.

Tony, I believe in your case with the diesel / kerosine, the main reason for use is the oiliness, which provides a small degree of lubrication to the MFIS, much like the ATF use, although I have not seen any physical evidence to prove the theory that additives actually clean an engine. This would actually be worth it.

Having said that, water/methanol injection is also reputed to have significant cleaning abilities, and prevents carbon buildup. Again, no 'before and after' bore scope footage to see if piston crowns and valves can actually be returned to 'as new' condition.

Over use of additives can actually increase the buildup on valves and piston crowns. I had a mate who used to run a tank of avgas through his standard engine (no mods) and the lead buildup on the valve faces and piston crown had to be sen to be believed. So over-use of the (back then high-lead) high octane fuel at low altitudes without any adjustments to fuel delivery resulted in a worse result than running normal fuel.

98RON fuels are the cleanest and best readily available fuels available in the domestic market, and it's arguable that a Mercedes-Benz is the best available vehicle in the domestic market, so put 2 and 2 together and you get to keep 4 wheels moving without any extra hassles.

[enginah]

Does 100ron exist?
While we're on the subject of additives, I read that adding a half litre of diesel or kero to a tank of gas increases the octane,

For mechanical fuel injection, it has been suggested that a little diesel added to the fuel will reduce issues with the mechanical fuel injection due to the improved lubrication it provides. I have no knowledge if this is fact or fiction, but it seems to make sense to me.

Regards,
Tony

Hey guys, In my industry we burn diesel and heavy fuel. One of the most significant problems we have had in the last few years is this stupid green issue of removing sulfur from fuel due to SOX emission limitations.
the problem being that sulfur is actually a lubricant. and when it is removed we end up having serious problems with fuel pump plungers wearing nearly twice as fast as they should. so it would make sense to assume that the same would be true that by upping the sulfur level by adding diesel to your car engine it would have the same lubricating effect.
however i would be very very cautious about doing this as the diesel also has a tendancy to carry things that can mix badly with the petrol and cause sludging and all sorts of residues that will block your injectors

[MBChris]

This is an interesting topic.

I only use 98RON in my cars, and have never had a valve seat recession issue.

One thing to be careful about with all these older cars, though, is the fuel system generally with these modern fuels.

None of our engines were designed to run on modern fuels, and while the valves will be OK on it, the rest of the system is not so sure.

A good friend of mine is a Mercedes-trained mechanic with his own workshop. He buys quite a few parts from me for his fleet (5 or 6 lovely cars, including a W128 Cabrio) and this has included carburettors (for his 5k mile 280S) and fuel tanks for an assortment of cars.

His advice is to only use 98RON, and to put in two stroke oil, into the tank. He uses about 500:1 ratio. The reason is that modern fuels are actively abrasive, and also relatively hygroscopic compared to older fuels. This trace of oil is not detectable in the exhaust, and prevents rust in fuel tanks (remember modern cars use plastic tanks for a reason) and abrasion in injection pumps and carburettor bodies, particularly the accelerator valves. His cars run better, he says, wit it in so I tried it.

He's right.

I have done over 20,000km's in the alst 6 months in my 420SEL, and it has never run better than now, with the oil in the tank.

Water injection is well known. A company I worked for in the early 80's was making systems to inject trace steam into the intake manifold to keep the engine clean and stabilise combustion. That particular system was relatively unsuccessful, but one only has to look at an engine with a water jacket head gasket failure to see that the cylinder with water in it is pristine (or rusty, depending on time-frames...) and this is good evidence of the benefits, despite the gasket failure!

Chris

[stumpjumper]

My 3.5 M116 has a compression ratio of 9.5 to 1. While the valves might be ok on 91 and 95RON, with normal timing it pings on acceleration with anything but 98RON in it. I've never tried it on 91E10. I don't think it would be any better than straight 91RON.

[Greg in Oz]

The 3.5 M116 probably needs a minimum of 98RON more than any other MB engine. It was very sensitive to correct ignition timing and any attempt to retard the timing to possibly allow for lower octane fuel will result in very lethargic performance and heavy fuel consumption. Back when they were new they were designed to run on "super" grade leaded fuel which was usually between 97 and 100RON. 98RON unleaded is the closest replacement available today.

I can remember back to the years following the introduction of 91RON unleaded before the introduction of 98RON unleaded when "super" leaded fuel was having its lead content progressively reduced with a resultant loss of octane rating. The result in my 350SLC at that time was pinging and lousy performance due to the need to retard the timing. When 98 became available, I restored the correct timing resulting in the performance I remembered from the days of genuine "super" leaded fuel.

[Mook1]

The documentation for my W109 3.5L states 96 RON. Ah well, no-one sells this and hence 98 is probably best...