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  #1  
Old 05-18-2004, 03:40 PM
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Can you use 89 octane gasoline in a 300E?

I was wondering with today's gas prices if I can switch to unleaded plus( 89 octane) for my 87 300E. Has anyone tried it?
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  #2  
Old 05-18-2004, 04:44 PM
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Talking

Octane Rating With Gas Prices
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  #3  
Old 05-18-2004, 04:49 PM
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Sure. You can run kerosene in it if you can get it to burn the stuff. You have a knock sensor that will retard the ignition timing if it detects spark knock to prevent engine damage.

That said, under most driving conditions this is false economy. When the ignition is retarded, the fuel mileage will decrease, negating at least some of your fuel savings.

The difference in cost between the three fuel grades is the same regardless of the gas prices. Premium is always about 20 cents higher than regular and about 10 cents higher than mid grade. That means that at $1 per gallon it is about 20% more expensive, but at $2 per gallon it is only about 10% more expensive.

You have a car that cost well over $40,000 new and has a high compression ratio. By running mid grade your performance and your fuel mileage will degrade. Spend another 10 cents per gallon and be done with it IMHO.

Good luck,
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Old 05-19-2004, 01:56 AM
Bud
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There is no knock sensor/ignition retard in the M103 (at least not on my U.S. version).

Larry's right, it's false economy to use lower octane fuel. In fact, when I go to states where I can get 93 octane fuel, my mileage and performance increases markedly.

Here in Arizona, you should hear U.S. cars pinging in the heat of the summer despite their so-called ability to use 87 octane fuel.
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  #5  
Old 05-19-2004, 02:36 PM
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I use 89 in the winter, but switch to the top stuff in warm weather.

The compression ratio is relatively high on these motors, so there is benefit to high octane. On the majority of vehicles out there, higher octane is a waste of money.
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  #6  
Old 05-19-2004, 03:23 PM
LarryBible
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The resistor determines the MAXIMUM ignition advance. The 103 has the knock sensor and the ignition module. The ignition module is where your diagnostic plug is located.

Have a great day,
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  #7  
Old 05-19-2004, 06:21 PM
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I have a 91 300E 130K pre-multivalve engine. All said here in general holds true from my experience. I run 90 Octane and have not experienced mileage nor apparent performance degration. Now I wouldn't try 87 Octane but 90 seems to work in my car. Maybe if it was 0 F or below outside. I have not driven it in very hot weather being up north(MN). The hotter the intake the higher the risk of pinging/knock/pre-detonation. So I will check again in August.. This works in my MB and my area, but I would not generalize for all cars. I have an Eagle Talon Turbo running 18PSI of boost and I would never use less than 92 Octane, this would melt my pistons.

Higher Octane can be a waste of money but I would follow manufactors recommendation. My MB model appears to have good knock protection so it will retard the timing as much as possible. This should show up in your gas milaege, so if your have a good baseline of mileage at X ambient temp and 92-93 Octane then you can drop the Octane to 90 and see if your mileage suffers. Conversly in a car that recommends 87 Octane if your mileage doesn't go up using 90-92 Octane then your wasting money.

P.S. Ethonal actually helps to prevent pre-detonation. All gas here is 10% Ethonal.
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Old 05-19-2004, 08:53 PM
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Sorry to disagree Larry, but I'm pretty sure the M103 engine does not have a knock sensor, at least not the early ones. I have an '86 and I've had two different ignition modules in it, and in both cases the engine knocks under moderate to heavy load at low to mid-range rpm even using 91 octane CA gas (with ethanol).
I wish it did have one so I'd have less to worry about as I can't seem to eliminate the problem. Eventually I think I'm going to have to pull the head and decarbonize everything. Where are you getting your info that there is one on these cars, and where is it supposed to be physically located?

Gary
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  #9  
Old 05-20-2004, 02:55 AM
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Larry, I'm not aware that my '88 M103 has a detonation sensor. Maybe the later models???

I run 87 PON in cool weather. If I short shift and get on the throttle too fast, I get a little transient detonation, so I've just adjusted my driving habits to load throttle a little slower on short shifts, or rev it up a little more.

87 might not work in the heat of the summer with the A/C on and 100 degree coolant temperatures, but I just use my 190 from about Thanksgiving to Memorial Day.

Duke
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  #10  
Old 05-20-2004, 03:54 AM
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'87 260E must use premium?

Hi All,

With all said so far, do all recs apply to the 2.6 M103s as well with regards to "must" use 91 or higher octane? Do I have the knock sensor?

thanks,
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  #11  
Old 05-20-2004, 08:41 AM
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I can't guarantee the other years, but my 103 engine in my '88 has KE III Bosch Injection. Page 34 of the "Bosch Fuel Injection & Engine Management" manual shows the knock sensor and its connection to the Ignition Control manual for the KE III system. So, don't argue with me. Contact the Bently Publishers.

I have a partial copy of the MB manual, but it is extensive and very difficult to research. If you insist, I will find references in that manual as well, but it will be awhile before I can have that much time to spend.

I too have run regular in my 300E due to that being the only fuel available when stopping on a trip. I never heard any knock. I have NEVER heard spark knock in my 300E under any conditions.

To check the sensor connect a timing light and observe timing while banging on the engine block with a hammer. Timing should retard.

Have a great day,
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  #12  
Old 05-20-2004, 12:01 PM
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When you get a chance, Larry, I'd appreciate it if you research you MB data and verify whether or not the '88 103 has a detonation sensor. Also, where is it physically located on the engine?

An automatic transmission will probably have less tendency to detonate being as how it is tough to load it at low revs due to converter slippage. I have only heard brief transient detonation on my car if I short shift and give it high throttle as I described above. I've never heard sustained detonation on 87 PON.

Thanks!

Duke
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  #13  
Old 05-20-2004, 01:56 PM
LarryBible
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It is located on the drivers side of the block.

Have a great day,
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  #14  
Old 05-20-2004, 08:28 PM
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I was under the car again, today, replacing a shift linkage bushing. I looked from above and below and could not find anything that might be a knock sensor in the block, but the driver's side has a lot of hardware, so I might have missed it.

Can you give a more specific location and a description of the connector or other physical characteristics?

Duke
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  #15  
Old 05-21-2004, 11:40 AM
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My understanding (limited.. did not sleep at Holiday Inn Express last night) is that the 103's did not have a knock sensor. My search of MB CDs indicates that also. With the advent of the 104 came the more sophisticated ignition and a knock sensor.

Can someone positively confirm all this unconfirmed speculation?

Larry, while I respect your constant and valuable help here, I believe you are wrong on this one.
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