Regular In A Premium 94 E420, - Benzworld.org

MBGirl2007 · 12-21-2007, 08:23 AM

I own a 1994 E420 model mercedes that calls for premium gas. I have always put in premium gas until today when I wasn't think and just wanted to get to work so the weekend could come. I ended up putting 11 gallons of regular in my car and 4 gallons premium. I was very worried and went into the store and bought and put in a bottle of octane booster. The car started fine but didn't drive as well. I haven't put my foot into it and slowly gained acceleration. My husband was saying that I need to run the car until it's chugging out of gas type empty, so when I do fill it up with premium the regular won't delute it. Is there anything else I should know regarding regular in a premium car?

Thank you
Sharlene

neanderthal · 12-21-2007, 08:38 AM

i run regular in my car all the time (1990 300E 2.6) and i've seen no performance detriment as a result. i used to have a misfire/ stumble when i used certain types of gas. so started paying attention to which brands/ stations caused it and avoided them. i do try to cycle in a tank of premium every once in a while

as long as you don't accelerate hard you should be ok. i really don't think there's any problem with just leaving it in there and running on it from now on. i don't profess to be all knowing however.

refill the tank with premium when you get to about halfway if the car is still running badly. the more higher octane fuel you have in the tank the higher the octane.
you do NOT have to completely drain it unless you poured in water or diesel or some other non combustible/ compatible fuel

edit. you might want to get a tuneup if the car is running badly regular. could be indicative of other issues.

MBGirl2007 · 12-21-2007, 08:45 AM

Thanks for your input. We just got in the set of new caps and rotors and spark plugs since the other's are dieing, probably another reason why it's not running as well. I feel better about putting regular accidently in and will pay a lot more attention. I got sidetracked as well when I was driving my husbands Jeep GC V8 that takes regular.

94Wagenmeister · 12-21-2007, 09:04 AM

I frequently run regular in my 94, But not more often than every 3-4 tanks of premium. No difference noticed, except the mileage suffers a little. I, too, need plugs, a new cap and rotor - I notice the miss/hesitation only on hard acceleration. Could also be dirty injectors (or a bad coil I suppose). Vamos a ver!

roycehof · 12-21-2007, 12:26 PM

I stopped at a station one time and they were out of Premium and Mid-grade. I put a tank of regular in and it was fine. Didn't seem to run as well and when it got down to half tank, I filled up with the super and it went back to normal. I have a 94 and I was told it has a "knock" sensor that retards the timing if the engine knocks. I didn't notice any knocking, but maybe it is my imagination, it just didn't seem to run as well. Since then I have only used Premium. For the few cents more, it just isn't worth possibly damaging something. I probably should have just put in enough to get me down the road, but being a "creature of habit" I just filled it up.
Bottom line - I really doubt if you would do any serious damage by using the regular gas, but it is probably best to use the recommended fuel. I feel fairly confident that the engineers have the engine "tweaked" for the premium fuel and that is where you get your best performance and economy.

imon_2nd · 12-21-2007, 01:36 PM

Hi, Sharlene.

There have been a few threads about this. Use the Search function and you'll find them.

Long story short; you won't damage the engine as long as you don't have a heavy foot. The knock sensor kicks in quickly. Nonetheless, some damage is done every time a knock occurs.

When you have a fresh tank of premium in the tank, disconnect the battery and then reconnect it. That will reset the timing to its normal advance for premium fuel. Don't count on the car to reset itself. Mine didn't. It wouldn't run properly until I did the manual reset.

Cheers,

Dave

94Wagenmeister · 12-21-2007, 06:01 PM

... but don't forget your Radio Code before disconnecting!!

Benz50602 · 12-21-2007, 10:37 PM

My mercedes mechanic almost encouraged the use of mid octane gas. He said it had some thing to do with the elevation. I usually never have a problem with it, the only difference i notice is that it starts easier with 91. Has anybody else heard about using lower octane because of elevation?

neanderthal · 12-22-2007, 10:12 AM

at higher elevations the air is thinner, less oxygen per cubic foot of air. i forgot how that ties in with the octane but what he said is true. you dont need as high an octane at higher elevations. only time i ever saw a 85 octane fuel was at high elevation.

White_Knuckles · 12-22-2007, 02:27 PM

Wikipedia:

For each 1000 feet of altitude the atmospheric pressure will drop by a little less than 11 kPa/km (1 inHg). An engine that might require 93 octane at sea level may perform at maximum on a fuel rated at 91 octane if the elevation is over, say, 1000 feet.

Fuel mixture needs to richen as altitude gains. Carburetors needed re-jetting for even slight changes. Octane is all about anti-knock. Pinging or pre-ignition is a bad thing. I think running low-grade is risky if ping is detected under acceleration.

My car loves Shell V-Power - yum!