AMGfan

I decided to mix some 100 octane with 93 octane Sunoco gas about two weeks ago. The day after when I started the car a CEL cam on and stayed on for the whole morning. The car was running fine and let it sit and went to work. When I went home...no CEL.

I filled up my tank this Thursday (50/50 mix of 100 and 93 again). The CEL light again turned on later in the day. This morning the car started with no problems and no CEL, but this afternoon the CEL light started to blink and then stayed on solid.

I'm getting it scanned on Monday to see if any error codes appear. I'm letting it sit overnight and check again tomorrow morning to see if the CEL is still lit.

The car seems to be running fine and strong. No leaks under my car or strange colored exhaust fumes...no stalling.

My question(s):

1. Is the mixing of different (higher octane) causing the CEL?
2. Am I doing any harm by mixing different octane grades?

I'll find out more once I get it scanned but my concern is what happens if the CEL disappears?

cls55

check your MAP sensor.

AMGfan

I've been doing a parts check for an OEM replacement for the MAP sensor but online I keep getting different results...either the online OEM parts store don't have them in stock or I get redirected to AEM Map Sensor (universal fit) for about $146.00.

I'll check on Monday at my local stealership. I didn't think it would be so hard to find an OEM MB replacement for the MAP sensor.

I'll get it scanned first to find out he CEL error code but I'd like to have an idea what part# and total cost for an OEM MB MAP sensor if I am led down that path.

AMGfan

I had the car scanned and the MAP sensor is fine. Evidently there are 3 misfires...1, 3, and 4...they are going to check the coils and wires. The car has just hit 30K miles (04) and the car has new injectors when it was rebuilt (they were flow tested by PTE at that time), so they are going to run the diagnostic on the coils and wires in 1, 3, and 4...

I'm holding off on the dyno run until this CEL is cleared up. From what they are telling me the dyno run may be compromised (i.e. lower numbers) if there are continual misfires...

Marcus Frost

Jon,

Don't run anything higher than 93 octane in your car. Our ECUs cannot take advantage of it, and it does cause the car to run rough, and possibly misfire. You're throwing money down the drain and possibly throwing codes with it.

When I ran 100 octane in my '03 E55, sometimes it would barely start - and this was stock. With all your complex mods you may be pushing the ignition system into territory it cannot deal with because the bottom line is higher octane is harder to burn and will cause a drop in horsepower unless there's an ability for the engine to take advantage of it. We do not have that luxury, even modified.

-m

Yacht Master

Jon,

The 93 octane Sunoco that you are using, is it 10% ethanol?

AMGfan

Bob...I just called the station...yeah it is 10% Ethanol. I usually use Mobil gas and didn't seem to have a problem before...

Yacht Master

Jon,
I ask because we are seeing some problems here.
This is what seems to happen, in a tank of fuel with 10% ethanol stored for 2 to three months then mixed with new fuel with out ethanol but with MTBE.
We find the mixture can form a gel like substance and can be enough to present problems, filters & injectors.

You may want to try E 10 only or a race fuel only. I think VP C-10 only would be good.
http://vpracingfuels.com/fuels_unleaded.asp

JamE55

Jon,

Isn't your ECU custom so it can take advantage of higher octane? Seems a bit weird if your ECU is custom programmed but not able to use higher oct?

As far as CEL i get that from time to time as well and am using VIP109. It would come on and go off but the cars performance has not problems.

AMGfan

I just filled up with Mobil 93octane...no CEL so far. I'll see if I get a CEL this weekend and if I do I'll get it scanned again to see if the same error codes get thrown again.

JamE55...I was under the impression that going higher octane on a tuner ECU could yield better performance. Hopefully, Victor or Vadim can shed more light on the subject.

Marcus Frost

Jon,

On certain platforms, yes, running higher octane allows for the engine to make more power. However, there is common misperception as it relates to octane. Higher octane does not give more horsepower, higher octane prevents preignition and/or predetonation and allows you to run more timing. Our cars do not have adaptation systems that allow the ECU to increase timing to match additional octane. The ignition tables are set, and the only way they adjust is when timing is pulled due to high IATs/etc.

In other words, running 100 octane on timing that is meant for 93 will actually ROB you of horsepower. The rule of thumb is that you want to run as little octane as possible before leading to knock/preignition/detonation. Running octane beyond what you can take advantage of requires more power to ignite and will yield negative results.

Good luck,

-m

JamE55

Jon,

Marcus is correct. The Kleemann custom ECU i have allows me to pull more timing so i actually get a little more hp when using a higher oct gas.

AMGfan

OK...now I get it...Day 2 and no CEL. I'll keep you guys posted to see if there is any change. If there is no CEL, then I'll proceed to the dyno run next week.

Thanks for all the input and knowledge!!! Marcus great talking with you, too.

RENNSTAGE

you get the same gas no matter what station you go to, most buy from same distributor and each brand puts there own dye in it to get there own color, plus gas is traded at the ports so you have no idea where its coming from

AMGfan

Florida Ethanol Label Law
http://www.fuel-testers.com/florida_..._news_e10.html

Marcus Frost

My pleasure Jon. If you were running this 100/93 mix a lot lately, it may also explain what we were talking on the phone about.

Good luck.

-m

str8ridin

So, it is getting harder and harder to find gas stations around here without "up to 10% Ethanol" in them.

From a stictly performace standpoint for our cars, how does ethanol mixed gas compare to non-ehtanol gas?

AMGfan

Yes it is...I've had to go through a couple of gas stations to find one that had at least the option of "straight" fuel. From a non gearhead point of view, supposedly it gives a little less fuel economy and less performance.

Someone on the forum might have a more technical aspect as to why above might be the case.

PS no CEL for over a month.

str8ridin

Interesting...

So, for our cars, what is least octane possible before leading to knock/preignition/detonation that we need? And, you say that is better than the greatest octane before our cars start to yeild negative power?

jangy

MB tunes our cars for ANY brand of gas, including ARCO , so that is where the gains of an ECU tune come from. I would be careful trying to control A/F ratios based on your gas purchase because it will not EVER be consistent enough. In an ideal world, yes you do want to run lean for max power. Anything extra and you are just dumping in fuel making it that much harder to vaporize, etc.

Having said that, I've seen MANY cars run higher dyno numbers from ONLY adding high octane gas. I think there is more to "optimal" than a simple number..

Marcus Frost

str8 - this conversation can get really lengthy/technical, so if you want to really learn a lot more there are many resources out there for you, and I'm always willing to answer some questions via PM.

To answer your questions as easily as possible, the least octane our cars can "run" would be 87-89 octane, however in order for the engine not to destroy itself with such poor gasoline the ECU would cut back on timing/etc SIGNIFICANTLY if it were to detect the presence of such low-octane fuel. These cars are tuned for 91-93 octane as their optimal level. They are NOT tuned to allow for additional timing beyond what they deem the ceiling @ 93 octane - meaning that even IF it is possible to run more timing, AMG did not leave that room in the tuning to take advantage of it. You can put 130 octane in there, it's not going to run more timing. Therefore, if the timing is tuned for 93 octane, using 100 octane at the SAME timing levels will actually hinder performance because the tune is not optimal and with the harder-to-burn octane fuel running through the cylinders the car will have trouble igniting it and will cause a drop off in power, not to mention a useless hole in your wallet. See Jon's original post as to the noticeable side effects excessive octane can have on your car runs.

ECU tuning from the aftermarket tuners, however, does change this and they CAN in fact program mapping that will allow you to take advantage of extra octane.

-m

Marcus Frost

He's talking about octane Jangy, not air/fuel ratios. They are two totally different beasts.

I've owned many cars that run higher dyno numbers and trap speeds from putting in 100 octane, however the 55s are simply not one of them. Not all ECUs are created equal.

Absolutely, but in stock form our ECUs are so tightly set that you really don't have much room to mess with things like octane to make any power differences.

-m

c32used

+1(I like this response best)

I recently had a misfire of multiple cylinders and it was cause I increased the boost to car by adding a bigger pulley..............now its not the pulleys fault! My Denso IK20s that were just over a year old took abuse from my taking then to the track thou not alot it did the damage and it wasn't obvious with the 178mm crank pulley...but the additional boost brang out the misfire. After checking plugs from cylinder 1 and 4 the gaps were different 1(.41), 4(.45) now if anyones doesn't believe that was the issue then once I replaced plugs(thanks AMG-Jerry) all was fine and misfire gone. In your case I believe you are misfiring as Frost said...you are Tuned I presumed for 93 Octane so I feel the higher Octane fuel and your tune are not working well together...run only 93 Octane and if you don't see any cells after 4-5 refills then you know its was the higher Octane and your 93 Tune. Keep us posted GL

AMGfan

I have not had an issue with CEL coming up since I switched back to 93. It's been about 3 weeks now and I've run through about 2 tanks of Mobil 1 93.