Could HIGHER OCTANE gasoline cause RICHNESS?

My 1993 Buick Regal with 3.1 MPFI engine has been running Rich for quite some time. Not enough to emit visual "black smoke" or something, but enough to 'kill' my catalytic converter in only 2 years. Not to mention the loss of mileage.

So, here's the thing. About 2-2.5 years ago, when my knowledge of gasoline was nil, I decided to run 91 Octane Supreme gasoline, and have always done so since then.

After reading that "higher octane gasoline is harder to ignite", I'm starting to think that MAYBE this is why my car has been running slightly Rich for almost 2 years. Perhaps some un-burned fuel is getting too far up the exhaust stream or something? I'm no expert...

Would it be OK for me to switch back to 87 Octane at this point? I thought I remember 91 making my engine feel/sound a little better, but maybe it was just wishful thinking?

All other possible causes of my slight Richness have been checked, and to this date, no culprit has been found. The O2 Sensor is 6 months old, I have a New EGR Valve, plugs, wires, coils, ICM, ECM, Fuel Pressure Regulator, etc. And no injectors are leaking.

Anyone?

The recent post regarding Regular vs. Premium got me thinking about this, so I'm glad I read up on that!

THANKS:)

Reply to
OldsFan4Ever
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It's not the fault of the excessively-high-octane gasoline you've been using. There's something the matter with the car's engine management system. Find it and fix it.

That's wrong. Higher-octane gasoline is more resistant to *spontaneous, heat- or compression-induced autoignition*. It is not "harder to ignite" when it's supposed to (when the spark plug goes "zap").

No.

"up" the exhaust stream is close to the engine. "down" the exhaust stream is close to the tailpipe. And running rich will indeed cause excessive unburnt fuel to be present in the exhaust, which will fail the catalytic converter. But high-octane fuel will not cause this.

Yes, but it won't fix the problem.

If *ALL* other possible causes of your slight richness (it is not capitalized) had been checked, your car would not be running rich.

Could be a 6-month-old bad O2 sensor. I've watched Bosch O2 sensors fail early and often in some applications.

Sounds like you're "checking possible causes" by buying and installing new(?) parts. That's the wrong way to do it. What is needed is

*diagnosis* using the correct tools, in this case a scan tool.

DS

Reply to
Daniel J. Stern

When I run high octane in my 88 Cherokee on trips as an experiment, my mileage does go down slightly.

This implies to me I am running slightly rich.

When I run regular in me CJ7, my mileage drops by 1/3, so I am for sure running rich.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

OldsFan4Ever wrote:

Reply to
Mike Romain

You are ignoring what the definition of "rich" really is. It's the mass of fuel being burned in a given amount of air. The carb or injectors don't know what the octane of the fuel is. They can't alter the amount they physically introduce into the cylinders because of something written on a pump somewhere.

Now if the fuel had a different density that would be another matter. The volume injected would stay the same but the mass would alter. However, on a closed loop system the lambda sensor would measure the oxygen concentration in the exhaust gases and automatically compensate. In any case, the additives that alter fuel octane are in such small quantities they make no material difference to fuel density.

More likely is that small differences in power output lead to changes in consumption and also that small changes in flame speed mean the ignition timing is not so perfectly correct with fuels of differing octane value.

Dave Baker - Puma Race Engines

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I'm not at all sure why women like men. We're argumentative, childish, unsociable and extremely unappealing naked. I'm quite grateful they do though.

Reply to
Dave Baker

I'll buy that. I bet it is the lower power I get in either situation that leads to me giving it more gas pedal to get the same effect so the electronic unit only sees a small shot of extra gas and goes lean for a moment until it catches up. I bet overall it still would be considered a 'rich' mix over a full tank of gas. Don't know if it would be enough to kill a cat though...

With the carb engine and no computer, octane changes are a radical thing.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's
Reply to
Mike Romain

So would you say that ALL of the following LINKS that have people posting about how running too high of an Octane gasoline WILL cause unburned fuel to exit the exhaust are wrong:(I put the quotes and then the LINK as to where I got it from)

"When you use a fuel with a higher octane rating than your vehicle requires, you can send this unburned fuel into the emissions system. It can also collect in the catalytic converter. When you over stress any system, it can malfunction or not do what it was designed to do properly. In the early 90's, an early warning symptom was a rotten egg smell from the tailpipe. Easy fix, go back to using regular 87 octane gasoline. The rude odor usually disappears after several tanks of gasoline."

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"Higher octane just means it takes a hotter temp to ignite. Due to this, the fuel burns slower. If the engine calls for 87 octane and you run

92, chances are the mixture won't finish burning before it's blown out the exhaust. Unburned fuel means less power, worse gas mileage, and excess carbon buildup."

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"Knowing this, if you run 93 in an engine spec'd to run 87, chances are the 93 won't burn completely before the exhaust valve opens and you blow out unburned fuel, which is unused energy and increased carbon. Your compression ratio and timing determine what octane to use to prevent pinging, so simply upping the octane does nothing unless you have modified compression ratio or timing."

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"Higher octane fuel will generally burn slightly slower than a lower octane fuel. This being so, where do these VOCs go? out int he exhaust... So thus you get vaporized compounds going out, where they cool. Thus you get unburned fuel in the exhaust. Some may get reacted by thge cat converter, but that reaction is not 100%, and in reality, the more unburned fuel goes through, the higer the load (and heat) on the converter, reducing its efficiency over time and ausing higher releases of unburned fuel, because too high of octane was used."

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I don't want to post the few hundred or so that I found, that would be rediculous.

Anyways, I like that you mentioned needing a Scanner to diagnose my problem, because I happen to have a Scanner myself.

I am thinking of doing some research. I've been running 91 Octane for

3 years now. I was thinking of first, buying some BG 44K(similar, but better than Techron) to clean out my fuel system/carbon build-up, and then put in half a tank of 87 Octane.

I would like to see the reaction of my ECM to this switch. I want to see if my Knock Sensor shows anything, and whether my timing is Retarded as a result. Also, I would like to see if switching has any effect on the Block Learn, Integrator and O2 Sensor readings.

I will re-post with anything I discover once this "test" has been complete.

:)

Reply to
OldsFan4Ever

Yes, because they *are* wrong.

There are lots of wrong things believed and stated by lots of people. That doesn't make the wrong things right, just widely-believed and oft-stated. And remember, there's no rule that you can only post correct facts on the web. Lots of people post incorrect factoids and assorted other idiocy. On the radio and TV, too! Ever listen to "Car Talk"? Know enough about automobiles and engineering to count the errors they make? I have, and I do, and the count's real high. Even books aren't legally required to contain correct information. Ever try to repair a car using a Haynes or Chilton manual?

None of the above is correct. The rotten-eggs odor is a result of excessive Sulfur content in the fuel, which is in no way related to the antiknock ("octane") level of the fuel. C'mon up to Ontario some time and sample some of our wonderful ultra-high-Sulfur gasoline. It stinks of rotten eggs regardless of the "octane" grade.

None of this is correct, either. Higher-"octane" fuel does not "burn slower". Common myth, though.

That's two quotes of the same incorrect statement.

That's three.

Yes, it would be, especially since once wrong, twice wrong and three hundred times wrong is still wrong.

The reading you *should* have been doing, instead of fartin' around trying to find support for your incorrect belief, is the Gasoline FAQ:

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It was written by someone who knows far more about the topic than you *or* I.

What's preventing you using it?

Seems to me you've already spent too much time doing "research" when you should be out there fixing your car's problem.

It's never too late to stop wasting money.

BG 44K is the only pour-in engine degunker that's worth a damn, IME.

DS

Reply to
Daniel J. Stern

I would add Techron to that list too.

Reply to
Bob M.

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