I am getting mixed responses on which gasoline to use in my 1989 740GL. Some people say that I should just use the regular, 87 octane gasoline. The car runs fine, but only gets about 20 MPG. Others say I need to use premium, 91 octane, the car runs better, and gets about 22-23 MPG.

The owner's manual says to use 89 octane or higher. So, what is the board's consensus on the matter? Normally I am prone to follow the manufacturer's instructions, but I am unclear in this case. Is there any harm in using 87 octane? What do others do?

Thanks,

Greg

1989 740GL

As you have noted and Don confirms, the engine has a knock sensor, and compensates for different grades of gas. If it detects pinging, more likely with lower octane gas, it will retard the timing, and the power and fuel efficiency willsuffer. I have used 87 in my 89 740 16valve and 89 octna and notice no difference, and i have tried to measure for better ga s mileage but there are too many other variables. By the way I have a manual trans and get 24 -26 mpg. mostly around town and highways at 65 mph.

My 87 turbo takes 91 RON (research octane number) which is equivalant to 87 AKI (antiknock index)which ,I believe, is the most common rating in the US. If your vehicle produces more power and gets better mileage on higher octane ratings it is likely to indicate some inaudible detonation may be occurring as the detonation in addition to damaging pistons,gaskets, valves,etc. will also reduce power since ignition and the flame front in the combustion chamber are essentially uncontrolled. This can result in the equivilant of overadvanced timing although it may have nothing to do with the static settings if they're even adjustable.

Higher octanes are resistant to detonation because they burn slower, they don't really unleash more power and there have been stories of cars not starting in the winter because of the slower ignition qualities of higher octane. If your manual says 89 AKI that's what I would use and the additive package might be just a little better too.

BTW AKI is a combination or average of RON and MON (motor octane number) RON tests low speed knock resistance while MON tests high speed resistance.

Use what the engine is designed for, 89. Claims, rumors, I thinks, etc from those who use 91, 94 octane that engine runs better, cooler, quieter, faster etc just are not true. Octane has nothing to do with power or mileage but rather the resistance to preignition.

David,

I hate to tell you, but you couldn't be more wrong on this one. The LH-Jetronic fuel system can (and will) advance the ignition timing as far as it can without causing a pinging condition. With this in mind, the better the gas, the more timing advance the engine can run. This translates to better fuel economy, a hell of a lot more power, and a more efficient running engine. As you had said, the higher octane does result in higher resistance to preignition, but with less chance of this, the timing can be advanced further with no problems. Sorry David, but you are a bit mistaken on your info...

Can you provide the source of your misinformation?

Which part of my supposed "misinformation" do you want a source for?

I am interested in the source for the following:

"The LH-Jetronic fuel system can (and will) advance the ignition timing as far as it can without causing a pinging condition."

David,

The LH-Jetronic system, in conjunction with the EZK-117 ignition system, can advance or retard ignition timing automatically, as necessary. It can even adjust timing per individual cylinder, not simply as a whole. This is why better gas will make the car run better and get better mileage, because the base ignition timing will automatically be advanced further with the higher octane fuel. I know this because my Father was a Volvo tech for 11 years and a good friend of mine still is a Volvo tech. Anything else?

You are correct in every repect except for a few key points. On the EZ....K systems the ignition timing is retarted upon detection of knock. If knock is still present the Fuel ECM will be informed to enrich the mixture. When further knock is not detected mixture is restored then timing is advanced in small steps back to the nominal ignition timing setting. This information comes from the Bentleys Manual, Haynes Manual and a book titled Bosche Fuel Injection and Engine Management, Author Charles O. Probst.

No publication refers to any kind of ignition advance capability other than a return to nominal after a knock detection retard event. The Bosche systems are not capable of any kind of automatic advance to the point of engine knock but rather will only retard when knock is present.

It is true that a low octane or poor quality fuel will affect engine performance. Timing must be retarded to prevent knock with such a fuel. It is also true that a specially designed or modified high performance engine will be capable of extracting more power from a high octane fuel. It really does surprises me that the myth lives on that high octane fuels will make a standard production car go fast. This despite much information available (even from the oil companies) to the contrary.

I believe this is caused but what I call "auto transfer placebo effect", you know like when you wash, polish and clean the car it runs better. Those who waste hard earned cash on the high test fuel want to justify it so there car really does appear to run better for them. That's just the way our brains are wired I guess.

With all due respect to your friend and even more to your father I have heard much misinformation and nonsense tumble from the mouths of mechanics, other dealer employees and many other "experts", so I should treat that kind of information with an element of skepticism.

David,

Thanks for the reply...I agree with what you are saying also. My point is simply that I have tried lower grade gasoline in my cars and the truly do appear to be "gutless". With that in mind, that is why I run the higher grade gas in my car. That is what I was trying to tell the person in the original post. If the car runs worse with lower octane gas (as he said his did), then why use it?? This should be a no-brainer. I simply prefer to go with experience (whether it be my own with my car or my Fathers experience in the dealership; These guys know from experience that these cars can advance ignition timing, reguardless of what these publications say) over literature (as I know for a fact that Haynes and Bentley aren't always accurate). Anyway, I guess that everyone should just run what works best for them in their gas tank. Thanks David...

Hey,

David is correct, i don't know where i read it, but the way it works is it monitors pinging, and advances the timing to as far as it can, i.e. just before pinging occurs, thats why, when pinging does occur, it NEEDS to retard the timing. Of course, there's limits to this, running 98 ocatane fuel will not give better performance than, say, 95, but 95 will be a large improvement over 87...... The car runs at it's most efficent *just* before the pinging stage, so thats where the computer tries to keep it at. Thats why you notice when it's colder (20 degrees celcius) the car will run quicker, after it's been driven for 30 minutes. Drive the car for 30 minutes in heavy traffic on a 35 degree celcius and take off, and it feels like someone's unplugged an injector..... Anyways, thats my .02 cents, but hey.....

Az

"Octane has nothing to do with power or mileage but rather the resistance to preignition."

True, true, true, I agree 1000%. You should use the LOWEST octane rating for which your engine continues to run well and not ping.

Here's the ringer (or maybe I should say, "Hear's the pinger..."). If your car pings slightly, you may not hear it but the knock sensor will -- and it will retard the timing to eliminate the unheard ping. So the retarded timing, induced by slightly downgrade gas, translates to degraded performance.

I use your method -- I opened the owner's manual for my '91 740 nonturbo and it says 89 octane. I also use 89 in my '82 245t, but 87 in my '86 245GL.

Hey Don,

I just posted about my 740 on the bb, i think it's a computer problem, (dry joint, soldering?!?, your area of expertise? haha), um, basically my car runs rough on regular unleaded ( i think it's 91 octane in australia) but fast (not smooth at all), and on 98 octane gas, it runs real smooth but lacks "real" power, as such, it's a non-turbo, but it really goes fast in regular! Thanks in advance Don!

Az

Hmmm...

The track record of the fuel injection control units appears to be very good, which is in contrast to the track record of the relays -- which is very bad. I've never resoldered a cracked solder connection in an FI ecu, although I have repaired an ignition module....... But this was when a ham-handed fool (me) broke a pin in the socket. (Can't blame that on the solder quality.)

Your description sounds a bit like a failing temp sensor (the one for the ecu, not for the gauge). It could also be a bad connection or plug at the sensor.

Also check the ground leads for the injectors -- the four heavy black wires attach to the intake manifold. Clean the connections, use a bit of grease to prevent oxidation, and tighten down the bolt.

You might have several problems -- crummy gas, a bad knock sensor, a bad injector ground, a bad injector...... the list goes on. Be absolutely positive your plugs, wires, cap, and rotor are in "new" condition. Perform the "midnight test" to confirm that you don't have high voltage leakage.

Greg,I have an '86 744 Gle 225k and I've been using Texaco premium (91) for the past ten or eleven years.IMHO our cars run better on 91 octane or higher.Have tried 89 a few times ( when gas gets expensive) and I'm not sure, maybe it's just me,but I thinks I can hear my 8 valves making ticking

sounds and I'm losing some power to my already old and under powered brick.

That's my $0.02 on the matter..Rey

Greg,

Personally, I prefer to use the highest octane that I can find (usually I use 94 octane from Sunoco) in my 1988 760 Turbo. While your car isn't a turbo car, it still would be beneficial to you to use the better gas. As you had stated, the car will run much better, have more power, and will get better mileage. Anyway, use what you want to (however, I don't recommend anything below what Volvo says to use) but I would use the best you can get. I hope this helps...

While it is a truism to tell someone to use the lowest octane that allows your car to run properly, what is properly?

I suggest that you try several tankfulls of each grade while on a trip.

We did 4996 miles traveling from Massachusetts to Saskatchewan (scratchy bum to natives) and back along a different route, and noticed a loss of almost five (5!) miles per gallon on regular with our 740 wagon (140k miles)

My Haynes manual tells me that the first thing that happens when a knock is sensed is the addition of more fuel! This stops the knock quicker than changing the timing.

Also, consider the changes that take place in an engine as it gets older, such as carbon build-up, running hotter, loss of efficiency of ignition and fuel systems, etc.

For the price difference, make mine high-test.