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2006 CTS
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Discussion Starter #1
As mentioned in the DeVille forum, the '93 DeVille I just bought is very different from what I am used to.

Listed next to the fuel gauge are the words "Premium Fuel Only". Trying to find out if it is necessary to continue using Premium or can I experiment dropping down to Plus Fuel. I know the concern is Valve Knock, but is there any other concern I need to consider.

Any info would be appreciated.
 

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76 Eldo (54k), 76 Eldo (52k), 78 Eldo (9k), 85 Deville (43k)
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668 Posts
You must use premium. This engine--unlike newer cars--does NOT have a knock sensor, and it won't retard the timing if you put a lower grade of gasoline in.

That being said, you might be able to get away with alternating between premium and mid-grade if you fill up at 1/2 tank, but I wouldn't do it. Just use premium.
 

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1995 Sedan Deville Spring Edition
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268 Posts
As shown in the display, the 4.9 is factory spec'd for premium, and it is highly recommended. I always run it in my 95 Deville, and have been pleased not to have any engine noise or troubles with 116,xxx on the clock.

That being said, I had a 1991 Deville before the one I drive now and I did sometimes run 87 (gasp) in it. I never had knocking from doing so, but I always worried it would come back to bite me. I sold the car when the fuel injectors started going south. Good thing too, the guy who bought it has had to do all kinds of work to keep it on the road.

Anyway, the timing on the 4.9 is set rather conservatively to allow the use of lower octane fuel in a pinch (if premium is unavailable for some reason) but it is to be used sparingly.

Since I assume that your question is prompted by gas prices, look at this the way I did a couple of years back:

If my tank is completely empty, and I need to put 20 gallons in the car, then I'm only paying $4.00 more total per tank for premium over regular (here in St. Louis, the difference for premium is always 20 cents more per gallon than regular, though this may vary where you are at). For $4.00, I get peace of mind knowing that I'm giving the engine the fuel it is designed to run.

If you are going to run mid grade, by my math you only save $2.00 per full tank fill-up, and that's if you run it bone dry every time. If you are trying to save money, this isn't going to make much difference when you get right down to it. If you fill up weekly, you still only save a max of $104 in a years time.

If you are running a lot of miles (more than one full tank per week) maybe you end up saving a couple of hundred dollars a year. Yeah, that isn't pocket change when considered as a lump sum, but I just buy one less fast food lunch a week and I more than make up the difference.

KDirk
 

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Cadillac 1993 Deville 4.9, 1991 Eldorado Touring Coupe
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370 Posts
A lot of gas station`s premium fuel is 93 octane. They jump from plus which is 89...all the way to 93. No in between, UNLESS you go to Sunoco where they have 91. 91 is the grade of octane needed by the 4.9 considered Premium.

In the very cold winter months i have used 89 as the colder denser air helps combustion and doesnt contribute to the gas pre-igniting (thats the ping knock)

But thats as needed in a pinch. The Sunoco 91 costs me very little more than the 89 and the savings is nominal.

I can see trying to avoid the 93 cost, But Sunoco 91 doesnt costs much more. Well in my area, And 91 is plenty sufficient.
 

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1993 Eldorado, 4.9L, 200k miles
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Ahhhh....Hogwash on the Premium Fuel. I have '93 Eldorado w/ the 4.9L. I feed it nothing but the low grade...at some stations its 87 octane at others its 89. I am not particular about where I buy it...sometimes Speedway or Road Ranger, other times Mobil.

I bought the Eldorado w/ about 70k miles. In the 6 years that I've owned it I've put on about 120k miles....it's now at 190k and going strong. It still has original engine, original heads, original valves, original trans...no engine rebuild.....absolutely no major engine problems. The 4.9L's are rock solid.

With all of that said, however, be smart....and know and respond to your engine's sounds. I've heard my engine knock on some extremely hot days in the summer...this only happens on about a dozen days per year. As my preventative measure, I reduce the engine load by reducing the gas pedal input...this always eliminates the knock. In your case, you may choose to feed it a higher octane during the higher risk times of hot summer days.
 

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2003 STS w/BoseNav, 1993 Deville, 1985 Deville
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It depends on where you drive it. If you're a flatlander at low altitudes, then you would be best advised to use premium.

If you live in the mountains like myself, premium and even mid-grade is a COMPLETE waste of money. At 6200 feet at standard conditions, the engine doesn't even make 80% power. On a hot summer day, it won't even make 65% or even 60% power. Modern passenger car piston engines CANNOT detonate at that power output on even 85 octane.

I've always fed my 93 Deville (4.9L) 85 octane unless I head for Salt Lake, San Fran, San Diego, or other low places like that.
 

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76 Eldo (54k), 76 Eldo (52k), 78 Eldo (9k), 85 Deville (43k)
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kdcing:

You're right about the knock, but it's really best to feed these engines premium. Your timing is likely set under the base level--probably due to some play in the timing chain at that mileage--and so you don't experience as bad of an issue as other people may.

If you really want to push it, fill up with mid-grade, then listen carefully for spark knock, if you hear none, then fill up with low-grade. I'd personally never do this, though.

I also only fill up my cars with Shell gasoline because its the only gas to meet "Top Tier" requirements set by GM, BMW, and Toyota (do an internet search if you've never heard of this). For no extra cost, and since there's a shell about a block from where I live, I always fill up there.

Agreed that 4.9Ls are also rock solid, except for some 1991's that have a really bad main bearing thump that wasn't taken care of under a recall.

thu:

You're right about the altitude. That's why many stations way up there sell regular as 85 octane, where in the midwest, regular is 87.
 

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85 Eldo Biarritz
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As mentioned in the DeVille forum, the '93 DeVille I just bought is very different from what I am used to.

Listed next to the fuel gauge are the words "Premium Fuel Only". Trying to find out if it is necessary to continue using Premium or can I experiment dropping down to Plus Fuel. I know the concern is Valve Knock, but is there any other concern I need to consider.

Any info would be appreciated.
I thought that premum fuel was indicated only in Northstar engines in 1993. But I think Northstar was only available in Eldorado Touring Coupes. So, I'm confused.

But if you do have non-Northstar car then whatever others stated will apply.

Peteski
 

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76 Eldo (54k), 76 Eldo (52k), 78 Eldo (9k), 85 Deville (43k)
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Premium is also indicated on any 4.x that came with port fuel injection (not throttle body).
 

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93 Sedan Deville 4.9L
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I thought that premum fuel was indicated only in Northstar engines in 1993. But I think Northstar was only available in Eldorado Touring Coupes. So, I'm confused.

But if you do have non-Northstar car then whatever others stated will apply.

Peteski
I think alll 93s came with 4.9s except the eldorado. 4.9 requires premium fuel usually, I get away with 89 midgrade often though. Retarding the timing will cancel the need for premium since it will stop any knocks, but yeild less power / mileage
 

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2008 SRX
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That's alot of "2 cents" we're gonna get rich here. Here's mine.
High altitude allows lower octane due to the decrease in oxygen available for combustion. There's a reason you can't get more than 91 in the Mile High area. It is not needed.
I've run 85 (low grade) in my 92 Seville for 90k miles, no ping, no problems. Set the timing to 6 instead of 10 and get 20mpg around town. Besides, a little.....very little ...... ping won't hurt. You'll know if your car is pinging too much.
 

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2003 STS w/BoseNav, 1993 Deville, 1985 Deville
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Exactly. I've run my 93 Deville to 298,000 miles on 85 octane and I thrashed the heck out of it often. Engine is still going strong.
 

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You shouldn't be driving a Cadillac if you can't afford to put premium fuel in it. It says "premium fuel only" for a reason.
 

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94 Fleetwood Brougham
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Factory Service Manual on the 1991 Cadillac Deville/Fleetwood states that you can reset timing to 6 degrees and run 87 octane. Section 6E-C2-17 if you don't believe me, refer to page 6E-C4-2 on how to properly et timing, if you see pinging issues with your 4.9L with stock timing (6 or 10) and appropriate grade of fuel, then refer to page 6E-C7-4 and clean the EGR stack tubes.

I did it for many many miles, compared 93 vs 87 and timing at stock (10 or 6) appropriately for each and saw 0% change in mpg. Power was down a tad, but 99% of the daily drivers will never notice. And cold starting 87 octane is slightly better than 93 octane.

By all means, don't waste the $$ unless you have the resources to do so. You can probably run 12-13 degrees on 93 (I am pretty sure that is what I ran) and see a boost in power over stock settings, but honestly, with the 35K miles a year I drive, it wasn't worth that $4 more a tank to run 93 over 87 when 98% of the time I didn't need the extra power.

85 octane at high altitude is not the same as 87 at lower. You require 1 less octane point for every 1000 foot altitude increase. So 85 @ 6000 feet (Colorado Springs for instance) is plenty fine for that engine. My 10.25:1 455 Pontiac ran fine on 91 octane in the Springs, never issues with pinging. But at sea level at Ft. Stewart, GA, 93 octane was NOT enough to make it run well, I always had to baby the timing back at WOT.

Remember, pinging isn't heard audibly till it is BAD. So run proper timing, run proper octane. Your pistons will thank you. Pinging is HELL on them.... It is a resonance that can shatter brittle metals like the high silicon aluminum pistons those cars (most any car) have.
 

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1992 DeVille
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You guys are bigger risk takers than I am. I've got a 92 DeVille with a 4.9 with 89K on the clock. With gas prices as high as they are I've been tempted to try putting 89 octane in the tank, but given my car's age and how well it runs, I'd kick myself if I put some cheaper gas in it and wound up hurting my baby. My advice is to pay the couple extra bucks and stick with the premium.
 

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Cadillac seems to think it is fine with the appropriate timing change. I trust Cadillac personally.

Is $4 a tank gonna kill you? Not likely, but for me driving 35K a year, yes, it adds up pretty fast.
 
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