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76 Eldo (54k), 76 Eldo (52k), 78 Eldo (9k), 85 Deville (43k)
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Discussion Starter #1
Hi everyone--

I know that fuel economy usually comes up in the forum about twice a year, but I thought I'd share a bit of information with everyone that I learned while talking to someone who worked on the 4.x program engines.

We've all wondered why our Fuel Data Centers are off by a bit--especially on the PFI engines--but here is the reason:

The fuel data center uses the injector pulse width to calculate the fuel used. It multiplies the injector on time by a specified flow rate based on a CONSTANT fuel pressure. For throttle body cars, where fuel pressure is always a constant 9-12 psi, this calculation is very accurate if the engine is in good tune--hence explaining why the TBI owners usually achieve mileage close to the readouts on the computer.

However, the constant fuel pressure assumption is a bit of a problem for the PFI cars because of the vacuum-controlled fuel pressure regulator. In high vacuum conditions, fuel pressure may only be 28-32 psi at the rail, whereas in low vacuum conditions, it may be considerably higher--40+ psi. The engineer I spoke with said that Cadillac simply used a constant 34 (or so) psi fuel pressure assumption to calculate the gas mileage. Hence, if you drive in a lot of high vacuum conditions (i.e. on the freeway going about 65-70 mph), you should beat the data center's mileage estimate. If you drive with your foot to the floor, or in the city, you'll likely see worse mileage than it estimates.

Of course, this is dependent on the fact that your engine has no vacuum leaks and is in top tune...other factors that could effect the readout discrepancy.

Hope this solves the PFI owners questions about mileage vs. data center.
 

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1995 Sedan De Ville (4.9 L)
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581 Posts
Good info! Thanks to share with us.

I have been wondering why it does not have a fuel pressure sensor (to calculate the fuel mileage). It is a fundamental parameter to "accurately" calculate the total amount of flow...

But now a new question comes up... Why didn't that programmer use the manifold pressure to adjust fuel pressure??? It could have been done, right?
 

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76 Eldo (54k), 76 Eldo (52k), 78 Eldo (9k), 85 Deville (43k)
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Discussion Starter #3
Ha, I suppose so, but I think they figured it was "good enough" for a crowd of buyers who likely didn't care about their fuel economy at the time (gas was cheap from 85-early 90s).
 

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1980 FBC
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3,215 Posts
This theory does not make sense. The reason for a manifold vacuum modulated fuel pressure system is to provide a constant pressure drop ACROSS the injectors. Port injectors must deliver fuel into a changing vacuum. Higher vacuum would deliver more fuel if not for the reduction in fuel pressure provided by the regulator.
There are plenty of other reasons for less than perfect fuel mileage calculations.
 

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'87 SDV
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1,296 Posts
You have a point ApeMan, but remember the computer is not measuring any fluctuation in pressure, it's relying a pre-programmed constant assumption.

I had a conversation recently with a retired GMI instructor who told me essentially the same thing. He added however that the FDC's psi assumption wasn't just pulled from the air. It was a mathematical compromise based on the idea that most Cadillacs (in the C-body era) are driven on a highway/city ratio of 65/35. The high-low variance of the FDC estimates almost flatline at that ratio, but skew considerably with less or more of either type of driving.

BTW - nice pull awadecki - this stuff is meaningless to 99.9% of society, but it's like gold to us. Does that mean we're weird?
 

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76 Eldo (54k), 76 Eldo (52k), 78 Eldo (9k), 85 Deville (43k)
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Discussion Starter #6
I agree with ApeMan's point--but I still believe the constant pressure assumption has something to do with it.

Having owned two 4.9-equipped cars (a 91 Deville and a 92 STS), and one 4.1 engine car (an 85 Deville), it's amazing to me that my 85 fuel data center is almost always dead on with respect to mileage and fuel consumption.

My 92 STS is not nearly as close. At times, I've filled up and achieved over 29 mpg when the computer said I only got 24...that was a great surprise! I've also noticed that, in town, and with the A/C on, the 4.9 just drinks gas, but on the freeway, it's a fuel mizer.

I don't drive very fast anymore...3 tickets when I was 16 cured that fast. It also helped that my next car was an 86 Ciera with all of 92hp, so I stopped drag racing, and now just drive leisurely. I usually putt along at about 65-70 mph on the freeway.

Here's my mileage summary:

85 Deville:

15-16 mpg: 100% city
25-26.5 mpg: 100% freeway, 65-70 mph

92 STS:

13-15 mpg: 100% city
25-30 mpg: 100% freeway (I've achieved 27+ many times)
 

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1980 FBC
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3,215 Posts
You have a point ApeMan, but remember the computer is not measuring any fluctuation in pressure, it's relying a pre-programmed constant assumption.

If the regulator works properly then there will be a constant pressure drop ACROSS each injector. That is the whole idea behind a mainfold modulated regulator. The compuker does not have to measure any fluxuation.
Now if you want to tell me that the mileage computation is inaccurate due to variance of pressure drop due to limitations of the regulator, I will buy that.
Maybe that's what the guy meant in the first place.
 

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1992 Town Car Cartier & 2014 Accord LX MTX
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Cool! I've always wondered why it's off by about 2.5 mpg on average.
 

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2008 SRX
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403 Posts
I hate those stupid-ass computers! Are you guys the one's that would come in years ago under warranty, with a three page tabulation of mileage obtained and calculated error of the FDC, and demand that the silly frick'n thing be fixed?!?!? Some of you bastards (LOL) would have pages of how many gallons it took to fill up documenting that the gauge was off by a gallon. It was always an old white haired guy with nothing better to do with his time.
 

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'87 SDV
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1,296 Posts
I hate those stupid-ass computers! Are you guys the one's that would come in years ago under warranty, with a three page tabulation of mileage obtained and calculated error of the FDC, and demand that the silly frick'n thing be fixed?!?!? Some of you bastards (LOL) would have pages of how many gallons it took to fill up documenting that the gauge was off by a gallon. It was always an old white haired guy with nothing better to do with his time.
Back in the 70s I was a parts man at a FLM dealer. At the time Lincoln made a Mark series called the "Cartier" which had a huge analog clock on the dash. We had a customer (about 80, retired, rich, bored) who brought his Cartier Mark in once a week to have a tech reset the clock so it matched his Cartier wrist watch!
 

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1992 Town Car Cartier & 2014 Accord LX MTX
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Are you guys the one's that would come in years ago under warranty, with a three page tabulation of mileage obtained and calculated error of the FDC, and demand that the silly frick'n thing be fixed?!?!?

Yes. Those are one of my favorite weekend activities.
 

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2008 SRX
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403 Posts
I~LUV~Caddys8792, c'mon you're way too young too give a crap about that stuff. You know, I can see that old guy with the Cartier clock thing every week.

Back to the point, I've noticed the '92 Seville is reading 22.4mpg avg. It always read 20.0 +/- .2 Recently I put a new O2 sensor in because of an S.E.S. light codes 13? and 46. I'll bet that raised my mileage. Interesting thread though.
 

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1992 Town Car Cartier & 2014 Accord LX MTX
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I~LUV~Caddys8792, c'mon you're way too young too give a crap about that stuff. You know, I can see that old guy with the Cartier clock thing every week.
Oh, lol, I was kidding! I need to make that more obvious! lol
 
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