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Old 06-18-2001, 10:21 AM
Registered User
Join Date: May 2001
Posts: 301

My 1991 420SEL is running strong and smooth but fuel econ is poor compared to what others report for their 420. On a recent 240 mile all highway trip, only 15.1 overall average is 12.2

air filter is clean,
no smoke at anytime,
plug wires are good,
cap and rotor are original

Whay could it be?
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Old 06-18-2001, 10:44 AM
Posts: n/a
Poor! I have a 560 and I get about 13.5 city, 18 hwy. Your 420 should do 19 or so on hwy, 15 overall.
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Old 06-18-2001, 11:04 AM
Registered User
Join Date: Jul 2000
Location: Sunny Florida
Posts: 199
My 560SEL get about the same as Bigshot's, although I do not do much highway driving. Sounds like you checked the car over pretty well, looking for common causes. Tire pressure ok, and transmission shifting right? Are you in an area with lots of hills, stop and go driving too? A/C on hurts too...This could be some of it......But the cars are not econo boxes, all models are luxury cars, not much thought given to conserving fuel. Sorry, not much help here.
1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)
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Old 09-04-2001, 11:15 PM
dpetryk's Avatar
Electrons can do anything
Join Date: Sep 2001
Location: Houston, Texas
Posts: 1,071
Check your oxygen sensor. I had the exact same problems and it was the problem.
I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k
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