My 84 300D was getting between 23 & 26 mpg. Lately it seems to have diminished to about 19-22 mpg. I replaced the fuel filters 6,000 miles ago and replaced the small primary filter 250 miles ago, still no help. What is the likely culprit? Valve job? Turbo/boost checkout?

Anxious to get some ideas and direction to troubleshoot.

Thanks,
John

Greetings John,

I picked up MPG by doing two simple things. Set the valve clearance back to spec, and checking and resetting the start of fuel delivery on the no. 1 cylinder. Was getting 23mpg before, now average 26-27.
Just a thought. If there's black smoke coming from it's behind, I'd check start of fuel delivery. It's also a good idea if you haven't run a can of diesel purge through the injectors, to do so. It will also help improve your spray pattern, hence burn rate increases and so does fuel mileage.

Charles

Thank you... I'm a little leary of the fuel injector pump and the valve adjustments. Never attempted anything like that. How difficult is it? I've got the Hayned manual so I guess I just need to crack the book on it as well as the Stu Ritter/Easley website... I have not run diesel purge yet but will try to do that first. Otherwise, runs great and I really enjoy the vehicle.

Thanks again,
John

John,

That is quite a drop in fuel economy, even with winterized fuel mpg should not drop that much. Are you using the same station and pump park the car in the same position???

Charles has some good advice. I have never attempted to adjust the start of delivery so cannot speak to that, but the valve adjustment is a good idea (recommended every 15K for the 617 diesels). The valve adjustment is not a difficult task just time consuming and you will imporve your technique each time you complete one. I was seeing 25-26mpg all highway at 70-75mph, two weeks ago I adjusted the valves all were tight, and replaced the air filter and now I'm getting 27-28mpg. I have also only replaced the air filter as routine maintenance and seen a 1mpg improvement. Check your tire inflation as well, FWIW I run Michelin MXV4s at 32psi front and back.

Good Luck,

Mark

__________________
1987 300DT 127K
1987 300TDT 283K (Sold)
1999 528iTa 75K
1991 300D 2.5T 214K - Dead Tranny
1984 300DT 375K (Sold)

Great advice, I had forgotten all about tire pressures. I also have seen the trip odometer not working from time to time so I keep a close eye on it now.... there is a rattle coming from the speedo cluster so there is something a little loose in there. The valves have not been adjusted in a while and I have already put 6k plus on it since buying in Feb. 2001. Otherwise, love it, love it, love it. I will read up on the ideas everyone has listed above. Thanks to all.

John

John, you say that you've seen your trip odometer not working from time to time. Can you describe your symptoms? In other words, does it stop working one tenth out of a full mile, or is it for a longer period of time, like a half mile or more? You don't notice the speedo being intermittent, do you? Just wondering, because some of my trips seem shorter than they really are, if that makes any sense.

__________________
Bob Roe
Lehigh Valley PA USA
1973 Olds 88, 1972 MB 280SE, 1978 Datsun 280Z, 1971 Ford T-Bird, 1972 Olds 88, 1983 Nissan Sentra, 1985 Sentra, 1973 230.6, 1990 Acura Integra, 1991 Volvo 940GLE wagon, 1983 300SD, 1984 300SD, 1995 Subaru Legacy L wagon, 2002 Mountaineer, 1991 300TE wagon, 2008 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon, all gone... currently
2007 Honda Odyssey Touring, 2014 E350 4matic

They've both been intermittent... didn't happen until I had put about 5k miles on it after buying it at 170k... and judging from the appearance of it all, the 170k was probably pretty much right on. The rattling under the dash cluster didn't start until the mileage slips started.