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  #1  
Old 08-12-2002, 01:44 PM
JHZR2's Avatar
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Join Date: Jan 2002
Location: New Jersey
Posts: 5,265
Poor mileage help

Hi,

My 83 300D has been slowly getting worse and worse gas mileage. I used to easily get 29+ on long trips and 27 if I was more in town. I havce been driving about 160 miles per day lately, and despiute lots of long-term cruising at ~70 mph, my mileage decreased to 26/27, and now it is down to 25. when it consistently dropped to 26/27, I changed the air filter, which was quite dirty. It didnt help, so just last week I got a valve adjustment done. but, after driving 1000 miles this past week up to Canada and back, I have been getting consistently 25. I did add some extra freon, so there are no more bubbles going through the receiver/drier, but even when it wasnt running at 100%, I would always drive with the AC on anyway, so my usage hasnt changed, and I would assume compressor load on the engine is the same regardless of freon level.

My car no longer drips oil or seeps out diesel fuel, except at the fille neck area, which I notice is sometimes a little damp, and definitely seeps out a tiny portion of the diesel fuel when I fill up. I dont know if this rubber deterioration is partially a culprit, but any advice on how to fix the rubber neck at the filler would be appreciated.

Any suggestions where to start now? I change my oil every 3000 miles with a diesel specific 15w40 oil, have a new air filter, oil cooler and lines, and also replaced one of the fuel hoses to the injector pump area. I use some fuel additive at most every fillup, and do not see somke or any mess out of the tailpipe, except when I floor it at night with a car right behnd me and I can see a cloud of exhaust. The rest of the time it runs fine, always starts right up with no problems, and seems to have great compression (because it starts easily, etc). I know some people say that 26 is about what the car was originally rated, and thats fine, but considering that I used to be able to get over 30 (even got 33 with the car packed 400lbs overweight, driving down I95 to FL), I suspect a problem.

Any help is most appreciated.
Thanks

JMH

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #2  
Old 08-12-2002, 07:20 PM
Old Deis
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Could be a few things. You might check the timing chain for excessive stretch and it seems to me that the injectors can wear, or get plugged up with accumulated stuff to the point where they need to be rebuilt/replaced. The injector service I took mine to said they seldom last more that 150k. I am not sure about that, but when I had them rebuilt the mileage and performance took a leap ahead, They were well worn though.
Timing chains can wreck your timing. Do a search and find the various ways to check the stretch there. Common enough problem.
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  #3  
Old 08-12-2002, 07:25 PM
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Join Date: Jul 2001
Location: Woolwich, Maine
Posts: 3,598
JMH,

I don't know if this will help, but I'll give it a shot anyway.

First, to lose about 10% in your mileage to a leak would mean losing nearly a gallon and a half of fuel per fill up. That would not be a little wetness here and there on the filler lines. If it is a leak, it will make a mess pretty quickly and should be pretty conspicuous.

There are some vent and drain lines to a chamber to deal with the foaminess of Diesel fuel that can get gooey if you overfill the tank regularly. The vent line is connected to the passenger side rear corner, under the bumper, vent hole. I have seen these lines collapse and seal off. This could result in poor fuel flow as the tank is subjected to a vacuum, which might simulate a clogged filter to the pump. I am not sure how this would affect fuel delivery and combustion efficiency, but it seems you would notice some reduced performance, as well as possibly reduced economy.

There are a number of items that change over time that can have nothing to do with the engine performance. For one, softer tires make higher rolling resistance and that will eat a bit of energy heating up the tires(solution is to run at the higher pressure listed on the inside of the fuel filler flap - I usually run 32 psi in the front, 36 psi in the back). So will wider tires, which will also affect the air and rolling resistance (adversely). Any changes in ride height can also affect drag, and it is feasible the overloaded experience you had, as long as the car was reasonably level, caused a reduction in drag as the car sat closer to the ground. At higher speeds lower drag can really improve mileage as the wind resistance eats power in cubic relationship to speed (twice the speed will use 8 times the power). New shocks can raise the car a bit, which would add drag.

There are other mechanical things, like dragging brakes, the center driveshaft support bearing failing, and so on, that could all conspire to add mechanical resistance. Once again, I would expect this to show up as some other performance problem before it ate 10% of your mileage. Like noise, or conspicuous heating.

In the end, it might be a little of a number of things. I know how this can get under your skin though, so keep us informed when you find out what it was. Good luck, Jim
__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
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  #4  
Old 08-13-2002, 10:00 AM
j shepardson
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It could also possibly be the quality of the diesel fuel.
I know in my car depending on where I get fuel and also the time of year reflects how many miles I can get for example:

Texaco diesel gives me about 298 miles to the tank{expensive cheap quality watered down fuel =) } , Citgo 330-345 m.p.t. (more in the winter)
exxon about 219-225 m.p.t., haven't tried Sunoco yet bet I get the best gasoline milage from them.

Other than that possibly your system needs a good purge. that would be the cheapest route to go and it would never hurt to give it a shot.

well good luck hope everything goes back to normal

Jeff
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  #5  
Old 08-13-2002, 06:00 PM
rebootit
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I would love 25+ MPG. I get about 22.5 at 75-85 mph highway, 21 mpg, city.No matter what I do.

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