Extremely bad mileage?

[JoeBobJr]

The 86 300SDL I just bought I finally got registered a couple days ago and was able to drive it around some. I drove about 45-50 miles at the most and it used over 1/8 of a tank. I divided the size of the tank by 8 and then that amount into the miles I went and got around 10-11 MPG that's insane. I noticed the oil is pretty black but from what I read in the owners manual if the oil is getting bad it tells you with a light.

I know I need to change it and I know that friction from the engine will make it use more fuel but that much more? I would imagine it has to be more then bad oil although I don't know very much about diesels other then what I have read here on the forums.

Does anyone have any ideas? It has plenty of power and runs great shifts fine. At about 65 MPH it runs around 3000 RPMs. There is no check engine light or anything of that sort on. It seems everything is working fine other then the mileage being ****ty.

[barry123400]

Not accurate enough way to check for milage. At least burning through one tank is required in my opinion to get any semi valid indication.

Gas tank dash gauges are not particularily noted for their linearity. You can have a look for heavy fuel leaks fron the injector hoses and underneath. It is just too premature in my opinion to try to get an estimate.

[pelon]

that doesn't seem to be a very accurate way to measure milage.
fill the tank, zero the trip odometer, then check it at the next fillup.
fuel gauges are not accurate enough for what your doing.
just my opinion...

[DieselPaul]

I wouldn't stake your engine on a 24 year old "idiot light" I would change the oil as the new owner and go from there, so from now on you'll know where 3k miles is.

I kick off my ownership of cars with a set of filters and a tune up. Do the canister and inline fuel filters. Change the air filter, and start using some cetane booster/injector cleaning additive.

[JoeBobJr]

Originally Posted by DieselPaul

I wouldn't stake your engine on a 24 year old "idiot light" I would change the oil as the new owner and go from there, so from now on you'll know where 3k miles is.

I kick off my ownership of cars with a set of filters and a tune up. Do the canister and inline fuel filters. Change the air filter, and start using some cetane booster/injector cleaning additive.

I plan on doing an oil change the second I can afford it. I have already changed the air filter out. The clear filter looks almost new. I want to change the big screw on filter soon too. I do realize that doing the mileage like I was doing it's all that accurate but it's not all that bad doing it like that either. My odometer doesn't work and I haven't pulled the cluster to find out yet. I have no other way to calculating it other then estimating. Still yet doing it like I'm doing I wouldn't imagine that I could get off by 1/3 of the actually mileage this car is suppose to be getting.

I haven't looked under the car yet but as far as under the hood there are no fuel leaks at all.

[JohnM.]

Don't have a working odometer either? ¡Ay, caramba!

The fuel gauge is supposed to be linear which means that it moves at the same rate from full tank to empty. But more often than not your fuel gauge can be quick for the first half of the tank and then super slow for the next half, etc. So your calculations are pretty meaningless. No odom plus an unknown fuel gauge (to you) is pretty caveman-esque. About as accurate as dropping a ball of yarn behind the car to measure distance traveled and knocking on the gas tank to hear how much gas is left....well maybe not that bad, but you get the idea.

[bustedbenz]

1. The only way to measure fuel mileage with any accuracy is to pretend the gauge does not exist. Go to a diesel pump on level ground and fill it right up to the brim. Let the foam go down, keep filling until there is literally liquid fuel right at the rim of the tank filler. Where you can see it, and know where that spot is.

Run the car until you get nervous, preferably down close to empty. Refuel at that same pump on that same level ground clean up to the neck again. Then divide the number of miles driven on that tank (write down odometer readings) by the number of gallons you put in the tank.

Repeat at least 3-5x. Then average your mileage. The gauge is inherently useless. I routinely drive 125 miles past when my gauge bottoms out on R.



2. Oil color is a useless measurement on a diesel. There is no "oil getting bad" feature on this car. Only a low oil level indicator light, which is no indication at all of oil change interval. The interval on this car is 5 thousand miles, not 3. Change it at a known time, write it down, repeat every 5k if you use "standard" oil, perhaps 7500 if you use synthetic.

EDIT: I realize now that the odometer isn't working. You'll need it working before any fuel measurement is successful.

[JoeBobJr]

Originally Posted by bustedbenz

1. The only way to measure fuel mileage with any accuracy is to pretend the gauge does not exist. Go to a diesel pump on level ground and fill it right up to the brim. Let the foam go down, keep filling until there is literally liquid fuel right at the rim of the tank filler. Where you can see it, and know where that spot is.

Run the car until you get nervous, preferably down close to empty. Refuel at that same pump on that same level ground clean up to the neck again. Then divide the number of miles driven on that tank (write down odometer readings) by the number of gallons you put in the tank.

Repeat at least 3-5x. Then average your mileage. The gauge is inherently useless. I routinely drive 125 miles past when my gauge bottoms out on R.



2. Oil color is a useless measurement on a diesel. There is no "oil getting bad" feature on this car. Only a low oil level indicator light, which is no indication at all of oil change interval. The interval on this car is 5 thousand miles, not 3. Change it at a known time, write it down, repeat every 5k if you use "standard" oil, perhaps 7500 if you use synthetic.

EDIT: I realize now that the odometer isn't working. You'll need it working before any fuel measurement is successful.

You are definitely right I was mistaken and I feel stupid for saying what I did about the oil light. I was thinking it was like some of the really new cars that actually tell you when it needs to be changed. This car only tells when it's low so that is useless to me. Thank you for pointing that out. Still yet the oil is pretty black so obviously it's broken down and it needs to be changed. All I'm saying is with a 26 or so gallon tank and 25 mpg I should be able to drive 45-50 miles and no matter WHERE the needle is on the gauge it shouldn't hardly move. That's a BIG tank. My 91 Mitsubishi Mirage has a 13 gallon fuel tank and I can drive that distance and the gauge hardly even moves and that's half the size of the tank on this car. You can't honestly tell me that driving 45-50 miles on an 1/8 of a tank there isn't something wrong. I've driven so many different types of vehicles and I have always been pretty damn good at guessing the mileage with such measures I am using. Not to mention every vehicle I have ever owned the needle has gone slower on the top half of the tank then it has on the bottom half. My needle was almost on the 3/4 mark when I started the drive and ended 1/8 of a tank lower then that when I was finished. Then I drove another 40 miles and it was on half a tank. So 1/4 of the top half of the tank and only gone 80-90 miles at the most? So that means if I drive another 100 miles my car will obviously be on empty. Is that really that hard to figure out?

I'm not trying to argue with anyone either I'm just saying just because you believe there is no way anything can be told by doing it this way doesn't mean that it can't be done.

All I'm saying is the mileage is extremely bad. If I can watch the needle move while I'm driving something isn't right.

[bustedbenz]

Originally Posted by JoeBobJr

You are definitely right I was mistaken and I feel stupid for saying what I did about the oil light. I was thinking it was like some of the really new cars that actually tell you when it needs to be changed. This car only tells when it's low so that is useless to me. Thank you for pointing that out. Still yet the oil is pretty black so obviously it's broken down and it needs to be changed.

No need to feel stupid on that one, it's not always obvious what features they've installed or haven't just by the pic on the dashboard. That said though, something you might not be familiar with on this particular engine is that there is no possible way to completely change the oil. There's always something like half a quart of oil that is retained in the oil cooler and the lines going to and from it. It doesn't drain back to the pan, and it is more trouble than it is worth by miles and miles to pull the lines off to drain the thing. And, just like a few drops of food coloring in a huge volume of water, that oil is enough to turn a fresh clean oil change black within 300 miles. Every time I change mine, it's black within a week. It's not an indication of oil quality on diesels the way it is on gas engines. They produce so much soot that the oil goes black long before it has "broken down".

That being said, there's certainly no harm in doing a feel-good oil change. Ever. It's always something the engine appreciates. But I'm just saying you have to go by mileage on these engines because there is *no* reliable way to tell visually.

Originally Posted by JoeBobJr

All I'm saying is with a 26 or so gallon tank and 25 mpg I should be able to drive 45-50 miles and no matter WHERE the needle is on the gauge it shouldn't hardly move. That's a BIG tank. My 91 Mitsubishi Mirage has a 13 gallon fuel tank and I can drive that distance and the gauge hardly even moves and that's half the size of the tank on this car. You can't honestly tell me that driving 45-50 miles on an 1/8 of a tank there isn't something wrong.

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So that means if I drive another 100 miles my car will obviously be on empty. Is that really that hard to figure out?

You may well be right. But in my own humble opinion, unless fuel is dripping to the ground constantly, there is no physical way possible for this engine, no matter *what* malfunction, to go through fuel that fast. It is literally impossible to put that much fuel through the six cylinders that are under this particular hood, under *any* circumstances. So, I choose to believe it's probably a faulty gauge rather than a miracle-engine.

I would make this suggestion. Get a long stick like a tomato stake or a broom handle, preferably one that is clean. Broom handle's actually a good idea; it can be wiped off with a paper towel beforehand and probably not have any dirt remaining on it. Stick it in the tank to use as a level checker. I could be wrong, as you've pointed out. But I seriously suspect that if you dip your tank when it's full with the broom handle, and then drive your 1/8 of a tank in 50 miles, and dip it again, you will NOT see a 1/8 tank change on the broom handle. The tank, so you know what you're dipping, is large, virtually rectangular, and goes from side to side across the back between the back seats and the trunk. Insert it carefully so you don't bash the strainer or the fuel sending unit or anything. It's not a muzzle loader; but you shouldn't damage anything as long as you do this carefully.

Like all electrical devices, a small error can have large results. I'm believing this to be the case with your fuel sender. It's a fairly simple resistance device; the resistance increases and decreases with the fuel level (can't remember if it's linear or inverse but it's one or the other) -- and so any simple electrical problem... short... dirty contacts... any of the thousand factors that could affect electrical resistance could be acting up in your gauge. I can very very easily imagine that your fuel gauge is malfunctioning in such a way that it... it assumes the tank is much much smaller than it actually is. I'm saying it's possible, presumably, for the float on the sending unit to be at 1/2 tank and report empty to the gauge. That's why you need a visual confirmation with the stick or something similar.

My $0.02. You may be right. But I consider 1/8 of 25 gallons to be more fuel than ANY 300SDL could burn in 50 miles, even if you had your foot on the floor up an equivalent hill to Mt. Everest, burning used motor oil and transmission fluid instead of good clean diesel, with an elephant buckled into each remaining seat belt, towing a filled swimming pool. I'm not trying to argue either, but that's how ridiculous that fuel consumption figure sounds based on the known characteristics of this engine.

For the record, I always use my odometer instead of the fuel gauge, and I refill somewhere between 525 and 560 miles, for safety. According to what I can put in at that time, there's still usually a couple gallons left, but I've run out once so once bitten, twice shy.

[JoeBobJr]

I guess if my gauge is really screwed up like bustedbenz's gauge it might be possible. I dunno it sure seems like something major is wrong. I know gauges can't be trusted sometimes but damn! lol

The bad thing is I don't want to run it till it runs out of fuel to find out how bad my gauge is. I've never owned a diesel but I've used diesel welding machines and equipment and know that it's a bastard to run a diesel dry and trying to get it primed again.

[filon102]

the oil light eh? Its the same with the bmw e39 models... 15k mile oil changes... ridiculous

[vstech]

IF it is actually going through that much fuel, then I'd look for a puddle!
look the engine over for fuel dumping on the side. the cloth return hoses are known for failing and really going through fuel. aside from that, I'd figure error on the gauge/your ability to fill.
these cars are really hard to fill with fuel. especially if the pump is a fast one. the fuel FOAMS really thick, and you have to barely trickle the fuel in to get it full. I can easily see 1/8 to 1/4 tank disappear if all you did was fill it with foam.

[barry123400]

At 10-11 miles per gallon and no fuel leaks. I would expect the exhaust to be a little black.

With no way to gather good evidence of the true fuel milage at this time about the only things that might pull it down really bad I can think of. Dragging brake calipers . I read the temperature of the rotors after a run with a laser type thermometer. The other thing that comes to mind might be a really poor set of fuel injectors. I would spend the effort to determine the actual milage first though.

[vstech]

with really poor injectors, you'd have fuel in the oil and the level would be very high.

[bustedbenz]

Originally Posted by vstech

with really poor injectors, you'd have fuel in the oil and the level would be very high.

I don't want to derail someone's important thread, but this intrigues me and I've never heard it before. Is this a one-post explanation, or is it something I need to search for because it would take a week to explain?