Poor diesel MPG - 1999 E300 TD, W210 turbodiesel

Hello,
I recently noticed that my MPG have dropped off by almost 5-7 MPG. No light on the dash. No loss of performance (at least none that I can notice). Recently changed all the filters. There is some VERY minor
leak near return air manifold (EGR stff?) that loops back on the driver side. Will appreciate any pointers/suggestions. I did change the brand of diesel as well. Thanks in advance.
-NK
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Check tires' pressure.
Return to using previous brand of diesel fuel, otherwise it's not a valid MPG comparison.
Only other obvious possibility is a fuel system leak - check filter connections.
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TGL,
Thanks for the reply.
1. TP - OK 2. Tried the old 'brand' fuel - no change 3. No fuel leaks anywhere
I guess, a trip to the dealer is in order :-(
-NK
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"3. No fuel leaks anywhere "
Have you looked carefully? On my 1980 300SD, I noticed the fuel economy had dropped from 26 to 21. On this car, they use an OEM crappy fabric covered hose on the fuel injector return lines that starts to weep after a few years. I had replaced all of them not long ago, except the last little stub, because that one was a capped off version, it wasn't leaking and I didn't want to try to figure out how to cap it off. Turns out, that was the culprit. It was leaking just a bit, but there were no drops on the garage floor. The only hint was an occasional whiff of diesel smell. It must have been evaporating off while the car was being driven.
Fixed that and the MPG instantly improved. So, I would try to take a real careful look around all fuel lines right after the car has been driven, as even a small leak that is not obvious can make this kind of difference.
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snipped-for-privacy@optonline.net wrote:

It doesn't evaporate and if it leaked even a drop every few minutes there would be wet fuel evidence somewhere and it would be obvious.
Fixed that and the MPG instantly improved. So, I would try to take a

It was more likely to be suching air into the fuel system and that this was somehow detrimental to economy.
Huw
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"It doesn't evaporate and if it leaked even a drop every few minutes there would be wet fuel evidence somewhere and it would be obvious. "
Oh, diesel fuel on a hot engine block doesn't evaporate? Are you sure about that? Want to recheck the laws of physics?
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snipped-for-privacy@optonline.net wrote:

I have over twenty diesel engines currently and have owned well over a hundred over 35 years. I know a diesel fuel leak when I see it and seeing it is easy. Finding where exactly it comes from and stopping it is another matter sometimes, especially on a black painted engine.
Huw
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"I have over twenty diesel engines currently and have owned well over a
hundred over 35 years. I know a diesel fuel leak when I see it and seeing it is easy. Finding where exactly it comes from and stopping it is another
matter sometimes, especially on a black painted engine.
Huw "
Then you should know that diesel fuel will evaporate off a hot engine, and small leaks can be occur without seeing a puddle on the garage floor or in the driveway. In my case, the injector return lines are well known for weeping small amounts, which never makes it to the floor. Which is why I suggested the OP take a good look.
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snipped-for-privacy@optonline.net wrote:

I haven't seen a diesel leak yet that does not remain wet on an engine. It only evaporates significantly off exhaust manifolds [headers] and certainly not off relitively cool injector return pipes. It is a relitively stable and oily fuel, not prone to evaporate easily, though it will eventually given enough heat and surface are in relation to volume. Anyone who knows diesel engines knows I am correct. Just because a leak does not make it to the floor does not mean it is not wet at the point of leaking. In essence if it is not wet at the point of leaking then there is no leak. However, I have known several old rubber return lines leak air *into* the fuel system and likewise on the suction side from the tank. Both air leaks might manifest themselves as either surging or loss of power.
Huw
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"Anyone who knows diesel engines knows I am correct. Just because a leak does not make it to the floor does not mean it is not wet at the point of leaking. "
I agree with that. What I was trying to point out to the OP, was that he needs to look carefully for leaks. One would think that a leak that could cause 5MPG difference in fuel economy would be easy to spot. I would have thought it would leave obvious spots on the garage floor, for example. In my case it did not. The last little 2" stub of the daisy chained fabric covered fuel return line on the injectors was the culprit. You're right, it was wet in the area immediate area on the engine. I even knew it was slightly wet. However these hoses are known to weep and you would think it would amount to much at all. I was gonna fix it when I had some time, but never thought it would lower my MPG by 5MPG. I had measured the MPG many times and it was stuck at about 20-21. Soon as I replaced that little stub hose, it went back to 25-26.
You think that was caused by air being sucked in, not fuel leaking out. But I don;'t think so. One day when I was having the car parked in the rain, I left it running for 5 mins until the attendant drove it away. When he did, I could see a pretty large oil sheen on the wet pavement. That return line is only pressurized when the car is running. So, what was happening was the leak was only active while the car was driven. Put it in the garage and shut it off and not even enough fuel made it to the ground to leave a mark. I think some of it likely evaporated off the engine before it made it to the ground. Even looking at it with the engine idling, there was no evidence that there was a steady stream coming out, it just appeared slightly wet. Yet, going down the road with the engine at speed, it was likely leaking a lot more than I ever thought.
So, my main point was, you need to look carefully for leaks, because there may not be as large amount of fluid around as you might expect.
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