higher temps, lower mpg over long trip - PeachParts Mercedes-Benz Forum

Jack None · #1 08-12-2013, 08:55 PM

She usually runs around 90 C or so, highway speeds, 85 F or so outside. She usually gets around 25 mpg.

I set out on a long trip (10+ hour days), towing a trailer (1300 lbs gross but getting lighter as I go). First day I was getting around 22 mpg, normal operating temps.

On the third day, I noticed she was down to around 18 mpg, and the temp had creeped up to 100 C. It wasn't particularly hot out, and this was on the flat, going 70 mph. No head wind. Trailer probably about 500 lbs gross.

I notice no other symptoms. Driving fine, exhaust looks normal, I haven't found a fuel leak (checking at idle), no hot hubs or axles. The age-old motor oil leak may have sped up a bit. On the third day I had to add about a quart. I don't usually have to add anything between changes. The water pump may be starting to weep a bit.

I'm not sure if this is relevant, but on the second day, I stopped by a friend's shop, he noticed the air horn seals were dead (the horn connecting air filter housing to turbo), we replaced.

Theories?

EDBSO · #2 08-12-2013, 09:06 PM

Could have under filled it one day and "burped" it the next and overfilled it.

I keep track of my mpgs and even though I do a consistent fill procedure they are al over the place.

mach4 · #3 08-12-2013, 09:24 PM

There are many significant factors that determine the mileage you'll get, especially on a long trip. Heading east out of LA you'll climb up over the continental divide which could be anywhere from about 5k ft to 12k ft. depending on route. Also prevailing winds are west to east which makes a huge difference.

Here's the altitude profile from a recent road trip in my 380SL diesel.

Just to highlight the extremes, on the outbound leg from SAN->PHX I got 33.8 and on the return leg I got 24.2. The difference was that outbound I literally made it to PHX without so much as tapping the brakes and almost no wind (a perfect run you might say), while the return trip was almost entirely into a 30-40 mph wind directly on the nose and the temps were in the triple digits (a most imperfect run). Uphill from PHX -> ABQ netted 30.6 while downhill on the return trip over the same track netted 36.4.

I wouldn't worry about it all that much, just be aware that conditions can have a profound effect and skew things drastically... especially towing a trailer.

Govert · #4 08-13-2013, 06:01 AM

High temperature indicates a bad cooling system, which would not necessarely increasing fuel consumption. It could increase fuel consumption if the water pump is dragging. The high temperature could also be caused by burning more diesel, typically if you are running at (nearly) full load, but I guess you should have noticed that (accelerator pedal must be fully depressed). The brakes of the trailer weren't stuck?

Fuel consumption is best measured over several tank stops.

tjts1 · #5 08-13-2013, 11:10 AM

Colder air is denser air which means leaner charge so you're burning more of the fuel being injected into the engine.

Jack None · #6 08-13-2013, 01:12 PM

Thanks for the ideas.

I'm pretty sure my mpg estimates aren't too far off, and that something has changed with the car itself.

Here are the things I'm planning to try, which indicate my theories. I'm not convinced any are correct.

1. Change air filter, check filter housing hookups.

2. Add some trans-x. I did not notice tranny slippage while driving, and I think I would have. However, it would seem logical that extra heat in the tranny could reduce mpg and cause the heating I'm seeing. Also, I am very sorry to admit that I did not get around to adding a secondary tranny cooler before I left.

3. Do a diesel purge. However, I can't imagine injector / combustion issues wouldn't be immediately apparent in the exhaust smoke. I am running on WVO. But I've been running on WVO for 30k miles without trouble. There's no trouble starting, no trouble idling. No loss of power.

After this, I'm out of ideas...

Doktor Bert · #7 08-13-2013, 01:27 PM

I have been driving my 300SD for over 15 years and it always gets better mpg the cooler it runs....

cooljjay · #8 08-13-2013, 03:06 PM

Regardless of what people say, wvo will eventually lessen the life of an engine by a significant amount....they car already had a lot of miles before the wvo was introduced....wvo produces a lot more carbon deposits then diesel does....eventually the rings will start sticking and wvo will start leaking past and mixing with the crank case oil....then you start getting gunk building up in the engine....the temperature of the oil doesn't get hot enough to burn off the wvo, so more periodic oil changes are necessary....plus it is normal for an injection pump to slightly leak diesel into the oil....so wvo can enter in via the injection.....yeh I will get flamed again....no surprise there, some believe wvo is gods gift to diesel engines....and I believe its only good for fish frys

Jack None · #9 08-13-2013, 03:18 PM

Yes, the WVO is an experiment. So far lots of fun, but admittedly not for everyone. Maybe I'll get up to 100k on WVO, maybe not.

As I said I think it's a red herring but I can't rule it out. I'm going to go for a drive without the empty trailer and see how she does. I'm wondering if an empty, bouncy trailer might be a bigger strain than I assumed.

Doktor Bert · #10 08-13-2013, 04:42 PM

Quote:

Originally Posted by Jack None

Yes, the WVO is an experiment. So far lots of fun, but admittedly not for everyone. Maybe I'll get up to 100k on WVO, maybe not.

As I said I think it's a red herring but I can't rule it out. I'm going to go for a drive without the empty trailer and see how she does. I'm wondering if an empty, bouncy trailer might be a bigger strain than I assumed.

I've ran thousands of miles on VVO with no effects...not sure about WVO, but if it's clean, I don't think there would be any mechanical detriment.

vstech · #11 08-13-2013, 05:04 PM

is it possible the turbo was pulling dirty air into the motor, severely wearing into the cylinders? you did say the seals were bad...

Jack None · #12 08-14-2013, 10:07 AM

Maybe, but wouldn't that cause blue smoke? The exhaust seems normal to me.

Maybe there are two separate problems. If a water pump needs replacing, can it fail gradually like this and cause slightly higher running temps (i.e. 102 C)? Or does it go all at once?

Jack None · #13 09-05-2013, 09:05 PM

I put in some injector cleaner and changed the air filter. Drove around town for a few days. Then back on the road. 500 miles with the same problem: higher temp, lower mpg.

Then it switched back to normal, running temp and mpg increased by about 3. I noticed no other change in environment or in the car. It just "fixed" itself on its own.

I doubt it was the injector cleaner or the air filter, though I can't rule them out. Anyone have an idea?

kerry · #14 09-05-2013, 09:22 PM

Sticking caliper? Test with an non-contact thermometer.

Jack None · #15 09-06-2013, 01:36 PM

I checked with my "hand thermometer" (by putting my hand on the hubs and near the rotors) after driving, during the low mpg state, and it felt normal. This was my first guess as well, as I did just replace the front calipers with remanned. If it goes back into the bad state I will check this more carefully.

OK, this will reveal my complete lack of understanding of these systems, but here are three guesses.

1. Could it have anything to do with the ALDA? Maybe I will follow Diesel Giant's cleaning procedure. Also, I know the vacuum system is funky. Car turns off fine, but door locks don't work and shifting is clunky.

2. Or the turbo? Is there a way I can test that the engine is getting enough air?

3. Could it be timing? I've never checked / adjust the timing. I could imagine something with the ALDA being intermittent, but not something with the timing.

In general, I don't understand a system until I've had to work on it.