Poor Fuel economy --> please help troubleshoot - Page 2 - PeachParts Mercedes-Benz Forum

yemington · #16 04-26-2011, 11:42 AM

Quote:

Originally Posted by Randy

correct tire size?

good question. original alloy wheels. just put Pirelli P4 195 70R14 tires. is that correct size?

Orv · #17 04-26-2011, 12:43 PM

The manual for my '81 says 195/70R14. I don't know if '84 got anything different. The previous owner of my car put on 205s, and they do fit, but I don't think the wider tires do the suspension geometry or fuel economy any good, so I'm going back to the 195s.

yemington · #18 04-26-2011, 02:29 PM

Thanks for all of the replies and advice.

Had the car on the lift over lunch: all of the wheels spin freely. And, on a flat surface the car does indeed roll forward. So, it would appear the brakes are of the list of potential culprits.

Also, over 10 miles the odometer was off by less than 2/10ths. And, the speed is just a mile per hour low (both measured vs. GPS). So, the Odometer also escapes the suspects list.

Fuel delivery seems to be the most likely culprit. I haven't yet tried any additive or cleaner. I haven't checked the fuel filter or the screen in the tank. Will do so this weekend. FYI, The car doesn't smoke unless you really give it the boot. Even then it is hard to see smoke (nothing at all like my 7.3L chipped powerstroke).

I have a lot of experience with mechanical injection systems from Spica (rumor has it the Italians stole the design from a Bosch diesel pump). In the Spica system there is an electric pump to get the gas from the tank to the injection pump. If this were the Alfa I would check the flow of fuel at the mechanical pump to see if I had a filter or screen problem. Does anyone know the expected fuel flow rate in to the fuel injection system?

jt20 · #19 04-26-2011, 02:42 PM

Quote:

Originally Posted by yemington

Fuel delivery seems to be the most likely culprit. .. FYI, The car doesn't smoke unless you really give it the boot. Even then it is hard to see smoke ...

I have a lot of experience with mechanical injection systems from Spica (rumor has it the Italians stole the design from a Bosch diesel pump).

Why would you suspect fuel delivery if there is no smoke? If you are getting bad mileage, you would be getting too much fuel, correct?

More likely, you need your head rebuilt if you have already covered the other suggestions to bring the vehicle up to regular maintenance standards. (valve lash, injectors, IP timing... etc..)

Compression test numbers?

yemington · #20 04-26-2011, 02:55 PM

Hmm. I see your point. I was thinking constricted flow of fuel would be like constricted flow of air and would mean I have to run at a higher rpm to get the same performance. But, it sounds a little strange now that you mention it.

Would a well maintained car need a head rebuilt at 164k miles? This thing has all of the stamps and all of the maintenance done by the local MB dealer since new. I need to check to see when the last valve adjustment was done but can't imagine it was very many miles ago.

Have not yet checked IP timing. Or, the injectors. Was going to start with a new fuel filter and work my way toward the pump, injectors, and valves.

Orv · #21 04-26-2011, 03:04 PM

Timing is definitely the next place I'd look. (You already did a valve adjustment, right?) It can be quite critical to good fuel economy on diesels.

Zacharias · #22 04-26-2011, 03:08 PM

I don't think anyone mentioned this before, but does it simply FEEL (seat of the pants, based on how far you're driving) like it's getting 15 mpg?

Before you try anything else, perhaps consider mapping out an established drive route where you know the exact distance, say about 50 to 100 miles, then top off the tank and drive the route and then top off again afterwards.

I really don't believe you are actually only getting 15.

jt20 · #23 04-26-2011, 03:23 PM

Quote:

Originally Posted by yemington

Would a well maintained car need a head rebuilt at 164k miles? This thing has all of the stamps and all of the maintenance done by the local MB dealer since new. I need to check to see when the last valve adjustment was done but can't imagine it was very many miles ago.

Have not yet checked IP timing. Or, the injectors. Was going to start with a new fuel filter and work my way toward the pump, injectors, and valves.

If you can verify that as the actual mileage, it would be unlikely that the head needs a rebuild. But it doesn't take much bad maintenance to loosen the valves up in the guides. Running these engines low on oil takes a toll very quickly.

One of my engines had a documented 195k on it.... all the exhaust valves were ovalized and the stems wiggled in the guides. This destroyed fuel mileage except on the highway.

It sounds like you have a bit of routine maintenance to do before inspecting the pump or the head. I also agree that 15mpg is rather shocking... even for a worn out engine.

whunter · #24 04-26-2011, 03:27 PM

Quote:

Originally Posted by yemington

Greetings,

Recently acquired a 1984 300D (164k miles) with an automatic transmission. Mileage in mixed driving is ~15mpg. Performance seems fine although I don't have a direct recent comparison to make.

I'm set to change the fuel filter and have read that the valve adjustment may improve the fuel consumption. But, I find it hard to imagine those two things will have a drastic effect. I would appreciate some recommendations for what to check in order of impact/importance.

Other thoughts include sticky brakes as the car does not roll when idling in drive--is this strange? At 62mph I am seeing ~2700 RPM.

Is there a thread or forum or resource for mods for improving fuel economy?

Thanks!
yem

Brake issues could account for most of your MPG issue, I suggest you read the following thread.

Brake: Brake:

A valve adjustment will not hurt, and may be desperately needed.

Valve adjustment OM615, 616, 617 FYI.
Valve adjustment OM615, 616, 617 FYI.

I suggest you study/learn the MB diesel fuel system

Diesel Fuel Delivery: Diesel Fuel Delivery:

.

kerry · #25 04-26-2011, 03:38 PM

If the mileage numbers are correct (and I'm suspicious that they aren't), and your troubleshooting results, I'd suspect a fuel leak.

yemington · #26 04-26-2011, 04:02 PM

Thought you all might want a look at the peach you're helping sort out.

Odometer checked out and i have seen the fuel at the top of the filler spout (not foam but fuel) so I am confident in the measurements (two tanks now) and calculations. There isn't a leak at idle. Or, sitting overnight.

I'm going to change the canister fuel filter this weekend and do a valve adjustment as soon as I can find some cheap wrenches (sorry PP but I can't afford retail). Also will check injection pump timing.

Thanks again! Will keep you all posted.

kerry · #27 04-26-2011, 04:08 PM

Personally, I don't see how any of those things could destroy fuel economy that badly. Maybe 10 or 15% but a 40% reduction in fuel economy is not likely to be explained by maladjusted valves, timing or a clogged fuel filter in my view.

whunter · #28 04-26-2011, 05:05 PM

Many brake issues only show up when the vehicle is HOT from driving.

#1. Most common MPG wrecking issue = an inboard brake caliper piston (front/rear) sticking, failing to retract.

#2. Most common MPG wrecking issue is bad brake hoses trapping pressure on a caliper (front/rear), failing to retract fast enough.

Please read the following thread.

Brake: Brake:

yemington · #29 04-26-2011, 05:10 PM

When I had the car up in the air I noticed the front passenger side brake hose was leaking. brakes work and don't pull to the side when stopping and the car doesn't pull coasting or under acceleration, either. that said, i'm not driving around with a leaking brake hose so I ordered 4 new lines from PP this morning.

Thanks.

amosfella · #30 04-26-2011, 05:11 PM

check exhaust. It might not be able to take away the old air fast enough...