Fuel economy plummeting, no loss of performance

any possibility you have a gas leak somewhere between the tank and the engine? Or maybe even a leak in the tank itself? That would cause lower MPG without affecting the actual running of the engine. Although I'd think you would be alerted to this by the smell of raw gas when the car is parked.... Still, might be worth checking, since there are many connections between the tank and the engine, and lots of opportunity in an older car for leaks to develop....

Or someone regularly stealing fuel from the tank, this is becomming more and more common around here with the rising prices.

I've checked for leaks, but now that you mention it the fuel gauge has been playing up. When i fill the tank it shows almost full, but not quite. It used to go quite a large amount above the full mark. I think this is just due to a faulty sensor in the tank, because the gauge has not been very accurate for quite some time. This said, i've been going purely on what the pump says at the servo, so even if the gauge was off by a mile it wouldn't affect my measurements. My brother suggested the odometer itself is out, is this possible/ likely? I don't think it is people stealing fuel, around here it does happen, but 10-20 liters at a time. Even my crappy gauge would notice something this dramatic. Is it possible the vapour catching system is simply letting my petrol evaporate? I can't smell it, but it still seems plausible. The lines going to and from the charcoal canister seem a little old. (it is the cylinder under the front left bumper yes?). I haven't bothered working on this because the car is going to be converted to gas in 2 weeks anyway. I was hoping to get all the little problems weeded out before then though. Thanks for the suggestions, i'll look more closely at the fuel lines today

A clogged air filter can cause an engine to run rich.

Gary

Before my odometer (gear) gave up completely, it worked fine... up to ~70 mph. At 70 and above it stopped ticking off the miles. Took me a while to figure out what was happening as my eyes are everywhere else but the odometer when running at highway speed. I knew something was up when my friends house (a 450 mile run) showed less than 400 on the clock. Very possible it is giving a false accounting. ('83 245)

With EFI? I know with a carburetor this will happen but EFI measures the air flowing through the intake.

actually, your suggestion about the odometer being inaccurate is a good possibility... In my '89 240, there's a small plastic gear that ultimately breaks, stopping the odometer completely. I don't know the specifics of yours, but it's certainly possible that it's just not ticking off the miles you are actually driving, and that would give you a reduced MPG calculation without affecting the way the engine runs.....

I checked all the fuel lines today, even on a full tank there were no leaks. At least that means another possibility ruled out...

K-jet injection is based entirely on air flow through the throttle body, clogged filter would just slow the car down.

I've written down my odometer reading on a fresh tank, and i plan to compare it with the trip odometer to see if they are running together. I'll have a look next time i drive at speed to make sure its still ticking. (100km/h is max speed limit, people have trouble driving at 50km/h let alone 70mph down here!...but thats another story).

I forgot to add that my transmission has recently been worked on, mostly replacing seals etc. But that wouldn't affect economy, not that i know of, anyway.

I'll keep you posted with any findings Josh

I'm not sure this will show you anything. They both run off the same gear so they will work, or not, the same amount.

hmm...well scrap that then. They were working together, but I didn't realise they were controlled by the same mechanism. Does the speedo run off this gear as well? That seems to be accurate.

Before i filled up on Tuesday i "un-tuned" the engine and started from scratch, i'll see how it goes during the week. Interestingly i haven't actually changed any parts, but the car seems to be a lot happier driving lately. Maybe i stumbled upon and fixed the problem accidentally? The accelerator pedal only needs to be held about half the distance to keep a steady speed now, hopefully that will result in more economy. Toward the end of the week i'll re-post with the mpg calculation i get.

A long shot, but does the temperature guage behave normally? I had a thermostat stick open about a mm in a 1970 145 and the fuel economy was cut nearly in half when winter came.

Mike

I suppose add to the mix an O2 sensor. I swapped my 3-wire for a $20 USD single wire replacement.

Is that the B230F engine? I have a 1985 245 automatic and I get 24mpg. I thought that was about 4mpg low. I was under the impression I should be doing about 28-30mpg on the highway and 22-24 in the city.

jb

I think he said it's an '83, 20 B23F. '85 was the first year of the B230 and it does a bit better in mileage.

The car doesn't run an O2 sensor at all, its mechanical injection based 100% on air flow. I think i've just about ruled out a bad mix though, because the temp is normal, the plugs don't seem fouled, and the car never struggles when warm. Its a B23E engine actually, which is the Australian model (K-jet injection, 2.4L, 3spd Auto, Bosch breakerless ignition. Acts like a stripped down B23F). I think its supposed to get slightly less fuel economy than a B23F, with a little more power. 18-20mpg i can deal with, 12 is a royal kick in the groin.

Mike; The temp sensor is right as far as i can tell. It sits at 9 o'clock when idling and just under when driving at a steady pace.

By the way, what is the little valve like thing on the top of the engine block. It has a large diameter vacuum hose going through it (almost an inch), and an electrical connector to the ECU. It seems to be measuring vacuum, but what for? It cant be for the injection or timing....maybe its for the EGR or PCV?

I was also wondering, how much economy would i expect to gain if i swapped out the transmission for a 4spd auto, or better yet, is it possible to retro-fit a 5spd manual? Has anyone any experience with the cost/time involved? I seem to waste a lot of energy due to torque converter slip.

Thanks for the involvement! Josh

I think its a BW55 transmission, which according to a couple websites has no torque converter lock up

Yeah 18-20 sounds about right for a B23E. I didn't know those cars ever came with an automatic, if you convert it to a manual you'll be pleasantly surprised, it really lets the B23E come alive, the auto wouldn't let it into the peak of the power band most of the time. I've done it, the conversion can be accomplished in a weekend, easier if you have a parts car on hand.

I did a highway run this morning, which should have showed 70km on my odometer. It actually read somewhere between 50 and 60, which is greater than 10%, which should account for the lower fuel consumption reading. This is just an estimate though, i forgot to check the odometer as i pulled out, but i am 75% sure of my guesstimate. Does anyone know of a good method to accurately test the odometer or how to check the gears in it? My only other option is to wait until a fortnight from now when i have another long trip. Fingers crossed that it is the odometer! Josh

Ok, did an accurate test this morning, the odometer is reading spot on, fuel economy was 6.5L/km over last week. Now i just dont know.....

Josh