Fuel economy

Sat Sep 15, 2012 12:07 pm

Hi all.
I've got a 1989 325is with 225k on the clock and 3.9 diff gears.
Urban fuel economy is currently 350K (215 miles) on 50 litres (11 gallons) of 91 octane fuel = 19.5mpg.
Is this any good? I've been told that 2.5 manuals should give 25 mpg around town. I don't usually drive like a loon...
I've given the car a bit of a birthday - new plugs, refurbished injectors, new O2 sensor, new blue temp sensor, oil and filter change, new fuel pump (longish story), new fuel filter, spray cleaned the AFM and idle control valve. Replaced fuel hoses, and checked for vacuum leaks. I'll check the C191 connector and wiring tomorrow.
I've run a multimeter over the AFM as directed in the Bentley's manual, but I don't seem to be getting linear increases or decreases as expected.
Given that new AFM's are hideously expensive and used ones are 25 odd years old, is there another option? Millers? Leave it alone? Black smoke from exhaust when giving it a rev, and cantankerous until it warms up.
Last question - my car has the 60 litre tank, however I ran out of fuel and when I filled the tank, only 50 liters went in. It is definitely the big tank.
Any ideas?

Sat Sep 15, 2012 12:36 pm

I'd add checking the blue temp sensor for correct operation to your list.

Sat Sep 15, 2012 12:54 pm

He's fitted a new blue temp sensor, so that should be OK, but worth checking it anyway, ideally at the ECU socket (that way the wiring to it gets checked as well). It should read around 3300 ohms cold, dropping to 600 ohms hot.

http://www.e30zone.net/e30zonewiki/inde ... emp_Sensor

The AFM can be repaired by moving the wiper to an unsweeped section of the carbon track.

Sat Sep 15, 2012 12:59 pm

sounds just like mine
the cantankerous bit

as for mpg i have had worse running the kids to school and back on the cold start

a run on the motorway gives me early 30s which seems ok to me

lpg?

Sat Sep 15, 2012 1:10 pm

The 3.91 diff is the least economical of the 325i diffs, while the 3.64 is the most economical at the expense of acceleration.

As an example, at 3000rpm in 5th gear you'll see 68mph with the 3.91, 74mph with the 3.64. So if you're obsessed with frugality, you might want to change that.

However, corroded wiring of the C191 plug under the inlet manifold will also lead to rich running do to faulty blue plug readings reaching the ECU, so check that as Speedtouch and daimlerman recommend.

Sat Sep 15, 2012 1:21 pm

Giddyup58 wrote:Urban fuel economy is currently 350K (215 miles) on 50 litres (11 gallons) of 91 octane fuel = 19.5mpg.
Is this any good?

If most of your urban driving is cold start rush hour queues and the usual stuck at every traffic light and roundabout then you're about right. Driving like a granny on the motorway for a tank full will get you high 20's possibly v.early 30's if engine is properly serviced and running correctly.

Drive with a heavy foot around town and you'll see mid-teens (calm down Pacer...) or less if you're invoking hooligan antics.

Sat Sep 15, 2012 1:37 pm

suchy wrote:
Giddyup58 wrote:Urban fuel economy is currently 350K (215 miles) on 50 litres (11 gallons) of 91 octane fuel = 19.5mpg.
Is this any good?
If most of your urban driving is cold start rush hour queues and the usual stuck at every traffic light and roundabout then you're about right. Driving like a granny on the motorway for a tank full will get you high 20's possibly v.early 30's if engine is properly serviced and running correctly.

Drive with a heavy foot around town and you'll see mid-teens (calm down Pacer...) or less if you're invoking hooligan antics.

i must be a geriatric then

Sat Sep 15, 2012 1:41 pm

Sat Sep 15, 2012 1:58 pm

The AFM can be repaired by moving the wiper to an unsweeped section of the carbon track.[/quote]

Um ..er ..OK .. What?
I checked the workshop manual and it doesn't say anything about "moving the wiper to an unswept section of carbon track".
I apologise for my ignorance, but I have no idea what you mean.
Help?

Sat Sep 15, 2012 2:18 pm

Wow, I wish I was getting that mileage out of my 325i! I would say you are probably ok with what you get at the moment, unless you have time to fiddle around with all AFM etc.

Sat Sep 15, 2012 2:19 pm

If you're finding the resistance readings, or output voltage of the AFM aren't linearly following the flap opening, then you can move the wiper to a different, unswept part of the resistive track:

A Mild chip can also work wonders on the 325i to boost low-down torque and give an extra 2-3 mpg in normal driving.

Sat Sep 15, 2012 3:27 pm

Had a look at the video - Now it all makes sense. I remember my mechanic mate pulled that cover off and ratted around with the lean/rich adjustment. It did idle and drive better, but that was obviously masking a problem with the worn out resistor track. The video says that the setting shouldn't be changed. Oops.
Also, I think the car has an aftermarket chip, as I have pulled 7,000 rpm a couple of times with no sign of an interrupter. Maybe cheap junk that is causing the black smoke...
Forgot to mention that the car has hottuning BTB copy manifolds full twin 2" system and cat delete.
I'll have a play in the morning with the AFM. Thanks for all the advice.
Any ideas on the fuel capacity dilemma, as that extra 10 litres would be useful. It definitely has the big tank.

Sat Sep 15, 2012 3:50 pm

With the fuel tank, the warning light should come on with 8 litres left. So if you brim the tank at that point, add 8l to what you put in for total tank capacity.

Sat Sep 15, 2012 5:04 pm

Worth opening up your ECU to find out what chip it is...

It's straightforward enough to do:
http://www.e30zone.net/e30zonewiki/inde ... ting_Guide

Sat Sep 15, 2012 8:45 pm

If someone has been playing with the spring tension in the AFM, it's probably time to replace it for one that hasn't been meddled with.
It may or may not have run better after it was interfered with, but that doesn't mean you've improved things. Chances are you've introduced a fault that acts, for some of the time at least, to counteract another pre existing fault. For example, if you had a car that was running a weak mixture under mid range acceleration, and made a modification that richened the mixture across the whole range, your car would certainly accelerate more cleanly afterwards, but would run rich at other times, causing a large increase in fuel consumption.
Most of the 'adjustments' on an E30 engine are preset at the factory, and only have one correct setting which will never change. If making adjustments improves matters, then this just indicates that there is a fault elsewhere which needs sorting.

Sat Sep 15, 2012 9:06 pm

I tend to agree that the set tune has now been destroyed and that the AFM needs replacement.
Any opinions on the Miller AFM, with or without the WAR Chip?

Sat Sep 15, 2012 9:39 pm

a couple of inputs here...

1/ fuel consumption: get the injectors cleaned
2/ fuel tank capacity: get under the car and check that the tank hasnt been dented from below... very common for a tank to get misshapped

Sun Sep 16, 2012 1:41 pm

Hi all.
I opened up the AFM and had a play with the wiper and moved it to an unswept portion of the carbon track. Checked resistance, and it is now linear.
Of course, it ran like a pig because the spring tension had been interfered with, but I moved it about until it stopped knocking and idled cleanly. Went for a drive and it was a bit doughy, so moved it again. WHOOT! Sounds better, goes better, feels faster (but that's probably wishful thinking).
Will see if economy gets any better.
Thanks for all your help and suggestions.

Fri Nov 16, 2012 11:18 am

Hi again all.
I've had a few weeks to play with the car, and the economy is actually no better.
Checked the resistance again and although it is linear, it is way too slow to react across the range.
80% of the increase comes in 25% of the movement.
I still reckon the AFM is ready for the tip.
Has anyone had any experience with the American Miller AFM (with or without the WAR chip), or with any brand rebuilt 325 AFM's?

Fri Nov 16, 2012 12:17 pm

I have used the miller MAF with chip on my 2.5 before I installed an S14 engine - I have it for sale if you are interested? Its a generation 3 so the latest version.

It comes with an airbox as well...

PM if you are interested!

Fri Nov 16, 2012 12:44 pm

I had a 1988 325iSE, which I fitted one of the first generation Miller MAFs to. I was impressed with it - it gave beautifully smooth throttle response, and you could easily trickle it along on a very light throttle most of the time, so you could easily achieve 30 mpg or more.

You didn't get the 'kangarooing' on light throttle in traffic that many AFM-equipped M20's seem to be cursed with.