MPG DROP (im stumped)

[MAURICE THE MARTIAN]

6 months ago on our usual runs noticed mpg dropped overnight by 10 -12%.
Since then changed o2 sensors .maf unit.plugs.airfilter .the tyres correct pressure.no brakes binding etc and still down.before i disconnected the old maf and it ran better and better fuel .so bought a new maf .today did the same run as on sat speed traffic etc all similar over the 30 ml journey country roads 50mph got 26.9 mpg same run tday with the new maf disconnected 27.9 no loss of power or performance so im getting better mpg no maf ????
Gotta admit im beat .this shouldnt be.
Any ideas anyone

 

[billz]

Have you checked for a perished hose. Their is a small pipe very hard to see it if my memory serves me right i think it is on the left hand side (as you stand in front of the car) at the back end of the engine. This caused a problem with our 3.0 Z3 and we were using more fuel. Found it had a small split underneath. My BMW mechanic said it was a common problem on that engine.

 

[billz]

I forgot to say it is about 3 to 4 inch in diameter.

 

[MAURICE THE MARTIAN]

Got a new n on. All hoses ok no airleaks or splits and no fault codes

 

[hard top]

"I may be old,but i`ve got a heavy right foot"

 

[FRANKIE]

"I may be old,but i`ve got a heavy right foot"

And a very light right arm........................................................Frankie

 

[MAURICE THE MARTIAN]

"I may be old,but i`ve got a heavy right foot"

Ha ha.
Nah .With Mrs M driving its still uses more.

 

[MAURICE THE MARTIAN]

I know its not a lot on short journeys but ive git a euro cruze wi Billz and i cant stand him gloating coz it cost me more evertime we fill up

 

[NZ00Z3]

Lower fuel economy is usually a sign of a vacuum leak. There will be one in there somewhere. A smoke test may help locate it. A common leak is the small hose on the "tube elbow". That's the rubber hose/boot that connects to the throttle body and idle control valve. The small off-take hose to the idle control valve tends to crack on the outer circumference.

When you disconnect your MAF. The DME uses some approx air flow numbers that are coded into it. The DME uses the air flow data and the O2 sensor data to trim the fuel mixture. Disconnecting the MAF does not really help you. The fact that you are getting better economy with the MAF disconnected may also point towards a vacuum leak. Would have to really put my thinking cap one to sort out the cause and affect of that one.

 

[MAURICE THE MARTIAN]

Hi Murray .the hose you mention is a new one .and i inspected the old one it was fine.ive checked and double checked and cant find any leaks ..
As i said im baffled.
Cheers M

 

[NZ00Z3]

Hi Maurice

The assumption I'm making is that you have a M54B30 engine in your Z4.

Have you checked to see what your adaptions (fuel trims) are? They should all be about mid scale (near 0).

If you do have rich adaptions, then a smoke test is the easiest way to find the vacuum leaks.

The following are a couple of the more uncommon vacuum leaks.

Do you have a Secondary Air Pump? If so, trace back the 3.5 mm rubber hose in the second photo back along the exhaust side, behind the engine and under the inlet manifold. There are plastic and rubber sections in this line.

The M54 and M52TU inlet manifolds have a couple of vacuum ports on the rear end of the manifold. Sometimes they are used, sometimes not. The 7 mm port is usually not used and its blanking grommet has a tenancy to split. Third Photo (M54B25 manifold that I have on hand). These ports are located under the manifold, just to the rear of the dip stick.

Also check the crank case ventilation (CCV) system. Breaks in the cam cover to separator or separator to dip stick pipes will cause a rough idle as well as a vacuum leak. The pipes that go from the separator up to the inlet manifold will only give a vacuum leak. The common rail to put the CCV air into the manifold has 6 O rings for seals into each inlet tube and are not normally a problem.

Hope this helps

Murray

 

[Althulas]

Have you changed where you fill up from just wondering as some of the super market fuels are not as good, just a thought.

 

[billz]

Have you changed where you fill up from just wondering as some of the super market fuels are not as good, just a thought.

No he always fills up at Morrisons

 

[raymond]

A long shot is that the car may be running a little cold. Thermostat may be at fault and perhaps you are using the air con more than usual. Had a similar thing with my Z4. No matter how carefully I drove it only 30 mpg. Gave up driving so gently and mpg went up to 34.

 

[billz]

A long shot is that the car may be running a little cold. Thermostat may be at fault and perhaps you are using the air con more than usual. Had a similar thing with my Z4. No matter how carefully I drove it only 30 mpg. Gave up driving so gently and mpg went up to 34.

Mozz never drives gently. He has a bigger than average right foot with the heaviest of lead weights in it

 

[MAURICE THE MARTIAN]

A long shot is that the car may be running a little cold. Thermostat may be at fault and perhaps you are using the air con more than usual. Had a similar thing with my Z4. No matter how carefully I drove it only 30 mpg. Gave up driving so gently and mpg went up to 34.

.

Not that bud needle bang on .tbh if anythng it runs slightly hot.
M

 

[MAURICE THE MARTIAN]

Hi Maurice

The assumption I'm making is that you have a M54B30 engine in your Z4.

Have you checked to see what your adaptions (fuel trims) are? Below is a photo of the trims on my M54B30 Z3 from INPA. They should all be about mid scale (near 0). A vacuum leak will pull the the Additiv and Multiplicadditiv rich (+ numbers). Adaptions need to get quite rich before they start causing error codes.

If you do have rich adaptions, then a smoke test is the easiest way to find the vacuum leaks.

The following are a couple of the more uncommon vacuum leaks.

Do you have a Secondary Air Pump? If so, trace back the 3.5 mm rubber hose in the second photo back along the exhaust side, behind the engine and under the inlet manifold. There are plastic and rubber sections in this line.

The M54 and M52TU inlet manifolds have a couple of vacuum ports on the rear end of the manifold. Sometimes they are used, sometimes not. The 7 mm port is usually not used and its blanking grommet has a tenancy to split. Third Photo (M54B25 manifold that I have on hand). These ports are located under the manifold, just to the rear of the dip stick.

Also check the crank case ventilation (CCV) system. Breaks in the cam cover to separator or separator to dip stick pipes will cause a rough idle as well as a vacuum leak. The pipes that go from the separator up to the inlet manifold will only give a vacuum leak. The common rail to put the CCV air into the manifold has 6 O rings for seals into each inlet tube and are not normally a problem.

Hope this helps

Murray

Cheers Murray. Will investigate your comments further.
The only thing ive not changed is the ccv pipes but didnt think that effected fuel .this will now get done as the z has done 102000
May have to take it in somewhere as my code reader is basic and so is my knowledge of reading em.
Cheers M

 

[Lee]

The wife's right foot

 

[Paul Rice]

No he always fills up at Morrisons

After breakfast in the Cafe

 

[TaffZee]

I get 36 to the gallon on the run from home to Porthmadog which is around 80 miles, this is taking my time doing no more than 60mph. Knocking around I get around 28-30. This is not using the computer but tank to tank fills. So something wrong somewhere Mozza.