Cause of high fuel consumption E420
 

Lately, I have been getting horrible mpg with my E420. I was used to 23 and on occasion even 24mpg. Now, I think I am lucky, if I get 15 or 16.
The difference between now and then, are a Rebuilt ETA, a rebuilt MAF (two, actually - the first one gave me a high/low voltage code), new O2 sensor, new plugs, caps and rotors.
My tech whom installed the O2 sensor (that thing is a pita to get to), said he took some air flow readings, and he thought they were high (20-21kg/hr) and thinks it ought to be more like 14.5/hr.
Does this point to another MAF? No CEL is present.

Thanks!

Is it in closed loop, is it adapted within range, what does O2 sensor control look like at 2500rpms and is your fuel pressure regulator leaking?

I will find out about the first 3 things. The fuel pressure regulator was replaced almost a year ago.
Thanks, Steve!

I read somewhere that a bad thermostat can increase fuel consumption too.

When the MAF/O2 sensor were replaced, did the tech reset/recalibrate the fuel adaptation?

Not that I know of.

How is this done, anyway?

I would like to know also. Anyone?

You can reset adaptation back to Mean Base Line by re-setting/re-activating the ECU memory . After you have cleared all existing codes [Single flash verification ], you then hold button for 6-8 secs. after the single flash. [wait 2 secs after single flash].. Turn off key for 2 secs, On for 10 and then start.
This is NOT a code clear, it is ECU memory reset. That gets your ECU back to Normal . From there the unit will adapt to the new parts and sensors, but rather then coming back from high adaptation extremes, it will come up/down, +/- as required from a Normal Base value.
The ECU can/will readapt from the old memory by itself with drive cycles [ relearn process] , but if the trims were way out , you can still get codes coming back while the process takes place..so, it is easier to just clear/reset the memory after you have changed parts and have cleared the diagnostic codes ..you are then assured of starting with an ECU that has a Mean Base Line setting value..[ ie..a Normal setting]..which should be closer to proper adaptations with the new remedy parts.
Just a short-cut........

Also, as SB mentions..FP regulator leaking diapragm will supply the engine with unmetered raw gas..not only will the engine suck raw, un-metered gas into the intake from engine vac , but the Fuel Pressure will go higher than spec and compound that problem...that would be a first check..
the same can be a vac leak..a vac leak of unmetered air is read by 02 as a lean condition , so th ECU tries to enrichen this false reading, resulting in wrong A/F mix..
Both reg and vac leaks will force trim levels +/- that it may not be able to handle [ beyond limit].........and result in crap MPG/Running condition.
If one does find a leaking/bad FP reg, it is also a good idea to change the oil.

Thank you :)

Thanks. I'm printing this one out for future reference. This is great stuff!

O2 Sensor voltage fluctuated between the mid 700's and 84 mv.

I had a hard time keeping it at exactly 2500 rpm...I kept having to adjust my foot.

Everything else is supposedly in working order. Only thing that came up, was a fault in the Idle speed contact (S29/3)

The fuel pressure was also checked, and it was fine too.

One of the cylinders was using more voltage than the other 7 - it was determined that a new wire end, would cure this (we swapped ends, and the cyl with the higher voltage moved). Monday, I will get new ends.

Bumping to top - I'd like to know, if possible, by monday morning, if there is anything else I should be asking for at my local shop.

Thanks a bunch!


Edit: Seeing the airpump thread, reminded me of this: My trip to work, is about 4 miles - I've noticed than when opening the gate to get in, that I can hear the air pump running. I thought this only came on for about two minutes after start-up (and, when cold).

Have you replaced the thermostat as someone suggested? It may be stuck open, causing the engine to run too cold. It may not be evident to you in Florida. I only figured this out about mine once the outside temperature dropped to below freezing.