Cause of high fuel consumption E420

[Larry Delor]

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!

[stevebfl]

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?

[Larry Delor]

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

[Texholdem]

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

[Zeus]

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

[Larry Delor]

Originally Posted by Zeus

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?

[emerydc8]

I would like to know also. Anyone?

[Arthur Dalton]

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.

[Larry Delor]

Thank you

[emerydc8]

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

[Larry Delor]

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.

[Larry Delor]

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).

[Andras]

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.