[V8] ABH using lots of fuel!

[Dag Bøsterud]

The problem seemes to be solved now. Cleaned the contact to the TPS, replaced the MAF and checked that the fuel lines crossing the manifold were correct. I also took the fuse to the ECU out for a few hours to clear fault codes.

Started her up and took her for a spin. Got her warmed up, drove her up to about 125 mph, and up to 6000 rpm a few times to clear out old particles, and down to about 55 mph. Got about 24 mpg on a steady run holding 55 mph. Drove it home and tried blinking out the codes. No fault codes.. so I would suspect the car is as good as new?
  ----- Original Message ----- 
  From: qshipq at aol.com 
  To: dag.boesterud at consulgranada.net ; V8 at audifans.com 
  Sent: Friday, June 20, 2008 11:18 PM
  Subject: Re: [V8] ABH using lots of fuel!


  Dag
  Not sure if you have solved this, but I just went thru this last week on a road trip.  26mpg highway, all good, then within the same trip, mileage dropped to 9 and black smoke on all heavy foot applications was so bad the rear bumper and trunk got stained.  I spent a lot of time on the electronics of the 92 diagnosing a bad ECU prior to this trip, so there were several things about the ABH I relearned in the process.   First, there are only 3 items which cause a fuel dump:  O2, MAF, FPR....  With the code of only 02 out of range, sounds like you still have an 02 problem.  In my case I disconnected the 02, which makes the ECU default to the MAF sensor/TPS for fuel injector Duty Cycle.  This works showing only <no 02 sensor present code>, but it still runs rich unless at highway cruise (where it runs slightly rich but also very cool engine temps).  The reason for this is that the software tables can use MAF only, but it really has a problem with any throttle application in the upper 1/3 of WOT.  WOT is a calculated percentage of closed loop operation, so without a closed loop reference, it will just dump fuel out the pipe as throttle is applied further.  However, just cruising down the highway slightly rich, I saw only about 3mpg drop.  Car ran fine, no stumbles no issues.

  In my case the PO had replaced the O2 sensor, but didn't tie the cable out of the way of the driveshaft, and it rubbed thru the casing, causing the O2 to short out to the heater circuit wires.  I field repaired the wiring but the O2 was shorted, so I just ran disconnected.  I replaced the O2 sensor on my return home, and all is well, gas mileage is back, and the O2 tends to run leaner than the MAF only signal.  WOT black smoke is gone now that the ecu has a reference table for closed loop operation.

  Given what I'm reading of your issues, I'd still be on the O2 sensor.  I suspect you need to trace the O2 wires from the computer out to the sensor itself.  If there is a short in the wiring upstream of the plug, you put in a new O2 and it shorts or is out of range almost immediately.  You need to address this immediately, as the raw HC dump can take out the cats pretty quickly on this motor.  The second diagnostic I would look at is to change the MAF with known good.  That said, IME on Ingo's car, the MAF will cause symptoms nothing like you are experiencing, the car is down on power, feels wheezy and generally runs like crap.  

  The third Diagnostic is to R&R the Fuel Pressure Regulator and (first) check the vacuum line going to it for leaks.  I don't buy the engine temp sender, as that gently lowers gas mileage, but doesn't have enough fuel correction in the programming to cause 9mpg.  

  Probably more than you wanted to know, but I wanted to give a complete picture of what the ABH motronic is doing when these sensors go whacky.

  HTH

  Scott J
  92 v8
  ex2ex
  91 v8





  -----Original Message-----
  From: Dag Bøsterud <dag.boesterud at consulgranada.net>
  To: V8 at audifans.com
  Sent: Sun, 8 Jun 2008 3:18 pm
  Subject: [V8] ABH using lots of fuel!


My 92 V8 4.2 got the head gaskets replaced and it's heads rebuilt during April 
2008.
After this, the fuel consumption has been like.. A Cadillac with a 500 cid V8!

The engine is very smooth during the whole range, fires on every cylinder and 
performance is not down
as far as I can tell.

I read fault codes and replaced the oxygen sensor just to make sure, although 
I'm quite sure that's not where the problem is.
The car still runs very rich.

Has anyone got any I as to why the engine suddenly startet to drink fuel?

/Dag
92 V8 4.2
202 000 miles

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