[V8] ABH using lots of fuel!

[Dave Saad]

Nice Detail!

Dave


On Jun 20, 2008, at 3:18 PM, qshipq at aol.com wrote:

>
>  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
>
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>
> -----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!
>
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> 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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