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Car Running Rich? Excessive Fuel Consumption etc


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Hi guys,

This will probably be a long one...

Last year, about 2 months after I bought my R, I experienced severe misfiring and hesitation problems. After 1 month, VW diagnosed the problem as being with the wiring harness for the #5 fuel injector, and repaired this. Up until a few months ago, I thought the car was running well.

Then, a few months ago, the coolant temp sensor died and the car refused to start. The temp sensor was replaced in one day by VW and everything returned to normal... or so I thought.

Lately, I have been experiencing a bit of hesitation, and have noticed that I am using significantly more petrol than other Rs in the area that experience the same traffic conditions (and indeed drive harder than I do).

So last night, a fellow 'texer kindly let me drive his car and we did some back to back testing.

The results are as follows:

* My car, in 2nd gear from about 1200rpm at full throttle, hesitates a little before finally pulling nearly as strong as Mike's (the control car).

* In 3rd gear, full throttle uphill (under load) the same thing happened, but not as noticeably.

* On overrun, my car burbled much more noticeably and for a longer period, particularly at low rpm. Also, on downchanges, the same thing happens - my car sounds deeper and louder.

* Then there is also the problem of my car randomly going into the super-rich mode at idle. This is happening more frequently now - about every second or third time the car is at idle. It lasts for about 10 seconds or so, then changes back to normal. If the instantaneous fuel comsumption readout is normally 1.1l/hr with the aircon off, it will jump up to 2l/h. AFAIK this never used to happen, but is happening more regularly now.

What could be causing this? What does the ECU use to determine whether it should be going into this rich mode?

Now, the car is going in for service with a very good independent VW mechanic (ex Aussie R32 race team) on Sunday, but I'd like to know what I should be checking.

Do the symptoms above sound as though the car is running excessively rich? Once it gets to 3800rpm or so my car is fine, and pulls like a normal R should, or at least very close.

Could the technically minded amongst you please tell me how likely each of the following components is to cause the problem, and how I can, with the help of my mechanic and VAG-Com, test each component.

* Coolant temp sensor (replaced, but is it possible that VW failed to reset the basic settings and do throttle body alignment etc when it was installed? Would this cause the problem?)

* Oxygen sensors

* MAF sensor

* Spark plugs

* Another problem with #5 injector

* Fuel pressure regulator

Any other parts?

I don't want to take the car to VW as it isn't throwing any fault codes, and the technicians are monkeys who wouldn't have a clue how to fix this problem - hell they took a month to fix the injector wiring harness!

Any help would be greatly appreciated.


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This initially sounds like a combination of ECU software problems and possibly the MAF sensor. The trick is to try to fix one thing at a time, drive it and see if it's fixed, rather than do everything at once.

If you've followed the threads on TsN, you've probably seen that the MAF sensor is one of the most common things to go. It's a £75 (about AU$175) replacement cost and takes 10 minutes to do.

The next place I would start is checking that the car is running the latest software revision. I am guessing that the ECU has a different software version in Aus, due to the different octane levels in the fuel. The European software version is #6463 - I can't find any info via Google, but check with VW which version yours should run.

You haven't said how many miles your car has done, but VW recommend replacing the spark plugs on the R32 every 40,000 miles (60,000 km), but I had mine replaced at 27,000 miles (44,000 km) and will be changing them every 10,000 miles from now on. They are relatively cheap items to replace at £50 (AU$120), so this is the next thing I would go for.

I am presuming that your "rich running" means that the car is revving to about 1200-1400 rpm, instead of the usual 700 or so rpm at tickover. This normally happens when you start the car from cold on mine, or when it's been standing for a couple of hours and kicks in for about 20 seconds before settling back down to normal idle speed. The fact that it does it randomly at tickover seems to indicate that one of the sensors is FUBAR'd, but I can't for the life of me remember what it's called!

The coolant temp sensor is nowhere near the inlet manifold on the R, so there is no reason why they would have had to play around with this. All they will have needed to do once they'd replaced the coolant sensor was to reset the ECU to clear the fault that was registered.

If the fuel pressure reg had failed, or was not producing as much pressure as it should, you wouldn't expect the car to pull as well as it seems above 4000 rpm from your description and it would also cause the car to run lean at the top end of the rev range, so the ECU would compensate for this and try to reduce the air/fuel mixture, hence knocking back the performance at the top end. Again, this is a cheap item to replace. It is a mechanical item, so I'm not sure if it would be covered under warranty.

I've not heard of fuel injectors going on very low mileage VWs and Audis. I've only ever had to have them replaced on a 180,000 mile (290,000 km) A4 1.8T. This isn't to say that it's impossible that one has gone, or is possibly blocked, it just wouldn't be first thing on the list to check.

I totally agree with using an independent specialist as opposed to a main dealer to do the diagnosis for you, as all of the independents I have used over the years tend to try to eliminate the problem, rather than just replacing black boxes because the diagnosis tools tell you to.

Loads of stuff that it could be Langers. Best of luck on this - let us know what you find out after it's been fixed.


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Mook, cheers for the response:

I'll be switching out the MAF with another car if I get the chance.

We actually have the same octane fuel as you guys in the UK (98 octane), but you are right, our software is different and we do not have any updates available.

Car has covered 15 000km.

Interesting thing about the rich idle mode is that it happens once the car is fully warmed up, but the idle speed DOES NOT CHANGE!

Not sure the dealer would even know where to start, as there are no fault codes!

Thanks again for the help Mook, really appreciate you taking the time. smile.gif

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  • 3 months later...

Hello Langers

How did you get on with the problems of misfire/hesitation in the R32?

*Spark plugs could have worked themselves loose if not tightened correctly

*Coolant temp sensor should be changed for the improved green one

*Have you looked at the knock sensor?

The most significant improvement I can think of is a software opgrade if your hesitation does not show up on VAG-COM.

For the manual R32 in Europe we have version 6463, where my MkIV DSG is on version 7831. It has recently been updated from 7000 and totally eliminated a very rare hesitation(800-4000rpm) that you seem to have.

Good luck 169144-ok.gif

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