Could this be a lambda issue?

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1993 Audi 80 2.0 single point (ABT engine).
It's my dad's car, had it from new & been well serviced over the years. I'm trying to diagnose poor top end performance and thinking it may be the
lambda sensor, or any other suggestions welcome.
Starts & drives well but gutless at top end. Up to about half throttle it has plenty of pull but put your foot down for overtake or approaching inclines and there isn't much there and takes a long time to build up speed - usually have to knock it down the gears. Feels as though it wants to go but a lack of fuelling at top end is holding it back. In fact if your cruising along say at half throttle and plant your foot down it feels like it actually slows down before very slowly starts to build up speed. TPS & various other sensors appear ok (within Autodata spec) & checked for any vacuum, air leaks. The injector unit to manifold rubber flange looked a bit worn & cracked but I've replaced that.
Only thing I can think of is the lambda sensor as the cars done about 120k miles on the original so was wondering if that could be on its way out and slow at sending the correct signal to increase fuelling at top end?
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Redwood wrote:

blocked cat would be my first thought
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It did have a new cat I think about 2 year ago. If I remember it snapped at the joint where it connects to the exhaust so had to replace the whole cat section, although I suppose even a newish cat can break up. It goes for an MoT later this month so will get the garage have a good poke about see if they can see anything. Not sure if this is also relevant but it idles ok with no lumpiness but I notice that if you try and hold the revs at a steady fast idle it fluctuates up & down slightly.
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On full throttle most cars go open loop so the lambda sensor no longer controls the mixture.
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wrote:

I guess that would rule out the lambda then. It does seem fuel related though and if you ease off the throttle you can feel it become more responsive.
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Hmmm Is there a full/wide open throttle switch on it? What sensors are on the throttle body since I am thinking two. I suspect that after you ruled out the cat. If the full/wide open throttle switch is not functioning then maybe the 02 sensor is not cutting out and allowing full power.
Again I am not familiar with the '93 Audi 80's there. Of course I ASSuME that you check for any obstructions in the air intake system. I have seen nests inside of air cleaner boxes along with leaves restricting air flow. ;-)
JMHO
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Worn fuel pump giving low pressure at WOT?
Does it get an italian tuneup every so often?
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Redwood wrote:

Has it got an air flow meter? Is it stuck?
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Blah wrote:

That was my thought- maybe a MAP sensor of a MAF or whatever it uses to measure airflow in. I'm not familiar with the setup on this engine though.
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On Wed, 15 Jul 2009 08:21:34 +0100, Chris Bartram

It's got one. The normal plug it into Vag-Com applies butodds are it's an air or fuel flow restriction.
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wrote:

Thanks for the replies, it does sound like a fuelling problem. I'll get hold of a fuel pressure gauge and check the correct pressure is reaching the injection unit. Autodata says to manually trigger the pump and reading should be between 0.8 & 1.2 bar and hold at 0.5 bar after 5 mins. Should at least rule out any filter, blockage or pump probs. It's got good pick up if, for e.g, I knock it into 2nd gear to turn into a side road with a steep incline. As soon as I press the throttle it will pull very well up hill from low speed but get to a certain point and nothing much happens at the top end of the throttle range.
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The fuel pressure effects the entire engine range. If it were wrong it wouldn't idle properly either. I suppose a blocked filter could have more effect at high demand, though.
The range of 0.8 - 1.2 bar seems awfully wide to me - on mine a couple of PSI makes a big difference. Although of course the pressure does change depending on engine vacuum. But I'd expect the static pressure to be much more accurate than that.
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On Wed, 15 Jul 2009 14:45:06 +0100, Dave Plowman (News)

Yup, if the filters clogged they're fine at low demand & bad at full bore.

1 bar sounds low anyway.
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Indeed. My old SD1 runs at 36 psi - and most modern vehicles are higher.
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wrote:

Got a fuel filter ordered for tomorrow. I'm assured it's had all filters changed at every yearly service, but with the fuel filter hidden away under the back end somewhere I will make sure & fit a new one anyway. The 1 bar figure is the regulated pressure when measured with the gauge fitted inline between the fuel inlet pipe & the injection unit - if that sounds more feasible.
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One Bar is about right for a SPI system. They run much lower than MPI.
Graham
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Didn't realise Audi ever used a SPI system - although the OP did say so.
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How old's the fuel & air filter?
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Blocked cat / exhaust is mine. NOT Lambda sensor.
Or cam timing incorrect.
Tim.

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I'd thrash the hell out of it for a while and see if it picks up...
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