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Matt
Matt, Mechanical Engineer
Category: UK Car
Satisfied Customers: 20590
Experience:  BEng hons Mech engineering, in auto industry 22 years
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I have a 2003 Audi S3. Under heavy acceleration in 4th, 5th

Customer Question

Hi Pearl.
JA: What is the model/year of your Audi?
Customer: I have a 2003 Audi S3. Under heavy acceleration in 4th, 5th and 6th there is a slight hesitation between 2900 and 3000 rpm. Otherwise the car is perfect.
JA: How old is the transmission? Have you checked the fuel filters?
Customer: Checked coils, cleaned MAF, just had 120 km service so new plugs and cam belt.
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: Has had some new vacuum pipes too
Submitted: 4 months ago.
Category: UK Car
Customer: replied 4 months ago.
New fuel filter installed. 122,000 km mileage
Customer: replied 4 months ago.
No codes, Audi stumped. They say they now need to go changing components for new ones which will be expensive in time and money.
Customer: replied 4 months ago.
I notice you said fuel filters. I have only changed one at the back of the car by the drivers side (RHD) rear wheel.
Customer: replied 4 months ago.
Any response yet guys?
Expert:  Matt replied 4 months ago.

Hello

sorry about the delay

This sounds like a lack of fuel and could be a lack of fuel pressure from the electrical pump at the tank.

1st thing to try is to remove the fuel filler cap and then flick the ignition on whilst listening for the brief ‘buzz’ of the pump

If the connections to the pump are corroded or damaged then the pump could stop at any time or could not be running at full speed. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse,

Ideally measure the pressure at the injector rail (should usually be around 3Bar / 45psi +) and if this is low check the pump as described and also consider changing the fuel filter as if this partially blocked this too will reduce pressure.

Customer: replied 4 months ago.
Hi Matt,
Thank you for the response. Things tried so far- Fuel filter changed by the rear drivers side wheel
- I can hear the pump starting when the ignition is turned on.
- Cleaned MAF sensor
- Vacuum pipes changed and system pressure testedThe performance of the vehicle is VERY good apart form this hesitation between 2900 and 3000 rpm only with foot flat down.
If it was the pump wouldn't the higher fuel requirements at higher RPM be causing an issue?The vehicle does have a large red air inlet pipe from the air filter and may have some software, previous owner, but there were no issues until recently.The car was not used very much for a couple of years and there was a broken breather pipe (under near side of the inlet manifold) for a while which could have allowed some dirt into the rocker cover ventilation system. Audi replaced pipes pressure tested and cleaned apparently.The pipe coming down to the large red air inlet pipe from the rocker cover ventilation system is very weak and can easily collapse under suction. Would affect anything?The problem is reproducible but gets a little better after the engine has warmed up. It doesn't matter if you test from low rpm or 2900 rpm you still have the same issue.Can you advise on the processes occurring between 2900 and 3000 rpm with foot flat down?
Expert:  Matt replied 4 months ago.

OK thanks for the extra info

that sounds like the fuel pressure is probably fine then

in that case I'd say to check the breather system next

if you loosen the oil filler cap a 1/4 turn and secure it with some tape so it can't fall out but leaving a gap so the engine can 'breathe' and then try to repeat the test ?

this will show if any of the breather valves etc are at fault with the hesistation

Customer: replied 4 months ago.
Hi Matt,
What does loosening the oil filler cap by 1/4 of a turn do?
Expert:  Matt replied 4 months ago.

Hello

that allows internal engine air / breather gas to exit instead of being piped back into the inlet manifold or pre-turbo

I'm thinking that you may have a faulty PCV thats not venting as it should

and by allowing the gases to exit all the time under the bonnet this will show if the breather system is at fault

Customer: replied 4 months ago.
Hi Matt,
Thank you for the explanation. I'll try the test you recommend. What results would you expect?Audi changed the PCV and some of the pipes recently when they did the 120k service as they had the inlet manifold off.Just out of interest I took the electrical connector off the N75 value and tried the test. Power was right down but there was no hesitation.
I'm wondering also if the potential collapsing of the old hose from the breather assembly to the air inlet pipe could be the issue.
Expert:  Matt replied 4 months ago.

Hello

if you have a blocked or sticking breather then opening the filler cap should stop any hesitation , as I'm thinking a sticking PCV will suddenly 'burp' a load of blow-by gas into the engine which will lean it out

so yes a the collapsing breather hose may be the issue

Customer: replied 4 months ago.
Hi Matt,Ok I tried the cap this morning and no difference. I'm going to order a replacement breather hose for the one I think is collapsing. However, it is odd that the problem is only at a certain RPM.The resistance of the N75 valve was around 32 ohms. I have seen reports by people that this test can be misleading and that the unit needs to be got hot with boiling water to test the resistance properly.Any more thoughts?
Expert:  Matt replied 4 months ago.

Hello

OK I'd agree that heating the N75 valve up to do its resistance check is a good idea

also I'd do a vacuum leak test

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattenedpiece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Customer: replied 4 months ago.
Hi Matt,
Apparently Audi did a leak test but that does sound like fun with the lighter fluid!
I'll change that pipe first, try cleaning the N75 and let's see :-)
Customer: replied 4 months ago.
Hi Matt,
Ok changed the pipe. It was encrusted with carbon deposits as well as being weak. Tested and now the problem is much less and is only really noticeable in 5,6.
Cleaned the N75. No change
I'm thinking I should clean all the pipes in the area.
Expert:  Matt replied 4 months ago.

Hello

Ok

good progress I'd check the rest of the breather pipes for leaks and blockages as you've already mentioned

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