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Discussion Starter #1
So a little history I bought an S5 that had a misfire on cylinder three running rich bank one I did a full carbon cleaning which was bad tested all the injectors had a bad leaking injector on cylinder three and two other leaking injectors replaced those had the others sonically cleaned and pressure tested cleaned everything in the intake and re-installed all components. Fired up the car and it ran excellent but I decided to take it for a drive and I logged long-term fuel trims short term fuel trims and fuel pressure. My fuel pressure seems to be excellent but I started pulling negative long term fuel trims even on both sides about -9 to -10 while driving and at idle they go as far as -23 on bank two and -17 on bank one. My short term fuel trims are within spec changing slightly as I drive no check engine light coming on yet car runs well drives well and the idle when warm just a little rough but not very noticeable. previous owner did change spark plugs and bank 1 hpfp. I am thinking it’s an upstream problem as both cylinders run even negative fuel trims while driving. So MAF? It’s not a vacuum leak because that would be positive fuel trims or lean condition. Could it be the other high pressure fuel pump on bank 2? Would that affect bank one also? Anybody experienced in fuel trim diagnosis. Thanks
 

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Discussion Starter #2
Sorry forgot To mention the particulars it’s a 2011 V8 4.2 6 speed manual transmission with 150000k
 

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I got a 3.2 from auction that was running rich, lumpy idle, -25 on the trims at idle, ok under load.

Carbon clean and the injectors sent away for testing and cleaning. Came back clear, put them back in, same problem.

Changed 1 bank with new injectors, existing on the other bank. The bank I changed out was now clear on the fuel trims, changed out the other side problem gone.

Read up GDI injector testing....they do not test at the high pressure they run at due to the safety of it, I think this is a big problem.

Mine had 118,000Km at the time.

Wish I had the V8...........
 

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A test for leaking injectors..........


Start car from cold so its not hot and the high pressure line fuel wonts to boil when you turn off car (and initially the pressure goes higher)

Run for 20 secs or so, monitor high and low pressure. Turn off car

Audi says high pressure side should stay above 2000psi for 20mins ( I think it was 2000psi....but it was 2/3rds of its running pressure for minimum of 20mins)

Mine dropped to the low side pressure of 5 bar within a minute

Take out the plugs use a bore scope to look for petrol on the top of the pistons.

The injectors in my car that failed were clean on the top, the 2 good ones were dirty i.e black and not stainless steel clean
Bad Injector Good Injector (ignore the brown stuff)
177160
177162
 

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Wow interesting stuff guys, and solution for the V8 yet?
 

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Discussion Starter #6
I
A test for leaking injectors..........


Start car from cold so its not hot and the high pressure line fuel wonts to boil when you turn off car (and initially the pressure goes higher)

Run for 20 secs or so, monitor high and low pressure. Turn off car

Audi says high pressure side should stay above 2000psi for 20mins ( I think it was 2000psi....but it was 2/3rds of its running pressure for minimum of 20mins)

Mine dropped to the low side pressure of 5 bar within a minute

Take out the plugs use a bore scope to look for petrol on the top of the pistons.

The injectors in my car that failed were clean on the top, the 2 good ones were dirty i.e black and not stainless steel clean
Bad Injector Good Injector (ignore the brown stuff)
View attachment 177160 View attachment 177162
will check leak down on fuel pressure. I replaced 3 of the 4 injectors on bank one with hitachi injectors from Rockauto. The injector service did say they leak tested to 2000 psi. I do no longer have as rough an idle or the hard hot starts I had previous with leaking injectors. Not looking forward to replacing more ??‍♂
 

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Discussion Starter #7
Will a failing hpfp cause a leak down of fuel pressure? Bank 2 the worst fuel trim offender at idle is original hpfp. Bank 1 was replaced by previous owner. I know they can push fuel into the valve cover. Just would like to rule out that before spending money on more injectors and not solving the problem. No problem spending money on parts to fix the problem but can’t stand wasting it.
 

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Discussion Starter #8
just an update I had someone hook it to vagcom and it turns out my lambda is only about -3 on bank one -4 on bank to so not running overly rich I suppose. I guess my OBD two reader isn’t very good. But something I have been able to confirm is I lose my fuel pressure in the rail quite quickly from 25K PA down to eight within 3 to 5 minutes. The car has no running issues no starting issues no CEL no misfires. All injectors where leak tested to 2000 psi. Where else can fuel pressure be lost? Hpfp?
 

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When I go to the auctions and look at the Audis, the higher gas KM cars all have -ve fuel trims. I use this to determine if I will be changing out the injectors, I also wait until the car is up to temperature and check if the idle is not butter smooth, if its a little lumpy with -ve fuel trims (-say -7'ish) this is my in the field indication that the injectors are original, has high miles and the injectors need to be changed out if you wont a butter smooth car.

All the GDI injectors across all manufacturers appear to have a 120,000km limit unit they are not what they used to be......in humans I have learned this to be around 42 years.

I have read a few people with the 4.2 FSI in the higher km/s chase an engine miss with plugs and coils first then carbon cleaning, car feels a little better but the miss remains. They change the injectors out and then all is cured.

Not saying you should just go out and buy injectors and hope for the best........I guess what I am saying is I think you 150,000km injectors are at their limit, so check the dates on the side of each injector and change them out if you intend to keep the car a while (once you have done the easy preliminary's like the plugs and coils, checked the crank case ventilation and the evaporation emission by plunging these off and watching the fuel trims). You could also check the O2 sensors by putting a little butane in the intake and the short term fuel trims (both banks) should go very -ve.

See the date on the injector below. Below the"HTC" ......"191107" means 19.11.2007 that was in a 2008 A5.

Good luck and keep us posted.

177202
 

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Discussion Starter #10
I appreciate the input but I have no missfire
Issues or poor idle I’m currently chasing. My only issue with the car is after I shut it off I lose fuel pressure quickly on the high pressure side from 25 kpa to about 8. I’m confident it’s not the injectors. What I would like to know, is there another point in the system that would allow high pressure loss. Would a failing hpfp cause pressure loss?
 

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You can check the low pressure side at turn off, it should go from the running 5kpa ( I think) to something much higher (say 8kpa)........but on the 3.2 the sensor peaks at 5.5kpa so in my case I would need to hook up a pressure gauge.

I was once looking for an ongoing problem of a lumpy start from cold (not butter smooth with a few miss fires)........it is still happening and I have stopped because it appears to be a software thing to help the cats get up to temp. In my efforts I noted that the rubber hose from the LP fuel line to the HP fuel pump would expand when you turned the car off, indicating fuel from the HP side going into the LP side.........so I purchased a brand new genuine HP fuel pump........same problem and the LP hose kept doing the same thing.

So yes the HP pump does relieve into the LP side at turn off and this is normal. However to better help you, you will need to go looking for fuel on the top of pistons with a good bore scope, or, clean looking injectors with the bore scope that should be all black if they are not leaking as per my first set of pictures.

PS. the pressure in the HP line is required to say high for longer than 20 mins (via Audi) if its low before you need to look at the injectors first via bore scoping as testing the HP fuel pump will be a replacement test (to my knowledge) and this will not be fun financially.

PSS. A good parts site for genuine and OE is Auto Parts online, Parts & Accessories catalog, Carparts store - Cars245.com so much cheaper then Australia where it is twice the price for genuine stuff.
 

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Discussion Starter #13
Ok thanks. I have tested the injectors and just installed them so likely a hpfp. Hate not being able to diagnose a part for sure before replacing. Also hard to replace a part when it’s not causing any real running issues at this point I wouldn’t even know about the pressure issue if I didn’t log data.
 

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But why did you change 3 injectors and not all 4......surely you should change the full set at a time as otherwise you are mixing new with old parts so not a good idea I would have thought.....
 
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