Car: Audi S5 V8 FSI, Direct Injection

I posted a question related to this but might not be the correct issue: Do injectors leak or injector seals leak?


2 months ago (1200 km) I performed a manual carbon clean of the intake ports to remove carbon build up (to get a tiny bit more performance).

I also changed the spark plugs while at it.

To clean the ports, I used Liqui Moly Fuel System Cleaner, which dissolved the carbon build up, and I scrubbed and sucked it away. I didn't removed the fuel rail or injectors, but some of the cleaner/gunk flowed down past the injectors onto the middle of the "V" (which I cleaned up, but some left stuck in injector support)


A few weeks later, after a 30 minute drive + 2 hours leaving the car off, I went to start the car. It cranked a lot more than usual, and limped it's way to starting. Normally it springs to life like any car. I didn't notice the issue again, until...

Last week, the same issue happened again, multiple times, with longer cranking times. Only after a few (1-2) hours. It starts fine cold, and starts back up fine within ~30 mins. I sometimes smell petrol, but not after start up and driven for a bit.

Thermostat was changed 2 years ago.


  • Scan tool - No fault codes, no dash lights, coolant needle stable at 90C (filtered)

  • The previous question suggested the ECT sensor. I had a look at the readings of it. The value didn't seem to spike anywhere, and the wires looked OK. I noticed that the value was a bit "slow". From a cold start, at idle, the temp didn't climb above 75C even after 10mins. After a quick drive however, it was around 100C. After a moderate drive, it was around 110C.

  • Having a browse of the measuring blocks, there was a fuel injection start adaptation. This remained 0% for engine temperature of 25C (cold start), 110C (hot start), 75C (after 1.5 hrs)

  • Fuel Rail Pressure was around 6 bar when off (25 bar when on, 70+ bar at WOT), it remained this even after 2 hours (I'm guessing it primes when you turn on the ignition)

enter image description here

  • Removed 1 spark. When I removed it, I got a whiff of petrol smell. The plug was dry, but looks black. I tried to look inside the cylinder with a torch, and I saw a bit of wetness. I'm not sure if this is normal or not after 1200KM?

enter image description here

Does this sound like an ECT sensor issue, a fuel injector leak, or something else? Obviously changing the ECT sensor is 100x as easy, but if it's not broken I'd rather not touch it.

  • Hi did you find a resolution to this problem please? I'm in a similar situation and interested to see what happened here
    – ajayel
    Commented Jun 11, 2018 at 9:44
  • 1
    @ajayel no I didn't. I had the injector seals replaced because the mechanic found them leaking. After putting it back together it still was hard to start. He reckons it was a leaking injector (not seal) - he said it was number 6 because with the scan tool it would show the misfire count rise rapidly before stopping. The other cylinders were fine. I ended up selling the car so can't know if that was the issue
    – tgun926
    Commented Jun 12, 2018 at 9:57

3 Answers 3


Fuel Rail Pressure was around 6 bar when off (25 bar when on, 70+ bar at WOT), it remained this even after 2 hours (I'm guessing it primes when you turn on the ignition)

I believe this is the root of your problem.

The fuel rail needs to maintain a certain pressure so that the vehicle can start up with the engine hot.

If the pressure isn't high enough, the heat from the engine will start to "boil" the fuel, making it difficult to start (think vapor lock). If you crank the engine for long enough, the fuel rail may be able to build up sufficient pressure to overcome this condition.

That said, here's a shortlist of possible culprits:

  • a leaky fuel injector or improperly seated fuel injector seal
  • minute cracks in the fuel line which leak fuel when under pressure
  • a bad fuel pressure regulator
  • a bad non-return valve in the fuel line which is allowing pressurized fuel to flow backwards
  • Are you saying the start up pressure should be higher when the engine is hot?
    – tgun926
    Commented Jan 7, 2017 at 20:54
  • @tgun926 The symptoms appear to suggest that, though I wish I had a number for you to cross-reference your reading against so that you could be sure
    – Zaid
    Commented Jan 7, 2017 at 20:56
  • Would that explain the black spark plugs though, or is that a separate issue?
    – tgun926
    Commented Jan 7, 2017 at 21:00
  • At this point I really can't say. It's black, so it looks like not all the fuel is burning as it should, but the plug isn't wet, so it's not that big a problem.
    – Zaid
    Commented Jan 7, 2017 at 21:14
  • The plug electrodes look fairly clean, and the insulator grey. I don't think those plugs are as terrible as others imply.
    – SteveRacer
    Commented Jan 9, 2017 at 3:13

Pull all your spark plugs, careful not to mix them up in reference to which cylinder they were IN.

Are ALL of them as BAD as That one you show us here?

IF that plug is the darkest, IS IT FROM THE CYLINDER that your FUEL PRESSURE REGULATOR gets its vacuum suction from on the INTAKE RUNNER feeding that dark over fueled spark plug. HINT undo the vacuum hose from the intake IF IT IS ON THAT PLUGS INTAKE RUNNER..Smell Raw Fuel in its hose? SEE any raw fuel drip from that hose..Thy Fuel Pressure Regulator..some have a thin rubber diaphragm & vacuum sucks IT Back further as thy throttle OPENS & airflow increases as RPM's Rise..which opens up a larger passage inside the Fuel Pressure Regulator.. FOR MORE FUEL or Fuel PRESSURE to Flow.. and IF the Diaphragm fails, It doesn't toss a CODE..well IT WILL but not until you get a dead Miss right after tha plug Fouls out from over fueling it obviously HAS from what i see in Your spark plug photo here.

If all the others look cleaner.. Tahhh Dahhhh.

If not, it could be a clump of rock hard CARBON you had coated all the way around an Intake Valves SEAT only got melted/cleaned in 1 spot..so the rest actually still has the valve seated except for in 1 tiny spot = hard start after she warms up. logic states.. What else WON'T toss a Code but makes my car engine hard to start after reaching running temperature?.. not MUCH. and a failing Fuel pump WILL ..but the plug sure WON'T be THAT overly Rich with Fuel like Yours Is.

Are you SURE you didn't split just the END of a tiny vacuum hose at the end where IT slips over its fitting at the intake OR most importantly: where IT fits to a Thermostatic type vacuum CONTROLLED gadget..that only OPENS once the engine is at normal operating temp..causing a VACUUM LEAK only when its warmed up= & harder to start than when its COLD and the split or left unconnected Vacuum hose wont create a vacuum leak.

So a split end on a rubber vacuum hose or leaving a tiny one OFF while fumbling around with the engine.. THAT will cause an engine to run RICH when a 02 sensor smells more air than IT Recorded going through the intake boot into the engine..the fuel control system is ignorant and only preset to Dump Less fuel OR more fuel into thy MIX.... NOT ACTUALLY CORRECT IT LIVE here in the real world (as it should) ~ Which Will Not toss a CODE immediately..and THAT will create your harder to Start *symptom..once she reaches normal operating temp too. one other thing.. I dig the way you think .. its great to see good diagnostic theory working to eliminate the few things down to get to the ROOT of thy problem.. because Band Aiding things never Cure thy ROOT of a engine not up to par in performance.. it merely resets the ROOT to create thy same problem AGAIN Later on. (like your 02 sensors that will soon toss a CODE IF she continues to run that RICH much longer..which a empty Windex container to fill with water..spray MIST into thy intake.. to Steam Clean those filthy black coated 02 sensors clean so they can READ correctly again..that is a sweet FREE Cure too). but keep in mind.. seek asking WHAT Few Things can cause an engine to run rich that wont toss a CODE! Seek and yee shall Find

  • I don't get the clump of carbon rock bit, wouldn't driving it hard at high temp scrape the valve seat and remove any tiny pieces?
    – tgun926
    Commented Jan 7, 2017 at 11:39
  • 3
    There is a lot of information here! Thanks for taking the time to share you knowledge and experience. I think it would help readers if it were edited with some paragraph breaks and lists of steps. If you're new to working with Markdown you can use the formatting buttons, or check out the gory details here.
    – dlu
    Commented Jan 7, 2017 at 17:46
  • I'm FAIRLY certain that the FUEL pressure REGULATOR doesn't GET vaccuum FROM one PARTICULAR cylinder. How WOULD it KNOW?
    – SteveRacer
    Commented Jan 9, 2017 at 3:08

NGK is backwards with heat range, meaning the lower the numeric digit is, the hotter the NGK plug is. This is unlike every other spark plug where you get colder in heat range when the number is lower.

So if you seek a band aid till you figure the problem out.. place a hotter plug in just the holes that show carbon deposits on the plugs, so it won't foul like the cold plug originally rated for your engine will once the car has aged or has high miles. Bending thy rules works too, with today's economy, we bend.

PS! A loose clamp on the tube at the intake from the air box could cause that too, as could a cracked tube between the air box and the intake manifold. Because all the air entering must be measured by the mass air flow sensor (MAF, the box that you see between the air filter box and the intake manifold). IF it sucks air in through a cracked tube or a loose clamp after the MAF which measures all the air entering thy engine, it creates a false lean condition which the O2 sensors read as lean, but due to the fact the engine isn't lean, and its just a leak in the intake tube, once it richens the ratio its programed to a set given amount, IT BECOMES WAY TOO RICH, and you see carbon build ups, like that plug in your picture!

  • A lot of information, thanks. Yes, I remember piercing the end of a vacuum hose at the intake, I also didn't put the clamp back on as it had lost it's spring, but it was on pretty tight to the nipple. Also one of 10 screws on the intake manifold wouldn't screw in, not sure if that's enough to cause a huge leak though... will investigate further.
    – tgun926
    Commented Jan 7, 2017 at 11:33
  • Is it ok to continue to use these plugs? Should I clean them in some fuel cleaner solution?
    – tgun926
    Commented Jan 7, 2017 at 11:34
  • Could you explain why you don't like ND plugs?
    – dlu
    Commented Jan 7, 2017 at 17:56

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