B7 RS4 2008 (JHM Stage 1+) - Long Crank Time when Warm!

Hi Everyone,
This will be my 1st post since the new look forum and from what I can see it is definitely a step up from the old one! Well onto my problem, need some guidance and hopefully a push in the right direction. Here’s my problem, started out of the blue, no warning just happened overnight between drives.

When the car is cold, parked overnight, it fires up as per normal no problem.
When the car is warmed up, sitting for 10-15min’s, fires as per normal no problem but when you let her sit for 1hr-2hrs it takes more than one attempt to fire-up. 1st attempt is a long crank and almost sound like its either flooded or not getting enough gas. Second attempt will almost fire up as normal. I guess if I turn the key OFF and ON a few times to prime the system properly it might fire on the 1st attempt, will test and confirm.

What makes it more difficult is the fact that there are no faults or errors displayed on the DIS or recorded in VCDS. As a precautionary step, because I happen to have one on the shelve, I replaced the fuel filter. I have confirmed with VCDS that the Coolant Sensor is working so that option is off the table. I have a suspicion that when the car gets turned off the high or the low fuel pressure systems doesn’t retain the required fuel pressure and hence the delayed start whilst the system gets re-pressurized when trying to start. I believe the low side should generate around 6Bar but I have no clue what the pressures in the fuel rails should be and what pressures it should maintain when the engine is turned off.

I have done some logging today whilst engine is cold before starting and record the following values.
LPFP/Filter - 4.6Bar
HPFP/Rails - 4.57(Actual) 33.23(Specified)
Unfortunately I haven’t logged the values again after I primed the fuel system by opening the driver side door and turning the key on before starting.

After letting the engine and fluids warmed up to normal temps.
LPFP/Filter - 6.26Bar
HPFP/Rails - 20.00Bar(Specified) 20.05Bar(Actual)

Hope it make sense will appreciate any guidance/ideas. I would like to diagnose and find the cause rather than just throwing new parts at the problem.

Thanks Rudi

You hit the nail on the head. This is actually a common thing on the RS4s… well semi common. The issue can be divided between two things. I think its the evap system that bleeds down the fuel pressure. or there is a check valve in the return fuel line at the tank that can go bad.

When you open your door after a long night the fuel pump primes the fuel system. This prime charge brings the fuel pressure up and you have all you need to start the car. If you leave the car for a short time that pressure is still in the lines and even tho it has bled down a little its not enough to stop the car from starting.

Take and now part the cars for long enough for the fuel lines to bleed down but not long enough for the fuel pump to prime the system. That is when you get the long first crank and fire on the second crank. Like you said. If you just primed the fuel system with the key 2 or 3 times you would probably never see this happen again./

So I have had to diagnose this on a few cars now and the 4.0Ts will actually rarely throw a fault code (that there is a TSB for) but sadly these older ECUs can’t recognize the problem because it happens when the ECU isn’t monitoring the fuel system since it doesn’t have electrical power from the key/ignition being off.

Do you do a lot of short trips where the engine oil doesn’t get up to operating temperature of like 80-100C or 180-200F? If so then fuel is likely getting into the oil and not getting a chance to burn off. I kind of doubt that is an issue for you but I have to ask.

Most likely the fuel pumps are not holding the fuel pressure or the injectors are leaking into the cylinders. Don’t know what your recent maintenance/replaced parts are on this topic so that would be helpful information as well.

Please do the following:

  1. With a cold engine (20-60*C) record coolant temps, low side fuel pressure, and high side fuel pressure specified vs actual (MVBs 3, 103, and 140) along with the engine oil level before starting the engine.
  2. Drive around normally to the point that the oil temperatures get up to at least roughly 100C or 200F.
  3. Go to a location where the vehicle can sit and be worked on.
  4. Shut the engine off and do the next steps quickly.
  5. Pull the low pressure fuel pump fuse.
  6. Turn on the ignition but do not start the engine. Look at and record the coolant temps, low side fuel pressure, and high side fuel pressure specified vs actual (MVBs 3, 103, and 140) for 30 minutes.

Step 6 explanation: you are looking for the low side fuel pressure to stay the same roughly 6 bar of pressure. The actual high side fuel pressure should raise up to probably 50-60 bar progressively over time from heat soak from the engine and then go back down close to the specified value of probably like 20 bar. If the low side pressure drops then the high pressure pumps or low pressure fuel pump are probably leaking pressure either into the fuel tank or engine oil. If the high side fuel pressure doesn’t go up progressively then the injectors are leaking down into the cylinders.

Easiest ways to check things from this point is to do the following:
A. Check the engine oil level with the dipstick and try to determine if you smell fuel in the oil.
B. Pull the spark plugs and check with a boroscope if there is fuel in the cylinders. Could also use a very primitive method of lighting a long barbecue/grill lighter or map gas torch as far down the spark plug tubes as I have done with proper fire safety equipment to see if the fuel inside the combustion chamber catches on fire but you assume all of the risk for using that method.

I totally scared the crap out of a bunch of other techs when I first had to test this on a Q5 3.2 FSI V6 and the injectors were leaking down. Lit the map gas torch in the hole for cylinder 4 and the whole side of the engine caught on fire. Good thing I was wearing a MIG welding glove, jacket, and face shield along with had an air powered blow gun in my other hand and a fire extinguisher on the floor behind me, lol.

Here is a link to AZ where they posted the TSB for the 4.0T to show context to what I am saying. Please keep in mind that the 4.0Ts are similar but different than these RS4 engines.


WOW that was a helpful write up. Great job Jimmy

Hi Jimmy,

Thanks, excellent reply and post, some good information!

Affirmative, I don’t do much if any short trips.

I have done most of the things you mentioned. I have logged the MVBs 103, 106 and 140. From what I have seen almost everything looks whilst the engine is running but I am a bit worried about the high side fuel pressures not going up that much after I have driven and parked the car.
I will do that again this weekend just to confirm everything but I have a suspicion that I have either a leaky HPFP or injector. Question, if an injector is leaking shouldn’t that produce quite a plume of black smoke on startup? But the low side pressures as far as I can tell is within spec so that would then rule out the HPFPs according to your Step 6 explanation correct?

Oh, and I think I will skip the gas torch down the spark plug tubes test, LOL!!

Do you have any idea by how much the high side pressure could drop when sitting over night or longer? I have found that it drop all the way down to the low side fuel pressure and stay equal to that. I don’t know whether that is normal or not.

Appreciate the feedback that will help me a lot to get to the bottom of this. Will do some logging and monitoring this weekend and report back with the results.

Justin, thanks. I have had to deal with this a lot.

Raudi, well check the values and let us know. I like to make notes every five minutes or so.

Pretty sure that the high side pressure will drop to the low side value overnight normally.

It is possible to get some black smoke at start up if the injectors are leaking but if you have any catalytic converters (stock or high flow aftermarket) then they will catch most of the fuel and burn it off especially if you don’t drive short trips. Kind of doubt that you still have cats but a bigger 2.75" exhaust has places for spare fuel to sit as well. Then the injectors don’t actually spray a lot of volume most of the time so it would need to be pretty far stuck open to put out noticeable black smoke.

The low side pressure drop isn’t much compared to the high side pressure change. In the past I have seen the low side drop from like 5.5-6.x bar while running to like 4-4.5 bar when the hpfp isn’t holding pressure. These cars need about at least 5.5 bar to start up on the first try consistently.

For example, my B7 A4 2.0T sedan currently has a dying hpfp and when I go out to start it in the morning after sitting all night the first crank takes two seconds to three seconds longer than it would if I turn the ignition on and wait for the low side fuel pump to build up the fuel pressure from 3.7-4.0 bar up to 5.5 bar. Occasionally throughout the day there will be a slightly delayed start up/long crank as well, especially if I go to a late lunch and the car sits for a few hours (2-3). The specified vs actual high side fuel pressure is close while driving but the regulator percentage (MVB 106 value) is pretty high to compensate for the dying hpfp. Been busy and haven’t replaced the hpfp yet because I want to put a higher pressure/modified one (Autotech) on with the bigger exhaust and tune as well.

Hi Jimmy,

Thanks, manage to do some monitoring and my findings based on your information and from what I have seen is that I might have a leaky injector. Hopefully not a HPFP but perhaps you will know better.

All the fuel pumps are still the original except I upgraded the HPFPs with the JHM upgrade kit when I did the Stage 1+ install. The fuel filter is brand new.

Here is what I found. Went out to an event yesterday and the drive back was quite long so all temps were up to required operational limits by the time I got back home.
Pulled straight into the garage when I arrived home and left the car sit idling whilst I connected the laptop and fire-up VCDS. Once setup I shutdown the engine, quickly pulled the fuel pump fuse and switch the ignition back on and recorded the fuel pressures GRPs 103, 106 & 140, as per Point 6.
From the evidence its clear that I have a leak down in the fuel system and probably its a leaky fuel injector because evidence points to the high side.
See the pic’s below, no increase in pressure on the high side, pressure start to drop off immediately on the high side and were down to the low side pressure within around 18 minutes! I am also not totally convinced about the low side though?

Another thing I did was to open the oil cap the moment I switched the engine off. Now normally you will get a distinct oil smell, instead there was a very prominent gas smell more so than the normal oily smell, if it make sense!

I can monitor the low and high fuel delivery specified vs actual live with the vFIZ system I have installed whilst driving and everything is within spec. The high side actually request & deliver more but that must be because of the upgraded HPFPs. If anything the low side might ever so slightly struggle to keep up and it might be a good idea to do the JHM LPFP upgrade sometime in the future. For now though my priority is to solve the annoying warm start problem.

Looking forward to your input.

If you get a prominent fuel smell. Check you LTFT data quickly If you can. Please post that. If you’re getting a fuel smell under the oil cap there is a chance your seal is leaking on your HPFP. That is something that is and can be bleading down your FP as well. Post your LTFT data if you can. that should tell you more

The thought did cross my mind that it might be a seal or O’ ring on one of the HPFPs but I just don’t know how to tell for sure I guess the only way is to take them off and do a visual check!. Well, I got some snap shots of the LTFT you asked for and I am posting 4 pic’s. All were taken over a period of time with the engine running.

First and 2nd ones are basically how it was since I last cleared them which is a long time ago. The 3nd and 4th ones are from after clearing the codes today so not sure what to look for really. As far as I can tell the LTFT seem to indicate a slight “rich” condition but as said I am no expert. Its something I need to educated myself on I do have a basic understanding of them but not enough to form any informed conclusions.

I also made some short video clips of the STFT as well so let me know if you need to see that.

Time: 1:22pm

Time: 1:32pm

Time: 1:40pm

Time: 1:48pm

Greatly appreciate the updates!

So a simplified explanation is that you are losing high side fuel pressure rapidly along with low side fuel pressure slightly slower doing the test I described along with a strong smell of fuel in the oil.

Those long term fuel trims are fine in my opinion. 0% is ideal but not likely to happen in real life application. Generally anything between from -10% to +10% is fine in my opinion. Pretty sure that the ECU/s have to see +/-25% to set a fuel trim too lean (positive percentage) or too rich (negative percentage). I should tell you that clearing the ECU fault codes even when there are no codes present will reset the long term fuel trims to 0% and turn the emissions readiness monitors to not ready. The emissions readiness monitors don’t matter to most people unless they have to pass an emissions test just like I had to get a client’s RS4 ready for just a few minutes ago.

The next best step is to see if there is fuel inside the cylinders to confirm if the injectors are actually leaking down. Get a boroscope and pull the spark plugs to visually check. I would also remove the high pressure fuel pumps to visually check if the internal seal looks like it is leaking. It is possible for the fuel to get past the piston rings or thru the cam follower opening of the cylinder heads for the high pressure fuel pumps. You need to determine the most likely leak source and this would be relatively easy diagnostic steps.

On a side note, I wonder if you could in theory do a cylinder leakage test on a direct fuel injection engine like a RS4 with sticking open fuel injectors to see the fuel pressure raise up to about 90-100 psi or 6.20-6.89 bar on the high side fuel pressure reading. Just a random thought.

Thanks for the feedback, everything make sense.

Did some reading on the Fuel trim’s and agree with you that everything looks normal and within spec.

I will chase the problem down systematically now that I have a good understanding of how the fuel system fits together and work. I have started to look for a boroscope yesterday and will try and buy one as long as it isn’t ridiculously priced but I figure a relatively cheaper one should work just fine.

I have pulled the HPFPs before when I installed the blower so that’s a fairly simple job. Can’t help but to cast my mind back to when I installed the JHM High Pressure upgrade kit and question whether I did some damage to one of the seals back then but I am sure it would have started to leak a lot sooner? Anyway will pull them and see if anything looks damaged. In your experience with these pumps are you aware of any after market seals? Thinking more about the bigger internal seal that fits over the shaft. O rings shouldn’t be a problem but I know there is a special seal that actually prevent fuel from leaking down the shaft and spring assembly into the cylinder heads. Will do some research on that can’t stomach the idea of having to replace a otherwise good pump if it’s just a seal that Audi doesn’t supply separately.

As far as the Fuel Injectors are concerned will do the scope inspection and should that proof to be the problem I will just replace all with new. I know I can have them cleaned up but would rather just install new ones with new seal kits.

Lastly, not convinced its the problem but I might just go ahead and order the JHM LPFP upgrade kit. I somehow feel its struggling to keep up with the upgraded HPFPs especially under WOT conditions.

Not sure about the cylinder leakage test, have never done anything like that so will probably leave for the last to do thing on the list and will talk to you in a bit more detail when the time arrive.