Car won't start after sitting for 30 - 50 minutes.

[Saugrim]

So I've got access to VAGCOM to check sensor values, and have scanned all codes to find that there are none. I have considered a few of the common culprits, but it's strange because most people have a consistent issue. My car will start every time without an issue if it has been sitting overnight, a few hours after stopping, or even immediately after stopping. The issue only crops up after I've had the car sitting for 30 - 50 minutes after stopping the warmed engine. I have tried to prime the fuel pump multiple times just to check if maybe that's an issue, but it doesn't change the outcome.

When I attempt to crank it sounds strong, it sounds like the car wants to start, but it just won't turn over. I also have had some luck by simply giving it additional gas while trying to start it, which will make it start up occasionally. The starter motor, alternator, and battery have all been tested and are good. I've also tested the coolant temperature sensor with VAGCOM and it seems to be displaying values that I'd consider normal.

I'm a little stumped on what could cause an issue like this to happen so irregularly. Any help would be appreciated, thanks!


2006 Audi A4 2.0T Quattro

TL;DR
After diagnosing the fuel rail, the LPFP was found to be operating at 68% at idle; far outside of spec. The LPFP was replaced. This didn't fix the issue.
Misfires were detected on all 4 cylinders, and a lean fuel code was given, spark plugs were replaced. Issue persisted.
Finally after running the oil cap test the PCV valve was suspected to be faulty. After replacing the PCV valve and clearing codes, the engine ran without misfires, and started without issue.

For further information read: https://www.audizine.com/forum/showt...tarting-issues
It has some good info on this exact problem, though the fix was slightly different.

[texadelphia]

I was having similar issues a few months ago. Ended up replacing HPFP and LPFP but it was kind of parts darts. But ever since changing these, the problem hasn't come back. Here's the thread:

https://www.audizine.com/forum/showt...tarting-issues

[Saugrim]

Funny enough I had been thinking about the HPFP. I know that the cam follower goes bad in these cars and can damage the HPFP. I had another 2006 where I had to replace those two parts, but the major difference between our two issues is that my car seems to run fine after it does finally start. There hasn't been any stalling or anything of that nature. Maybe the problem is still in its infancy and that's why it's not as bad but... I'm not sure. It does sound like we had very similar issues though.

[texadelphia]

Well that stalling thing happened a couple times and there were a couple times that I had the same exact issue as you. Stopped the car, sat for 30 minutes, tried to start and it would just crank and crank. It would finally fire up and run fine. It was all very inconsistent and unrepeatable.

[Saugrim]

Sounds about right, I'll have a shop test some of this stuff including the HPFP. I don't have the equipment required to test the fuel pressure through-out the system, so unless there's some way to test for the issue without some fancy tools I'll probably have no choice. With that said, I'm definitely going to narrow my search on the fuel system, everything I've been reading, and everything I've been experiencing seems to hint at some issue within that system.

[Saugrim]

A quick update, I used VCDS to look at the pressure values along the fuel line when the car is running. As expected they all seemed okay. The only thing that was strange was the adjusted value for the high pressure fuel system, but I was guessing that due to it being an adjusted and not actual value it probably doesn't matter much. That value was running around 0.5 at the maximum and 0.02bar at the minimum.

The fuel rail adjusted the PSI as expected when giving it extra gas, and the actual fuel consumption also increased in a way that'd be expected.

I didn't find any way to check the pressure coming right out of the HPFP with VCDS. I also doubt it'd help too much since I also couldn't find any way to check the values when the car wasn't running, which if it won't start the computer doesn't seem to log?

Anyways, that's just a quick update! I'll probably bring it to a shop next and have proper diagnostics run on it.

[EvolutionArmory]

What’s your coolant temp sensor reading when it won’t start? If it’s reading really low it could think that your car is colder and is dumping fuel thinking it’s below zero and it’s flooding. Too rich to start.

VCDS block 004.

Say you check the temp when it’s been sitting overnight. Your temp reading should be around ambient temp. It should also be 90 degrees warmed up. Now check it after it’s been sitting for a while and won’t start. It should be somewhere between ambient temp and 90. Not to temp but not ambient either. If your reading is way colder or way hotter than where it should be, you need a new coolant temp sensor.

This might not be what you’re chasing but any time I see a weird no start condition, I check the temp sensor because the temp sensor helps adjust cold start fuel trim.

[EvolutionArmory]

Another odd thing you could try is when it won’t start, crack your gas cap and see if it vents pressure and will now start. If it does, you got EVAP system problems.

[Saugrim]

Another odd thing you could try is when it won’t start, crack your gas cap and see if it vents pressure and will now start. If it does, you got EVAP system problems.

So, I decided to try both of your pieces of advice. I checked the engine temperature before I left for an errand today, this was about 2 hours after shutting down the car. It was at 53c, started just fine, ran just fine. I had a passenger check the fuel pressure the entire trip, and everything seemed to respond to throttle as expected. Fuel rail was between 50 - 140bar, and overall fuel pressure was at 5 - 6bar. The high pressure fuel pressure according to that sensor was sitting at 0.2 - 0.8.

Now, I just tried to start it after that errand. I set a timer for 30 minutes, went down there, and the temperature was registering at 90c which made sense considering it was still toasty. The car refused to start several times, and I decided to try your gas cap trick to no avail.

https://1drv.ms/v/s!AsprRHq974kcg9wc...EOAEw?e=8cMEep
This is a video that displays the last few attempts. It finally started but it sounded pretty bad, like an exhaust leak or something... I don't know... maybe someone will know better than I. I should mention I forgot to put the cap back in when this finally did start, so I'm not sure if that effects the reading at all. What stood out most to me was the fact that the high pressure fuel sensor registered at 0.00. I'm not sure if that's normal or not. I know once it gets started properly it seems to hover around 0.5 at first.

I should mention this is the first time it has ever sounded like this after starting up. All the other times it started up more or less normally once it finally did.

Thanks,
Saugrim.

[Theiceman]

I dont tnink..many will.sign up.for one drive to view your vid. Post it up to YouTube if you really want people to watch
.

Sent from my SM-G973W using Audizine mobile app

[Saugrim]

I dont tnink..many will.sign up.for one drive to view your vid. Post it up to YouTube if you really want people to watch
.

Sent from my SM-G973W using Audizine mobile app

Strange... Most have been able to view that without logging in. Despite that fact I uploaded it to YouTube per your mention.

https://youtu.be/Q6ofQkZvlig

Thanks,
Saugrim

[texadelphia]

Strange... Most have been able to view that without logging in. Despite that fact I uploaded it to YouTube per your mention.

https://youtu.be/Q6ofQkZvlig

Thanks,
Saugrim

I was able to view online without OneDrive sign in.

[Saugrim]

I was able to view online without OneDrive sign in.

Which is how it's setup yeah. :)

I don't suppose that video offered any insights though eh?

[texadelphia]

Which is how it's setup yeah. :)

I don't suppose that video offered any insights though eh?

Not really....sorry. FWIW, it seems very similar to how mine was behaving.

What block are you looking at that's showing the 0.0 reading for the high pressure fuel sensor? Is it 0.0 before you turn the key, while it's cranking, or while it's running?

[Saugrim]

So that's in the adv. measurement area. I was looking at block 003 for checking the temperature, and simply pulled that sensor up in adv. measurement along with these others. I'd have to check and see where that value came from.

The 0.0 measurement is while cranking, and before I turn the key in the video, and even while running for a bit. Typically I saw that sensor reading around 0.2 - 0.5 during normal operation. I wasn't sure if that's a sensor for the HPFP or not though.

What's odd to me is that the fuel rail seems to still be getting a fair bit of pressure, if the HPFP was broken would that still be the case? Especially to an extent of where the car wouldn't start? I obviously don't know enough about these vehicles to know off the top of my head. I think what I'll do is actually go and fire up the other car that I own, same year, same engine, same model, etc... see what these values toss up in that one. Might at least give me a comparison to draw from.

[Saugrim]

I decided to check that value on the other car and it ended up going negative. So I'm not sure what that value is or if it's important at all.

I decided to look up how to diagnose a HPFP and found this thread: https://www.audiworld.com/forums/a8-...valve-2985928/
There was also a video that suggested using blocks 106, 140, 141, and 230. : https://www.youtube.com/watch?v=k50Z5JLfuJs

With the video it was displaying the high-pressure adapted system at -0.33bar, which is right around the same value that the other car was reading at. That car had a HPFP replaced about 20000 miles ago.
My car reads the high-pressure adapted system at around 0.1 - 0.65bar. I have not driven the other car due to some other issues it has, just old brakes essentially. But, I would imagine that big of a discrepancy might show an issue?

If these sources are accurate 141 is the block for the high pressure fuel pump.

Seems to be a HPFP then?

[texadelphia]

Not sure. I believe adaptation is just how much the computer is trying to correct the fuel pressure so I don't know if those numbers are problematic or not. I think in theory the ideal number is 0.0 but maybe someone can correct me. Looks like your fuel rail pressure is good so I wouldn't suspect the hpfp based on that value at least.

[Saugrim]

Not sure. I believe adaptation is just how much the computer is trying to correct the fuel pressure so I don't know if those numbers are problematic or not. I think in theory the ideal number is 0.0 but maybe someone can correct me. Looks like your fuel rail pressure is good so I wouldn't suspect the hpfp based on that value at least.

Yeah and see that's where my confusion really sets in. Because if it were the HPFP I'd imagine that my fuel rail pressure would be low, but according to the values that I'm seeing, it seems like the pressure it right where the computer wants it to be? But that just creates an even bigger mystery because obviously if it's not something with the fuel system, and it's not something with the electrical system, and it's not something with the temperature sensors... I'm not sure where to even check next.

I also believe that you're right. Pretty sure that number is just how much the system is trying to correct it. But, I'm not sure how to diagnose a failing HPFP either. Because yours was at least partially to blame, but you didn't notice any lack of power or anything right? So that means at some point the HPFP started operating normally even after the starting issues? Kind of similar to mine?

[texadelphia]

Well I also replaced my low pressure fuel pump around the same time because it was running with a high duty cycle. I suspected that maybe the low pressure pump wasn't supplying enough fuel to the high pressure pump. Also I never checked the fuel rail pressure before replacing it to diagnose the hpfp. I was just going off the Audi tech's diagnosis that he suspected it was bad. Have you checked the duty cycle on your low pressure pump? Block 106 I believe.

Other things to consider are the coolant temp sensor and the crankshaft position sensor. If these are going bad, sometimes elevated temperatures can exacerbate the problem. Mine were both recently replaced which is why I didn't suspect them.

[Saugrim]

After reading your post I had been keeping an eye on the duty cycle of the LPFP and found that it sits around 11% when the vehicle is first turned to the ignition on position, and once it's started up at idle it's about 56% if memory serves. I believe that was in spec though? The actual fuel pressure through the LP line jumps up to about 5bar which again I believe is within spec?

As for the coolant temperature sensor, that has always been reading where I'd expect it to read unfortunately. I haven't considered the crankshaft position sensor though, that might not be a bad idea to check. Do you know what block that is by chance? Else I'm sure I can find it.

The other thing I did was a fuel pressure rail test per the earlier linked video, and the pressure rose after the car was turned off. Which they say is expected and within spec, since the heat soak causes the pressure to increase. Essentially with the engine off the fuel pressure rail would start at 50bar and then slowly rise as time went on.

[esandes]

How do the coil packs and plugs look? Do you see oil leaking around the valve cover gasket? Look for oil and brown spots.

- - - Updated - - -

Oil and brown spots along and on the spark end of the coilpacks.

[Saugrim]

How do the coil packs and plugs look? Do you see oil leaking around the valve cover gasket? Look for oil and brown spots.

- - - Updated - - -

Oil and brown spots along and on the spark end of the coilpacks.

I attempted to pull out the coilpacks tonight, however one of the tabs is broken and I wasn't able to figure out a way to get it off in the limited light I had. Didn't want to break anything. I'll have to look into pulling the packs tomorrow.

[RickB_7]

Have you tried a new fuel pump relay? Any warning lights flashing when it won’t start?


Sent from my iPhone using Tapatalk

[Saugrim]

Have you tried a new fuel pump relay? Any warning lights flashing when it won’t start?


Sent from my iPhone using Tapatalk

No warning lights flashing when it won't start, and I have not tried a fuel pump relay. But, if the fuel pressure seems good even when it won't start would that have anything to do with it?

[EvolutionArmory]

Trying a new fuel pump relay couldn’t hurt. It also wouldn’t be a bad idea to look at the fuel pump controller to see if it’s getting a little melty or has burnt connector pins.

[EvolutionArmory]

I attempted to pull out the coilpacks tonight, however one of the tabs is broken and I wasn't able to figure out a way to get it off in the limited light I had. Didn't want to break anything. I'll have to look into pulling the packs tomorrow.

His solution for everything is coil packs. If it was a bad coil pack, your car would be misfiring. And if it was a bad coil pack, your plug would be wet with fuel from not igniting.

The fact that your car runs fine when it actually starts basically eliminates spark issues from coils or plugs.

I have never seen a coil pack work sporadically. When they fail, they fail hard and you get misfires.

Trying a new crank position sensor isn’t a bad idea either. It’s actually a pretty good idea.

Your duty cycle at idle is technically in spec but at 56% at warm idle (has to be up to operating temp) 56% seems to be on the higher side that could be problematic for some people and not a problem at all for others. If you checked the duty cycle right when you started the car cold, drive it up to operating temp and check it again. It SHOULD go down. If it goes up, replace it.

If you throw in a new crank position sensor and it doesn’t fix the problem, maybe focus on the 2 or 3 main engine control relays under the ECU or the low pressure fuel pump and the fuel pump controller.

[Saugrim]

His solution for everything is coil packs. If it was a bad coil pack, your car would be misfiring. And if it was a bad coil pack, your plug would be wet with fuel from not igniting.

The fact that your car runs fine when it actually starts basically eliminates spark issues from coils or plugs.

I have never seen a coil pack work sporadically. When they fail, they fail hard and you get misfires.

Trying a new crank position sensor isn’t a bad idea either. It’s actually a pretty good idea.

Your duty cycle at idle is technically in spec but at 56% at warm idle (has to be up to operating temp) 56% seems to be on the higher side that could be problematic for some people and not a problem at all for others. If you checked the duty cycle right when you started the car cold, drive it up to operating temp and check it again. It SHOULD go down. If it goes up, replace it.

If you throw in a new crank position sensor and it doesn’t fix the problem, maybe focus on the 2 or 3 main engine control relays under the ECU or the low pressure fuel pump and the fuel pump controller.

Alright, so I decided to follow your advice. I ignored the coil packs because those seemed like an oddity in the first place. Instead I returned to where this all began. I tested the fuel rail last a few days ago, and since then I have done a lot of testing, messing about, and finally today I got a value that I believe indicates that it's either the LFPF or the HPFP. The trouble is, I've heard that a failing HPFP can cause the LPFP duty cycle to increase too, is there a way to determine which of them is failing?

https://1drv.ms/x/s!AsprRHq974kcg9wj...jFrGQ?e=Zf1kYr

This is the log I took with VCDS, can see the duty cycle has now risen to 68% with a warm engine. I should note that the coolant temperature was registered at 90c so this was a warm engine and this reading was taken less than 5 minutes after returning home from my commute.

Edit: I should mention during this log it started pretty anemic, and certainly sounded rougher than usual.

Thanks,
Saugrim

[EvolutionArmory]

At 68% duty at operating temp idle, you need a new fuel pump. No question.

[Saugrim]

At 68% duty at operating temp idle, you need a new fuel pump. No question.

Alright, so presuming the LPFP is the issue... https://www.1aauto.com/audi-a4-quatt...5/1170421/2006
This one will be adequate/correct?

I presume that the actual control module should be fine to switch from one pump to the other then?

Thanks,
Saugrim

[Jeevan]

Alright, so presuming the LPFP is the issue... https://www.1aauto.com/audi-a4-quatt...5/1170421/2006
This one will be adequate/correct?

I presume that the actual control module should be fine to switch from one pump to the other then?

Thanks,
Saugrim

I wouldn't trust any brand other than OEM manufacturer VDO. I think FCP and ECS has them listed just below 200$

There are very few things you are supposed to use proved Genuine or OEM parts. Fuel pump is one of them. You don't want a tiny spark to be generated inside the tank because of a cheap Chinese knock off. All you need is tiny light source to ignite petrol. Watch this

https://youtu.be/gct1BmKNvU0

Sent from my iPhone using Audizine

[EvolutionArmory]

Even the VDO pumps can be iffy even though they’re suppose to be the same exact thing.

Get a VDO pump from FCP Euro because if that pump fails, they’ll just give you a new one.

Or suck it and buy one from the dealer counter or ECS. ECS usually has the best prices on genuine Audi parts.

And yes, you only have to adapt the fuel pump control module if you replace the ECU or the pump module, not the pump.


Sent from my iPhone using Audizine

[Saugrim]

I wouldn't trust any brand other than OEM manufacturer VDO. I think FCP and ECS has them listed just below 200$

There are very few things you are supposed to use proved Genuine or OEM parts. Fuel pump is one of them. You don't want a tiny spark to be generated inside the tank because of a cheap Chinese knock off. All you need is tiny light source to ignite petrol. Watch this

https://youtu.be/gct1BmKNvU0

Sent from my iPhone using Audizine

Yeah, I'd certainly agree with that proposition. I wasn't sure who the OEM manufacturer was in this case, but I'll search for the VDO OEM part in this case.

Thanks,
Saugrim

[Saugrim]

Ended up purchasing:

8E0 919 051 CQ - Fuel Pump Assembly with Fuel Level Sending Unit CONTINENTAL VDO
8E0 201 263 G - Fuel Pump Adapter CONTINENTAL VDO
4F0201511E - Mann Fuel Filter
1J0919133B - GENUINE Sealing Ring

So, hopefully when these parts arrive that'll fix the issues. I'll update then, just encase anyone has similar issues in the future.

Thanks so much for all the help,
Saugrim

[texadelphia]

Ended up purchasing:

8E0 919 051 CQ - Fuel Pump Assembly with Fuel Level Sending Unit CONTINENTAL VDO
8E0 201 263 G - Fuel Pump Adapter CONTINENTAL VDO
4F0201511E - Mann Fuel Filter
1J0919133B - GENUINE Sealing Ring

So, hopefully when these parts arrive that'll fix the issues. I'll update then, just encase anyone has similar issues in the future.

Thanks so much for all the help,
Saugrim

That's exactly what I put in mine. Bought from FCP Euro. So far so good.

[Saugrim]

That's exactly what I put in mine. Bought from FCP Euro. So far so good.

So, I finally got a chance to get all the parts put in yesterday. Everything seemed good, but then today it started sounding anemic right after starting, and during some starts it'd die again. It didn't have as many troubles starting, but it's still not perfect. I'll do some deeper diagnosis in the coming days, but it seems that all of that did NOT correct the problem entirely. HPFP or fuel controller? Not sure at this point what could be causing it.

[esandes]

How's that other advice working for you? How do the plugs look? Try a fresh set. No codes are showing? If that doesn't work try replacing the cam position sensor. It's a cheap part that when faulty or going bad can cause bad starts. You'll need a T30 torx L wrench to get access.

[Saugrim]

How's that other advice working for you? How do the plugs look? Try a fresh set. No codes are showing? If that doesn't work try replacing the cam position sensor. It's a cheap part that when faulty or going bad can cause bad starts. You'll need a T30 torx L wrench to get access.

Tonight I replaced all 4 spark plugs after I noticed that the car was misfiring and now showing a lean fuel code. I have checked the new LPFP and that's sitting at 51% usage at idle, and fuel pressure is hovering around 5bar. The fuel rail pressure also seems to increase as expected when providing throttle.

While replacing the LPFP I also replaced the cam follower, and that was actually okay, but I threw a new one in regardless. I have also replaced the N80 valve less than 200 miles ago. I am getting misfires on every cylinder, so I'm doubting that it would be coil packs. The actual coil packs also seemed perfectly fine, nice and clean, no burning, no browning, etc.

I haven't done the PCV valve in this car yet, perhaps that would be a good place to start?



Edit: I decided to diagnose further. The vehicle failed the oil cap test, and it also had the characteristic hiss after shutting down that failing PCVs often exhibit. I've bought a new PCV and the corresponding gasket. They'll arrive Wednesday, I'll update then on whether or not it corrected the problem.

[Saugrim]

Tonight I replaced all 4 spark plugs after I noticed that the car was misfiring and now showing a lean fuel code. I have checked the new LPFP and that's sitting at 51% usage at idle, and fuel pressure is hovering around 5bar. The fuel rail pressure also seems to increase as expected when providing throttle.

While replacing the LPFP I also replaced the cam follower, and that was actually okay, but I threw a new one in regardless. I have also replaced the N80 valve less than 200 miles ago. I am getting misfires on every cylinder, so I'm doubting that it would be coil packs. The actual coil packs also seemed perfectly fine, nice and clean, no burning, no browning, etc.

I haven't done the PCV valve in this car yet, perhaps that would be a good place to start?



Edit: I decided to diagnose further. The vehicle failed the oil cap test, and it also had the characteristic hiss after shutting down that failing PCVs often exhibit. I've bought a new PCV and the corresponding gasket. They'll arrive Wednesday, I'll update then on whether or not it corrected the problem.

Replaced the PCV today. Car is running without any misfires now, starts up first turn, and otherwise seems to be fixed. I'll add a TL;DR in my initial post for anyone that ends up having this issue in the future.

Thanks so much for all your help everyone!