Help me - Diagnose my intermittent 'hard crank'/no-start

[Dasfinc]

So my 2001 Audi AllRoad (2.7t Quattro A6 Avant basically) is more or less fully re-assembled, and I am trying to wrap up its issues before the weather gets too nasty.

One issue it has is it seemingly randomly has a hard or no start, where it just cranks and cranks but doesn't start. Sometimes it seems that a dab of the throttle seems to help, and it seems to happen commonly while low on gas, or shortly after a fill-up. It seems to happen more commonly on cold start, but happens while still warm as well on occasion.

Today it full on stalled out at a stop light, and again as parking while nearly completely empty.

The CELs it has are all emission control related, which would not point this at a sensor failure. I also don't think it's wiring or power related, especially after it cutting out today at a light while super low on fuel, and again as I came to a stop parking.

It has not cut out while under load, or experience any kind of miss.

Leaves me thinking Fuel Pressure Regulator, Filter, or Pump.

The filter I'll try tackling tomorrow as its cheap insurance and I have no idea when it was last done, and I will take a look at the pump since it has a fairly easy to access panel, but as its so randomly intermittent, and doesn't have issues under Load AND I hear it prime and it's not making any loud or 'bad' noises it makes me think FPR since that's tied to Vacuum, which may be related to some of my emission CELs?

Thoughts? Things to test/check?

Click to read more...

 

[OffshoreDrilling]

problem: it's Audi


but, I'd guess FPR bleeding down pressure.


any correlation between when it does it and how long it's been sitting since it was last started?

 

[Dasfinc]

problem: it's Audi


but, I'd guess FPR bleeding down pressure.


any correlation between when it does it and how long it's been sitting since it was last started?

Doesn't seem like it, I parked it for 30 minutes the other day and it took like 6 tries to start, sat over night and it has a hard start, but started.

And yes, I call it my little German problem child, but it rides REALLY nicely, which may not be saying much compared to what I normally drive. It's the absolute perfect car for the Dogs and for bad weather also.

 

[OffshoreDrilling]

can you leave a gauge on the rail and take note of it when you shut it off and start it
up?

 

[Dasfinc]

can you leave a gauge on the rail and take note of it when you shut it off and start it
up?

I'll have to check and see if I can or not, that's a good question though.

 

[wolfe]

Filter would be my first replacement. I would also check plugs just cause. Why are you running a car low on gas that you believe has fuel problems?

 

[wolfe]

Would also clean the maf and throttle body. If pressing the gas a little helps it could be the throttle body. At least I'm assuming it's drive by wire. I know the throttle bodies can get touchy if dirty when drive by wire. It could be something to do with that as well, either pedal or tb. I would also run some injector cleaner through it.

 

[Dasfinc]

Filter would be my first replacement. I would also check plugs just cause. Why are you running a car low on gas that you believe has fuel problems?

Been letting our friend drive the car that's staying with us, plugs are fresh. I tanked it today after it was having issues and it's been fine for the rest of the day.

Would also clean the maf and throttle body. If pressing the gas a little helps it could be the throttle body. At least I'm assuming it's drive by wire. I know the throttle bodies can get touchy if dirty when drive by wire. It could be something to do with that as well, either pedal or tb. I would also run some injector cleaner through it.

Ran injector cleaner when I first got it, might toss another bottle through.

It IS drive by wire, very early at that being an 01. I'll clean both, couldn't hurt, but I'd be hard pressed for that to explain it cutting out at idle, and not throwing a CEL code (such as a miss or lean/rich condition).

 

[Mike K]

Check if it has the same fuel system as the other cars like the A8. If it does then a single fuel pump failure will manifest itself in all sorts of funky ways.

On my A8 there was a pump in each side of the tank with a balance line that went through a tunnel to feed from one pump to the other and then two Venturi pumps at each side that fed the opposing pump. So in theory your driver's side Venturi pump would feed your passenger side fuel pump which in turn would feed the driver's side fuel pump.

It's a really simple system if by simple you mean completely fucked.

 

[Dasfinc]

It's a single pump system with some kind of cross over, SUPPOSIDILY the previous owner replaced the Pump, but I don't necessarily believe it (neither do I want to replace it if it was already done)

I've been told that fuel pump failures on these cars are usually Loud, and/or 'all or none'

Not having issues under load makes me not think the pump is shitting the bed, but I agree that pump issues can manifest in weird ass ways.

 

[Turbocharged400sbc]

keep in mind that by its nature the carbon track tps/APPS are wear components.
it used to be youd have to replace just the tps on the tb, now you have two to 3 in the pedal and 2-3 of em in the TB.
the redundancy of 2 to 3 different ramp/slope rates letting the pcm compensate for just one traces failure is nice, however they all end up worn most at below 25% and can end up with trace fractures causing millisecond/pico second glitches that cause all sorts of funky issues.

I am actually monitoring the failure of the APPS traces on my 04, which is only a two trace system, it leaves quite noticible intermittent dead spots/stutters at 7 and 11% throttle, 7% glitch point is actually hit while using the cruise...I noticed the issue via using the cruise to tell you the truth....it never did it via the cruise acceleration, only via the pedal assembly.

have fun computer boy

 

[OffshoreDrilling]

ever think about the fuel pump relay sticking? swap it around with another one in the fuse box and see if it changes anything

 

[Dasfinc]

keep in mind that by its nature the carbon track tps/APPS are wear components.
it used to be youd have to replace just the tps on the tb, now you have two to 3 in the pedal and 2-3 of em in the TB.
the redundancy of 2 to 3 different ramp/slope rates letting the pcm compensate for just one traces failure is nice, however they all end up worn most at below 25% and can end up with trace fractures causing millisecond/pico second glitches that cause all sorts of funky issues.

I am actually monitoring the failure of the APPS traces on my 04, which is only a two trace system, it leaves quite noticible intermittent dead spots/stutters at 7 and 11% throttle, 7% glitch point is actually hit while using the cruise...I noticed the issue via using the cruise to tell you the truth....it never did it via the cruise acceleration, only via the pedal assembly.

have fun computer boy

None of the behavior I've described I feel points to TPS issue, Under load, and normal driving conditions, it doesn't have any issues, I'd expect all kinds of funkiness during driving if that was the culprit. I think most things point to a simple fueling issue that is just hard to pin down.

ever think about the fuel pump relay sticking? swap it around with another one in the fuse box and see if it changes anything

I've read relay issues are not uncommon in these cars, but again, normally its a "its Dead or its Not" kinda thing. I have to figure out where its even located on this car, there are only like 3 serviceable relays on this car (one of which is Fuel), everything else is in the ECU. I don't think there is another relay I can just 'swap out' for that test unfortunately.

 

[Turbocharged400sbc]

just remember that wear increases nearly exponentially towards the idle position/angle of the throttle blade.

im just saying don't discount a potentiometer issue until you put it through its paces with a DSO recording with ms glitch capture and not find anything

 

[Dasfinc]

just remember that wear increases nearly exponentially towards the idle position/angle of the throttle blade.

im just saying don't discount a potentiometer issue until you put it through its paces with a DSO recording with ms glitch capture and not find anything

I can actually do TPS Positioning tests with the VAGCOM (where it will read me back the position of everything based on sensor readings), The ECU and utilities behind the ECU of this car are VASTLY more powerful than most of what you work on . Regardless, these cars aren't known for those kinds of electronic issues to be honest. They are more prone to micro-switch failures in the doors/windows/sunroofs/etc, the ECU is Bosch ME7, Pretty sharp stuff.

 

[Turbocharged400sbc]

lol ive yet to see a pcm that can diagnose or capture a glitch and trip a code.

I don't even think the newest systems do even...

 

[Dasfinc]

lol ive yet to see a pcm that can diagnose or capture a glitch and trip a code.

I don't even think the newest systems do even...

The VAGCOM will read me back where the throttle position is at in clear text? If the issue was severe enough to cause the symptoms I'm having, I'd imagine I'd see a dead spot jiggling the pedal, or some kind of unusual reading going through the range on the sensors. A difference in reading between the pedal sensor and actual TPS would trip a CEL on these cars... There is a reason why VAG products need a whole different set of tools to Diagnose issues than about anything else on the road.

Either way, its such a long shot for this specific issue, I think you are over thinking it.

Back to Fuel Pump, FPR, or Filter...

 

[bikrboy128]

you have a 15 year old shitbox audi, not a nasa space shuttle. your car doesn't require a "whole different set of tools" to diagnose.

i just had a jag come through with similar symptoms and no codes. it ended up being the connector at the throttle body, one of the wires was pushed back in the pigtail. it would make connection most of the time, but cut out on occasion. i would be checking connectors on the topside of the engine, especially considering how much was apart to take care of the oil leak issues.

it does sound like a fuel issue though. is there a PID for fuel pressure?

 

[Dasfinc]

you have a 15 year old shitbox audi, not a nasa space shuttle. your car doesn't require a "whole different set of tools" to diagnose.

i just had a jag come through with similar symptoms and no codes. it ended up being the connector at the throttle body, one of the wires was pushed back in the pigtail. it would make connection most of the time, but cut out on occasion. i would be checking connectors on the topside of the engine, especially considering how much was apart to take care of the oil leak issues.

it does sound like a fuel issue though. is there a PID for fuel pressure?

I know its not a space shuttle, I haven't experienced an ECU and tool set that lets me see so much detail in it before to be honest, its pretty slick stuff. I'm sure its nothing new to full-time mechanics such as yourself . Make no mistake, I absolutely know this thing is a shit-box granted its age and known problem areas, but its the nicest 'winter beater' I've owned by far. I'm used to Explorers/Rangers/Expeditions for winter, having a car with a heated steering wheel, heated front and rear seats, and a million speaker stereo and a built-in dog barrier is a substantial improvement.

I don't THINK it has a sensor for that, so I don't think it'd have a PID for it in the ECU (or if it did, I'd imagine it would throw a code if its dropping below parameters).

This issue existed before we started trying to tackle the top-side oil leak, seems to be worsening/becoming more frequent lately (or specifically when low on fuel).

You have given me an idea though. I'm going to Pop the pump out of the tank tomorrow and take a look to see if the PO fussed up a connector if he actually did replace the pump. He said it was 100% dead when he replaced the pump, and that it had the intermittent issue after the replacement, could be a Pin pushed back, or crimped wire.

 

[Turbocharged400sbc]

the scan tool is giving you data glipses of the actual data. no you will not see it, the data bus is incapable due to the time interval of the glitch (in ms)

often since inputs are weighted/rms'd what the pcm see's is a downtrending in data of those thousands of samples per second, your scanner is sending you less than 100, and that data is often weighted/smoothed.

the only way to capture is via a direct digital storage scope that can divide dow to the ms scale. a 3-7ms glitch of a pcm's 10ms sample rate can mean 20-70% error in a sampling affecting engine running for 1/10th of a second as the pcm is momentarily in a different area of its tables than expected.

no you wont see it, it'd be like expecting to see the microscopic brain of a 3800 owner using a 10 mp I phone camera. you wont find shit till you break out the microscope bitch.

but yeah check fueling, just be aware that ive seen the bosch as well as the Delphi/hitachi shit failing, never count it out till you confirm the wave form. never.

 

[Turbocharged400sbc]

make sure you try priming the fuel system and not priming it to see. iirc that's a deadhead car that could be ending up with empty fuel rails after sitting overnight

btw nice fender, car needs moar daspink tho

 

[Dasfinc]

make sure you try priming the fuel system and not priming it to see. iirc that's a deadhead car that could be ending up with empty fuel rails after sitting overnight

btw nice fender, car needs moar daspink tho

THAT is a good idea, Next time it gets pissy, I'll give that a try.

Lets say I try firing it up in the morning and it gets like 8 cranks and doesn't start. I'll just try priming it 5 or 6 times and see if it fires up.

I'd imagine that would point to FPR issue if fires up with no issue, where it having trouble holding pressure. Next steps I'd imagine would be to check for vacuum leaks to the FPR if that's the case? If no leak, then replace the FPR itself?

If priming it 5/6 times makes no difference, I'd have to assume its pump/filter related?

 

[Turbocharged400sbc]

most of the time its an old worn checkvalve in the fuel pump or the pressure regulator seat.

cranks are you talking revolutions or as in KOff/Kon?

if engine revolutions it should be priming though a weak electrical system means even less voltage at the pump to prime the system.

from now on prime with 4-5 key cycles before every start, if it doesn't have an issue, try not priming, if issue exists, deal with it or replace the pump and regulator

 

[Turbocharged400sbc]

check pump voltage drop at engine cranking, your starter may be drawing too many amps if its near the ned of its life.

 

[Dasfinc]

The battery is some brand I've never even heard of, so that's another wrinkle...

Wouldn't explain the car randomly cutting twice today with low fuel though right?