Stutter rough idle when first starting car and clears up after 3-4 seconds.

[NitroViper]

107° lol.
I dont know, sometimes things look normal when it stutters and sometimes things look abnormal when it starts normally. The only video that looks wacky to me with the one with the timing way off. Ill keep trying but I might just throw the HPFP on and put in a new temp sensor.

[dalmation53]

Hey Nitro check this out. i tried something today actually been doing this a week. before starting the car push the accesories on and off 3 times. let the fuel pump prime 3 times on the 4th start it. it should not misfire on cold start up this makes me think is either HPFP or low pressure fuel pump.

[NitroViper]

Hey Nitro check this out. i tried something today actually been doing this a week. before starting the car push the accesories on and off 3 times. let the fuel pump prime 3 times on the 4th start it. it should not misfire on cold start up this makes me think is either HPFP or low pressure fuel pump.

I am going to change the HPFP and coolant temp sensor this week when I put the k04 on. Im sick of trying to figure it out.

[Benji16]

Lol i mean dalmation has been saying to change the HPFP for quite a while.

I am going to change the HPFP and coolant temp sensor this week when I put the k04 on. Im sick of trying to figure it out.

[NitroViper]

Lol i mean dalmation has been saying to change the HPFP for quite a while.

Ive had the HPFP for a while... I wasnt changing it because we were testing... to maybe find the issue for future and current users with the issue. Im about to do it anyways.

[dalmation53]

please please do. i have been waiting for you to do this lol...

Ive had the HPFP for a while... I wasnt changing it because we were testing... to maybe find the issue for future and current users with the issue. Im about to do it anyways.

[NitroViper]

please please do. i have been waiting for you to do this lol...

I can almost guarantee it wont fix it because of the timing readings... I am only putting it in because I am going to have more power and want reliability. Ill be changing the coolant temp sensor as well so if it does get fixed I wont be able to tell you which it was.

[NitroViper]

please please do. i have been waiting for you to do this lol...

I changed it today, wont know for a few days I guess. Looks a little strange installed but it runs fine. Ill post a pic later. It looks a little crooked but that bracket just wiggles all over the place.

[dalmation53]

yeah take a pic of it i'm not sure what you mean?

I changed it today, wont know for a few days I guess. Looks a little strange installed but it runs fine. Ill post a pic later. It looks a little crooked but that bracket just wiggles all over the place.

[NitroViper]

Seems fine I guess, just didnt expect that gap.

[Traptalk]

Hey Nitro check this out. i tried something today actually been doing this a week. before starting the car push the accesories on and off 3 times. let the fuel pump prime 3 times on the 4th start it. it should not misfire on cold start up this makes me think is either HPFP or low pressure fuel pump.

I think I told ya to try this a couple pages back lol

[NitroViper]

So much for that experiment... $250 wasted.

[JLAllroad]

So much for that experiment... $250 wasted.

I wouldn’t say wasted, I consider these HPFPs a maintenance item at 100k.

Additionally, once you get a tune you quickly find the weak links, and this can be one of them.

Although, would normally want the car at 100% before the tune to limit variables.

Do you have any misfires when driving or wot?



Sent from my iPhone using Audizine

[NitroViper]

I wouldn’t say wasted, I consider these HPFPs a maintenance item at 100k.

Additionally, once you get a tune you quickly find the weak links, and this can be one of them.

Although, would normally want the car at 100% before the tune to limit variables.

Do you have any misfires when driving or wot?

In the 2 cold starts since installing the HPFP its stuttered so bad the check engine light comes on... Then runs fine after 30 seconds or so when the car revs down.

Car runs perfectly fine after it revs down from its high idle at starting. It is only cold starts and until the car idles down that it messes up. I cant believe in all these years no one has a fix for this when it seems to be common. Ill be changing the coolant temp sensor when I change the turbo.

[Traptalk]

Try building prime pressure. It’s free

[NitroViper]

Try building prime pressure. It’s free

Yea its free but will never tell you whats wrong. It gotta be either the LPFP or Coolant temp sensor...

[Chillaxin]

Maybe I should clean the MAF.

On cold start up the engine is running in open-loop control since the O2 sensors are not warmed up and giving good info. Doubt the MAF would cause stuttering on the first 3-4 seconds of cold start. Not a bad idea to clean it once a year though.

[old guy]

On cold start up the engine is running in open-loop control since the O2 sensors are not warmed up and giving good info. Doubt the MAF would cause stuttering on the first 3-4 seconds of cold start. Not a bad idea to clean it once a year though.

Agreed. If you watch my cold start video you can tell when the ECM goes into closed loop. About 30 seconds after the initial start the STFT begins to make adjustments. That occurs when the ECM goes into closed loop.

For the past week I have been closely observing the residual fuel pressure at every cold start. If the car sits overnight the LPFP will engage and the pressure is typically 110→114 psi when I start cranking the engine over. Within less than one second the fuel pressure will peak over 1,000 psi and then drop to 860 psi along with the fast idle.

Cold starts are always smooth with no issues.

Nitro's cold residual pressures seem to be a little lower than mine. Once his engine starts his fuel pressures also seems to level out in the first second. However, his cold start pressure seems to level out at a lower psi than mine on a cold start.

I have no idea if this is a significant difference or not. My gut feel is that it is something other than fuel pressure that is causing your cold start stutter. Did you replace your fuel injectors when you did your recent rebuild? I wonder if you have an under-performing injector? Cold start fueling is pretty critical and having one of your injectors providing a crappy spray pattern could possible be a contributing factor.

[JLAllroad]

My gut feel is that it is something other than fuel pressure that is causing your cold start stutter. Did you replace your fuel injectors when you did your recent rebuild? I wonder if you have an under-performing injector? Fueling is pretty critical and having one of your injectors providing a crappy spray pattern could possible be a contributing factor.

This, in part, is why I was inquiring on misfires(logged) as I believe coils/plugs/injectors are new(er) doesn’t necessarily mean that they are all “good”.


Sent from my iPhone using Audizine

[NitroViper]

Did you replace your fuel injectors when you did your recent rebuild? I wonder if you have an under-performing injector? Cold start fueling is pretty critical and having one of your injectors providing a crappy spray pattern could possible be a contributing factor.

Plugs are new and gapped, injectors are new bosch, intake/dividers and valves all perfectly clean no carbon.
This was happening before I ever removed the engine.

[old guy]

Plugs are new and gapped, injectors are new bosch, intake/dividers and valves all perfectly clean no carbon.
This was happening before I ever removed the engine.

Damn.... I'm running out of ideas.

How long does the stuttering last when it occurs? 20 seconds? All 30 seconds of the cold idle?

[NitroViper]

Damn.... I'm running out of ideas.

How long does the stuttering last when it occurs? 20 seconds? All 30 seconds of the cold idle?

The bad stuttering lasts maybe 2-3 seconds, once in a great while itll trigger a check engine light, obvious misfires. Light goes right off tho. After that sometimes the idle will have little hiccup here and there, once the engine revs down its completely normal.... I swear it has something to do with the engine reading the block temperature wrong, thats what the timing told me in that one video.

If its not cold its perfect, to me it seems like if it was the HPFP it would do it no matter what the temp is, it doesnt change RPMS or pump speed on different start temps. The engine changes the speed but the pump doesnt function any different.

[old guy]

Well lets see what happens when you change the CTS.

[JLAllroad]

How old is your battery? Has it and the charging system been tested?

Guessing the voltage would be lowest and demand would be highest when cold.


Sent from my iPhone using Audizine

[NitroViper]

How old is your battery? Has it and the charging system been tested?

When I bought the car I wondered about the battery because when you start it and the dome lights are on they sort of flash. I had advance auto test it and they said its fine. I dont trust the battery since ive owned it to tell ya the truth. Im not about to throw another $200 at it guessing at the moment tho. We will see how it goes after a few days and the new turbo and CTS is in. I know the turbo and stuff has nothing to do with it but ill be changing the cts when the antifreeze is drained.

[Traptalk]

Yea its free but will never tell you whats wrong. It gotta be either the LPFP or Coolant temp sensor...

If building prime pressure stops the issue than your pump is obviously at fault...

Which would also explain why the warm starts are ok. When the car sits for a long time you lose pressure and fuel leaks back into the tank.

I’m telling you man just try it and see.

[NitroViper]

If building prime pressure stops the issue than your pump is obviously at fault...

LPFP? My car has been at the correct pressure many times cold and still stuttered. I guess we cant go off app readings then if priming it fixes it. I was the guineapig and bought and installed the HPFP like everyone wanted. Where is the guineapig for the LPFP?

[dalmation53]

Me i am the guinea pig. I replaced my LPFP when i first got my A5 becuase it was starting to show signs of failure with loss of power. that fixed it 100% but not my cold start misfires. still on original HPFP.

LPFP? My car has been at the correct pressure many times cold and still stuttered. I guess we cant go off app readings then if priming it fixes it. I was the guineapig and bought and installed the HPFP like everyone wanted. Where is the guineapig for the LPFP?

[Novarider]

Have you changed the pcv? You can use a vacuum gauge to test if it's functioning properly.

[Traptalk]

LPFP? My car has been at the correct pressure many times cold and still stuttered. I guess we cant go off app readings then if priming it fixes it. I was the guineapig and bought and installed the HPFP like everyone wanted. Where is the guineapig for the LPFP?

I already offered you a free one lol

[NitroViper]

I already offered you a free one lol

Yea but we never talked about sending it, how much you think shipping would be? a pre paid box from usps? If the coolant temp sensor doesnt fix it thats the only other thing I could think of. Youre in Buffalo, just drive 2 hours and install it for me because im lazy lol.

[Traptalk]

Yea but we never talked about sending it, how much you think shipping would be? a pre paid box from usps? If the coolant temp sensor doesnt fix it thats the only other thing I could think of. Youre in Buffalo, just drive 2 hours and install it for me because im lazy lol.

I bet I could fit in the $7 or $8 flat rate box. Would have to stop in and check. Post is super slow right now. Have a priority mail usps package going on a week late right now.

Yeah I’m in buffalo lol pump is free but i didn’t mention labor was

Maybe if can get some free time one weekend soon, have some extra clips for the back seat as well and a tool to remove the union nut

[NitroViper]

I bet I could fit in the $7 or $8 flat rate box. Would have to stop in and check. Post is super slow right now. Have a priority mail usps package going on a week late right now.

Yeah I’m in buffalo lol pump is free but i didn’t mention labor was

Maybe if can get some free time one weekend soon, have some extra clips for the back seat as well and a tool to remove the union nut

I can do it just being lazy. Of course you need a special tool, audi... wtf lol.

[Smac770]

Just rent it from AutoZone.
https://www.autozone.com/loan-a-tool...ack/605758_0_0
Unless you plan to buy the real Audi one with four prongs, which will no doubt be worlds better. But the job is still easily done with the AZ tool.

I've never replaced the bench clips; maybe it's a "might break" part. You can certainly pick them up after the fact if it turns out you need them.

[NitroViper]

Well, the k04 is in and while doing that I replaced the coolant sensor. Even after the k04 tune and a new coolant temp sensor it still starts like crap on cold start. I didnt expect the tune to fix it but was hoping there was a chance lol. I logged it below, listen to how far the engine bogs down.... My car's exhaust also sounds different in videos now becuase of the high flow cat.

[old guy]

The only significant difference that I see between your rough cold start and my smooth cold start is that your fuel pressure is about 150 psi less than mine. You reach your operating pressure immediately but it seems to level out in the low 700 psi range whereas mine stays around 860 psi.

Once dropping to low idle both of our pressures stabilize around 570 psi at idle.

I have no idea how you would increase your cold start fuel pressure. I'll poke around a little and see if I can find any info on the subject. Who knows, maybe Smac770 will be bored and need a new challenge

[Chillaxin]

Just rent it from AutoZone.
https://www.autozone.com/loan-a-tool...ack/605758_0_0
Unless you plan to buy the real Audi one with four prongs, which will no doubt be worlds better. But the job is still easily done with the AZ tool.

I've never replaced the bench clips; maybe it's a "might break" part. You can certainly pick them up after the fact if it turns out you need them.

Both of mine definitely broke when removing my rear seat...FYI

[Smac770]

I only see a burst to 50 bar (5 MPa, 5000 kPa, 725 psi) during engine start, and just 40 bar (4 MPa, 4000 kPa, 580 psi) during high idle as well as normal idle.

As for sitting cold fuel pressure, mine will be 4.25 bar (425 kPa, 61.6 psi) before a cool start and as low as 3.25 bar (325 kPa, 47 psi) before a cold start.

What I notice in Nitro's latest video is that ignition timing is really advanced during the stalling out. I don't know if the timing is a reaction to the stalling, or a cause. But on mine, I see only a minor blip to the positive advance just as the fuel pressure hits 50 bar and starts to turn south towards 40 bar. This lasts for only a quarter second. All the rest of the time, my ignition timing is mild negative. As it slows the idle down under 1k, the timing goes and stays mild positive.

I see the timing mild positive but it blips high positive (13.x) each moment the idle drops below 750 (climate system on, A/C off) as it appears to be maintaining rpm in the 750-770 range.

Even once I'm driving, I never see double digit negative timing adjustment except rare blips just above idle (8xx rpm). I do get positive values up to 30.

No idea what A THR is (total harmonic resonance?). Also, how is it attempting to compute vacuum? Did you install a pressure gauge on the manifold?


So I don't think fuel pressure is an issue. It looks fine to me. I might log more with different starts. But it takes an overnight wait to get a cold start with long high idle. Also note, I have a B8.0 CAEB. They might have changed runtime parameters in B8.5 CAEx, or also on the CPMx motors.


fuel temperature - any thoughts on where the ECM is making up this value from? Other VAG vehicles have a G81 fuel temp sensor, but like they removed the G410 low pressure line pressure sensor, we also have no G81. I see nothing in the connectivity of the fuel pump to imply a fuel temp signal is overlaid on the fuel level signal from the pump to the J285 (instrument cluster). I notice that the fuel pressure value is available at all times, but the fuel temp value is only active while the engine is running. Engine not running, value is an immediate -48°C. I assume it's the same kind of phantom bologna as the exhaust temp and exhaust temp before cat, since the engine has zero means to measure those. Unless the 02 sensors somehow rely that value.


As for residual pressure on the high pressure rail after shutdown, 2-3 times idle is not even close. It's over four times. I was at 172 bar (~2500 psi) when I stopped logging at around 25 minutes.

[JLAllroad]

When I bought the car I wondered about the battery because when you start it and the dome lights are on they sort of flash. I had advance auto test it and they said its fine. I dont trust the battery since ive owned it to tell ya the truth. Im not about to throw another $200 at it guessing at the moment tho. We will see how it goes after a few days and the new turbo and CTS is in. I know the turbo and stuff has nothing to do with it but ill be changing the cts when the antifreeze is drained.

You should be able to log voltage both pre start and running, compare to @OldGuy. If found to be low you could always do a cold start with a jump pack if you have one.


Sent from my iPhone using Audizine

[old guy]

You should be able to log voltage both pre start and running, compare to @OldGuy. If found to be low you could always do a cold start with a jump pack if you have one.

I will note mine at my next cold start. My battery is a about 1 year old.