Random misfire, intermittent hard start

[Matt E.]

Hi, I'm new to the forum. My son has a 2015 A3 2.0t premium plus. It has a random misfire under load. It's usually right after it shift into 2nd or 3rd around 2000rpm, give or take. It'll misfire a few times but go away if you continue to accelerate. I work on heavy trucks and have a heavy truck scanner. I was able to check codes with it and no active or inactive codes are set. When I look at the misfire parameters all four cylinders show 3 or 4 misfires on the last drive cycle. My son also said that intermittently he has a hard start condition (I have not experienced the hard start). It's very intermittent but seems like it only happens when the engine is warm. Has anyone had similar issues or any ideas what might be causing this?

Thanks!

Matt

[cliffcoggin]

Matthew.

Not casting aspersions on your particular code reader, but many are incapable of correctly reading Audi codes. It may pay you to get it done professionally, or to get a OBD11 reader which seems to be successful amongst knowledgable users here.

[Matt E.]

I am aware of that issue which is why I pointed out the type of scanner I was using. There is no check engine light, nor has there been. I have pulled misfire codes from this car before with that scanner (that time it just needed plugs and coils) and it is showing the misfires on the previous cycle. I think it just doesn't misfire enough times to trigger the code.

[Stevey Y]

4 hours ago, Matt E. said:

I am aware of that issue which is why I pointed out the type of scanner I was using. There is no check engine light, nor has there been. I have pulled misfire codes from this car before with that scanner (that time it just needed plugs and coils) and it is showing the misfires on the previous cycle. I think it just doesn't misfire enough times to trigger the code.

Hi with the correct scanner I have no doubt there would be codes of some description, these vehicles log everything including the day/date of the incident plus the times and frequency of the misfires, you wont get a light on until whatever is causing this goes terminal, there are a lot of scan tools on the market that for not much outlay will retrieve the resulting codes, come back with them and we may well be able to help.

Steve.

[Matt E.]

Do you have a recommendation on what scan tool I should use? Or, does anyone have a recommendation for a reputable Audi shop in central New Jersey? I've dealt with a couple and was not very happy.

Thanks

[Steve Q]

Obd11, vcds, vagcom or Ross tech are all good. 

[Matt E.]

Ok, thanks

[Matt E.]

On 11/12/2024 at 1:44 PM, Stevey Y said:

Hi with the correct scanner I have no doubt there would be codes of some description, these vehicles log everything including the day/date of the incident plus the times and frequency of the misfires, you wont get a light on until whatever is causing this goes terminal, there are a lot of scan tools on the market that for not much outlay will retrieve the resulting codes, come back with them and we may well be able to help.

Steve.

Ok, so I got my hands on an OBD11 and on VAG (I think that's what it was). They both, and my snap on scanner, show no active or inactive codes. I experienced the hard start problem today. I drove the car for about 10 minutes. Shut it down for maybe 10-20 minutes and when I tried to start it again it cranked for a cycle without starting. After a couple revs into the second attempt it fired up and ran fine. It seemed like the fuel line drained back but it only seems to happen when restarting after sitting for a short period. If the car sits overnight it starts fine.

[cliffcoggin]

Matthew.

The only thing I can think of that causes electronic problems without yielding error codes is a failing battery. Typically such a battery will cause problems like aircon failure, cruise control failure, loss of radio presets, false warning lights etc. I have never heard of it interfering with starting. Nevertheless I suggest you get the battery tested with a deep discharge tester, and if it is over four years old I would renew it anyway. Ensure the new battery is coded to the car if you change it.

[Matt E.]

Yeah, the battery is good.

[cliffcoggin]

Fair enough. Do please let us know if and how you resolve the problem.

[Matt E.]

28 minutes ago, cliffcoggin said:

Fair enough. Do please let us know if and how you resolve the problem.

Ok, will do. Thanks

[Stevey Y]

15 hours ago, Matt E. said:

Ok, so I got my hands on an OBD11 and on VAG (I think that's what it was). They both, and my snap on scanner, show no active or inactive codes. I experienced the hard start problem today. I drove the car for about 10 minutes. Shut it down for maybe 10-20 minutes and when I tried to start it again it cranked for a cycle without starting. After a couple revs into the second attempt it fired up and ran fine. It seemed like the fuel line drained back but it only seems to happen when restarting after sitting for a short period. If the car sits overnight it starts fine.

Hi thanks for the info, I think you could have a fuel rail pressure sensor problem or a lazy lift pump in the tank the regulator is not diagnostically monitored but the lift pump is but only when it fails completely.

Steve.

 

[Matt E.]

1 hour ago, Stevey Y said:

Hi thanks for the info, I think you could have a fuel rail pressure sensor problem or a lazy lift pump in the tank the regulator is not diagnostically monitored but the lift pump is but only when it fails completely.

Steve.

 

Steve, I was leaning toward regulator or pump. Seems that they're similarly priced. I was hoping one was considerably cheaper and I'd try that first, but no luck. Have you ever heard of either of those two components causing a random misfire under load?

[Stevey Y]

21 hours ago, Matt E. said:

Steve, I was leaning toward regulator or pump. Seems that they're similarly priced. I was hoping one was considerably cheaper and I'd try that first, but no luck. Have you ever heard of either of those two components causing a random misfire under load?

Hi the regulator is normally the cheaper option try eBay Europe/uk, yes either could cause a temporary drop in fuel pressure which would manifest itself as a misfire, the regulator is electro mechanical and if the spring is weak it will lose pressure when hot, the only way to know for sure is monitor the live data and check any fluctuation in fuel pressure when starting from hot.

Steve

[Matt E.]

1 hour ago, Stevey Y said:

Hi the regulator is normally the cheaper option try eBay Europe/uk, yes either could cause a temporary drop in fuel pressure which would manifest itself as a misfire, the regulator is electro mechanical and if the spring is weak it will lose pressure when hot, the only way to know for sure is monitor the live data and check any fluctuation in fuel pressure when starting from hot.

Steve

Ok, so I was planning on doing a full pressure leak down test today but it doesn't look like there is a port to tap into, probably because of the high pressure pump. Obviously I can't do it with the scan tool because the if the ecm is powered up the electric pump is maintaining pressure. In your last post Steve you mentioned monitoring the pressure on a scan tool during startup. How do you do this? My scan tool takes a little time to connect to the vehicle. I would imagine that by the time it links up the pump will have overcome any pressure loss.

Thanks again for all the help.

Matt

[Stevey Y]

22 hours ago, Matt E. said:

Ok, so I was planning on doing a full pressure leak down test today but it doesn't look like there is a port to tap into, probably because of the high pressure pump. Obviously I can't do it with the scan tool because the if the ecm is powered up the electric pump is maintaining pressure. In your last post Steve you mentioned monitoring the pressure on a scan tool during startup. How do you do this? My scan tool takes a little time to connect to the vehicle. I would imagine that by the time it links up the pump will have overcome any pressure loss.

Thanks again for all the help.

Matt

Hi if you connect up then start the car, go to engine and then select live data, you will get a white box come up with select channel put in 106 this should alter the display so you have the fuel pump pressure second display down the top display should be the fuel rail pressure  if the pump display is doing anything over 45% at idle and rises way up when you rev it the pump is working overtime and is probably knackered, also watch for massive drops on the fuel rail pressure .

Steve.

[Matt E.]

1 hour ago, Stevey Y said:

Hi if you connect up then start the car, go to engine and then select live data, you will get a white box come up with select channel put in 106 this should alter the display so you have the fuel pump pressure second display down the top display should be the fuel rail pressure  if the pump display is doing anything over 45% at idle and rises way up when you rev it the pump is working overtime and is probably knackered, also watch for massive drops on the fuel rail pressure .

Steve.

Ok, so there's no way to do an actual leak down test? I want to monitor fuel pressure after the engine is shut down. This is to see if the fuel is bleeding back into the tank and causing the hard start/long crank time issue.