Warm start issue, car almost dies, no rattle?

[FulhamFTW]

So, occasionally, when my car is warm, it has a weird starting issue. It usually happens if it's only been a few minutes since the car has been shut off. It sputters like it wants to die, then just kicks in and is fine. I have no rattle, plugs and coilpacks are good, no CEL, and no performance loss. It has never happened on a cold start, only on warm starts and it doesn't always happen. I am due for a new battery, but what else could it be? Fuel issue? Filter is relatively new, but pump is original and I'm at 144k miles. I would think there would be codes related to fuel if that was the case though. Anyone have this issue or any idea of what it could be?

[VinnysS4]

Can you check your fuel pressure ? It's hard to tell without a proper diagnosis, but reading the fuel trims can shed some light on this. I was watching a video of a diagnosis of a bad/weak fuel pump. VERY interesting stuff. i'll dig up the link if you would like. Might help tremendously!

[FulhamFTW]

Thanks man, definitely appreciate it. I just don't understand why it only happens on some warm starts and no other times.

[VinnysS4]

Yeah I have no clue about the intermittent part. When you have nothing to go off of, it's hard to diagnose an issue. . I watch some videos from time to time from a guy on youtube Called Schrodingers Box. He is a scientist that does mechanics on the side and he explains fuel trims extremely well. Highly recommend watching some of his videos. I like how he goes about his diagnosis. .. it has helped me tremendously.

[QIKRNU]

Hope you get your problem resolved quickly.

[Lucca M]

My buddy told me in cases like that it might be crank sensor going bad. Id look into that. But not sure. Just an idea

[FulhamFTW]

The problem is, it's so intermittent and reliant on specific warms start situations that I can't really call it a problem yet. I don't want to start throwing money at it just yet, but I'll definitely consider the crank sensor if it gets worse.

[Sodium]

Seems like the thing to do is some data logging using the VCDS software and see what the ECU thinks is going on. There's a whole host of could-bes. timing chain tensioner, fuel pressure sensor, fuel pump, crank position sensor, throttle position sensors, throttle valve, leaky injectors. Stumbling at warm start... hm. Well, you've ruled out plugs and coil packs though those are really are subject to wide temperature differences.

Time to bite the bullet and invest in software that can perform data logging. Your basic code reader isn't going to give you enough information.

[VinnysS4]

iirc, the crank sensor is a potentiometer. They are tested very easily:
http://www.1aauto.com/content/articl...sensor-testing
dont just buy one and throw it at the car in hopes that is the problem. One of the only ways to figure out what is wrong is to reproduce the symptoms. THEN you can start a proper diagnosis. Other wise you are just looking for "something" and who knows what that may be. If you have a scan tool just crank and look at the RPM's. If it shows nothing, this is your problem. If it cranks and still wont start but the scan tool shows RPM's, move on to the next component. Back to the basics. Air, Fuel, Spark, Timing, Compression

[JimmyBones]

So, occasionally, when my car is warm, it has a weird starting issue. It usually happens if it's only been a few minutes since the car has been shut off. It sputters like it wants to die, then just kicks in and is fine. I have no rattle, plugs and coilpacks are good, no CEL, and no performance loss. It has never happened on a cold start, only on warm starts and it doesn't always happen. I am due for a new battery, but what else could it be? Fuel issue? Filter is relatively new, but pump is original and I'm at 144k miles. I would think there would be codes related to fuel if that was the case though. Anyone have this issue or any idea of what it could be?

I think that this sounds like a warm start vacuum leak. These engines get hot as hell after they have been driven for a little while, then they cool back down while being shut off, and reheated when you are out running errands. During that time the metals heat up/expand, then cool down/shrink, and then heat back up/expand back again.

To start with do some simple things and check some other things. This is stuff that you can do easily for very little money/time. Check that your MAF boot is on the throttle body really tight because sometimes the clamp isn't that tight and a slightly loose MAF boot at the throttle body can make the engine run like crap. Next check that the plastic 90 degree elbow that goes into the MAF boot is not broken or cracked. Take a look over the hoses from that go from that 90 degree elbow to the Y-shaped suction jet pump and check that the suction jet pump isn't broken. Then check that the 5 mm allen bolts for the intake manifold are tight. Those bolts back out because it is a steel bolt going through a magnesium intake manifold into an aluminum cylinder head. All those different types of metals heat up and cool down at different rates so the bolts tend to back out and thus there was a TSB for that issue. Have you taken out the fuel injectors out recently? I ask because the pintle caps tend to break off and I wonder if the bottom o-rings are not sealing correctly.

Have you done any work to your car recently?
Also if you can then scan for fault codes. Even if they are intermittent codes that would be helpful to know.

Yeah I have no clue about the intermittent part. When you have nothing to go off of, it's hard to diagnose an issue. . I watch some videos from time to time from a guy on youtube Called Schrodingers Box. He is a scientist that does mechanics on the side and he explains fuel trims extremely well. Highly recommend watching some of his videos. I like how he goes about his diagnosis. .. it has helped me tremendously.

I agree that it would be helpful to see the long term fuel trims. He can check them with a vagcom in measuring value block channel 32 if he hasn't cleared the fault codes in a while.

[FulhamFTW]

I think that this sounds like a warm start vacuum leak. These engines get hot as hell after they have been driven for a little while, then they cool back down while being shut off, and reheated when you are out running errands. During that time the metals heat up/expand, then cool down/shrink, and then heat back up/expand back again.

To start with do some simple things and check some other things. This is stuff that you can do easily for very little money/time. Check that your MAF boot is on the throttle body really tight because sometimes the clamp isn't that tight and a slightly loose MAF boot at the throttle body can make the engine run like crap. Next check that the plastic 90 degree elbow that goes into the MAF boot is not broken or cracked. Take a look over the hoses from that go from that 90 degree elbow to the Y-shaped suction jet pump and check that the suction jet pump isn't broken. Then check that the 5 mm allen bolts for the intake manifold are tight. Those bolts back out because it is a steel bolt going through a magnesium intake manifold into an aluminum cylinder head. All those different types of metals heat up and cool down at different rates so the bolts tend to back out and thus there was a TSB for that issue. Have you taken out the fuel injectors out recently? I ask because the pintle caps tend to break off and I wonder if the bottom o-rings are not sealing correctly.

Have you done any work to your car recently?
Also if you can then scan for fault codes. Even if they are intermittent codes that would be helpful to know.



I agree that it would be helpful to see the long term fuel trims. He can check them with a vagcom in measuring value block channel 32 if he hasn't cleared the fault codes in a while.

Lots of good info here, thanks. The last thing I worked on was replacing my sparkplugs and coolant reservoir, but nothing serious. I did mess with the MAF though so I'll check to make sure everything is tight there. I'll see if I can find someone local with vcds to log it for me. I'll let you guys know what I find out.