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Discussion Starter #1
My wife's car will start, and drive until it warms up. Suddenly it will stall. If you try to restart it, it starts right up, runs for a few seconds, then stalls. Engine speed makes no difference.

I'm thinking it has to be a sensor, but there are no DTCs. Any hints as to where to start?


Some History:

My wife's been towed home three times over the last year because the car has stalled on her. The first couple of times it wouldn't start at all when it got home. I replaced the ignition module/coil and the car ran fine for another 3 months or so. I've been driving it for the last week to try to experience the problem personally. This is the first time that I've experienced easy starting, followed by a stall.

It's had a persistent P0171 - system too lean code, but it did not return after the last time I erased codes.

Over the past year I've replaced the ignition module, spark plugs, wires, distributor cap, rotor, fuel pressure regulator, fuel filter, fuel pump relay, and fuel pump. I also cleaned, and finally swapped the MAF sensor, but the lean code comes back. I can't find any vacuum leaks, but I haven't had it smoke tested.
 

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The precat o2 sensor could be the cause of the lean code, but I don't think it is causing the stalling unless it is stalling right when it goes into closed loop operation.

A common cause of stalling right after startup is the cam position sensor. Although I cannot say for certain that you engine has one.

Another item to check is the crankshaft position sensor. When they go bad, they will cut out when hot, but work ok when they are cold.

I see that you state that there is only the p0171 code. Check for a tear or a leak in the air intake boot, check the clamps and look between the bellows of the hose for tears. Any unmetered air entering will cause a p0171, and if too much Unmetered air enters the engine will stall.

Cam and crank sensors do not always set codes. If the do it will be something like these:
P0340 or p0341 fro the cam position sensor
P0335 for the crank sensor
 

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Discussion Starter #4 (Edited)
I'm not sure exactly how the Motronic software works. It seems logical that it would always start open loop, then close after a couple of seconds...which is exactly when it stalls. Unfortunately my scan tool loses its connection when the car starts, and it stalls before it can reconnect, so I can't confirm the exact timing.

I looked at the signal from the crank position sensor when it was both hot and cold, and it looked normal. I thought if the CPS wasn't working the engine wouldn't start at all. The fact that it will start immediately after it stalls makes me think it's not the sensor. I did get a crankshaft position sensor code once, but it was a long time ago and it's never returned.

I dropped it off at a shop with a smoke machine last night. It's cheaper than a CPS and an O2 sensor.
 

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did you try unplugging the Mass air flow sensor... and seeing if it stays running? possible the MAF is telling the car to lean the fuel / air ratio out to an extreme lean condition causing the stall. and hence throwing the o2 too lean code.

my car was running.. then when it would reach running temp would die.. and wouldnt start til completely cooled down.. several hours later.. ended up being the crankshaft position sensor.. if that helps you out too..

sorry to hear it went to a shop hopefully they fix you up.
 

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Discussion Starter #7
I had it towed home the next day, and it ran fine.

I did not try unplugging the MAF when it was stalling. I've read to try that on several sites since, but didn't know it at the time.

I only sent it to the shop to look for vacuum leaks. They found and fixed two. It idles like crap now, but at least I can move forward knowing those are out of the way. They also told me there's a TSB, that several circuits are grounded through the fuel pump relay, and VW recommends replacing it. Anyone care to share the full text? The relay's only a week old, but it's possible I got a faulty one.

I'm back to driving the car until it stalls again. I've got a spare relay in the glove compartment, and a scan tool stashed under the seat.
 

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hm..

the rough running could be caused due to the plugs being old / fouled / gapped wrong.

could always be a bad ground.. bad injector.. dirty injectors.. bad gas.. bad ground.. .. do you have a corroded battery terminal (neg)? theres a big ground on top of the trans.. undo it.. wire brush it a bit.. then screw it back on.. see if that clears anything up..

did you ever replace the fuel filter? that could also be a culprit.. does it start right up after it stalls? if so.. then i wouldnt think fuel pump to be the culprit here..

also id try unplugging the maf sensor when it does it again..
 

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Discussion Starter #9
Cleaning the grounds is on my list of things to do. The injectors sound fine when I listen with a stethoscope. They probably are a little dirty, but the car doesn't stumble when it stalls. It cuts off like you've turned off the key. I'll probably clean the grounds this weekend.
 

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yeah.. sounds more of an electrical cut.. then an actual fuel cut from clogged injector etc.. usually a fuel issue the car shakes, stumbles, chokes, then dies.. not just instant dead.

although a bottle of lucas fuel injector cleaner wouldnt hurt..
 

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Discussion Starter #11
I put a bottle of Lucas in and fed it on a steady diet of Shell V-Power back when I started working on the car for a "simple" misfire. No help.

My best guess is a sudden sensor failure that the computer can't handle. Or computer itself is flaking out. Either could also be caused by a bad ground, so thanks for reminding me. I think the mixture problem is a red herring, but it has to be dealt with anyway.

I pulled the codes this morning before going to work. Now I have:
P0172 - System rich and
P0455 - EVAP large leak
So I guess P0171 is fixed :|

I started the scan tool recording while I was driving about 40 @ 2200 RPM. The MAF jumped around from 8.2 - 10.9 g/s. I'll do a more controlled test this weekend to confirm, but it looks like the MAF is on borrowed time. I have VAG-COM, but I prefer to drive around with the cheap scan tool instead of my laptop in the car. I can't afford a new MAF right now anyway since I just paid the shop.

In the meantime, I guess I'll be tracking down an EVAP leak.
 

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Discussion Starter #12 (Edited)
It's stalling again.

I left work for lunch, and as soon as I got out of the parking lot, it lost power and stopped responding to accellerator input. Before I could pull into the next parking lot it started to respond again and drove fine.

On the way back to work, it lost power again, then cut off. Same symptoms as before--it would start back up immediately, run fine for a few seconds, then cut off. No sputtering, it would just cut off, like you'd turned off the key.

Disconnecting the MAF, swapping the fuel pump relay, and giving the grounds and battery connections a good wiggle didn't help. I had my scan tool, so I hooked it up and found two pending codes:
- P0102 - MAF or VAF circuit low (expected since I disconnected the MAF)
- P0121 - TPS/Pedal position sensor ckt. range/perf.

As a last ditch effort before calling for a tow, I reset the codes. The car started, and stayed running. I made it about a mile to a stop light, and it cut off again. Same thing--restart fine, then cut off. I reset the codes again, and it started. I made a quick U-turn and got it home. It idled fine in the driveway for several more minutes. I just turned it off to let it cool down and try to reproduce the problem.

I'm thinking the throttle control module is acting up. I'm going to clean the throttle body and run the learn procedure.

Does anybody know where the learned data is stored? Please tell me it's not in the PCM.
 

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its in the computer..

but before you go that far.. there should be a little sensor.. held in by an allen bolt.. sticking in the throttle body.. thats the TPS..

could also just need a throttle body. since they are electronically driven.. instead of mechanically / cable driven..

run the learn procedure via vag com if you can get your hands on it.. could always try a junk yard throttle body and see if it ends the issue..
 

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Discussion Starter #14
After I got home it ran fine until I disconnected the electrical connector from the throttle control module and was able to reproduce the exact problem. I gave the throttle body a good cleaning and ran the adaptation. The car's running fine, and no codes yet.

The stalling problem seems to be hitting me about once a week, so time will tell if it's fixed for good. Still unsure about the MAF, but I don't have time to check that out thoroughly tonight.
 

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Discussion Starter #16
The connector looked good and was on there nice and tight. I didn't see any corrosion, so I don't think water's getting in there, but I put some dielectric grease on it just in case. It's possible moisture's making it's way into the sensor itself, or it's just plain worn out.
 

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Discussion Starter #18
It seems to me like a more appropriate response to losing the throttle signal would be to inject enough fuel to fast idle with no throttle response. It's obvously capable of starting the engine. That should lead even the most inexperienced techs to suspect the throttle before even pulling the codes. At a fast idle, you'd at least be able to get the car out of the road.

I suppose there's probably at least one guy at the dealer who's seen the exact behaviour and could solve it immediately (the second time).
 

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Discussion Starter #19
Three stalls on the way home tonight. :cry: I was able to restart and keep driving. It looks like cleaning up the throttle body and running the adaptation helped, but was not enough.

I plugged my scan tool in while I was stopped at a light, and was able to get a recording. I accellerated to 45 mph, with the throttle held steady. The data from the TPS (orange line) is all over the place:



It stalled right at the end of the chart and I coasted to a stop at the next light. I just put a new throttle body on order :sad:
 
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