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Author Topic: Random acceleration when cruising along Tdi?  (Read 552 times)

Offline ginettaman

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Random acceleration when cruising along Tdi?
« on: December 26, 2020, 03:51:11 PM »
My old faithful mk2 Gal has started doing odd things on a run. If running a constant speed of say 70 mph occasionally it suddenly starts accelerating so I have to brake. Throttle not moving when it goes so would  that discount the pedal sensor? I've noticed it will do it with or without cruise control activated.
I presume a speed sensor or something must be getting false readings? Any ideas where to look first?
1.9 tdi 150bhp.
Thanks

Offline SirDavidAlhambra

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Re: Random acceleration when cruising along Tdi?
« Reply #1 on: December 26, 2020, 04:06:24 PM »
Oh no, could this be excessive pressure in the crank case forcing mist of engine lubricant into the intake manifold, which is then burnt in the same fashion as diesel??

Fuel and oil leaks causing engine runaways can have both internal and external causes my friend. Broken seals or a broken turbocharger may cause large amounts of oil mist to enter the inlet manifold, whereas defective injection pumps may cause an unintentionally large amount of fuel to be injected directly into the combustion chamber.

The experts on here may have some answers but please put ur safety first here, there is nothing - and I’m sure everyone agree when we say literally nothing - more important than ur safety and that of other road users even though the galaxy does have good brakes


 cya
I drive a Seat Alhambra 1.9Tdi which has 115bhp and an automatic gearbox.

I am happy to help you with all your questions. I am not a qualified mechanic but seem to be better at fixing my car than even the most experienced garages.

I have lots of friends here and very much enjoy talking with you all.

Always remember, a motor car is a serious tool and should be treated with respect. Put your safety first, always.

Offline ginettaman

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Re: Random acceleration when cruising along Tdi?
« Reply #2 on: December 26, 2020, 04:16:28 PM »
So where can I check crankcase breathing? Touching the brakes switches the cruise control off so I think it also switches off my accelerator fault which makes me think its a sensor fault?
Hope it's not the turbo!

Offline SirDavidAlhambra

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Re: Random acceleration when cruising along Tdi?
« Reply #3 on: December 26, 2020, 04:25:46 PM »
Hmm, puzzling, does it do it only when on cruise control? If so it does sound more like a sensor fault my friend although tbh I wouldn’t listen to me too much, I just try and figure these things out using common sense whereas others on here have much more experience and will give much better advice I am sure
I drive a Seat Alhambra 1.9Tdi which has 115bhp and an automatic gearbox.

I am happy to help you with all your questions. I am not a qualified mechanic but seem to be better at fixing my car than even the most experienced garages.

I have lots of friends here and very much enjoy talking with you all.

Always remember, a motor car is a serious tool and should be treated with respect. Put your safety first, always.

Offline brianh

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Re: Random acceleration when cruising along Tdi?
« Reply #4 on: December 26, 2020, 04:56:59 PM »
Does it behave normally at idle? If so, I'd suspect the throttle pedal sensor has a bad spot (and it would make sense any bad spot will be where it often sits around for a motorway journey).

If its a manual box, does it do it around the same speed in a lower gear? Or around the same rpm in a different gear for that matter.

If you can borrow or obtain a cable and laptop for VCDS lite, that might shed some light on whats going on, either by checking fault codes or by monitoring the throttle pedal (I would guess you somehow do that through the measuring blocks function on it, but have never tried that myself, others on here may well know what your best option there is).

If the diesel crankcase feeds to the same place as the petrol engines, it ends up on the outside of the air filter (the lower side of it, connected to the bottom of the box), If your losing oil that way, the bottom side of the air filter will be covered in oil. A simpler test (though its hard to judge very well this way) is to remove the oil filler cap and check for excess pressure with the engine running. I'd think the crankcase breather unlikely with what you describe, as you'd expect problems at all times/rpm if that was where your problem was. Plus a lack of power possible as well as likely blue smoke from the exhaust.

Does it smoke from the exhaust once its warmed up? Blue smoke would suggest burning engine oil, either through the crankcase breather or more likely a turbo. Does it even appear to use much if any excess oil between changes?

Offline Chrispb

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Re: Random acceleration when cruising along Tdi?
« Reply #5 on: December 26, 2020, 08:53:15 PM »
That does sound the engine starting to run away (running on engine oil aswell as diesel) has the oil level dropped rapidly recently.

Check with diagnostic to case anythings been logged.

Engine smoking from exhaust?
2016 Vauxhall Insignia Elite Nav in White 2.0CDTI Manual.
2009 Kia Sedona GS In Black 2.9CRDI 183PS. 5 Speed Manual WAV.
2003 MK2 Galaxy Ghia In Solid Black 1.9 TDI 115PS. 5 Speed Automatic.

Upgraded Eberspaecher booster heater  to independent heating with 7 day timer.

With VCDS lite (full version) need a code clearing or want to scan for faults in the north kent area, PM me.
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Offline johnnyroper

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Re: Random acceleration when cruising along Tdi?
« Reply #6 on: December 26, 2020, 10:50:12 PM »
Does it behave normally at idle? If so, I'd suspect the throttle pedal sensor has a bad spot (and it would make sense any bad spot will be where it often sits around for a motorway journey).

If its a manual box, does it do it around the same speed in a lower gear? Or around the same rpm in a different gear for that matter.

If you can borrow or obtain a cable and laptop for VCDS lite, that might shed some light on whats going on, either by checking fault codes or by monitoring the throttle pedal (I would guess you somehow do that through the measuring blocks function on it, but have never tried that myself, others on here may well know what your best option there is).

If the diesel crankcase feeds to the same place as the petrol engines, it ends up on the outside of the air filter (the lower side of it, connected to the bottom of the box), If your losing oil that way, the bottom side of the air filter will be covered in oil. A simpler test (though its hard to judge very well this way) is to remove the oil filler cap and check for excess pressure with the engine running. I'd think the crankcase breather unlikely with what you describe, as you'd expect problems at all times/rpm if that was where your problem was. Plus a lack of power possible as well as likely blue smoke from the exhaust.

Does it smoke from the exhaust once its warmed up? Blue smoke would suggest burning engine oil, either through the crankcase breather or more likely a turbo. Does it even appear to use much if any excess oil between changes?

Diesel breather feeds from rocker cover in to inlet pipe between maf and turbo. To rule runaway in or out check oil level if it’s been running on that then it will have dropped. Personally I would not be thinking that as touching brake stops it. If it’s starting to runaway even braking you would still feel car trying to pull. I would be more inclined to think electrical issue going on the description.
As said vcds may shed some light on to it

Offline brianh

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Re: Random acceleration when cruising along Tdi?
« Reply #7 on: December 27, 2020, 12:07:40 AM »
Diesel breather feeds from rocker cover in to inlet pipe between maf and turbo. To rule runaway in or out check oil level if it’s been running on that then it will have dropped. Personally I would not be thinking that as touching brake stops it. If it’s starting to runaway even braking you would still feel car trying to pull. I would be more inclined to think electrical issue going on the description.
As said vcds may shed some light on to it

I don't think its runaway type issues either as you'd expect to notice the stink of burning oil if so, and can't see it being noticed only under some conditions (plus your point regarding the brakes). As you say, if the oil level isn't dropping, then its a good sign that burning oil from whatever source isn't the issue.

Offline regw

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Re: Random acceleration when cruising along Tdi?
« Reply #8 on: December 27, 2020, 12:00:43 PM »
We know its 15 years old but what's the mileage?
Bearing in mind the throttle pedal sensor is basically
a variable resistor, it's not inconceivable that the track/wiper
are pretty worn by now.
As I also work on bikes which are notorious for getting the weather in,
I like to go round the electrical connectors from time to time to check
they're sound clean and bright.
All related terminals and contacts get a squish of switch cleaner in them too.
I solved an erratic throttle position switch on my old beemer K100 by just doing this.
 
Whatever, 'test don't guess' is my usual mantra so I suggest as others have
that you get vcds on it first

Offline ginettaman

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Re: Random acceleration when cruising along Tdi?
« Reply #9 on: December 28, 2020, 06:38:39 PM »
Thanks I've tried plugging it in to vag.com but no codes. It happens with cruise is on or off very occasionally? This made me think it might not be the throttle pedal sensor as it off when on cruise. Something else must monitor the speed to make adjustments too? Do they have crank sensors? Don't think it's oil loss related but its done 135k miles and always been looked after by myself. Never uses oil between service intervals. DM flywheel was my last big task as it stated vibrating so it had a clutch too then . I've never cleaned the maff  or manifolds as I've had no issues before.  Since lockdown usage has dramatically dropped as working from home which won't help the car.
How to instructions would be useful from those of you who do your own spannering?

Offline brianh

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Re: Random acceleration when cruising along Tdi?
« Reply #10 on: December 28, 2020, 07:39:14 PM »
There is a crank sensor on the front side of the engine near to the gearbox end. Did this start around the same time you changed the flywheel? Just wondering if it is the crank sensor causing the problem if you could have bent one of the teeth?

Good point on the pedal sensor, doing it both on and off the cruise setting suggests the issue is elsewhere. Does the speedometer show the increased speed when this happens? I'd wonder if its possible one of the abs sensors isn't detecting correctly though not convinced that would explain exactly whats happening. Its not unknown for the reluctor ring on the end of the driveshaft to snap though would expect to see an error being logged if it was.

Offline insanitybeard

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Re: Random acceleration when cruising along Tdi?
« Reply #11 on: December 28, 2020, 08:08:46 PM »
This is just a bit of straw clutching on my part but the cruise control switch and speed setting on the left hand indicator stalk is just requesting a desired speed to the engine management by feeding it a signal which is interpreted by the ECU (at least on the fly by wire version used on the TDI). If the cruise control switch or associated wiring is defective (such as an intermittent short or wiring fault) could it possibly be enabling the cruise control without driver input and then the same fault causing the ECU to see a request for a higher speed hence the acceleration? If applying the brake corrects the issue then that tends (to my understanding which may be wrong) to point towards a fault with the cruise control, especially if it also happens with cruise control engaged and no input from the throttle pedal sensor.

Do you have a manual or automatic gearbox? There is a speed sensor fitted to the gearbox, though I'm not sure if the engine management uses the input from this sensor, the wheel speed sensors, or both when gauging vehicle speed for cruise control.
Always learning..... Often by mistakes!

Offline brianh

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Re: Random acceleration when cruising along Tdi?
« Reply #12 on: December 28, 2020, 09:08:35 PM »
This is just a bit of straw clutching on my part but the cruise control switch and speed setting on the left hand indicator stalk is just requesting a desired speed to the engine management by feeding it a signal which is interpreted by the ECU (at least on the fly by wire version used on the TDI). If the cruise control switch or associated wiring is defective (such as an intermittent short or wiring fault) could it possibly be enabling the cruise control without driver input and then the same fault causing the ECU to see a request for a higher speed hence the acceleration? If applying the brake corrects the issue then that tends (to my understanding which may be wrong) to point towards a fault with the cruise control, especially if it also happens with cruise control engaged and no input from the throttle pedal sensor.

Do you have a manual or automatic gearbox? There is a speed sensor fitted to the gearbox, though I'm not sure if the engine management uses the input from this sensor, the wheel speed sensors, or both when gauging vehicle speed for cruise control.

If it is that, then it looks like you should be able to disable the cruise by disconneting the clutch pedal switch, if I'm reading the diagram correctly (as the switch should be closed when the clutch is engaged/pedal up and open circuit when the clutch pedal is down) then this should inhibit the cruise control. There doesn't look to be any simpler way of doing it (brake pedal switch not a good option as it would stop it controlling the brake lights).

This is assuming you have the controls on the wheel rather than on the indicator stalk, which i think is the more likely setup given the info in TIS and the age of your Galaxy. obviously if you have an auto box, you won't be able to use that option.

Offline KingyMAK

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Re: Random acceleration when cruising along Tdi?
« Reply #13 on: December 28, 2020, 11:17:27 PM »
Funnily enough;
I’ve just started encountering this when I replaced the hub & wheel bearing, but left the second hand ABS sensor in the hub instead of my old one.
It accelerates, if I dip the clutch it revs itself, regardless of the gear and braking slows it down.

The ABS light then flashes a few times, then usually goes out, sometimes it just stays on.

I think the sensor that’s on is a dodgey/knackered one, an older model or maybe even one designed for a different ABS ring, possibly?

Offline brianh

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Re: Random acceleration when cruising along Tdi?
« Reply #14 on: December 28, 2020, 11:34:32 PM »
Funnily enough;
I’ve just started encountering this when I replaced the hub & wheel bearing, but left the second hand ABS sensor in the hub instead of my old one.
It accelerates, if I dip the clutch it revs itself, regardless of the gear and braking slows it down.

The ABS light then flashes a few times, then usually goes out, sometimes it just stays on.

I think the sensor that’s on is a dodgey/knackered one, an older model or maybe even one designed for a different ABS ring, possibly?

Think its more likely knackered than wrong, though I've known abs sensors to be fine before you start swapping bits then refuse to work afterwards until you swap them out. More often you end up swapping them anyway as trying to remove them to clean or reseat them usually fails to work with the sensor being stuck in with rust and dirt.

Assuming you still have the old one on the bits you removed, it might be an idea to see if you can remove it from the housing to look to reuse it. Don't know if you'd be able to see a part number on either sensor that might help prove if its a different sensor?

@ginettaman you mentioned checking for codes, have you just checked the engine? It might be worth checking ABS as well if you didn't? I'd suspect you can tie it down further with looking at actual sensor values with VCDS, but not sure what values to suggest.

Offline ginettaman

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Re: Random acceleration when cruising along Tdi?
« Reply #15 on: December 29, 2020, 08:03:51 AM »
It's a late 2005 manual with cruise on the left stalk. No abs lights coming on and the clutch was done last year so it's run ok until the last couple of runs.

Offline brianh

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Re: Random acceleration when cruising along Tdi?
« Reply #16 on: December 29, 2020, 10:39:15 PM »
I'd try disconnecting the clutch pedal switch then. At least that rules out clock spring issues.

If it does appear to be the cruise control coming on when it shouldn't, then it would be an idea to check for any broken wires in the usual suspect places (the doors and boot) as I've known some of them to play havoc with stuff you wouldn't expect (like the backlight coming on behind the instruments when you press the brakes for example), though never the cruise control (though mine doesn't currently work even with a new pump, thats on the to-do list).

Offline ginettaman

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Re: Random acceleration when cruising along Tdi?
« Reply #17 on: January 04, 2021, 10:17:41 AM »
What are clock spring issues???Its a TDI.  The car has only done 135k miles so just run in! Door wires are regularly repaired but they only tend to effect the window auto functions. Recently Broke my arm so not able to do much or drive now sadly but can try the clutch switch at some point.

Offline brianh

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Re: Random acceleration when cruising along Tdi?
« Reply #18 on: January 04, 2021, 10:33:14 AM »
Clock spring is the bit that connects the steering wheel to the signature and steering wheel controls. Since your buttons are on the stalk, that doesn't apply to yours.

Its usually the boot that causes the odd issues with broken wires. But worth trying the clutch switch as that should disable it. I'd take it turning it to off doesn't make amy difference?

If that does stop it, I'd start suspecting the control stalk. I think TIS had a test procedure for it, but my copy of Tis only covers the earlier models. Don't know if the pinout changes but worth a look. Given its intermittent may be easier to replace stalk to rule it out.

 

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