Extremely Wierd Revving Problem

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dequinn
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Extremely Wierd Revving Problem

Post by dequinn »

Firstly, after much searching its great to see an active Polo community on the net. Kudos to everyone smart enough to use this board and the people who take to time to help others!

I am having a really strange problem with my Polo MKIV. It is a 1998 1.0L 4 door. Its my first car and I don't know a lot about car systems yet, but I have 8 years experience with bike engines so I know roughly whats going on.

I took the Polo to get serviced (with a trusted and competent mechanic) and when I got it back I found the revs where very high. More specifically I found that when I pushed in the clutch or slowed down the car to a stop, the revs would stay stuck at 2-2.2k for a good 10 or so seconds.
This went on for a number of weeks until last week I noticed something important- the revs stay high until the exact moment the speedo reaches zero. The instant the speedometer hits the 0 mark, the revs drop down to normal 800-900 idling speed.

To confirm whether or not the speedo had anything to do with it I ran two tests:
1.I took the car to the top of a hill, took it out of gear, took my feet away from the clutch and accelerator, and waited until the engine had completely settled to idle speed. I then took the handbrake off, and as soon as the car started to roll, the revs went up, quickly maxing out at 2.2k. They stayed that way until the car stopped. Again-This was with the engine in neutral and no acceleration pedal used.

2. I took the car to a long flat stretch of road and brought it up to 50mph.I then took the car out of gear and let it roll. The revs stayed stuck at 2.2k for the 30 or so seconds the car was rolling. The car came to a stop, and a second or two later, when the speedo had registered 0, the revs dropped straight down to normal idle speed.

I called the mechanic who serviced the car and he said it was extremely unusual. We cant figure out why the actual road speed would have an effect on the ECU (which is what I presume to be causing it). I understand the ECU would look at things like temp sensors, air flow etc... but actual wheel speed?

What I can't figure out is 1. Why would the road speed be a measurement the ECU uses for anything? 2. Why is the ECU increasing the revs to 2.2k when it senses the car is moving? 3. What could have happened during the service (when the problem started) that could have caused this.

I'm also open to the possibility that I don't know what I'm talking about! and I should stick with bike engines! It may not be the ECU, but the fact remains that I have done the two above mentioned experiments which at least tell me the road speed is causing the revs to go up and stay up until the car stops.

One other thing, it has been suggested to me the reason the road speed is measured is for gearbox management. Again, I don't know a lot about car gearboxes, but I could understand why the road speed would be a factor there.
Anyway if anyone has ever had a similar problem or a small clue as to what the hell is going on, I would most grateful!
Last edited by dequinn on Sat Oct 06, 2007 8:24 pm, edited 1 time in total.
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alexperkins
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Post by alexperkins »

The ecu is connected up to speed sensors, so possibly one of them is faulty. Throttle body could be related to this, but i find this unlikely

What was done on the service?
The_phat_controlleruk
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Post by The_phat_controlleruk »

Mmmm, sounds like it could be the TB, exactly what was done during the service??
dequinn
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Post by dequinn »

The_phat_controlleruk wrote:Mmmm, sounds like it could be the TB, exactly what was done during the service??
It was just a "standard" service, Im not too sure what exactly was done but I would assume oil change, sparks etc. One thing that I know was changed was the timing belt which was on its last legs. I know changing the timing belt is fairly extensive, but I would certainly trust the mechanic who did it as he is a family friend and long time professional mechanic. That said I'm not closed minded to the possibility that a sensor connection was broken or something like that.

I forgot to mention- I have had the car up to the mechanic a few times since the problem started and he has cleaned and checked the TB a few times but did say it was a possibility. I just cant see how it could be since the car throttles and returns to idle perfectly if the car is stopped, but the problems only occur when the wheels are rolling, even if it is out of gear.
warrmr
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Post by warrmr »

have you read the ECU for faults with VAGcom or similar. this would be the first port of call to see why its acting up. if theres no noteable codes then reset the ecu and see what happens then change sensors appropriatly.
The_phat_controlleruk
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Post by The_phat_controlleruk »

I have had the car up to the mechanic a few times since the problem started and he has cleaned and checked the TB a few times but did say it was a possibility.

Yep, the old TB, get her VAG-COM'd as per the previous post. Sounds like you either have errors in the ECU (as i did and had the same probs) or you need the TB "basic settings" set, again using VAG-COM.
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Post by Rmachines »

I had a problem similar to this. I couldnt get my head arround it so i decided to have a fiddle, I pulled off a vacucme hose from the Throttle body to the carbon cansiter ( i think) while it was running then reconnected it and it was fine again after that.

Also try resetting the ECU, unplug the battery for 10 mins or so.
dequinn
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Post by dequinn »

Rmachines wrote:I had a problem similar to this. I couldnt get my head arround it so i decided to have a fiddle, I pulled off a vacucme hose from the Throttle body to the carbon cansiter ( i think) while it was running then reconnected it and it was fine again after that.

Also try resetting the ECU, unplug the battery for 10 mins or so.
Thanks I'll give that a shot tomorrow. I would agree with everyone that hooking up to a VAGCOM could better reveal whats going on. I tried disconnecting the battery for an hour and the problem remained, but I read someone suggesting to disconnect both terminals and touch the leads together (presumably to discharge any latent charge on the circuit). I might try this tomorrow as well.
Failing that, I have priced getting someone to look at the ECU and this would cost about €80 (£55). I have seen a connector on the net called Scantool thats priced about £30
http://www.scantool.net/products/produc ... ucts_id=41
that would let me look at the readings myself. Would anyone recommend doing it this way? I work in the electronics and computer field, and have a fairly good understanding of engine mechanics. If I had to change a sensor or something I would probably get a mechanic to do it but should I try and save money and check the ECU fault codes myself?
The_phat_controlleruk
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Post by The_phat_controlleruk »

I do, fairly simple, just follow the guides on Ross-tech when using VAG-COM. I don't take my car to a mekanik for anything unless I really CANNOT do it. I bought myself a lead off fleabay for £9 and got hold of a copy of VAG-COM, I used these to clear my errors and re-align my throttle body, I was charged £30 last time someone did this so money well spent.
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Re: Extremely Wierd Revving Problem

Post by jorgan202 »

dequinn wrote: Sat Oct 06, 2007 6:23 pm Firstly, after much searching its great to see an active Polo community on the net. Kudos to everyone smart enough to use this board and the people who take to time to help others!

I am having a really strange problem with my Polo MKIV. It is a 1998 1.0L 4 door. Its my first car and I don't know a lot about car systems yet, but I have 8 years experience with bike engines so I know roughly whats going on.

I took the Polo to get serviced (with a trusted and competent mechanic) and when I got it back I found the revs where very high. More specifically I found that when I pushed in the clutch or slowed down the car to a stop, the revs would stay stuck at 2-2.2k for a good 10 or so seconds.
This went on for a number of weeks until last week I noticed something important- the revs stay high until the exact moment the speedo reaches zero. The instant the speedometer hits the 0 mark, the revs drop down to normal 800-900 idling speed.

To confirm whether or not the speedo had anything to do with it I ran two tests:
1.I took the car to the top of a hill, took it out of gear, took my feet away from the clutch and accelerator, and waited until the engine had completely settled to idle speed. I then took the handbrake off, and as soon as the car started to roll, the revs went up, quickly maxing out at 2.2k. They stayed that way until the car stopped. Again-This was with the engine in neutral and no acceleration pedal used.

2. I took the car to a long flat stretch of road and brought it up to 50mph.I then took the car out of gear and let it roll. The revs stayed stuck at 2.2k for the 30 or so seconds the car was rolling. The car came to a stop, and a second or two later, when the speedo had registered 0, the revs dropped straight down to normal idle speed.

I called the mechanic who serviced the car and he said it was extremely unusual. We cant figure out why the actual road speed would have an effect on the ECU (which is what I presume to be causing it). I understand the ECU would look at things like temp sensors, air flow etc... but actual wheel speed?

What I can't figure out is 1. Why would the road speed be a measurement the ECU uses for anything? 2. Why is the ECU increasing the revs to 2.2k when it senses the car is moving? 3. What could have happened during the service (when the problem started) that could have caused this.

I'm also open to the possibility that I don't know what I'm talking about! and I should stick with bike engines! It may not be the ECU, but the fact remains that I have done the two above mentioned experiments which at least tell me the road speed is causing the revs to go up and stay up until the car stops.

One other thing, it has been suggested to me the reason the road speed is measured is for gearbox management. Again, I don't know a lot about car gearboxes, but I could understand why the road speed would be a factor there.
Anyway if anyone has ever had a similar problem or a small clue as to what the hell is going on, I would most grateful!
Hi
Hopefully you are still around, quite an old post. I have a 2015 R6 diesel polo and the symptom's are exactly as you describe. As soon as the car is rolling the revs pick up to 1200. This is most annoying when driving, with car in gear and your coming up to a corner or even a stop the car does not actually slow down as you would normally expect, as it should be braking on the engine as it slows down, but no it carries on at the speed appropriate to the rpm and with the diesel torque that can be quite disconcerting. Its amazing at what it can do in 3rd with no intervention. You have to change down and have the clutch in a lot earlier than what you would normally do.

So long story short, Germany want nothing to do with it and refer it back to VW UK. VW UK will not do anything with it if the dealer ship don't report it as a fault. All the dealership did was find another car with same symptoms and said that's how it is designed so its not a fault, end of story. Tried to tell them that maybe its a "type" fault and it affects the driving style of the car. My wife can't get used to it and keeps stalling the car because she has to use the clutch much earlier that one would normally do!

Response from VW UK eventually, that it is there to prevent stalling and to protect the clutch plate from wear. That is an absolute insult to Engineering, absolute load of crap!! If there engineers can say that with a straight face.... unbelievable! Its causing the stalling and once the car has stalled, how do you prevent it from stalling??? Really. As for damage to the clutch, the clutch is not slipping, so not sure what other damaged it would cause. Riding a clutch causes wear and tear, but solid (fully engaged) clutch is going nowhere.

So if its a design feature, can this be VCDS's and changed?
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