Jerkiness And Power Loss At 1500rpm

[dt1]

Hi. My 2001 auto Gal MkII (60,000miles) has become jerky at what appears to be one engine speed of about 1400-1500rpm. This is probably because most of my driving seems to be at that rpm. It is most noticeable when the motor is cold and when driving slowly. I suspect that something mechanical is worn out and is sticking. At low speeds and gentle acceleration the torque of the engine makes the car lurch a bit unless I either back off or accelerate through it. At higher speeds I can sometimes feel a slight 'oscillation' but it is damped by the inertia of the car.

 

It's rare for the issue to cause further problems, but sometimes it cuts power until I stop and restart the engine. Having cleaned sticking turbo vanes some months ago I recognise the symptom and I don't believe it is a turbo problem (I believe turbo vanes open at a higher rpm).

 

I suspect now that it is either the boost control solenoid or the vane mechanism in the inlet manifold. VagCom gives the errors below. I've had the 17664 for some time so I think the main error is the good o' 17965. I assume either the solenoid or inlet manifold vane sticks. Would both cause an over-boost situation? Advice please.

Thanks, David.

 

17664 - Engine Coolant Temp Sensor (G62): Open or Short to Plus

P1256 - 35-00 - -

17965 - Charge Pressure Control: Positive Deviation

P1557 - 35-00 - -

[big_kev]

Sticky Vane Actuator mechanism ?

[tim-spam]

Presumably, the Charge Pressure Control: Positive Deviation fault code is intermittant?

 

You mentioned that the car has tripped into limp mode occasionally - at what engine speed did it do this? I would guess that it was over 2000rpm, and is the cause of the fault code.

 

As regards the lurching at low speeds, this could be related to the transmission or could indeed be a boost control issue. The first thing to check is that your turbo vanes are still operating freely and that the ring of 5 bolts holding the VNT mechanism are still tight. If all appears well, the next possible culprit is the N75 valve (or boost control solenoid valve). You could try removing and cleaning it to see if this makes any difference - a garage would just replace it. The same goes for the TMAP sensor - if this is contaminated or faulty, it could be giving spurious information to the ECU. For example, if the car was tripping into limp mode at 1500rpm due to an alleged overboost fault, I would suspect the TMAP sensor as it is very unlikely that the turbo would be capable of overboosting at this engine speed.

 

If you have VAGCOM, you could always try the graphing functionality and plot actual boost pressure against requested boost pressure to see if this shows up any oscillations or deviations - it would be better to have someone else driving, although I believe that you can start recording, go for a drive, and then stop recording. If you monitor engine rpm at the same time, this may make the results more meaningful. I have only played with the graphing functionality very briefly, but it does look good.

Edited April 3, 2007 by tim-spam

[dt1]

The limp mode occurs at 1400-1500 which is why I don't think it is the turbo. It's my understanding that the turbo will be at full boost until over 2000, some say over 2500. If I accelerate firmly to move through the troublesome spot quickly, the car performs correctly and does not trigger limp mode.

 

The jerkiness appears to happen at 1400-1500 irrespective of ground speed. So does this suggest it could be something mechanical in the engine but less likely to be transmission? If transmission, would it be more likely to relate to ground speed?

 

I pull out of my drive onto a twisty residential road on a slight decline. So other than establish motion I travel quite slowly with little power applied for a few hundred yards. Under low load or consistent cruise at any speed, the auto seems to always try to select a gear that runs the engine at 1400-1500 rpm. The car surges and slows and can get into a non-violent but nevertheless noticeable oscillation which rocks me back and forth a little if I let it stay at that rpm at low speed. Something that controls acceleration seems to stick. I think this implies something physical that moves rather than electronic.

 

I don't have the full version of Vagcom so I doubt if the free version does the graphs?

Thanks, David.

[tim-spam]

If your car is going into limp mode at 1500rpm and the DTC shows this to be due to excessive boost pressure, then I would suspect that the TMAP sensor is giving the wrong information to the ECU. From your description, this sounds to be an intermittant problem, so the first thing I would try is to remove the sensor and clean it. Alternatively, as they are not massively expensive, you could try "doing a main agent" and just replace it and see. The other thing main agents tend to do is to replace both this and the N75 valve. However, from what I've read on other VW forums, it appears that N75 valve faults usually result in a negative deviation DTC.

 

I would doubt that a sticking VNT or a faulty/blocked N75 valve could cause sufficient overboost at 1500rpm to cause the car to trip into limp mode. However, if the TMAP sensor was telling the ECU that the boost pressure was excessive, this could be causing the problem.