3

I've got a '05 VW Caddy 2.0 SDI here that didn't run at first. Scanning the OBD (with a chinese knock-off ELM327 scanner, and the Torque Pro App) gave me a P0726 fault code. The description says "Engine Speed Input Circuit Range/Performance". My bet was that the crankshaft sensor was at fault, so I ordered a new one and installed it. The Caddy runs again now, but it won't ever rev past 3500 rpm. Not at standstill or while driving with a warmed up engine. I erased the error, but it reappeared again after a while.

Next to the rev limit thing, it starts terribly bad; it takes a second or 6. The glow plugs light is blinking, but it's still hard to start when the engine is warm, so I guess it isn't caused by the glow plugs.

The car does not hesitate under acceleration, it doesn't smoke, nothing is out of the ordinary. It is a manual, by the way. Could it be that there's something else wrong, or that my chinese scanner misses/doesn't recognise a fault code?

Improve this question
2
  • Are you saying the rpm hits a hard stop at 3500 rpm (a possible electrical/ control issue) or never makes enough power to get up to 3500 rpm? If you place car in second gear and smoothly accelerate does it go smoothly up to 3500 and immediately level out there? – zipzit Mar 7 '17 at 23:02
  • @zipzit It just limits the rpm, doesn't matter whether I accelerate hard or smooth. It all works fine until 3500rpm. It just rather abruptly cuts off power at that point. Probably because a certain incoming signal isn't trustworthy enough to use for revving over 3500. Normally such a condition would likely be caused by a incorrect crank sensor signal, but I just replaced the sensor, cleaned the connector etc. – Bart Mar 8 '17 at 7:35
1

P0726 fault code can be caused by:

  • A failed engine input speed sensor
  • A failed transmission output speed sensor
  • A short or open in the wiring harness to one of the sensors
  • A loose connection to one of the sensors
  • A faulty crankshaft position sensor
  • A faulty shift solenoid
  • Dirty, contaminated or low transmission fluid
Improve this answer
5
  • Well thanks, but this information shows up at the first google. I already diagnosed the error, but none of the other causes should make the engine limit its rpm to 3500. – Bart Mar 8 '17 at 7:42
  • @Bart Why are you assuming that "none of the other causes should make the engine limit its rpm to 3500"? If the ecu is missing an input signal or it is unreliable (poor / broken connection) then the software / ecu is most likely doing what it is designed to do. – Solar Mike Mar 8 '17 at 9:24
  • @SolarMike I worded my comment rather poorly; but I already diagnosed the cranksensor sensor and circuit as said, so only the shift solenoid or transmission fluid are left. There is no shift solenoid because it's a manual, and there is no transmission fluid sensor that measures level or contamination. I'm beginning to suspect that the MAF sensor may be dirty, causing the measured rpm differing from the rpm that should be achieved at a certain air flow. – Bart Mar 8 '17 at 10:18
  • @Bart - a fault with any of the intake sensors can cause the ecu to limit rpm - intake temperature, maf, turbo pressure etc. – Solar Mike Mar 8 '17 at 13:30
  • @SolarMike But those sensors aren't mentioned in the error code description, I suppose other faulty sensors would throw their own error code. However, only the P0726 code shows up. But maybe there's another error that my cheapo scanner doesn't recognise? – Bart Mar 8 '17 at 14:11
0

To diagnose the problem easily, you have to use a tool like VCDS to read VAG specific values and error codes which are much more detailed. The common ELM327 readers will only read the OBD values which are rather limited and generic. VAG tools can read the actual problem codes. You can also read codes from other components which OBD scanners can not read. There are other tools than VCDS which can read VAG codes. Fortunately you can buy chinese cables less than $20 still.

Your car (2K) uses CAN-bus so you can't use VCDS lite.

For example: http://www.ross-tech.com/vag-com/tour/dtc_screen.html The OBD code is P1282 in the example, but the VAG tool can read the actual status of the engine when the error was logged, and also the sensor which gave faulty reading and also possible reason of the sensor malfunction.

Here is VW caddy specific info page which shows the actions you can perform: http://wiki.ross-tech.com/wiki/index.php/VW_Caddy_(2K)

Improve this answer
2
  • Thanks, I thought a vagcom was really expensive, but if those chinese versions also work decently, i'll try that I think. Thanks for the info. – Bart Mar 14 '17 at 15:16
  • You can buy for example from AliExpress, just make sure that your car model is supported. If it doesn't work, you can open dispute and get your money back. I had to do it many times, usually you can at least get 50% because the shipping fees are too high for sending back. I think the prices float around $20-$40 so it is worth trying. It is nonsense to rely on OBD for diagnosis, most of the time it doesn't give enough information. – Evren Yurtesen Mar 14 '17 at 15:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.