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Limp mode on track - any thoughts?


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  • Gold FFM

On Thursday at Brands Hatch my car had a problem in one of the sessions, and I wonder if anyone had experienced similar.

Coming out of a corner, I tried to accelerate and there was no power at all. I slowed down slightly and the car would accelerate back up to 4000rpm and then stop, so it was obviously in some sort of limp mode.

There were absolutely no lights on the dash to indicate that anything was wrong. I cruised back to the pits, and after I'd left the car a while and restarted it the fault was gone. I did two more sessions, and a 120 mile journey home with no problems.

A fellow owner reminded me that the brake light switch can cause problems....BUT, my car was still in Sport mode when I got back to the pits. I thought issues with the brake switch kicked you out of Sport mode? Or is that not always the case?

I've connected a standard OBD reader to the car, and there are no fault codes stored.

Here's a video of it happening:

Any thoughts? 

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I had something similar on mine but the red redline lights came on simultaneously at 4,000rpm and wouldn’t let me rev higher.

Fortunately at Silverstone and Ben got  it in straight away to do diagnostics, but nothing came up. He wasn’t sure why and it’s never done it since.

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On the analog dash you typically get the 3 red lights at 4k RPM when the car is in limp home mode.  I have no clue what the "AIM dash version" of that looks like; same goes for the CEL.  Only thing I can think of is that the code cleared after restart (so not a persistent code thus not emissions related) - fuel surge is possible, but it would throw a code, albeit maybe not a persistent one; would need to look that up.

Anyway, it is fixed now he 😉 

 

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@Paul_D can't see what was your fuel level at. Was it less than 50%? If less than 50%, could be the fuel surge. But I guess car would go back to normal after a few seconds of a slow straight drive.

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  • Gold FFM

It’s a possibility, but I don’t think it was likely to be fuel surge.  The tank was over 50% full, and the track was still a bit damp so I wasn’t cornering particularly hard. 
 

I don’t know if I’ve been lucky, but I’ve run the tank much lower than that before on a dry track and never had a problem with fuel surge. 

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Was your AIM dash in track mode and recording? If so you will have a data log of a load of ECU parameters you can search through to try and identify the issue. I diagnosed a faulty yaw sensor on my car doing exactly this:

79774600_Yawsensorfaultdatalog.thumb.png.4d9b6501aea8faefd715ec3c1d0420a5.png

You can see in the graph here my car dropping out of sport mode (blue line) to normal and the ECU Yaw flat lining at -163.8 degrees/sec of rotation (like the car was spinning) when I was travelling in a fairly straight line. 

You can also use the AIM to diagnose brake switch issues by looking at the brake switch output (2 of them) and also the cruise control value which triggers a fault code when the switch is not setup correctly (think it switches to value 3).

Have a look through the graphs and see if you can see any odd values. Accelerator position and revs should allow you to pinpoint roughly the time when this happened as revs will stop increasing despite you pressing the accelerator... Then work back from that time period.

 

 

Edited by rmaurer
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  • Gold FFM

@rmaurer Unfortunately, for the first time since I got it I didn't actually have the dash in track mode for those sessions. 

One of the first things to go through my mind when I got back to the pits was 'damn it, I'm not in track mode so I can't even analyse the data when I get home'

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