Limp mode - what is it?

Hi Guys,

Today my 51-plated Audi A3 TDI had a sudden loss of power whilst in 4th gear on the motorway.

I have done some research on here and it turns out it probably went into limp mode (i.e. I couldn't get above 3,000 revs no matter what).

However, what I would like to know is what is 'limp mode' and what causes it? Can anyone answer me this?

My theory is that my car went into 'limp mode' as I was very low on fuel at the time (I'm someone who always waits until the reserve light comes on before re-fuelling). If this low fuel level is what is the likely cause of the 'limp mode' then I will without a doubt change my attitude to fuel levels!

Thanks in advance!

I wouldn't have thought that low fuel would have put the car into limp mode. You're best getting the car read for fault codes which should give you a better idea.

I am with Hagis. I have had my low level petrol light on more than once in my Passat and never encountered any problems.

Well I have to say I have had the fuel light come on and never had this problem. I can't begin to wonder what causes it.

Can anyone help?

Cheers.

Its probly low fuel. Its happened to me 3 times now:biglaugh: becasue Im a scruge! Ill drive for about a week after the fuel light comes on. It will go into limp mode about 5 miles before it runs out. Its so hard to drive, It happened to me yesterday whilst waiting to turn right on a busy main road witha HUUUGE sainsburys HGV behind me.When I pulled away and couldnt go over 5mph he wasnt a very happy bunny! A good old stop and restart sorted it untill the petrol station.

However RE your origional question, the car will go into limp mode if it reaches any kind of critical state were it 'thinks' anythign other than light driving will harm the engine.It sets a rev limiter on the engine so it can only be driven slowley, Iv found that the fow fuel limit is about half of the regular error limit. Normal lim pmode is around 3000 RPM but my A4 with no petrol will only do about 1500/2000 RPM. In the case of low fuel, It helps preserve that last little bit and also give you a last REAL reminder that it really will run out of fuel! Other limp mode reasons are things like low oil lever/pressure, high temp, low water level, brake system faults or any critical engine/saftey system error. PLus for thsoe who arent in the know, theyre more likely to take the car to a garage to get it checked out if its in limp mode,as they wont have a clu whats going on with it, and the more serious fault (should) get diagnose and maybe even cured!

So ther you have it, limp mode ....DONE!

:beerchug:

Thanks for the advice/help!

After it happened I went to the petrol station and put some fuel in, together with a full can of Redex - not had the problem since (touch wood!).

Cheers!

Limp mode is a generic 'reduced power' mode, that results from any number of potential engine problems. (resulting in reduced fueling & N75 operation)

On a TDI 'limp mode' is usually the result of real time boost pressures being out of spec for a given period of time. i.e Overboosting or underboosting.

Low fueling resulting in reduced combustion & charge pressure can cause underboost limp mode directly before running out on a VE engine.

I would not have thought the PD engine would suffer from this, as the injector pressures are very high.