Diesel engine over reving on tickover, MAF problem? SOLVED?

rory_perrett · Post by **rory_perrett** » 16 Jun 2011, 22:25

First, apologies for the long post

I have a 2000 Xantia Hdi (110), about 2 years ago it started “driving itself” in that for no reason (every 10 mins or so but apparently quite randomly) the revs would increase, sometime to as much as 2000rpm, usually somewhere between 1100 and 1500rpm, quite disconcerting if you were on the overrun or driving slowly.

It would rev up for 10 or 15 secs and then drop back but to a fast idle between 1000 and 1100prm. At the time I concluded that it might be something to do with the MAF sensor as the plug was a bit broken and fiddling with it seem to get the revs back down.

I got a replacement plug from a scrap yard that came with the MAF attached so I just fitted the lot, problem solved – for about 12 months when it started again although it never reved to more than 1300/1500rpm. This time I just cleaned the sensors with brake cleaner and again the problem disappeared.

Now its back again, cleaning the MAF does not appear to have worked this time. Each time I take it out and put it back the problem seems to disappear for 2 or 3 days and then it returns. Reading other posts, MAF problems seem to be associated with reduced performance, presumably the MAF is under reading flow and under fuelling. My symptoms suggest over fuelling on tickover, as there is no problems with the way the car runs and drives otherwise. It seems strange to have two MAFs fail in this way so I’m a bit concerned the MAF is a red herring and it is due to something else. Ok a new MAF is not too bad at £35 off e-bay but I don’t like throwing money away. Does anyone have any thoughts?

mongoose100 · Post by **mongoose100** » 17 Jun 2011, 13:00

Throttle cable/potentiometer thingee sticking maybe?

Or, are these 'fly by wire'?

spider · Post by **spider** » 17 Jun 2011, 13:06

Check the breathers are clear too and not full of oil, some residual 'mist' traces are to be expected. Look at condition of air filter / air box as well.

rory_perrett · Post by **rory_perrett** » 01 Oct 2011, 18:52

Having lived with the revving problem for a while it suddenly got worse about a month ago. While messing with the MAF sensor and air box would stop it for a while it only lasted a few minutes rather than the previous days and it was making the car very unpleasant to drive.

I found a split in the small rubber vacuum pipe to the EGR valve at the back of the engine and a small hole in the pipe from the vacuum pump to the Recycling regulation electrovalve (EGR) and Boost pressure regulation electrovalve.

In the 4 weeks since fixing these pipes the problem has gone away. Is this just coincidence (like changing the MAF sensor earlier) or is there a logical reason why these air leaks may have caused the over revving?

torq · Post by **torq** » 01 Oct 2011, 19:43

Air leaks usually cause uneven idling.

Once bought a bmw e30 318i which was hunting on the idle and wouldn't run properly. While looking at i guessed it was a split vacuum hose. Sucked air between my teeth, made all sorts of this could be dodgy comments and offered the owner €450 instead of the €950 he was looking for. He reluctantly accepted, I spent €30 on a new vacuum hose and the car was perfect.

cachaciero · Post by **cachaciero** » 01 Oct 2011, 22:44

Absolutely! leaking vacuum pipes cause all kinds of problems depending on where they leak. In your case the EGR valve was effectively out of control probably intermittently, but would have affected the rpm at low to mid revs.

cachaciero