karen_karen Posted July 31, 2008 Report Share Posted July 31, 2008 I paid a friend who is a mechanic to do some work on my car. She changed the rocker gasket cover and also replaced the radiator. She had to disconnect the battery to do the work. I have driven the car for about 1 hour since getting it back and have noticed that the mpg reading on the trip computer is all over the place, it changes randomly every couple of seconds. There is no sequence or order to the numbers, it just jumps around from anything as low as 20 up to 200. Can anyone help me understand why this is happening? All other readings on the computer seem fine such as the journey time and temperature. Has this happened because the battery has been disconnected and reconnected, do I need to somehow reset the computer and if so how? I don't understand how the trip computer calculates mpg, are there sensors throughout the engine and if so does anyone know where? Could it be that she has disturbed one of these? She said she will take the car back and try and sort it. She does have a comp with diagnostic software on, does it have to be vagcom? I'd appreciate any help as although she is going to sort it, I'd like to understand it for myself. Thanks Karen Link to comment Share on other sites More sharing options...
Cuprabob Posted July 31, 2008 Report Share Posted July 31, 2008 I don't want to sound patronising but are you sure you're not just looking at the instantaneous reading as opposed to the average reading? Most VAG cars display both Link to comment Share on other sites More sharing options...
karen_karen Posted July 31, 2008 Author Report Share Posted July 31, 2008 Yeh I am talking about the real time/instantaneous reading. Before I had the work done on it the instantaneous reading would usually start off at around 15mpg and then gradually creep up to about 35mpg depending on the length of the journey. The real time reading is now just jumping all over the place. It doesn't make any sense to me for the car to be performing at 20mpg for 3 seconds and then jumping up to 200mpg and then to 90mpg etc. Surely this is not normal? Link to comment Share on other sites More sharing options...
Cuprabob Posted July 31, 2008 Report Share Posted July 31, 2008 Hi, I'm sure there is nothing wrong with your car. The real=time reading does go all over the place, mine varies from about 5mpg upto 200mpg on the overrun. Apart from the instantaneous mpg reading you have two average mpg readings depending on what the memory setting is. The first is the trip mpg which will start low at the start of a journey and gradually get higher. The other is the combined average since the computer has been reset. Link to comment Share on other sites More sharing options...
burble Posted July 31, 2008 Report Share Posted July 31, 2008 Surely this is not normal? Yep, it's completely normal. Is it a MkIV or MkV you have? Link to comment Share on other sites More sharing options...
karen_karen Posted July 31, 2008 Author Report Share Posted July 31, 2008 I've got a MKIV 2.0 GTI Thanks for all the help guys. It appears I've probably been a bit silly. I'm going to have a look on my drive home today and check. I didn't realise the real time mpg value would change so much. Just out of interest does anyone know how the computer calculates the mpg, are there sensors somewhere? Also, can you connect a VW up to diagnostic equipment that isn't vagcom?? Sorry if that's a really stupid question but I'm quite new to this all. Link to comment Share on other sites More sharing options...
Mook Posted July 31, 2008 Report Share Posted July 31, 2008 Welcome to Tyresmoke Karen The mpg is worked out by measuring the volume of fuel passing a sensor and, as the car knows how many miles you've done, it can figure out the rest. VAGCOM is by far the most common after-market diagnostics tool on the market, but I can understand if the price might put you off. Whatever you do, don't buy fake VAGCOM cables off of the Internet or eBay - they don't work. HTH Link to comment Share on other sites More sharing options...
burble Posted July 31, 2008 Report Share Posted July 31, 2008 I'm doing this from memory as I sold my MkIV in November last year. Look at the windscreen wiper stalk and there should be a rocker switch on the end of it. Push the switch and the highline computer will cycle through the display options. One of those options will be the MPG that you wanted. The reason the reading is varying so much is that it is looking at the instantaneous reading. So if you're driving along at a constant 70mph it'll be a fairy constant reading. Boot it and obviously more fuel goes past the sensor so the readin goes down. Conversely take your foot off the gas and start coasting and the figure will climb upto a maximum indicated of about 200mph and then display ---. Link to comment Share on other sites More sharing options...
benjie Posted August 1, 2008 Report Share Posted August 1, 2008 As above,. and there's a little button on the underside of the windscreen wiper stalk which will change the little number in the display from 1 to 2 and back again. If 1 is in the corner, the readings you are looking at are from your current journey and are reset automatically when the car sirs still for 2hrs. The little number 2 in the corner is the average mpg/speed/total time/total miles for all the journeys you have done. Note - when you are displaying current mpg (the engine takes a reading every 30metres), the little number1 or 2 won't appear. Also, as your battery has been disconnected, the info in number 2 will have been reset too. If you want to reset your mpg readings etc manually, press and hold the little button on the underside (that switches from 1 to 2). If you're on 1 and press & hold the button, 1 will be reset. If you're on 2 and press & hold the button, 2 will be reset. Hope I've been clear! Link to comment Share on other sites More sharing options...
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