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Is there anyway you could try another MAF with an airbox? My 3B coupe suffered dreadful heat soak problems with a cone filter, the MAF that had been conected too the cone also caused me a load of bother.
1990 B3 2.0 80 quattro.
1992 C4 2.8 Avant quattro
1994 MTM RS2.
1998 B5 TDI quattro Sport.
2000 B5 TQS.
Get a meter and measure electrical resistance of the 2-pin IAT sensor... its just a thermistor... resistance changes with temp... and ECU adapts fuel & spark accordingly. It decides a nominal number in the event of disconnection / total failure... but funny running could arise if it gets haywire values when hot vs cold. Try and measure it cold and after a run... you can drive car without it too and see if substitution is better.
I had a 3B loom in here this week for repair and pin for pin examination... The MY91 3B DOES use MFTS-R pin signalling into ECU pin #36. This appears on schematics and was reflected in reality on this MY91 loom - there is easy way to spot it.
MY92/93 schematics for 3B however show that MFTS R-pin is unused as far as ECU is concerned... just connects to aircon where applicable.
Just measured mine and unlike normal thermister, temp goes up and resistance goes up, is this correct. 512ohm cold running engine , versus 540ohm (and climbing)after left standing idling until fan cutting in and out and inlet manifold and pre throttle pipewirk hot to touch.
Will drive home tonight with it unplugged and see if any difference or perhaps stick a 470ohm resistor across lead ends instead of IAT.
1986 Coupe q - 2281cc 3B, chargecooler, 2.75" exh & downpipe, ported head, water injection, CGR box, modified MD357 turbo, 440cc injectors, high power coil.
2004 A4 2.5TDi convertible, re-mapped to 180bhp
2011 A5 sportback 3TDi-Backbox delete, remap
some thermistors are designed to be 'positive' (PTC) in that resistance increases with temperature
Some are negative (NTC) in they reduce resistance when temp rises...
IAT on 3B is indeed a PTC device but that seems a very low range of values... let me try to dig out the graph
An unplugged IAT is detected by ECU and substitution of 40 celsius is used for boost regulation... aims to minimise boost levels for safety but not shut down active boost of course
NOt found graph yet but OFRM states that air temp of 20 degrees should give resistance of 450-650 ohms... real tight spec there - NOT... So nominally should be around 525ohms.
If thats right my 540+ohms tells the ECU my intake air temp is at 50degrees, no wonder it cuts boost as soon as ambient temps go over 10-12degrees. Just straddled a 5k6 resistor accross it, be interesting to see if it makes a difference. Brought it down from 540+ to just under 500ohms which is about right for todays 22degrees outside temp.
1986 Coupe q - 2281cc 3B, chargecooler, 2.75" exh & downpipe, ported head, water injection, CGR box, modified MD357 turbo, 440cc injectors, high power coil.
2004 A4 2.5TDi convertible, re-mapped to 180bhp
2011 A5 sportback 3TDi-Backbox delete, remap
That's interesting. Perhaps this is something I should consider as a suspect? A couple of questions though; would this components failure show up as a fault code? Mine appears to have been soldered to the wires, does it matter which way round the wires go? Can i check whether this little sensor IS actually used by the ECU? With the parameters being so wide, and if a new sensor is NLA, can one viably just fit a resistor across permanently to give a steady readout?
"Its made with bits of real S2, so you know its good."
That's interesting. Perhaps this is something I should consider as a suspect? A couple of questions though; would this components failure show up as a fault code? Mine appears to have been soldered to the wires, does it matter which way round the wires go? Can i check whether this little sensor IS actually used by the ECU? With the parameters being so wide, and if a new sensor is NLA, can one viably just fit a resistor across permanently to give a steady readout?
ECU only sees it as fault if it is open or short circuit.
Bodging IAT with a fixed resistor is not ideal long term... ECU does various adjustments for fuel & spark in cold vs hot air for starting as well
Nice trick... will certainly be interesting to see how that goes
Well with the 5K6 resistor in parallel with IAT, I drove home in a different animal to the one I drove to work in.... despite the 23degrees outside temp the car performed as it did during Aprils cool mornings. So it looks like my IAT is reading far too high. Checked after the fun drive home and with my resistor across and wire disconnected it showed 502ohms which is what I'd say it should be using the chart. Saves having to buy a new IAT.
Might do some experimenting now to get best results but not dropping below 480ohms when engine cold and 20deg ambient.
1986 Coupe q - 2281cc 3B, chargecooler, 2.75" exh & downpipe, ported head, water injection, CGR box, modified MD357 turbo, 440cc injectors, high power coil.
2004 A4 2.5TDi convertible, re-mapped to 180bhp
2011 A5 sportback 3TDi-Backbox delete, remap
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