0.1bar at idle - 996 turbo

Afternoon, Noticed today driving to work that my boost gauge is flickering between 0.0 and 0.1 bar at idle. Is shows 0.0 if the engine is off but ignition on, ruling out a faulty MAP sensor. No other symptoms. I can't tell if its boosting well or not as i'm currently running it in after a rebuild and am having to snail it around under 3krpm, however it is boosting as normal up to that point on long 4th gear pulls. I'm also running a mafless tune before anyone blames the maf :grin: Anyone had a similar thing? :?:

With the engine idleing does it continually flicker between 0 and 0.1 or does it hold at either for a while ? For it to be mechanical then the pressure would have to alter at the Map .. a fast flick to me would lean towards it being an electrical fault .. possibly .. it might not actually be a fault . A slow switch could indicate a pressure increase then drop off .. kinda like something opening and allowing the pressure to drop off . Does it idle ok .. no rpm fluctuation ? Difficult for any diagnosis on here for me im afraid so just throwing out there what i can think of. Its a 5 volt sensor so that could be checked .. the voltage varies but i have no set readings for what it should be at idle .. somewhere between 0.5 and 1.8 off the top of my head . Ill have a rumage to see if i have any info here about it . You havent got a tester that can see the voltage of this sensor have you ?

Hi Demort, thanks for the ideas. It can hold the reading for a while, but can also flcker from one to the other as if there is 0.99 bar of pressure, briefly touching 0.1 and then back again. Either that, or as you say it is electrical. However it doesnt move at all with the ignition on but engine off. It idles fine, not fluctuating. I haven't put it on the durametric yet to see if I can see the voltage as I'm at work and its only just started happening. I suppose another question could be - should there be any pressure in the intake at idle?

Rags I'd suggest at idle the reading should only ever be 0 as you'd expect there to actually be a vacuum at idle. Have you got a durametric?

Cheers Tom, yes I do, but I'm about 40 miles away from it at the moment :roll: The more I think about it, I had a similar problem a few years ago when one of my new turbos had a seized actuator flap. Whenever it was freed it would stop happening.. I checked the actuators last night though and they are as free as a bird. Still, I'm not sure how a seized actuator could cause this symptom. Puzzled from Barking.

My gut feeling is electrical as im struggleing with a mechanical fault at idle . I can say that testing a maf with ignition on and not running will give a reading between 0.9 and 1.1 volts and is supposed to be ok at anything in that range .. Doesnt mean to say the sensor is actually ok though and im kinda thinking this about the Map atm . Get a tester on it and see if you can check the voltages and rpm .. also any fault codes and adaption values if possible.

I don't think it would be wastegates / turbo related as they would not spin fast enough at idle to create boost. Also you'd expect the springs to be holding the wastegates closed any way. I think it is likely to be the map sensor or the wiring to it. Could be a high resistance joint or a core partially damaged shorting to earth. I'm not sure how the sensor works, as in whether it would expect a higher or lower voltage as boost increases. Could even be a high resistance connection on another sensor causing it to back feed through the loom. My first step would be to check durametric when you can. Then I'd clean the plug to the map with contact cleaner. Also the two main engine loom plugs. Has it deffo only appeared recently? Could be the way es have written the tune... might be worth a call to them in case they say it's normal.

Cheers lads. Seems like both the spark's reckon its electrickery then! My gut feeling is the same tbh. Yes its deffo only started today. I've been using it as a daily for the past couple of weeks to get the miles on her and it hasn't happened before. I've got a long drive down to London and back in it tomorrow to get the running-in service done by the engine builder so the drive back will kill or cure it! I'll let you know how it gets on :thumb:

The trouble is the dme uses this sensor to addapt the fuel mixture so its going to need fixing i would have thought . Ive also been mulling it over .. i just cant see anyway for it to be a mechanical fault .. im certain the revs would be fluctuateing if it was . Turbos are at idle .. waste gate position wont matter and cant open close at random .. any split pipe wont be random .. any increase/ decrease of pressure .. hmm .. i cant see it atm . Drive it and see .. we dont know what the boost is actually like untill its run in atm . EDIT .. opps i should have read Toms post first .. just repeated what he said Edit no 2 .. Okay a test here useing a meter set on Hz .. https://www.picoauto.com/library/automotive-guided-tests/digital-map-sensor/ If i understand it correctly a non running engine should have a Hz of 80 so ignition on and measure the Hz of the map sensor .. Tom does that sound correct as this is not something i use to test items .

God I love this forum at times :thumb: :thumb: :thumb: :thumb:

So a digital map sensor forms a varying frequency ay, you learn something everyday! The example above is on a N/A car as it only shows vacuum. You need to hook up to durmatric and do a live read out from the map sensor. Circa 1000 would be atmosphere pressure, at idle you would expect lower than atmosphere pressure there would be a vacuum within the intake. It maybe that you have a (big) leak at the throttle body so the sensor is reading atmosphere pressure and not seeing the negative pressure it expects to see therefore assumes there must be some boost. I think testing the sensor with a meter would be tricky as we do not know what readings a good sensor will give.... Using durametric will tell you what the ecu is seeing. Be the fault a leak, faulty sensor or wiring. If the sensors reads atmosphere pressure (around 1000) and it does not change engine running or not then I would say you have a leak (after the sensor). You could also try blipping the throttle as this should make the vacuum bigger, so long as you do not give so many revs the turbos start to boost of course. Could just be your ecu deciding what is normal atmosphere pressure... If things were a long way out I think you would get a light come on. With maf monitoring air flow in, map monitoring pressure and lambda sensors checking the results at the end after fuel has gone in. A big difference it would throw a code I am sure or it will be adapting to suit. At the end of the day atmosphere pressure does change. you would only need a 50mb increase in atmosphere to get 0.1bar on the dash more than you are used to seeing (I am assuming the dash display follows normal rules of rounding up and down). I bet it will just settle down. In the small chance there is an issue it will likely get worse which will make it easier to find. That's my sparkies view...

I'm also very surprised that a 'digital' MAP sensor exists, as it's transmitting an essentially analogue range (varying pressure). Looks to rely on a rapid on/off signal to indicate the variance in pressure, something I've never encountered in industrial instruments (my job), have seen it on flow meters however. Interesting stuff. :thumbs: Oh, and the analogue gauge in the Cossie shows VERY negative off boost, as you'd expect.

So a 500 mile round trip to/from London for the running in service and it has sorted itself out! Completely back to normal. I wonder if it was the inclement weather yesterday :?: Now running some proper revs (and therefore boost). I'd forgotten just how quick this car is...

There could have been one more explanation. If, as Tom has said, you had a vacuum leak on idle, this would not have been picked up due to the maf delete. In certain circumstances the idle mixture could be slightly richer than ideal, the extra air being induced then causes the idle to raise which the ecu sees, then retards the ignition to try to maintain idle target value. With retarded ignition the burn happens late, this causes some minor spool on idle. In other words, this is the basis of anti lag systems the world over, it's just that in this case the air may being supplied by a temporary vacuum leak rather than a jacked throttle or a bypass valve. The same thing can be attained by playing with the overlap via the variocam, but this would happen at all times. I'm not saying this has happened, but it is possible, especially if after driving the car harder today has helped seal any potential leak due to a heat increase. ~But that is a long shot!

Can i just add .... :worship: