Standard MEMS at altitude

[Andy Lef]

I am off to Italy for 3 weeks in the recntly converted 1.4ss - 1.8ss with a K15 upgrade all from DVA. Standard MEMS remains fitted on a VHPD and all works fantastically.

 

I have just been trawling (idling) through the sites and have come across this here. Where the chat revolves around poor performance when tavelling at altitude. The chat is not really conclusive and quite frankly I was baffled more than informed. Ludite obviously!

 

Should I be worried when thinking about starvation as I cross the alps (will also be going to Tignes 3100m for a couple of days. Or am I worrying about nothing?

 

Andy

[Colin Mill]

I would be seriously surprised if the ECU couldn't cope at 1500m. The ICAO standard air density for 1500m ASL is 1.058 kg/ cu.m compared with a sea-level density of 1.225 kg/cu.m so we are looking at a 14% shift in density. You get the same shift in density going from 0C to 38C air temperature (at constant pressure) so a system that would run into difficulty at 1500m would be in trouble in Spain in the summer.

 

You can get a 10% pressure variation between extreme cyclonic and anticyclonic weather conditions as well.

 

 

 

Edited by - Colin Mill on 25 Aug 2009 11:24:26

[oilyhands]

The difference with air temperature is that the ECU has an air temperature sensor and adjusts accordingly, the problem with altititude is that there is no ambient pressure sensor only a manifold absolute pressure sensor.

 

Oily

[Ian B]

I have a 1.6ss 'standard' K-series with rover Mems, and have toured in the Alps and the Rockies without a problem

 

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Ian - Slightly Vider SVelte SuperVeloce model 😬 - Joint AR for North Kent

 

Edited by - Ian B on 25 Aug 2009 12:01:58

[Andy Lef]

😬

 

Just spoken to Dave and am happy that my fears are for nought. Thanks Dave et al.

 

Andy

[Willie.]

What fuel pressure regulator are you running, the 1400 or the 1800s one?

 

On my 1.4-1.8 SS conversion, I ran the 1800 (MGF) FPR, which made the car very rich low down, but spot on at high rpm.

 

Touring over the Alps, including Stelvio, I had no problems whatsoever.

 

Willie

[Colin Mill]

the problem with altititude is that there is no ambient pressure sensor only a manifold absolute pressure sensor.

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But so long as the fuel pressure regulator is referenced to the manifold pressure (which it is on the standard set-up) this is not a problem because the mass of air entering the cylinder can be characterised from the absolute manifold pressure, manifold air temperature and rpm without need for the absolute air pressure.

 

On my 1.4-1.8 SS conversion, I ran the 1800 (MGF) FPR, which made the car very rich low down, but spot on at high rpm.

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This is the effect you get if the fuel pressure regulator is referenced to ambient pressure rather than manifold pressure (unless the system has been mapped for this eventuality) With the regulator referenced to ambient the pressure drop across the injector increases as the manifold vacuum increases (i.e. at light loads).

 

Edited by - Colin Mill on 25 Aug 2009 15:32:07

[oilyhands]

The FPR is referenced correctly and although in theory this should cope with much lower air densities, in practice it doesnt actually work that well, the system tends to go rich at altititude, in some cases causing misfiring and bogging down.

 

Oily

[Colin Mill]

Given that Ian has had no problem at altitude I wonder if the systems that show altitude sensitivity have something amiss (i.e leaked MAPS vac. reference??). I know we don't always have much faith in Rover engineering but given that the Bosch D-jetronic system got altitude correction right back in the 1960s how could Rover get it so wrong in the 1980s?

[Simon.Rogers1]

I have seen at first hand what can happen to a VHPD engine on MEMS at altitude.

 

It was a very good friends new 7 and had run perfectly for the first part of a trip throught Northern Spain. As we climbed one very hot day into the mountains it began to run rough as a dog and then cut out. Turn off and restart - sweet and then after another mile or so still climbing the same would happen.

 

We changed everything on that car that evening under torch light.

 

Once we were back down to fairly constant levels everything was fine.

 

The answer was that MEMS only senses Barometric pressure at start up and not constantly. Therefore everytime we restarted everything would be fine.

 

I would point out that my own supersport and all the other less tunned K's were fine. It seemed to only affect the VHPD. This was 2001.

[Colin Mill]

Interesting what they say here about the MAPS - that you can basically drive the car around with a duff one without noticing it by the ECU using a reserve mapping based on rpm and throttle butterfly position! (but of course that would be making assumptions about the pressure situation)