MAP sensor test

14th November 2005, 17:19

Does any one know how to test whether a MAP sensor is working correctly?

My VTS often idles poorly but is always smooth at engine speeds above idle and I have been wondering whether the MAP sensor is to blame. This morning in the frost it didn't want to idle at all well for the first minute of running but it isn't normally that bad.

14th November 2005, 17:23

get a multimeteter on it from cold to warm, write down roughtly the figures and report back here and ill tell you, although you will find they change all the time, but on idle it should be anything from 2.5 or less volts going below 1v eventually.

14th November 2005, 18:36

Thanks Ratty, I'll do that.

Does it sound to you like it could be the MAP sensor? If that is what's causing the poor idle, would it still pass the emissions test on the MOT?

14th November 2005, 19:20

The voltage changes with pressure, so it will be different on idle depending whether engines cold, atmospheric pressure, etc...

I have some 'normal' readings written down somewhere too.

Be interested to hear your figures.

If the revs are hunting (up and down) and you can hear a change in the pitch of the engine, this is usually due to somethign making the fuel weaken, then richen up,

I was watching mine do this on tickover yesterday. Revs were wandering by +/- 250rpm and I could see the fuelling cycle down on the AFR meter when the revs went low, and the fuelling richened up when the revs picked up.

Theres probably a way to speed up lambda scanning on idle... Hmmm.

14th November 2005, 19:29

normally takes a few seconds (30 or so) for lambda to kick in on idle until sensor is up to operating temp. Would need a wideband sensor to make it pickup faster, they alter about every 120ms or so roughly.

17th November 2005, 20:30

Right, I've got some readings for you:

Ignition on but not running (i.e. atmospheric) 4.41V

Idle, cold 0.4-1.6V (very erratic for first 30 secs or so)

2000rpm, cold 0.60V (steady)

Idle, warm 1.49-1.55V (mainly 1.54V but a bit erratic)

How do these look?

17th November 2005, 20:37

can you get a reading from your coolant temp sensor from ign on only and then when idle cold.

All the rest appear correct, so could be dodgy information from one of the sesnors causing it

17th November 2005, 20:43

Sorry, I meant to say, all of those readings are from the MAP sensor.

The coolant temperature sensor was changed recently because of the poor idling but made no difference.

Is it true that all VTSs are a bit lumpy on the idle?

18th November 2005, 00:15

can be but its normally for a reason, mine is perfect always has been.

Basically i dont think its your map sensor, i think something else is causing your dodgy idle.

From first turn on, how long does it take for the revs to settle to normal idle?

18th November 2005, 12:55

It idles at about 1050 initially and settles to the normal 950 after a couple of minutes I'd say.

I don't know if this helps but the only time it idles perfectly is when its fully warmed and I switch off for about 5 minutes and then restart. On the restart it will run perfectly.

I have been wondering about the other sensors like the lambda and throttle position sensors.

The idle tends not to fluctuate, although that has happened occasionally, it is just a bit lumpy pretty much all the time.

18th November 2005, 16:37

you could check the values on TPS and lambda on idle, but it takes lambda about 30 seconds to kick in as it doesnt work until tip is warm.

18th November 2005, 17:02

The lambda looks probable then. I'll check both though.

Thanks for your help so far