14 replies to this topic

[jemo]

Hi,

My mpi has been idling badly for some time - at times it seems ok and at other times the revs are too high at about 1200-1500 but not as high as some posts I've seen in here.  I'd say it always seems ok when cold - the revs are higher as expected but once full warmed up it doesn't always drop to below 1k.   I've read a lot of the other posts on what seems to be a fairly common problem but I still have a few questions:

 

1) On a healthy setup, how quickly should the revs die back to idle after you blip the throttle?  Say I blip the throttle to 2-3k and then take my foot off.  The revs seem to quickly drop off to about 1500 and then there is almost a pause before they drop more slowly to normal idle speed (when it is idling normally)  - is that expected behavior or should the revs drop almost immediately to idle?

 

2) After reading various posts here I stripped down the throttle body and idle control valve and cleaned both thoroughly.  This does seem to have improved things but I'm not entirely convinced.  One thing I noticed when I had the throttle body off the car is if you hold it up to the light you can see a glimmer of light around part of the circumference of the throttle blade when it is closed.  I actually have two throttle bodies and they are both the same in this respect - we're not talking much of a gap but enough for light to get through.  Is this normal or when the throttle is shut should it seal completely?  I've read about warping etc but don't know how severe that is and the fact two bodies are the same made me think maybe it's not supposed to be a perfect seal?

 

3) I've seen that there was a mod issued for the water temp sender which can have some influence on idle. - I assume this was just to alleviate any pulling on the harness which is quite tight?  If I remove the connector when the engine is running the fan comes on immediately so there is clearly a signal there.  Is that a good enough test?

 

4) Does the oil temp sender have any part to play in the engine management or is it just there for the gauge?  I believe it is routed through the ECU but I don't think it is an active control signal.  My gauge hardly moves so I suspect there may be a sensor issue hence my question.

 

Any help or suggestions appreciated?

 

Thanks, Jeff

 

 

[Mervyn]

I'm unsure as to the points you've raised as im unfamiliar with mpi ,but there's a few simple checks that apply to most cars

Check for vacuum leaks around the inlet manifold (spray with wd40 and see if idle changes is my method) and check any vacuum hoses for splits or loose ends

Ensure the throttle pedal and cable are free to move. Could be frayed internally

Checked any throttle stops / fast idle adjustment if applicable

Cheers, I hope you get it fixed


Merv

[Sprocket]

oil temp sensor is purely for the temp gauge, it is not wired through the ECU. Has noffect ony anything other than the displayed temperature.

 

yes in the main, you should see some daylight around the throttle plate as the idle air is divided between the fixed volume through the throttle and the variable volume through the stepper motor. the volume of air and ultimately the gap around the throttle plate at idle is small, i doubt you could make a good enough assumption just by looking at it.

 

Maybe there is something wrong with the throttle body, or throttle cable adjustment, or a cracked idle control valve body (common), or just a plain manifold air leak or two. if for what ever reason there is more air at idle where ever it is coming from, when you raise the idle speed with the throttle, the stepper motor follows it, opening up. this is to catch the idle speed as you lift off the throttle. the stepper then brings the engine back down gently to the pre defined engine idle speed. this is a pre defined stepper value based on coolant and air temperature. if there is more idle air than intended, the stepper will still react normally initially, but the ECU will then sense that the idle speed is not correct and adapt, it will reduce the ignition timing below the lower threshold to reduce idle spped and activate the stepper to reduce idle air further reducing idle speed. if the stepper eventualy reaches closed and the idle speed is still not correct, the ECU then can only use the ignition advance to reduce idle speed, at which point idle stability can suffer. (a point you made earlier that everything appears normal when the engine is warming up could be an indication of the above, since the stepper will be open further to provide more air during this period)

 

without knowing the commanded stepper position, ignition idle advance, coolant and air temperature it will be hard to pin point, which is why everything mainly revolves around connecting a diagnostic tool to the ECU as a window into these parameters. wthout this information, everything else is pure speculation

[jemo]

Thanks for your replies - it does sound like an air leak somewhere.  The throttle body itself isn't cracked as I thoroughly check it over when I had it off but I didn't check the mounting piece of idle control valve so I'll try that.  I do have access to an ACR4 so will try to get the diagnostic readings as well and see if I can make sense of them

[cian]

Anytime an idle issue arises with an mpi always check the iacv or vacuum pipes

[jemo]

Ok, latest update.  As Sprocket suggested I've now had the code reader on it with the following results.  I took readings at cold idle, hot idle and hot fast idle as follows:

 

Cold Idle

RPM 1300 pretty stable

ECT 15

IAT 14

MAP 46

Throttle Pos 4.3

Throttle SW Off

Batt V 14

02 Volts 4.1 – stable

FBACK BNK1 ST OFF

Coil 1 Charge T 1.48

Coil 2 Charge T 1.53

Injector 2 P/W 1.48

Injector 4 P/W 1.42

Stepper POS 30

 

Hot Idle

RPM 960ish but quite unstable jumping between about 910 and 1000

ECT 96

IAT 77

MAP 46

Throttle Pos 5.4

Throttle SW Off

Batt V 14

02 Volts 0.2->3.3 jumping about considerably

FBACK BNK1 ST ON

Coil 1 Charge T 1.48

Coil 2 Charge T 1.55

Injector 2 P/W 1.05

Injector 4 P/W 1.04

Stepper POS 0

 

Fast Hot Idle

RPM 1900 approx as hard to hold steady

ECT 96

IAT 77

MAP 41

Throttle Pos 9

Throttle SW On

02 Volts 3.6 pretty stable

FBACK BNK1 ST ON

Coil 1 Charge T 1.5

Coil 2 Charge T 1.55

Injector 2 P/W 0.85

Injector 4 P/W 0.82

Stepper POS 27

 

I also noted that the fan kicks in with a coolant temp of 105deg and back out at 97 which seems reasonable.  I should mention that the car has the Cooper Works S setup fitted.

 

Can anyone comment on whether the other readings appear 'normal'?  

 

The inlet air temp seems very high when hot but the car was obviously stationary and the manifold is going to get damn hot.

 

One question is whether it is normal to observe the lambda sensor voltage chopping and changing so much at hot idle while it appears much more stable at cold and fast idle?

 

One thing I have discovered was that after a prolonged fast idle when I took my foot off the revs only died to about 1500 but if I lifted the pedal they died pack to 950ish so clearly something is sticking a bit.  The return spring seems pretty strong on the throttle body so not sure where the snag could be but I'll get the WD40 out.

[Sprocket]

What about ignition timing?

Check hot idle ........ stepper position 0

That there is telling me that there is far too much air as i predicted. What you will find with that and the high idle speed is the ignition advance will be very low and the reason for an unsteady idle.

You now need to find out why there is too much air.......

Edited by Sprocket, 18 April 2017 - 09:22 PM.

[jemo]

OK thanks for the help.  might be the weekend before I get to investigate further.  You mention ignition timing - should I be able to see that on the ECU reader?  Thought I had stepped through all the readings.

[Sprocket]

I think ive heard this before, but i really am surprised that it doesnt show. With the SPi and the Crypton ACT it gives ignition advance values. Maybe a peculiarity of the ACR?....

But still..... that stepper position is still way way too low.

[jemo]

OK, I'll double check but pretty sure there is no ignition reading.

 

Do you think the cold idle stepper value is also out?  

My understanding is that at cold you want a richer mixture - so less air relatively - and the idle valve would have a low position then at warm it would be higher?  The fact I am seeing 30 at cold and zero at hot, could this indicate the 'rogue air' is only present or getting worse once everything is warm which in turn could indicate something warping or opening up with heat expansion?

 

Thanks

[Sprocket]

I would expect a higher step at cold, reducing down to about 40 at normal hot idle

[jamesquintin]

if your oil temp gauge isn't working, then it has broken (very common apparently, but most people don't have the oil temp gauge. This will fit it 9worked for me!)

http://www.minispare.../YCB100320.aspx

 

 

Q

[jemo]

Thanks, I'll look at repairing the gauge when I get the air leak fixed.

[FlyingScot]

I think ive heard this before, but i really am surprised that it doesnt show. With the SPi and the Crypton ACT it gives ignition advance values. Maybe a peculiarity of the ACR?....
But still..... that stepper position is still way way too low.

Yes ACR system with Rover Pod V3 or later won't show timing on SPi or MPi

FS

[jemo]

Update: Tried a few things at the weekend - thought I had made a difference with adjusting the throttle linkage and stop but alas no.  Think I have tried everything around the inlet manifold for air leaks except the manifold gasket itself, so that's next.

Quick question - the gasket has a silver metal side and a black side.  Looks like it could go on either way round so which is correct if it matters?