BX 16 Valve Idle Problems

[frog]

I've had an ongoing problem with my BX 16valve since I bought it 5years ago:
When the car gets hot (especially in summer, stopped in traffic, at a drive thru) it will start to idle around 2000-2500 RPM.

I have tried pretty much everything I can think of short of aftermarket engine management.
This problem has persisted over a complete engine/intake swap, so it has had a known working TB, intake, AFM, ICV etc swapped on.
I've tried different ECU's, cleaned all the engine wiring loom plugs.
Is there anything that could affect the idle if it became heat soaked?

I think the key is that it only happens when the car is hot, eg Australian Summer.
It is particularly bad if I say drive the car to the shop (10 mins) park, then come back to the car 10 mins later..
The problem will resolve after I have had the car up to speed in no traffic and it has a chance to cool down.
Edit: It almost never happens in Winter too!

Any ideas? I'm at the end of my tether with it.
Happy to hear any and all advice!

[Tinkley]

On older pre injection ie carb stuff, you get very weird effects IF there is any leak in the inlet manifold. If the manifolds are 'rubber' they often degrade and become porous. Not sure about other things in the injection system, though recently ran into an outboard (2014 manufacture) that had been over pressurised in the fuel injection part so would not start - too much fuel!.

Note the following info taken from the Wiki, though I'm sure others with injection systems may know more.

''Problems with IAC

Although the IAC is supposed to last the vehicle's lifetime, various reasons may cause it to fail/malfunction prematurely. The most common failure mode is partial/complete jamming of the actuator (due to dirt/dust or even oil) where it cannot be smoothly controlled. The result is an engine that fails to maintain idle RPM and frequently stalls. A jammed actuator may be freed simply by cleaning it. However an actuator that has stopped working due to a fault in its servomotor will need replacement.

Air leaks in either the stepper housing or pipes will cause elevated idle RPM.''

[frog]

Thanks for your thoughts.

I haven't had the intake manifold off this engine, it was replaced around 10 years ago and had no problems in the previous car.
It does sound like an air leak some where, like though a sticky/worn TB or malfunctioning ICV. The TB is a known good one that has been left untouched though. It's had a new throttle cable and sheath too.

Is there something relating to the control of the idle stepper motor/ICV that would open it up more if malfunctioning?
Like a temperature sensor somewhere that is on the way out, telling the ECU the engine is cold and letting more air through the valve?

It's just strange that the problem is so sporadic. We had a reasonably hot day today, up to 29c, though the car behaved it self.

[Kitch]

You've renewed the coolant temp sensor, right?

[frog]

Hi Kitch,

Haven't replaced it formally, but the problem has occurred on two separate engines/sensors.
Worth a shot though.

Should i replace the 112 deg switch as well? Or is that just for the warning lamp?

[Way2go]

It sounds very likely that you have a sticking shaft on the second butterfly in the TB. preventing full closure under its independent spring pressure at idle.
A squirt of 3in1 penetrating spray will likely solve the problem.

[frog]

Hi,
I'll give that a shot too, is that likely to happen due to excess heat?
This TB was fine on the other car.
Cheers,
Chris

[Kitch]

Sounds unlikely that it's the sensor if it was ok on a different engine, in fairness. I was going to suggest that the TPS isn't set correctly, but I'm guessing this was also ok on other engine?

Jump on the 16v forum, download the ECU diagnostics PDF and get your multimeter out. At least that way you eliminate any electrical issues. It's quite likely to be a leaky hose or something, but the only counterpoint to that is that it'll be unmetered air, so you'll be running lean at idle if it does turn out to be a leak. Is this a 2row or 3row car?

[Way2go]

Hi,
I'll give that a shot too, is that likely to happen due to excess heat?
This TB was fine on the other car.
Cheers,
Chris

Possibly, especially if the shaft has become dry & dirty with age.

[frog]

Sounds unlikely that it's the sensor if it was ok on a different engine, in fairness. I was going to suggest that the TPS isn't set correctly, but I'm guessing this was also ok on other engine?

Jump on the 16v forum, download the ECU diagnostics PDF and get your multimeter out. At least that way you eliminate any electrical issues. It's quite likely to be a leaky hose or something, but the only counterpoint to that is that it'll be unmetered air, so you'll be running lean at idle if it does turn out to be a leak. Is this a 2row or 3row car?

Hi Kitch, 3 row car, I can only find the 2row service manual on the 16v site.
Yep TPS was fine in the other car.
Could the O2 sensor cause any problems? I haven't changed that.

[frog]

Maybe the O2 sensor is failing when it's too hot and motronic is using a substitute value? Surely I would have an error code for this though.

[Kitch]

Your emissions would be all over the shop. It's possible though.

[frog]

Economy isn't too bad, so probably not the O2 sensor. I doubt it would fail so sporadically, and no fault code so..

Interestingly the car was doing its thing today so I pulled over and unplugged the ICV, idle dropped from ~2500 to 1500. Plugged it back in and got a huge rev, then it resumed idle at 2500, calmed down after 5 minuets of driving on open road.

So I'm convinced it's an electrical problem, something telling the ICV to let more air through.

[MULLEY]

I had a very high idle, turned out the multiplug which is connected to the afm had a damaged wire, once repaired the idle was correct. I've also been led to believe that the tracks within the unit can get dirty which can lead to erratic idle.

[frog]

Thanks Matt I will investigate that lead (pun intended)..

New o2 sensor on the car has made no difference. Though the car has a different exhaust now and feels a lot faster, I think my cat may have been stuffed.

Also took the pedal box out and refitted it as the clutch cable broke, nothing wrong with the throttle cable etc.
Intake piping was also taken off and checked during this.

Fun and games continue, coming into autumn here now so the problem is happening less and less and will likely disappear until next October/November.