Hunting and stalling and idle - Cold start

[German-Whips]

I've been having a few issues with the idle recently and after having the HT leads changed, due to them being physically damaged (they needed replacing anyway), the problem still persists. It's an M20B25 engine.


This morning when I went to start the car, it started up and the revs seriously started fluctuating around idle and the engine hunted really badly and stalled.
I started it again and tried pressing the throttle a bit and it died again.
3rd time around the car fired up and stayed idling.

After popping the bonnet, I checked the TPS switch and couldn't hear a click, which means the TPS switch is not working/not adjusted correctly, so the car is not "idling" at start-up. Does this mean the car is not "idling" and therefore too much fuel is being pumped into the car at idle immediately after starting, and causing overfuelling and rich running?

Can rich running lead to the problems like hunting and stalling when cold and starting?

[Grrrmachine]

Yes, a disabled TPS can lead to all rhe issues you describe. So can a small air leak, such as a loose brake booster hose on the intake manifold, or a decayed rubber bung under the throttle body.

Take the throttle off, clean up abd reposition the TPS, and inspect the rubberwork and gaskets while you're at it.

[Brianmoooore]

Does this mean the car is not "idling" and therefore too much fuel is being pumped into the car at idle immediately after starting, and causing overfuelling and rich running?

Can rich running lead to the problems like hunting and stalling when cold and starting?

Hunting and stalling like this are likely to be the result of an excessively lean mixture, not rich. Why do you think it's rich?

[German-Whips]

Does this mean the car is not "idling" and therefore too much fuel is being pumped into the car at idle immediately after starting, and causing overfuelling and rich running?

Can rich running lead to the problems like hunting and stalling when cold and starting?

Hunting and stalling like this are likely to be the result of an excessively lean mixture, not rich. Why do you think it's rich?

I checked the TPS and it's not clicking, which means the car is not registering idle so I thought the ECU would then pumping in more fuel than required because it's thinking the throttle is open?

Before i turned the car off last night, momentarily before turning off the ignition the ECU would have been pumping in more fuel than neccessary due to thinking throttle is open?

Is this correct or am I thinking of it wrongly? Would the ECU pump in more fuel than usual if the TPS is not registering idle?

[Grrrmachine]

All the TPS does is tell the ecu to listen to or ignore the input signal from the AFM. If there's no idle signal but the throttle is closed, then there's no reason for the engine to inject more fuel.

[German-Whips]

All the TPS does is tell the ecu to listen to or ignore the input signal from the AFM. If there's no idle signal but the throttle is closed, then there's no reason for the engine to inject more fuel.

Makes sense!! So as soon as the throttle is opened, and the TPS clicks open, the ECU then collects data on the amount of air entering via the AFM, and the ECU then adjusts fuelling according to the amount of air entering. So an incorrectly adjusted TPS, which is stuck OPEN, will always tell the ECU to go by the AFM data. On closed throttle, no problem as air flow is the same as it would be on idle anyway, (for an engine which hasn't had the screw and factory settings messed with). However, on an engine at idle where the throttle adjuster screw has been screwed in to raise idle to counteract a low idling engine issue, with a non functional/incorrectly adjusted TPS, throttle will be open at idle, and too much fuel air mixture entering. This would cause high idle right?

BUT, in the case of the above problem, AND, an air leak causing unmetered air to enter, the RPM could be dropped because the unmetered air is being introduced, and this can counteract the high idle, and cause rough running/hunting.



I think the throttle adjuster screw has been incorrectly screwed in on my engine to temporarily raise the low idle, so maybe this is causing problem on turn of the key on a cold engine?

Can air leaks cause a low idle? The elbow connected to the throttle body from the ICV is very loose. The pic below shows what it was like, it was very free and had no resistance. Could this be sucking in air and causing unmetered air from entering? The splines are very worn and no longer seem to create a tight seal.

[Brianmoooore]

You have a significant air leak there. Refit the elbow with araldite.

[German-Whips]

OK, an update! The car is still idling too low!

I took off the ICV, rubber boot, and AFM and air filter last night.
I cleaned out the dirt and sludge using carb cleaner.

The throttle cable was also adjusted, wound back the screw which had been previously screwed in. The TPS now clicks, and also works, as when lifting off the throttle the MPG needle shoots straight off the the "infinity" range, which it didn't before.

Refitted elbow with this sealant and it now fits perfectly without any movement. I made sure the adhesive was spread evenly over the contacting surface.
http://www.granvilleoil.com/product_inf ... rod_id=150

I couldn't take off the throttle body because it seems like when it was fitted, they used some sort of silicone gasket. There wasn't any gasket, so I didn't want to take it off and have no way of sealing it up, as I had no gaskets or silicone sealant that they used.


The car is still idling low!

On cold start-up this morning, it didn't stutter, stall or hunt like yesterday. It started right up, but during the warmup phase, it only idled at around 700, then the idle RPMs dropped to about 550-600.

What could be causing such a low idle? I couldn't find any split pipes, brake booster hoses on the RHS were all perfectly sealed too!

[Grrrmachine]

If it's been driven for ages with the TPS set in the wrong position, then the engine probably doesn't know what it's idle speed is supposed to be.

With a cold engine, disconnect the battery and ground the positive wire to the shell for a minute. Reconnect and start the car. Let it idle for ten minutes, then shut it off, restart again, and take it for a proper hoon through the rev range. You'll probably find you have a healthy 700rpm idle when you're finished.

[German-Whips]

If it's been driven for ages with the TPS set in the wrong position, then the engine probably doesn't know what it's idle speed is supposed to be.

With a cold engine, disconnect the battery and ground the positive wire to the shell for a minute. Reconnect and start the car. Let it idle for ten minutes, then shut it off, restart again, and take it for a proper hoon through the rev range. You'll probably find you have a healthy 700rpm idle when you're finished.

Thanks! Will try this tomorrow morning.

Is this a different method to simply disconnecting the negative terminal of the battery?

Yesterday whilst working on the car I only disconnected the negative cable of the battery only, I left the positive terminal on whilst working on the car; for about 1 hour in total.

What's the method for grounding the positive cable to the shell, simply making contact? And what will this do?

Also, for how long should I disconnect the battery?

Should there be a delay between shutting the car off and starting again for the 2nd time?

Thanks!

[Grrrmachine]

The ECU has a very rudimentary way of storing charge, so disconnecting it for an hour *should* reset it, but shorting the terminal to ground is guaranteed (however, if your SI batteries are nearly dead, this will probably kill them dead).

There's no "method" - just make metal contact and you've formed a circuit. Stop overthinking things