Cold Slow running

[DeTomato]

So yesterday it was -5 degC. I took the car to work as usual, but time when I sat at a junction with the engine warming to roughly 40 degC on the thermostat (half way to normal 80 degC midway so the auto-choke was not engaging any longer), the engine was struggling or hunting for fuel or air on idle, dropping to about 500-600 Revs until I pumped the throttle where it went back up again to normal.

Not seen this before. Any ideas?

[oldcodger]

Looking at your other post about the users radio preset stations being lost when the Battery is disconnected, the same can happen to the ECU, so it enters learning Mode and gives symptoms of erratic tickover for the first few miles.

Is that what you have done /experienced ?

Re the Radio codes, when the radios logic is programmed its  presumably given some base radio stations, but when the user enters new ones there is no method to store them when the power is totally removed; power is clearly being used even when you take the keys out, hence it appears to save them, but not fully.

Expect some more modern and costlier units have such a feature built in to save them, for our older car you can find a variety of Power Saver devices that can be used when you need to disconnect the main Battery, loads on eBay etc

[DeTomato]

I hadn't disconnected the Battery... that was only to advise on the other post

Yesterday it appeared the Revs were lower than normal to 500-600 Revs on engine warm up but the auto-choke was fine beforehand, as if the severe cold was setting the warming engine to run the Revs lower when the auto-choke was no longer needed. Until I blipped the throttle, it then came back up to 850 Revs. This happened a few times until I got the engine warmed up to 80 degC. 

But since I've been out this evening and the Revs are seeming to be act strange. When I am coming to a junction and engaging the clutch, the Revs temporarily go up 500-1000 Revs before then setting down immediately to the usual 850 Revs for idle. But it doesn't do it all the time! This is weird... I wonder if there is an air leak somewhere?!?!

Any other ideas?

[oldcodger]

Not really, could be so many things, eg temp sensor failing in the cold , no engine check light etc  coming on ?

Think it best if you wait until this sub zero weather has past and see what its like then ..?

[DeTomato]

Yes, good point. There's no dash lights coming on, no engine check light, and nothing showing on the OBDII Diagnostics. Really weird... will have another look tomorrow. It's too cold tonight!

[DeTomato]

So I checked over the possibilities of what the cause could be. Today I put some new clips on the Air Hose from the Air Filter box to the Throttle Body. It ran a little better. I then checked the slow running at 87 degC. Interesting I noted the radiator fan doesn't appear to come on at all, and it's not really all that cold today. Apparently this happens with VVTL-I engines if there is air in the system which then makes the Rev's do odd things when its trapped at the idle control valve area which has a water flow in it. I had flushed the Heater Matrix a while back.

So question is, how do I get trapped air out of the system? What's the best way to do this? Any ideas?

[DeTomato]

So I squeezed a few hot water hoses and looked on the internet to find the best way to expel air in the coolant it to park it facing up an incline and let the engine warm up with the radiator cap off. The air will then rise to the top and bubble out. Nothing seemed to come out... so not this.

Meanwhile noticed the idle position blips up before it comes to settle down at 870 Revs to idle. Revving it up stationary and letting the Revs fall, the idle hovers temporarily at about 1400 Revs before dropping further to settle at 870 Revs.

And at about 1400 Revs the Revs glitch when slowing down and a slight temporarily rise when engaging the clutch and changing gear. Wondering if the Throttle Position Sensor has worn out at around 1400 Revs so it can't immediately settle and throwing out a false reading.

Also, when putting the headlights on, the Revs go up to 960 Revs from 870 Revs on stationary idle - is this normal for a Corolla?

Something weird is happening - no air leaks I can tell and no codes indicated. Any ideas to help please?

[oldcodger]

Hi,

No idea what could be wrong, but have you tried using a obd2 scanner to see if any codes are stored and also, from cold, you could use its Live Data function to check the main engine parameters as it warms up ( while stationary).

Even better , if you have one, an obd2 data logger that can record said parameters as you are driving along and you play back onto a laptop afterwards.

[DeTomato]

yes, checked the obd2 info and no codes. But I'll have a look at the data logger to see if there are any blips are seen when I rev the engine and see what codes come up then - good idea  I should have thought of that.

[DeTomato]

So I think I've worked it out. The throttle stop screw was broken in half midway! Looks like the nut underneath was pretty loose because of this too.

When it was really cold, the screw contracted and shrunk enough in the screw housing that it ran at lower revs. When hot, the metal screw expanded and the screw eased out a bit so the revs rose slightly.

I got another stop screw from a mate and we fitted it yesterday. The revs now remain steady at 885 rpm with a bit of thread lock on the replacement screw. Hopefully this will fix it.

[Notoyboy]

Nice work, well done tracking down the problem, and an easy solution.  Always very satisfying  ☺️