Uneven idle issues

[nsa]

My Twingo has an uneven idle and is failing emissions because it's running slightly rich. I've changed:

Coolant temperature sensor
O2 sensor
Coil
Spark plugs and leads
IAC valve
Air filter
MAP sensor
Fuel filter. Pump comes on fine and delivers good pressure.

I've also changed the gaskets in the throttle body. Unfortunately the car doesn't have OBD2 (I'll never buy another car with injection but without OBD2) and I don't have access to the old Can Clip diagnostic machine. There's no engine light. Checked engine block earth. The injector is clean and I checked the O-rings. I replaced one with a green O-ring, which probably isn't right for the fuel system. I'll change it. According to the new coolant temperature sensor the engine is running slightly too hot, because the temp light on the dash comes on but it's quite faint. It's only a one pin sensor and the resistance curve might not be the same as OEM.

Compression is 160-175psi on all four cylinders. I checked when the engine was cold. Vacuum is OK; drops slightly when the engine slows but still within good range according to the dial.

The video is when the car is warmed up. This is the C3G engine, not the later D7F. The idle used to stumble more than this but since I changed the IAC valve it goes up and down slowly. There's also a burble from the exhaust but no smoke. The spark plugs seem OK. I've only posted one picture because they all look the same.

The engine starts and runs well, apart from a slight occaisonal misfire under load at light revs. Fuel is economy is still good. These use hardly any petrol anyway so it's hard to tell.

My thinking is that there might be leak in either the intake or exhaust so I've bought a smoke test machine to check both.

Any ideas what might be wrong with it?

Engine running: https://youtube.com/shorts/oUU77w4VuGQ
Vacuum test prior to new IAC valve installation. New test would be similar, not as dramatic a drop: https://youtube.com/shorts/fqHnGR_Pcr8?feature=sha...

Embed links don't appear to work.

Edited by nsa on Friday 10th April 12:24

Edited by nsa on Friday 10th April 13:39

[E-bmw]

Before you mentioned it I was going to say, first job is to check inlet for metered air leaks.

Exhaust is possible but less likely, both should be pretty easy to check without a smoke machine.

[nsa]

E-bmw said:

Before you mentioned it I was going to say, first job is to check inlet for metered air leaks.

Exhaust is possible but less likely, both should be pretty easy to check without a smoke machine.

Thanks. I've tried with a vacuum gauge (video link in the post). The vacuum drops when the idle is lower. It's still within the green area of the gauge. The intake is at the back of the engine, under the throttle body. I've sprayed it with brake cleaner. If there's a leak underneath I doubt it will have any effect. Worst case is that it's a stuck valve or spring.

Otherwise all the lines seem OK. I also replaced the MAP sensor.

Could it be something to do with the PCV system? There's a bit of blowby when I remove the oil cap; nothing dramatic. Hardly anything when I remove the dipstick.