Mi16 running lean

[davemar 16]

My Mi16 (XU9j4) seems to be running lean. When it had an MOT a couple of months ago the tester said it appeared to be very lean when he tested it. It also feels a bit flat when driving, though seems to idle OK.

I know many things can cause this from fuel pump, AFM, sensors, etc. But any tips on what I should focus on checking first? I don't have any kit for testing fuel pressure, but can probably test most of the electrical things OK.

 

[Tom Fenton 1,524]

Probably not what you want to hear, but

 

Check fuel pressure first!

[petert 549]

I’d service the injectors if pressure and flow checks out ok.

Edited June 10, 2022 by petert

[SRDT 146]

On the electrical side you can check if you really have +12v at the pump, with a bad ground or a badly installed alarm / kill switch it's easy to end up with +8v or less.

There is a CO pot to pass emissions so even if you are lean in general it can be compensated at idle.

Lean at high loads should be obvious around 6000-7000rpm with a reduced max speed.

[davemar 16]

It lacks that typical pick up of power at higher revs, so I think it's a more general leanness.

I'll try and check the fuel pump voltage. I've had a few problems in the past with the connector on the pump being a bit iffy when it simply wouldn't conduct well enough to power it at all, so maybe it's gone a bit crappy again.

In the meantime I had a quite check of the resistances of the coolant temperature sensor and the AFM air temperature sensor while the engine was cold.

The CTS measure 6k, and the ATS measure 2.8k. Not sure if they are in the expected range, but don't seem like completely broken values.

 

[welshpug 1,621]

Resistance on the cts seems a little high, managed to find this https://www.bosch-motorsport-shop.com.au/temperature-sensor-130-deg-c

 

theres a pdf showing  the resistance values expected at 10°c increments,   0° should be 5800 ohms, 2500 at 20°

 

ATS sounds like its ok as they run a similar range I believe.

 

From those inital numbers it may actually  be going rich.

 

The Co pot in the afm can fail, I have had one on an MP3.1 pot fail and they are the same variable resistor, repaired with a part from RSonline.

[davemar 16]

I finally got round to measuring the fuel pressure after getting hold of a gauge. After some fun and games trying to get various adaptors onto the hoses and squirting fuel everywhere, I managed to get a reading that varies between 35-40 PSI (2.4 - 2.7 bar) depending on whether its idling or the throttle is being blipped. Does this sound like sensible values?

 

[Tom Fenton 1,524]

Sounds ok. The fuel pressure is vac referenced. Better way is to check with fuel pump running, engine off. You should see circa 3 bar.

[petert 549]

Pressure is just one check. You also need to check flow. ie it should be approx. 2L/min, as illustrated. I'm still tipping dirty injectors however.