Escort Work Log - Fuelling Recalibration, AFR & CO%

Work took place in August 2024.

AEM Air/Fuel meter + wideband O2 sensor
Replacement Idle Speed Compensation Valve

Since replacing the fuel pump and changing the fuel delivery to a lift-pump setup, the car accelerates more cleanly than it ever did since completing the conversion, however it is still subject to a faulty/useless aftermarket ISCV valve and an idle that is either adjusted to be okay when hot (and far too low when cold), or adjusted to be okay when cold (and far too high when hot). The cooling fan thus always played havoc with the running of the car since the ISCV valve couldn't properly adjust the idle when the extra load was put on the engine.

Since the idle couldn't be set correctly, it was also a real trial to set the base fuel delivery via the ECU adjustment screw, ending up with the car needing to run with far more fuel than is necessary.

Put simply:

ISCV valve didn't work, so…
… a correct idle speed for all running modes (cold start, hot running) cannot be achieved, and…
… the correct base fuel mixture (AFR/CO%) can't be dialed in

So although the car can drive through the gears much better than before, when it comes to starting, pulling away, or moving the car at car-park speeds, it's still a chore and frustrating having to keep the revs up manually and slip the clutch to avoid stalling.

The first thing we did was fit the AEM air/fuel ratio gauge into a pod and mount it on the side of the tunnel next to the additional instrumentation.

The wideband O2 sensor which comes with the gauge is mounted into a second boss which I had welded into the exhaust downpipe back when we fitted the exhaust system up several years ago:

At the moment the data cable from the sensor to the gauge is just trailing out from the back of the bonnet through the drivers window, but it will eventually be fed through the bulkhead when the fuel calibration is finally able to be set correctly.

Outcome

After installing the AFR gauge we tried to set a reasonable idle speed (around 1100-1200rpm is the best we can achieve with the ISCV unplugged, otherwise you need to tickle the throttle constantly when starting the car from cold) and then proceeded to let the car warm up and then adjust the ECU fuel trim screw.

The car was initially running at an AFR of ~11.5:1, corresponding to a CO of around 8%, which is way, way too rich, and I then backed off the fuel trim screw slowly leaning the mixture out until it approached around 13.0:1 (which is still quite rich, around 4% CO; at least 1.5 points above where it should be), at that point the engine wanted to die and we had to wind back more fuel on to stabilise things.

It became really difficult to try and set the base fuel level as the car got hot and the fans came on - since the ISCV valve didn't function correctly.

It's a vicious circle of not being able to set the correct base fuel trim, because it's difficult to set a nice idle, because the ISCV doesn't work properly. ARGH!

The Idle Speed Compensation Valve is attached to the front of the throttle body on the standard (2wd & 4×4) Sierra/Escort Cosworth intake system. It fastens via two screws and is sealed against the throttle body with two rubber o-rings.

The ECU activates the valve which energises a solenoid inside, opening a plunger which opens additional air into the intake, thereby raising the idle speed. The problem is that you simply cannot buy an original Magneti Marelli valve, which is what the Cosworth was originally fitted with. All of the parts on the market now are either copies, or are equivalent parts from different manufacturers… this would not be an issue normally, but the internals of these newer valves are not made to the same tolerances as the originals, so they don't work quite the same.

In my case, the valve which came with the donor Sierra Cosworth 4×4 engine was bent and seized solid. I had no choice but to buy a replacement - and sadly it has never worked correctly. Most of the time, depending on what you do with the internal springs, it either does nothing, or it ends up in a bouncing idle; revving up to 1200-1400rpm for a second, then dropping back down again (too far), then bouncing back up to 1400rpm again. Since it can't generate a relatively static idle, setting the base CO level or using the idle screw on the throttle body itself is an exercise in futility.

I bought (another) replacement ISCV valve from Brand Hatch Performance which was listed as follows: