Converting SE to closed loop ECU

From WikiLEC

It is possible to convert the UK standard SE with an open loop ECU (9929 or 9239 PROM id) to run a closed loop program.

The open loop/closed loop terms refer to the way that the ECU controls the mixture. In closed loop the ECU usually tries to achieve an a/f ratio of 14.7:1 - this is complete combustion of the mixture (or as close to it as the engine can get).

The open loop ECU bases it's fuelling on the characteristics measured on a typical engine - there is essentially no loop. The a/f ratio is set slightly rich, at 14:1. This is a safety measure to prevent the mixture going lean.

The closed loop ECU follows these steps:

1. sets up the injectors
2. reads the output of an oxygen sensor mounted in the exhaust flow
3. adjusts an injector correction term if the sensor reading is lean or rich
4. loops back to step 1

At high throttle openings the ECU uses fuel enrichment to suppress knock and in this event the narrow band oxygen sensor can no longer be usefully employed, as the mixture moves out of its measurement range, so the ECU operates in open loop mode until the normal 14.7:1 mixture is next required.

Why do it?

Changing an SE to closed loop improves fuel economy (unless you go everywhere at full throttle) and removes any worry from the emissions tests at MOT time.

Parts Needed

You need two things:

1. A standard narrow band (aka binary) oxygen sensor. They can readily be found on US car parts sites; look for an oxygen sensor listed under the '1991 Isuzu Impulse RS' application. DON'T pick a universal one, get one with the right plug on it.
2. One of the following:
   1. A 1499 (S2 or 49 states federal) closed loop ECU or MEMCAL
   2. An upgrade chip based on the 1499 program (in the interest of full disclosure I should mention that I sell these!) [As an independant contributor, I can thoroughly recommend them - Geoff]

Instructions

1. Under the bonnet you must remove the bung in turbo housing, the one you can see through the hole in the turbo's heat shield. The sensor is fitted there instead. It will be tough to get out, use penetrating oil in advance and be prepared to apply a lot of force, damage to the casting is a possibility. After the bung is out its all easy;
2. Fit the sensor and plug it into the empty socket in the harness just by the engine mount (4 position connector in a 2x2 configuration with only three wires connected).
3. Fit the ECU and you're done!