Converting SE to closed loop ECU

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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. If you're getting a "Mountain Chip" ECU upgrade and you've got a car with the necessary wiring, then for the cost of an O2 sensor (£22 to £90+ depending on the supplier) you can choose a closed loop chip and improve fuel economy by about 5mpg.

What's the catch?

O2 sensors are usually quite pricy, because they're made of expensive things like platinum. They can also be a little fragile; a failure of the heater circuit or some silicone in your fuel can easily kill the sensor. They'll probably need replacing every 60k miles or so at most. Not all SE cars have the necessary O2 sensor wiring in place, so unless you check your main wiring harness you may find that you need to do some electrical work before your sensor & ECU will talk to each other. See the Fed O2 Sensor Wiring Diagram

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. The Bosch 15701 is known to be suitable.
  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]


RHFHC.jpg IPHC.jpg

  1. Remove the driver's side front wheel arch liner and follow the right hand front wiring harness back to the bulkhead. It should end in the "RHFHC" wiring harness connector where it joins to the main harness. If you can't see the connector it might be on the other side of the bulkhead, above the driver's footwell. Inspect the connector; pins 4 and 8 serve the O2 sensor. If there are wires connected to these pins on the main harness side then the necessary circuits are probably all in place and you can skip the complicated bits. If not, you'll have to install your own wiring to serve the sensor:
    1. Remove the glovebox, dash binnacle ("half moon" thing above the instruments) and driver's side front wheel arch liner. Pass the end of some suitable signal wire from the passenger footwell behind the radio and over the back of the instrument panel. now take a length of 3A or higher rated supply wire and tape it alongside the first wire. Pass the pair down to the right of the steering column and into the right end of the driver's footwell.
    2. Take about 8" of thin steel wire, bend it double and push the bent end from the wheel arch through the bulkhead grommet alongside the main harness. From the driver's footwell, pass the two wires through the loop, then use the loop to draw the wires through the grommet into the wheel arch.
    3. Splice the new wires into the RHF harness. The signal wire connects to the wire on RHFHC pin 8, and the supply wire connects to the wire on pin 4.
    4. Disconnect the battery, return to the passenger footwell and unplug the ECU harness from the ECU. Connector pin D7 can be located using the markings on the connectors. The wire from this pin should be spliced to the footwell end of the signal wire.
    5. Return to the open binnacle cover. The "IPHC" harness connectors are on the left of the opening. Inspect the large black connector, disconnect it and identify the wire on the IP harness side of pin A15. Splice the binnacle end of the supply cable in here.
    6. Congratulations, you now have the necessary wiring! Check for good continuity from O2 sensor connector pin A to ECU harness connector pin D7, from O2 sensor connector pin B to IP harness connector pin A15 and from O2 sensor connector pin D to any earth point. Investigate and correct if necessary. Replace ECU (now's a good time to install your closed loop chip) and battery.
  2. Under the bonnet you must remove the bung in the 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. Another risk, particularly after applying extreme heat & cold, is shearing the head off the bung. In this case you'll want to fetch some good cobalt drill bits and start drilling it out, being careful to try not to damage the threads. One method that has worked when penetrating oil failed is to warm up the engine, then (with long pliars and safety goggles) hold ice cubes against the bung until they stop boiling violently. If you can't shift the bung with an 18" bar then you probably need to encourage it with some more heat/cold/oil treatment. You'll need a 27mm socket. After the bung is out its all easy;
  3. A wire brush can be useful to clean the threads in the housing. Fit the sensor (use a proper O2 sensor socket and the correct torque) but don't add anti-sieze unless you know what you're doing - the sensor should come with some already applied and the wrong type could damage the sensor.
  4. Plug the sensor lead into the empty socket in the harness just by the engine mount (4 position connector in a 2x2 configuration with only three wires connected).
  5. Fit the ECU/chip and you're done!