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MG MGB GT V8 Factory Originals Technical - EFI tuning

Now that I have my car running & driving- see fuel injection, start no idle- it is time for some tuning & a bit more help from the list.

Plugs & injectors are all good. I have the timing set at 6' BTDC. Finally got the initial idle set high enough- thank you Justin- but I get the occasional backfire when mashing the peddle, which makes me think I am running lean. Don't really know how to adjust this without some sort of CO2 meter, which I don't have.

The car runs much better when hot, just before the electric fan cuts in, which I think means I should be running a 195' thermostat in place of the 180', but I would rather fool the computer & run the 180'. Any downside to this? Anyone know how to fool the computer?

Thanks to all who have helped with advice. This has been a bit frustrating, but all in all, a lot of fun. Next step is to install the O2 sensors.
Jim Stuart


I would advise getting the system working as well as possible without trying to "fool" the computer. Mine runs very crisp and clean after much fiddling with settings. But I did not try to "fool" anything.

You are probably aware that there is a "tune" resistor in the 14cux harness. Figure out which one you have. I'm not sure what mine is now (I was told "European" tune but that doesn't match any independent info I've found), but I plan to install both into a switchable configuration to see which works best for emissions and power. I have O2 sensors installed and connected, with a cat, but it is my understanding, which may be incorrect, that one resistor value causes the ECU to ignore the O2 sensors.

BTW, if you think that running a bit richer will give more power, I think adding a 150 ohm resistor in series with the coolant sensor will make the ECU think the car is running about 20*C cooler.
Edd Weninger


I have the green tune resistor, which is non-cat. I also have a white tune resistor which is for a cat car, that is, one running O2 sensors. They are so easy to switch I don't know if I would bother with the wiring you mention.

Specs call for a 195' thermostat & the temp sensor changes the mixture when that approximate temperature is met, but with the high ambient temps & humidity we have in the summer here in DC, I much prefer to have the thermostat open at 180' & let the cooling system have a little more room.

I will try the resistor this weekend.

Jim Stuart

Jim are you using oxygen sensors in the system with the green tune resistor?



I am using the green resistor without O2 sensors. As soon as I get the O2 sensors installed- maybe this weekend- I will change to the white tune resistor.

Jim Stuart


It won't be hard to put in a switch. I want to do a A=B test under identical conditions, perhaps eventually even on a chassis dyno. I also have to meet smog requirements in SoCal and it will be interesting to see if there is a difference. Might be a while though, the car is running good and the weather is warm.
Edd Weninger

Installed the O2 sensors & changed the tune resistor this weekend & what a difference it makes! Anyone that is using EFI without using the O2 sensors is definitely short changing themselves.

Thanks again to all who have suffered through my problems & offered help & encouragement.

Jim stuart
Jim Stuart

What O2 sensors did you use?

I bought a set of used O2 sensors from a 1990 Range Rover on E-bay. They were nice & clean, wires in good shape & they just plugged into the harness.

I had bungs put into the headers a long time ago so I could run an O2 meter. Only when I got the Rover sensors, I found they did not match the 18 MM bungs, so I had to make a set of bushings from 18 MM oil drain plugs. I just drilled & tapped the plugs, ground the threadded ends to 3 threads, installed them in the headers, then ground away the hex.

Per the Rover manual, I carefully used anti-seize on the threads only, none on the tips.
Jim Stuart

Jim, A while back I believe I suggested you try 1.7V on the air flow meter, this has a draw back.
As you know I was playing around with the voltage across the two outer wires on that hot wire air flow meter and found that the 1.7 V recommend for cat' cars gave an increase in power. However I went for a drive to anearby town the other night and used most of a tank of petrol in less than one hour!. I've reset the voltage to 0.18V and am getting about 6 times the milage. Also on the 1.7V setting the engine ran way too cool, well below normal.
At least I now know that increasing the voltage enriches the mix. As an experiment I might try a different voltage ever time I fill up and find the best trade off.

I finally got around to checking my air flow meter, and I was at 1.9!! I backed it off to 1.6, changed to a cat tune resistor, installed my oxygen sensors, AND backed off my timing by 6 and it's running like a champ. It still stalls at a cold start, but I'm more than willing to attribute that to the air flow sensor being at 1.6 volts between the outside wires. Idle is much more stable, power feels improved, everything feels pretty good right now, and that makes me happy. :) My fuel gauge is on strike at the moment, but my trip odometer is working perfectly, and I know how many miles I get (got) per tank, so probably in a few days I'll have an update on the fuel mileage thing. I had been getting around 14 per gallon in city cruising, and I'm hoping to improve that substantially. An added bonus: running temps dropped 20 for me with all these changes. I had been sitting at 200-205f consistenly, and now it's down to 180F

Lots of changes, and lots of success. =)

This thread was discussed between 04/04/2004 and 19/04/2004

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