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MG MGB GT V8 Factory Originals Technical - EFI Hotwire tick over problem


I have a 3.9 with Hotwire FI. It had been running fine all summer but over the last 6 weeks or so it has been off the road while I have been making odd changes and updates (rad, fan, heater, hard-top etc.) but nothing (intentionally) to the engine or ignition.

Now when I start the car up it has a very high tickover (circa 2000rpm). I have taken it for a drive but is doesn't fall back to normal(except under load). I have checked that the thotttle is closing on to its stops and have had a general inspection of the wiring, all seems to be in order. Does anyone have any idea what I might of messed up? (I don't have speed or CO sensors connected). All of the archive issues seem to deal with problems of too low RPM's!


R Weston

The idle air control provides a path around the throttle plates to control idle. The throttle plates merely provide a base airflow which is trimmed with the IAC valve. I believe it is a stepper motor that controls a rather large needle to correct the idle to that commanded by the computer.

I suspect that yours is stuck open, either by failure of the stepper motor or by gummy oil that is introduced into the intake system by the crankcase ventilation system, which takes engine blow-by and introduces it back into the intake manifold.

You can check the stepper motor by removing it from the intake plenum and then turning on the ignition to power up the motor. You should see it move slightly.

If the idle air control motor appears to be funtioning, you will need to clean the valve and seat to restore correct operation.

Having cleaned it or replaced the IAC, you need to follow the procedure for setting the base idle in your manual.'

Good luck.


Hey Phil! Long time no see! How's that sand rail project going? Still putting a Northstar engine in the back of it?

There's an EXTREMELY easy method for adjusting the base idle (Evan Amaya taught me this)

Step 1: shut off the motor.

Step 2: Remove the rubber hose from the back of the plenum where the stepper motor is.

Step 3: put duct tape around the hose fitting on the plenum and reattach the rubber hose.

Step 4: Start the motor and just the idle screw till you get a nice smooth idle.

Step 5: shut off the motor and remove the duct tape and put the hose back on.

Step 6: Drive away happy.

Evan: If I missed a step, correct me!


I don't know what it is about this website and the MGB-V8, but I sold my car over a year ago and I still read this bulletin board and every MGB I see is viewed as a potential conversion candidate. A sickness, I guess.

Evan's method is a lot more straightforward than the factory manual and works even if the air passage is gummed up. The factory manual had you turning on the ignition, removing the IAC connector, turning off the ignition, plugging in the IAC connector. You had to do this multiple times, then leave the IAC connector off and start the motor. Set the "base idle" with the screw under the anti-tamper cover, reconnect the IAC and you're done. With either method you'll have to clean the passage and valve/seat and make sure the IAC is working.

Justin, I did have a Cadillac Northstar, but it would have required an aftermarket computer that could control ignition and fuel to make it run and that was big dollars, so I sold it. I bought the all aluminum Chevrolet LS-1 and used the factory computer and never looked back. Great engine with gobs of torque everywhere. LS-1 + MGB would be a heck of a combination. (I hate making headers, however)


Hi Phil,

Your car is doing well. It is in good hands. Thanks for doing a good job on it

How is the airplane project coming along? I just got another project to work on.

Its an old '71 work-horse which will be a 2 year rolling restoration project.

Rain today, so the MGB is in the garage while my girl friends SC430 is getting wet.

Edd Weninger

Thanks Phil & Justin, your diagnosis proved to be spot on. Over ther weekend I tested the stepper motor as suggested and the idle went back to a steady 800 RPM. I then took off the stepper motor and cleaned the valve, reassembled and the rest is history.
Took the car for a blast on Sunday afternoon and all is back to normal.



PS. Phil, keep looking at this website!
R Weston

This thread was discussed between 01/12/2004 and 06/12/2004

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