Thread: Idle and backfire problem 89 3.9

1. Idle speed adjustment if needed is best done with the allen key (3/16 inch) bypass screw in the top of the manifold.
2. Idle mixture adjustment is a screw in the end of the MAF, search the forum for adjustment procedures. Also MAF cleaner spray from a parts store might help.

If you back off from cruise to stop at a set of lights it drops to about 400 RPM and then hesitates and dies, I've wound the throttle cable up a bit just to test if I could get it to idle at around 900 RPM at the lights to keep it running which it will if when you back of you give the throttle a stab just before it gets to 400 but no good with the throttle cable set correctly.

Seeing it now runs well other than at idle, I suggest that it is just a dirty or dead IAC valve. The 14CUX takes a little time to stabilise idle to target 600-700 in N or about 600-650 in D. The ECU manipulates the IAC to achieve this.
The IAC should hold the idle at about 1100 coming to a stop, then drop the idle to the correct value on stopping. It may overshoot the closing especially if dirty ( and therefore slow), to a level that the ECU cuts the fuel( about 450RPM).
The IAC is not as powerful as in later ECUs and the base idle of the car needs to be correct at 550-600 RPM in N. To do this you block both ends of the tube from the throttle body to the IAC and open or close the air bypass which is adjusted by turning the allen key on top of the throttle body( out for more air =faster in for less =slower.)

Take the IAC off and clean the plunger and the bore in the housing with carby cleaner.

Make sure you clean all around the throttle butterfly, and the oil vapour lines.
Make sure the IAC housing is tight on the plenum and teh vacuum hose to the fuel pressure regulator is not split..

Do that and tell us what the result is.
Regards Philip A

Thanks heaps guys, I will try that either tomorrow or Thursday and let you know.

Cheers Casper.

Well I took the info that you guys provided with me today and had a look at the Rangie.

I cleaned out the Airflow meter and throttle body and adjusted the IAC and amazingly enough it seems to have worked.

The IAC doesn't seem to be working properly though as the idle seems to stay at about 900 to 950 Rpm in drive but about 1100 Rpm in Neutral.

I'm going back tomorrow to pull that back down to about 650 as I left it high for him to test drive on his way home this arvo.

Just want to see now how it starts in the morning now as it took him about 10 mins of cranking this morning to get it started but when he left this arvo he said that it started easily.

It still has a bit of a miss at idle but I think this may be plugs or maybe rotor button so I will investigate that tomorrow as well.

I would have liked to have finished it today but I was time poor today so I will have a quick look and chuck some plugs in it tomorrow.

Thanks heaps guys, from me and Dennis (Blue Rangies Owner).

Old Dennis had a bit of an issue this morning to get it started.

It took ages to start so I had another look at the timing and advanced it to near on 10deg and also noticed that it's developed a bit more of a miss at idle and when you stab the throttle hard it sounds like it's got a bit of a miss fire.

I'm replacing the rotor and cap on Monday as they both looked a bit rough and see how that goes but I'm getting to where I'm running out of things to look at.

My weak ignition fixing thread here:

Lucas ignition amplifier replacement by Bosch 024

Don't bother replacing the cap and rotor unless they are obviously damaged. Weak spark is a common isue with Lucas electronics. Check the plug lead and cap boots for tightness too, cold and damp conditions will cause voltage leaks if the boots are not tight enough.