Weird idle 1984 308 GTS

I’ve drove my car since getting the belt service and fluids changed, put about 70 miles on it - at cruise and at idle at the end of the day the car was doing a weird idle pattern - it’s idling between 1100 and 1300 rpms, it cycles back and forth and the cycle takes about 3 seconds. I figure it’s a sensor for fuel to oxygen sensor- have any of you had this happen? Find a solution? Thanks.


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Yes, had this happen with my old 84 GTB QV. Turned out that a hose had come off a (valve?) under the radiator expansion tank on the left side on the engine bay. Slid it back on and all was good. Can’t remember what the item it is connected to is called at the moment. (Hope that helps. Need more coffee this morning)

 

Normal idle and lower gear low throttle setting cruise surges a hundred or 2 RPM. The 02 sensor constantly adjusts mixture from a little rich to a little lean and the RPM responds. If it is a US 328 or 84-85 QV and not doing it something is set wrong.

 

Your car is hunting to find the correct fuel mixture. Most common issue would be an air leak, check all the vacuum hoses, maybe something was disturbed during the service procedures. Can also be a failed O2 sensor, you can disconnect the connector on this item and see if the idle smooths out, when disconnected the computer defaults to a static, slightly rich mixture.

 

It's the O2 sensor adjusting the mixture - but don't assume the problem is the sensor itself and run out and buy a new one.

 

Hunting is 100% normal

 

I will check all these things as soon as I can. Thanks for the help!!!!


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One function of the throttle microswitch on the US version K-Jet with Lambda models at idle is to "tell" the Lambda ECU to limit the hunting at idle. An easy test to do is just unplug the throttle microswitch at idle - one of two things will happen:

1. The idle RPM should increase, and this shows that the idle switch portion of the throttle microswitch is working correctly, and the hunting is already being minimized at idle (when the throttle microswitch is plugged in), or

2. The idle RPM doesn't change = bad thing because this means that the idle adjustments were made with the throttle microswitch not working at idle = you'd need to fix whatever is wrong with the throttle microswitch (could be bad or just misadjusted) and then retweak the idle airflow and/or mixture adjustments.

 

Thinking about it more.. my car was surging from 1000 -2200 rpm. Not the same symptoms.

 

When My car does this I feel it when I am driving. If I am holding steady at 40-45 mph it feels like I am very slightly tapping the brake pedal

 

Check the auxiliary air valve and if the hoses under the coolant catch tank are connected- also check the fuse in the relay on the rear right side ignition module located under the carpet in the trunk


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