engine light comes on and off???

[jakewaze]

Hi
Last week I picked up a blue 1990 VN S Commodore for 2300buks. It seemed to run fine through the test drive and te owner explained the engine has been on fore ages and he doesn't know why.
anyway the only problem I know of is the leads are a bit dickie and r4ading the service report from 3 months ago nothing was picked up.
The car still starts fine and runs fine, but every now and then the engine light will come back on with a bit of a jolt and if i'm idling (at the traffic lights) the car is unstable and has sometimes stalled. I'm not sure what this is but really want to get it fixed coz i am gettin license nxt week. could it be the CAS or should i just do a compter test thinghy???
Also does the engine have to have been on right before the computer test.
Sorry for the long write up but want to be specific.

Also when the light isn't on the car runs really smooth. Also whne the light is on it does miss occasionally while driving but is worst when idling.

[fu2]

ill be thinking its the oxygen sensor. a compter fault test would show you the code thought.
got here if you dont know how to do it

Checking your Computer for Faults (VN - VP)

[jakewaze]

thanks fu2, one of the leads had actually fallen off and was loose. i replaced all the leads and the problem seemed to go away 50pc of the time but is still there along with the occasional engine light.
i did the computer test and got code 42 (before new leads) but haven't tried again. should i try again or reset the puter???
cheers

[fu2]

wont hurt. if it still does it after that then check for faults again

[Cheap6]

The code will stay in the computer until a certain number of starts (how many? = ?????) or until cleared. If you don't clear the codes, and confirm that they are cleared, you won't know whether the fault has recurred or is the code from before.

A code 42 is an ignition system fault. My guess on a VN is that the ECM is suspect. One check is to clear the codes and tap the ECM case (behind the LH kick panel, under the glovebox) with the tips of your fingers with the engine running. If the code 42 triggers (the Check Engine Light comes on) with that, the ECM is probably faulty.

Other code 42 triggering faults include poor connections to the DFI (ignition) module. This is on the LH front of the engine underneath where the ignition leads go too. Wiggle the wiring harness to this with the engine running and watch for the CEL again.

An ignition system fault, like a lead fallen off, could set an O2 sensor or lean exhaust code but you would have other symptoms (like an engine miss) with it.

[jakewaze]

cheers cheap6,
i'll try the ECM tonight and c how it goes