Hello! I'm back here again looking for advice.

Previously the engine idle was way too high, but that was caused by a fault EGR valve which I have since replaced.

The high idle seems to have been hiding the issue of the car being unable to hold an idle or stay on without throttle input.

I originally thought it was the MAP sensor as it wouldn't start with it plugged in, but I replaced that and it only ran a little bit smoother.

Other work I've done has been: new water pump, new oxygen sensor, new spaek plugs, new temp sensors all around (gague, fan; ECU temp sensor off of a later model with new connector spliced to the wires, cleaned and reconnected grounds, securing vaccum lines.

Seems to be running rich as when I pulled out the plugged to gap them correctly to 060 (I had them mistakenly at 045 before) they were black.

Once again I am lost as to how to proceed and where to go from here, any advice will be appreciated!

quote Originally posted by Csell2: Hello! I'm back here again looking for advice. Previously the engine idle was way too high, but that was caused by a fault EGR valve which I have since replaced. The high idle seems to have been hiding the issue of the car being unable to hold an idle or stay on without throttle input. I originally thought it was the MAP sensor as it wouldn't start with it plugged in, but I replaced that and it only ran a little bit smoother. Other work I've done has been: new water pump, new oxygen sensor, new spaek plugs, new temp sensors all around (gague, fan; ECU temp sensor off of a later model with new connector spliced to the wires, cleaned and reconnected grounds, securing vaccum lines. Seems to be running rich as when I pulled out the plugged to gap them correctly to 060 (I had them mistakenly at 045 before) they were black. Once again I am lost as to how to proceed and where to go from here, any advice will be appreciated!

Hi! I just noticed that no one responded to this. My thought is that... with the extremely high idle that you had before, my guess is that someone tried to adjust the idle screw lower before in order to compensate. As a result, your idle is likely adjusted too low, and now that you've fixed the vacuum leak problem, the car is pumping out too much fuel and not enough air. Rich is better than lean... haha... so that's a positive, because Idle can burn and crack stuff. Rich just makes it really messy.

I would adjust the idle so that you get a desireable rpm at idle. The Duke likes to idle around ~900 hot, ~1100-1000rpm cold.

I don't know where the idle screw is... unfortunately. I have a duke that I can go outside and take a look at, and I also have the 84 Factory Service manual... let me know if you want me to take a look and I can try to take a picture.


Hope that helps!

Hey there! Sorry I didn't get a chance to respond until now, life's busy!

I'd love to say that someone had adjusted the idle screw but the idle screw cap on the duke is still there so there's no way it's been tampered with (I found a diagram of the TBI which has been really helpful in getting to where I am right now)

I did a little bit of working cleaning the injector which had resulted in a marginal improvement? It still doesn't idle when warm and quite literally dies while off the throttle in gear driving down the road. Its frustrating because she starts up instantly, has no trouble accelerating; she just doesn't want to keep running.

Drilling out that cap has been on my mind, but clearly this issue is caused by something else and I don't want to just be masking the problem; so really I'm torn at what I need to do.

quote Originally posted by Csell2: Drilling out that cap has been on my mind, but clearly this issue is caused by something else and I don't want to just be masking the problem; so really I'm torn at what I need to do.

No, you don't want to be messing with the idle stop screw, as it's not an idle speed adjustment screw. There's a very slight chance on a very high mileage car that whatever surface the end of the idle stop screw rests against has worn away (allowing the butterfly valve to completely close), but that's highly unlikely.

Regarding the rich condition, one thing I would check is to make sure that the single injector in the throttle body isn't leaking fuel past any of its O-rings when the system is pressurized (engine not running).

I might also add that diagnosing an engine without the ability to see what information the sensors are feeding the ECU is like operating in the dark. You currently have no idea if one or more sensors are faulty... and unfortunately, faulty new parts are a common occurrence. You need either a scanner, or a cable that allows you to run scanning software such as WinALDL on a laptop.

[This message has been edited by Patrick (edited 07-02-2023).]