'83 944 Suddenly Erratic Idle
 

I have an ’83 944 that I’ve had since ’84 and I do most of my own work on it. At 170,000 miles, it has never run better. About 3 ½ months ago, I replaced the spark plugs (Bosch Platinum+4) and plug wires (Clewitt Engineering), distributor cap and rotor (Beck Arnley), and air filter (K&N). I borescoped the valves and pistons (through the spark plug holes) and they’re all clean, with just a thin layer of carbon.

About 2 weeks ago, the idle went wonky. On any given drive, it will idle rock solid at one stop and vary by +150RPM at the next stop. It may idle perfectly for several stops and then idle wonky for several stops. I’ve not discerned any pattern. Off idle, it runs perfectly.

Any ideas?

Does this happen when the engine is both cold and up to operating temperature? If you start it cold and wait, does the car immediately exhibit this uneven idle? Thanks,
John

Start simple. A vacuum leak might cause this. Does the idle variation go high instead of low?

I agree with start simple. I chased an idle problem with my 944s2, spent money on expensive parts like throttle position switch and idle control valve and still no fix.

For me, the problem ended up being that the grub screw on the throttle body needed to be turned just a few turns to increase the idle speed. Sounds crazy simple, but for me what was happening in the engine would drop below the minimum required idle, and then would bounce higher to try to recover. I had this crazy idle pattern from fast to slow, and sometimes the engine would even die if it was able to bounce off the low.

Once I turned the idle screw to increase minimum throttle position it idled perfectly.. not more erratic bouncing idle.. you might give this a try.

I disconnected the O2 sensor and there was no change in the erratic idle. It was time to replace the O2 sensor and fuel filter anyway, so I did so with no effect on the erratic idle. When it idles erratically, it drops about 150 RPM then immediately bounces back up to about 50 RPM above base idle, and repeats. The only pattern I’ve noticed is that idles normally when cold (presumably when the aux air valve is open) and does not idle erratically until warmed up, but, as I mentioned initially, at one stop it will idle normally and the next not, with no discernable pattern. Ambient temps have been as low as the 30s since I first posted, with no change in the erratic idle behavior. I’ve tried turning on the AC when it was idling erratically, and, most of the time, it smooths out, but sometimes not. Last summer, I adjusted the base idle up to about 1050 RPM, as the AC has always dragged the idle down to 800-900 RPM, so, when the AC’s on, it will idle to spec of 950 RPM. I have been unable to catch it idling erratically in my driveway to get any clues, including looking for a vacuum leak.

How's the fuel pressure? Have you checked/changed the filters recently? What kind of pressure do you get at the rail? sometimes a failing pump can act wonky, or if there's a bad ground for the pump could also do it..just throwing some stuff out there. How about the spark plug wires? could one of them be grounding out? Does it idle smoothly after the drop in RPM? Plug wires should be good since they are new..

Is the throttle switch working properly? you should hear it click at closed throttle and at full throttle. The platinum plugs are probably not the problem but I would ditch those and run stock ones. The early ECU's can sometimes be an issue and you can swap it out for a later one as long as you swap the AFM from a later car as well.

+1 on a vacuum leak. Time to round up the usual suspects.

Too true on the vacuum leaks and easy to check. Don't forget to check the idle screw on the throttle body, I have had those work their way out a few times.