Volvo 940 2.0 High Idle When Warm

[ArlenM]

Hi Everyone!

So I bought myself a lovely 1994 Volvo 940S 2.0 Auto a few weeks ago, wanted a Volvo for years so needless to say I'm pleased! It has a very irritating issue however...

When you start it from cold, it idles absolutely fine in P/R/N/D, it sits at about 800RPM. Once the engine has warmed up to operating temp however, the idle rises to about 1000RPM in R/D and 1250RPM in P/N. The vehicle has no fault codes at all.

So far I've tried the following:

I've measured the resistance of the ECT sensor both hot and cold, this matches with readings someone had posted online so guessing no problem there.

Adjusted the TPS as it wasn't "clicking" when throttle fully closed, and I've also verified that the internal microswitch is actually making a connection, this did actually drop the idle from 1500 to 1250 so it's definitely helped.

I cleaned the original IAC valve, which didn't help, so I bought a new one and that also made no difference.

I've smoke tested the whole inlet system and this revealed a hideous leak from the rear of the rocker cover gasket so I replaced that, it also showed up a few whisps of smoke leaking from the throttle butterfly pivot, is that of concern?? I've tried dripping thick oil into the pivot to seal it and the idle doesn't come down.

I've fitted a new fuel filler cap as the seal was perished, wondered if it was drawing air in through the EVAP system, again no difference.

I've verified that none of the PCV system components are blocked up, although when messing around with this I did notice that if I disconnect the PCV pipe from the intake pipe between the MAF and throttle body, thus leaving a leak of unmetered air, the idle will drop to about 850RPM, so that made me think back to a car with a carb and a manual choke, if you have the choke engaged with a warm engine, the idle rises as it's running too rich, introduce more air and the idle drops, going back to the Volvo if adding more unmetered air makes the idle drop does that mean it's running rich?? I have noticed it's very thirsty on fuel, about 25mpg at best.

Clamping off the hose to the IAC drops the idle to about 1000RPM, I've read on here that it should drop to about 500RPM under this circumstance, so I have a theory which could be total rubbish (and feel free to tell me if it is), that for some reason the fuel system is overfuelling (leaky injectors, stuck pressure reg etc) and the O2 sensor is telling the ECU that the mixture is too rich, so it's opening the IAC slightly to "lean out" the mixture, but in turn this is causing the idle to rise because the MAF sensor is detecting greater airflow and therefore increasing the injection pulse width, however when I'm introducing unmetered air, it's leaning out the mixture without increasing the airflow through the MAF, and as such the idle drops because there's more air with the same injection pulse width. Please feel free to tell me I'm an idiot, I can take it!

The stop screw on the throttle body doesn't appear to have been adjusted, and the butterfly does actually stop against it, surely it can't just be as simple as adjusting this screw??

Any help with this would be greatly appreciated, I'm at my wits end with it!