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!

[TonyS9]

Seems you at having a good go at diagnostics but some of your basics are a bit off.

A rich mixture cannot cause a high idle in a spark engine. If you add air and the idle drops it means the mixture is correct or weak. Adding air to a rich mixture is what happens when you open the throttle.

If the idle is high with the IAC pipe clamped,, its because you have too much air getting in. A smoke test is the right thing to do but it' doesn't seem to be showing up the problem. Try again look harder.

The clamp test does indicate its a leak and not the ECU doing it deliberately, it can do this if it thinks the auto drive or AC has been engaged.

[GreenBrick]

I know you have tried the blocking off the IAC, but are you sure it hasn't stuck? Only a 10 minute job to clean it out and pop it back in.
I had this all 3 of my 900s. The IAC gets sticky and once warmed up the RPMs don't drop.
I used carb-cleaner to flush it out then a drop of engine oil to lubricate it. Don't use any silcone based oil like WD40 as this can contaminate the cat.
It should be free enough to rattle back and forth if you rotate it.

Feel free to ignore this, but don't overlook the obvious, I have in the past and wasted a lot of time, and some dosh.

[TonyS9]

It was a long post but he did fire the parts Canon and replace the idle valve already.

However I wouldn't rule out further throttle body issues.