240 1991 240 idle issues

Hi all,
I am 17 years old and my 91 240 wagon is my first car. Ever since i picked it up in September of last year, its always had an issue with a rough idle when the motor is cold, and would typically stall if I didn't help it with the throttle. Once I did some looking, I found some common things that cause problems with rough or high idle. I've cleaned the throttle body and IAC and the car no longer stalls, but the idle is now much too high. Car has a check engine light on and here's where the weirdness starts. One day I cleaned and replaced all the fuses and once i reconnected the battery, the check engine light was off but the car ran terribly, but once it was up to temp idled okay with no check engine light. After a few days of driving however, the idle slowly crept back up and the check engine light returned. If i simply take the fuse out and put it back in, light goes off and the cycle repeats. Any pointers would be greatly appreciated, I want to take care of this car as best as I can, as it's only at about 120,xxx and will probably outlast me.
Thanks!

When the check engine light is on. You can use the diagnostic port led to flash you up to three stored codes. The procedure is straight forward. There is also a mode where you can test the idle motor. Get a serice manual either the Bentley or the Haynes. Or you can search on youtube how to read Volvo LH2.4 diagnostic codes. The code definitions are in those manuals or online at brickboard.

Since this car is still fairly new to you. I would suggest planning on a full service of all fluids and filters along with a tuneup. That way servicing of the car is on your schedule.

There is a switch on the throttle body that lets the computer know the throttle is closed so it will regulate the idle. You can check with the engine off by slowly opening and closing the throttle. You should hear a click as the switch contacts open and close. If that's not what you hear. The switch may need adjustment or replacement. Without that switch working you will experience a high idle.

91 should be an LH 2.4 car, use the diagnostic box to read the flashing light codes, I'm guessing coolant temp sensor issues because all of these cars have something going on with the harness and that sensor now.

Also, make sure the throttle body and position switch is set up according to the green books, do you have a copy for your model year? They're floating around the web now, but will take some digging unless someone saved a copy over time.

Another thing to check is that all of the Bosch connectors on the intake side of the motor are in the correct spots, There are at least three that I can think of on that side, IAC, ECU Coolant temp sensor, knock sensor, and possibly EGR valve.

They all have the same connector so getting them mixed up if someone pulled the intake is a very real possiblility. Peel back the connector and check the wire colors as compared to the green book for YOUR model and year. Volvo was all over the place with their wiring colors over the years.

I’m thinking coolant temp sensor too. My 93 240 had a similar problem where it would idle terribly when it was cold but when it got to running temperature it was fine. It’s like a $10 part from ipd

Vacuum hoses. I just bought an 89 with a check engine light, throwing a bunch of codes. Seller told me AMM was bad. Checked for leaks with a 20 dollar hand held vacuum pump off amazon, and found 3 hoses with splits. Replaced them, disconnected battery to reset computer, Reconnected battery, all good now. Full tank of fuel used and not a hiccup. 5 minutes of rough running while lh re-learned and I was good to go. LH2.4 will relearn and can run poorly for a bit after fixing something and resetting the computer, It has to relearn for the new situation
I'd bet beer you have a vacuum leak.
Pull off every vacuum line, inspect the hose for cracks and make sure it fits tight. Pay attention to the ones running to the charcoal canister.
Way too many people on here jump to bad parts right away. Never replace parts until you are sure there are no vacuum leaks.
Check the intake manifold and throttle body gaskets for leaks as well.

Thank you all for your advice! So far I have no vacuum leaks at any of the gaskets but I plan on giving all the hoses a good check as well. I do not have the green book for this car myself (Will try and dig one up) and will have to check the wires and connectors. My throttle body position switch does appear to be working and there is an audible click when the butterfly valve opens and closes. I had already ordered a coolant temp sensor so I'll be throwing that in tomorrow when I replace my motor mounts. Thanks again guys and I'll post an update once i go through the remaining stuff.