I just recently bought a 1991 740 turbo wagon, my first Volvo and I love it, it's in great shape I'm just trying to iron out the bugs. I'm going to give all the details since day 1 in hopes it gives you guys a better idea what could be wrong.

Known issues:
So first off, my temp gauge on the cluster isn't working after I had blown a radiator hose on the highway. I plan on replacing the sensor.

My 4th cylinder spark plug (closest to the firewall) is corroded where the wire connected to the end. I'm replacing plugs and wires.

I have a service light on that turns off after about 3-5minutes of driving, I don't know how to get service code.

These is a hissing in the center of my dash, presumably a vacuum leak, that stops when the AC indicator is turned to defrost. Getting into the dash seems like a nightmare so I haven't even considered it yet.

Unknown issues:
Occasionally when I turn the car on in the morning the idle is very low, under 500rpm, and if I shift to reverse the engine dies. This doesn't happen regularly.

Today its first morning start was fine, but when I hit the gas the car would bogg at anything above 2000-2500rpm. After driving about 2 miles the problem went away like nothing ever happened.

 

The hissing from the center of the dash is most likely caused by a bad vacuum check valve in the engine compartment. This can be easily checked. The check valve is located on the top of the intake manifold towards and is a black & white plastic cylinder with a vacuum hose coming out of each end. The larger vacuum hose goes to the intake manifold while the skinnier hose should go to the firewall and into the passenger compartment. Remove the valve and see if you can blow through it both ways. If air flows both ways then it needs replacement.

Is it the service light that is on or the check engine light. The service light is a reminder that comes on every so many miles to remind you to get the car serviced. The check engine light is tied to the ECU and is the one that would go on if there were any codes to read. If it is the service light that is on you can reset it. There should be a small recessed button on the instrument cluster that can be pushed using a stiff wire or a pencil.

If you had a radiator hose bust, I'd be concerned about too much pressure in the system and would perform a compression test just to be sure there aren't any head gasket issues that may be causing back pressure in the cooling system.

 

remove the glove box and check out the bellows that change the airflow for the AC. If any are torn or disconnected it will make a hissing sound, and that would be consistent with the symptoms you are describing. Some are by default closed (vac on) therefore only hosting when its set to normal vs defrost

check the codes, there is a box on the left strut pillar in the engine bay

put the ignition into the ll (2) on position and put the probe into the sockets one at a time (i believe there is only #2 and #5 although there are 6 hols)

when the probe is in the socket, press the button one good press (about 1 second long) and wait, the light will flash. the numbers are in 3's so, if you get something like

- = light blinking

- --- -- = 1-3-2
-- -- -- = 2-2-2
--- --- -- = 3-2-2

so on and so forth, we can look up what those numbers mean



next, as for the idle issue
sounds almost like a bad MAF sensor, unplug it (in line of your throttle and air box) then while its unplugged try to start your car. I have a 940 turbo that acted the came when it went bad

 

Thanks guys, great help. I'll get to it tomorrow and update what I've found

 

I unplugged the MAF and the car started. It ran rough and seemed to increase the idle problem dramatically. Is that was I was looking for? I haven't removed the glove box and inspected the bellows yet

 

I tested the vacuum check valve like you said and it's fine, also reset the instrument cluster and the service light went away. Compression test is next