I've got a 1994 940 Turbo with 98k miles. Here's my problem guys. It's actually two problem and I'm not sure if they are related or not...

Problem 1: When the engine is cold, it idles lower than it should be, at about 600 RPM. The low RPMs cause the whole car to vibrate and "buzz", especially when I put it in gear. When the engine is fully warmed up after driving around, the idle rests high at 1,000 RPMs when I put the car in Park. It never idles down to proper RPMs.

Problem 2: When the engine is cold and I try to accelerate, the engine surges badly. The more warmed up the engine gets as I'm driving, the less apparent the surging becomes. Even when the engine is fully warmed up though, the surging is still apparent, just less so.

I've done a full tune-up and used top grade parts. I changed the air filter. I've cleaned the throttle body. I ran fuel injector cleaner through it. My Volvo mechanic checked for engine codes and replaced the coolant temp. sensor and throttle position sensor as a result of some codes that came up. Nothing has helped. Someone told me that it's possible the O2 sensor is bad, which makes sense to me since the engine temperature seems to affect the problem. I'd like some more feedback though from mechanics and other Volvo enthusiasts who might know what's going on with my poor Volvo. I love her! Help me get her back in shape.

 

You might want to check the IAC valve; I'm surprised, given your symptoms, that the Volvo mechanic didn't at least test it.

Other thoughts:

The idle on these cars is supposed to be computer controlled, non-adjustable. The computer gets inputs from various sensors and, based on these inputs, issues commands which result in a smooth idle and good drivability in general. Logic tells us that only four things can go wrong: (1) The computer gets bad info from a bad sensor and gives a bad command (2) The component controlled by the computer is faulty and does not obey the command (3) The computer itself is faulty. (Possible, but least likely) (4) Leaks (Fuel, vacuum, etc.)

Replacing a suspect part with a known good one is a perfectly legitimate method of testing and is sometimes more effective than testing the part itself. I've ultimately solved problems numerous times by replacing a part that was "within spec" on the ohms resistance test, but which nevertheless didn't work. I'm more likely to do "process of elimination" parts replacing in the case of items that are maintenance anyway, like spark plugs.

I'm thinking your O2 sensor probably isn't the problem because it would most likely set a fuel related code, like 221, 231 or 232. Bosch would like you to replace it every 50K, though, so if you decide to replace it and it doesn't do the trick, console yourself with the thought that "it's a maintenance item, anyway"

 

Get an injector light set and see if you have a weak injector. Check spark strength from coil and check fuel presure. You have 2 fuel pumps and your intank pump may be weak or non working. Anything diiferent as fuel level changes? Whatkind of gas mileage do you get. Your problem sound fuel related.

 

Thanks for the info. BIG help. Forgive me, but what is an IAC valve? Is that something I can check/replace myself? If so, where is it?

I've owned 2 940 Turbos (a '92 and this '94). I've also sold quite a few 940 models when I had my used car dealership. This 940 I bought has had a lot of very weird problems that I've not experienced with the 940s before. I don't know if mine was built on a Friday or what. I don't know if my Volvo mechanic just doesn't want to deal with the problem or if he honestly has no clue. He's an honest guy. He replaced, what I think he said was the throttle position sensor (?) with one from a junked Volvo. The car did run a little better after the replacement, but still very rough and the surging persists. I don't think he'd be stupid enough to install another faulty sensor and leave it like that. Surely he tested it. So I'm assuming it's something else causing the issues. I have to assume also that he was smart enough to check for vacuum leaks. I don't know if he investigated fuel issues.

I will go ahead when I can afford it and replace the O2 sensor, since like you said, it's supposed to be changed every 50k miles. I doubt it's ever been changed. But judging what you're saying, sounds like it won't fix my rough idle and surging issues. Would be nice if it does though!

 

I'm not a mechanic, just a light duty DIY'er. So I don't have all kinds of tools to test all of these things you mentioned, nor do I have knowledge how to go about any of this. I'd have to leave it up to my Volvo mechanic. I did a full tune up with good name brand parts (Bougicord and Bosch). It didn't help the running condition at all. The car seems to be getting good mileage right in line with the EPA rating of 19/24. I'm averaging 22mpg in mixed driving. The car fires right up without hesitation, and there are no strange noises coming form the fuel tank. This leads me to think the fuel pump is fine. As for it being a possible fuel injector issue, I'll have to talk to my mechanic next time I get the car serviced in 2k miles. Thanks for your help.

 

Mine was a bitch to figure out but it was a dirty cold start valve and a really bad fuel pressure regulator. I got a new(er) regular at the junkyard for 15$