Someone please help me before I go insane. I bought my 240 turbo a few months ago and thought it was in great shape, but now has turned my life into a living hell! A week ago after getting some gas my car started to buck in any gear, but ran fine at idle. So I thought I had gotten bad gas and put some injector cleaner in it, the next day I drove it home and it was fine, but I noticed my fuel pump was making a noise, so I spent the next day replacing it. So on a short test drive the car started bucking again, and after talking to a few people and looking in the manual, I find out that it was making the noise because the in-tank fuel pump wasn't supplying it with enough gas so it was starving and making a noise. So my bad, I replaced that one too. Now however my car is running fine at idle, but whenever I give it some gas and release the accelerator the rpm's drop to zero, and the car barely keeps running, then goes back to the correct idle speed shortly after. What could this possibly be? I asked a couple people and they told me air filter, so I replaced that and it didn't help. Then they told me fuel filter, so I replaced that, didn't help. Then a Volvo mechanic told me it might be ignition trouble, so I went and got new spark plugs, distributor cap, and rotor. But guess what, same problem... I have spent over $300 and week of frustrating work on a car I bought for a $1000. Can anyone tell me what's going on before I go insane? What is the constant idle system??? Maybe that's it but can't find it listed on any parts sites...
Thanks
Inzerowetrust...

This is an easy one for a turbo guy. Set your base idle as per the Bentley and you will cure this.

Definitely could be a vacuum leak. Check for cracked or unhooked hoses everywhere. IAC sensor is possible too. they're pretty easy to clean out. I would also advise investing in the Bentley manual, and also I wouild suggest either asking here or some other volvo expert before replacing things like the main fuel pump, which is a stupidly expensive part! I've saved a pile of money over the years by waiting to spend money on what I THINK is broken until someone here tells me otherwise and I discover that they're right. Spending money on parts when you're not sure what's wrong rarely fixes the problems (been there). At least now you still have a working main fuel pump as a spare (you didn't throw it away, did you?). So there's my statement. Don't lose heart over this, it doesn't sound like a big issue. If it runs fine except when returning to idle, I would suspect a minor air leak first. Good luck on this...
Nate Gundy
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'86 240DL, 260K miles, M46, K cam, 25/21mm sways, and now failing engine...

Check the turbo and vacuum hoses, more than likely there's something unpluged. Also the pcv hose going to the turbo hose.

What is the constant idle system???"

"The CIS system maintains idle speed within a 40 rpm range by regulating the amount of air bypasing the closed throttle."

That's from the beginning of 7 pages in the Bentley 240 Service Manual. It has text and pictures of how to locate, test, and adjust these components:
> Throttle Switch
> Idle Air Control (IAC) valve
> Electronic Control Unit
> and related circuits

One quick check is to listen to the IAC when the engine is idling. It's down under the intake manifold, so you may need to use a long dowel or something as a "stethescope". It should be pulsing or whirring.

The IAC can get gummed up internally. Try removing it and the attached air hoses, and flushing it out with something like WD-40. Here is a picture of its location. Yours is probably not as easy to get at, but it looks like this:

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P.S. I urge you to get the Bentley manual ($40-$50, try online) before spending more money on parts you may not even need.
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Bruce Young,
'93 940-NA (current)
'80 GLE V8 (Sold 5/03)
'83 Turbo 245
'76 244 (lasted 255,000 miles)
73 142 (98K)
'71 144 (track modified--crusher bound)
New 144 from '67 to '78
Used '62 122 from '63 to '67