Here is my issue with my 1994 940 turbo wagon. The car runs great, starts no problem but as soon as you come to a stoplight the idle is very lumpy. Start off away from the light no problem. The motor was freshly rebuilt one year ago with no issues what so ever for about 8 months. I have ordered silicone vacuum hoses and have replaced all of them except the one that runs into the interior which I imagine is for the gauge cluster. I have replaced distributor cap, rotor, plug leads,plugs, and fuel filter. Any hints or solutions from all the good people on here who may have gone through this issue are greatly appreciated.

 

with a turbo, there's two into the dash, one is vacuum for the 'turbo boost' gauge, the other is vacuum for the ventilation controls. the one for the turboboost gauge is right off the intake manifold with no one way valve, the one for the ventilation is after a one way valve.

idle speed is controlled via the IAC (Idle Air Controller) which is under the intake manifold. The engine management uses the IAC when it sees the throttle is at idle according to the throttle position switch (which is just on/off at idle). ANY air leaks around the IAC, throttle body, or really anywhere downstream of the MAF/AMM can cause lumpy idle. sticky IAC can cause lumpy idle.

bunch fo stuff in the 7xx/9xx FAQ re debugging this. https://www.volvoclub.org.uk/faq/Eng...#Idle_Problems

 

Thanks for that info. Pierce. I will look into that. Lot of information in the link.I am going to clean the throttle body and see if that helps.

 

i'd start looking for small air leaks in the intake. tiny ones will only affect idle. you can verify the throttle idle switch is working with a multimeter (see that FAQ) without taking anything apart.

these engines should idle rock steady just under 800 RPM once they are warmed up, and barely blip when you put in gear, or the a/c kicks on.

if you're going to clean the throttle, you need a new throttle gasket. I would probably clean the IAC at the same time, and use fresh rubber tubes for it.

 

Good education here. I had no idea about how the IAC worked. It does sound like things are pointing towards that as if you lightly touch the gas pedal it will idle properly. I imagine this activates the idle switch and you are kicking out the computer activated idle air. I will keep looking into it. Much appreciated

 

Lumpy idle is solved, thanks for the input. I started at the start of the list in the link provided. I poured in fuel injector cleaner into the gas tank on my fill up and then took the throttle body off and thoroughly cleaned it inside and outside so it looks good too. I also disconnected the battery while I did it to give the computer time to reset. Upon starting and a bit of a test drive for the first 4 or 5 blocks it wouldn't go well at all. Put on the gas and nothing would happen then all of a sudden it would surge ahead then it all cleared up and now all is good. It was almost like the computer had to learn some new settings or information for a few minutes. is that a reasonable guess? Now it idles nicely at about 750rpm and runs good too.

 

Well I thought I had solved my problem but it turned out not to be so.Letting the computer reset would last for a couple of months and then it was back to the lumpy idle. It sounded like an old big block v8 with a lumpy cam in it. The fix was replacing the O2 sensor Quick fix. At least I got a bunch of maintenance issues taken care of while I was trying to solve the problem.