ok...I'm now starting to understand why there are so many "rough idle" threads. I've gone through roughly 29 threads via the search function and they're all just different enough to warrant their own threads!

Here's my contribution. 1991 Volvo 240 sedan.

About a month ago I started getting terrible gas millage. Around the same time, the idle would not come down from 1500 RPM. I pulled the codes. There was a bad injector code and an ECU compensating for extremely rich mixture code. I went ahead with the obvious routine stuff: changed the spark plugs, cleaned the throttle body, cleaned the MAF, replaced the flame trap, and fixed what appeared to be a vac leak in the small hose that runs from the intake, over the valve cover and into the firewall. I'm assuming this is for the HVAC? I also attempted to clean the IAC (it's not a rebuild-able one). After all that, the problem got worse. (well...no bad injector code this time though)

After all that, i'm loosing idle RPMs to the point of stalling. Figuring this was a result of a bad IAC valve I went ahead and tested the resistance...dead. Just to be sure, I tested the voltage past the throttle body. 12 volts. I went ahead and purchased a new IAC Valve.

Separately, after fixing that vac leak, I now hear what I guess is vac pressure building inside the cab somewhere. Building or releasing...i don't really know which. I know HVAC systems sometimes do this but I've never heard THIS car do it. What I noticed was a direct correlation in the idle after this pressure build up or release or whatever the hell is going on.

I know the new IAC is doing what it's supposed to. I'd say I have a vac leak causing the bad idle. Oddly, for as loud as it is, I can't seem to locate it anywhere in the cab?!? Just as odd...I can't for the life of my locate the EGR? Where the hell is that supposed to be?

Any help would be appreciated.

 

In basic terms .... The vacuum source is from a line connected to a check valve on the intake, which goes a white vacuum reservoir behind the center console. From there a line feeds the three climate buttons. Each button feeds the associated circuit of vacuum actuated "flapper" air valves.

 

A simple test would be removing the vacuum line from the firewall side of the check valve and plug the check valve end temporarilly and drive her around the block a few times and see how the idle reacts. If it's better I would then start the console removal to find the leak. I would also check the injectors and the wiring to them as that bad injector CEL code could indicate an injector getting ready to go out.

 

so simple!...thanks!
I fixed that one already.

 

check valve....that's the word I was looking for. After reattaching this I noticed that the metal portion of the fitting rotated pretty freely in the rubber gromet that attaches it to that check valve.

old cars eh?

 

Any older vehicle with a vacuum actuated A/C system, you should suspect teh A/C plumbing if bad idles occur.
I like the disconnect supply test at the engine, will eliminate teh issue.
I had an older Valiant once, would idle really badly with the A/C on and windscreen defrost activated. The diaphragm that controlled the distribution valve on the A/C unit had a small tear that only opened up at the end of it's travel, a nightmare to find!
I had to disconnect each vacuum line from teh A/C controller under the dash to isolate it.

Regards, Andrew.

 

THAT sounds like a PITA...i'd just as soon not have A/C or defrost! To bad I fog up the windows in my car. Sigh, I'll tackle that particular issue when the snow melts a little more. till then, I got heat. That's all I care about.

However, I STILL have a periodic high RPM idle when sitting in park. I don't know that I would notice it so much when the car is in drive with the breaks on....the idle seems to hang out around 800 RPM in that case. But once she's in park, it's 1200 RPM.

I was just out driving. Got home, put it in park, noticed the high idle again (because it's not all the time anymore). Shut the car off and imediatly turned it back on. It did it's rutine start up high idle, registered the car as warm and came right down to where it's supposed to be...S.O.P. What am I missing here? I'd go straight to the EGR but I still can't find the damn thing!

 

What brand spark plugs are in there and were in there before? I had Bosch super plus spark plugs in mine and my engine idled to rough it was awful. A Volvo mech. told me that the 240's don't like those and so I switched them out for just the Bosch supers and it fixed the problem.

 

It had Volvo plugs in it. It has Bosch now. I could see the plugs causing a rough idle but this isn't rough at all...it's just really high. When I do have a normal idle, it purs like a kitten.

 

I would be planning a trip to the junkyard with my idle control valve part number written down on a piece of paper. And a screwdriver

 

I just replaced the IAC with one of Groton's rebuilt units.

 

There is a possiblity that it could be your throttle position switch.

 

yeah....ok.....I guess I only checked to make sure the thing was clicking. I never checked the thing on ohms or volts. Are you thinking maybe there's a dead spot on it?

 

Did you see if plugging the vacuum line to the climate controls made a difference?

It looks the Blue/white wire on the throttle position switch is tied to ground during idle.

The green book says a deffective temp sender can also cause a high idle.

 

plugging the vac line got rid of the vac leak noise in the car. I left it plugged at the intake manifold until I get a chance to tear into the dash and find the leak
I see. So if I go out there and test this thing at a normal idle it'll read out 12 volt? High idle would read out less than 12 volts? Or do I have that backwards?
really? there are supposed to be two right? I already know the one controlling the temp gauge in the car is bad. I have a replacement for it but it was just different enough to cause me hesitation on forcing it in.

 

normal and fast idle ... there should be no voltage on the blue/white wire while the switch (throttle) is closed. But if you do see voltage during fast idle that would suggest the switch needs to be adjusted.

also after the fast idle I would turn the car off and make sure the throttle plate is closed all the way (cable bind, etc)

There are three - one for the gauge, one for the fuel, and one for the ignition. The igntion and fuel are contained in one package.

 

Ok great! I always look forward to getting under the hood as long as I have an actual direction to follow.

 

new development. after the last round of fixes I cleared out the trouble codes. Drove it around a little bit, pulled the codes again: 3-1-1. The speed sensor is on top of the bell housing right? That's gotta be unrelated to this idle issue though?

 

Well shut my mouth!

 

The brickboard FAQ says:
3-1-1
Signal missing from speedometer
Speedo wiring or instrument cluster wiring faulty
Idling problems

This looks like connection "32/2" in the diagram I posted.