IAC problem......I think
 

My '91 740, non turbo, Regina starts fine, idle's fine and runs fine. But when I let off the gas and it decelerates the rpms dip so low it either dies or almost dies. My first thought was the idle control valve, which I cleaned and checked for smooth movement by manually forcing the plunger down. No problem and the spring loading seems to work when I release it. Hooked it back up and same problem. Applied 12 volts to the terminals, and could hear it click, so I didn't think that was my problem. All the threads I've searched about IAC's seem to be stuck open and have high idles. Pulled mine out again and gave it 12 volts while I looked down the valve and again it clicked but just barely and quickly moved a little and returned to the close position. Question: as long as power is applied should it continue to stay open? Or is it a pulsing action - open...close...open?

as long as 12V is applied, it should stay open. did you apply the right polarity? the wiring diagram shows pin 1 as +12V and pin 3 as grounded to actuate it.

Yes, I made sure the polarity was right. In fact, the valve has + and - markings near the terminals. So I guess mine is shot. Makes sense. Thanks

Well this is embarrassing, but good news in a way. I was using my 12v test light as my positive jumper lead and, like I said, getting a click with very little, temporary movement. Out of curiosity I used a straight wire right off the battery and it made a loud click and stayed open. I guess running through the light dropped the voltage enough to affect the valve. I checked the voltage coming from the connector and I'm getting 12 volts but when I hooked it up with the ignition on, engine not running, again I got just a little movement and wouldn't stay open. So I tried using the positive lead from the connector and grounding the negative lead to the engine. And it worked, which told me the negative lead of the connector wasn't a good ground. So, finally, I plugged all the vacuum hoses, connected the valve to it's connector and started the engine. I watched inside the valve as I revved and slowed the engine and it responded the way it should. I guess it's very sensitive to having the right voltage going to it. Anyways, it seems ok now but I will have to check that rpm drop during deccelaration tomorrow when there's more light.

with the idle valve disconnected, the engine fully warmed up, and all accessories shut off, it should idle about 500rpm in neutral/park. turning on a/c, putting it in gear, etc can stall it without the IAC connected. connecting the IAC should bring the idle up to 600-700rpm on a fully warmed up engine, and it should maintain that 600-700 when you put it in gear, switch on the a/c, etc.

Have you checked your engine codes? I had similar symptoms on my '90 240 and found a 3-1-1 code which was missing speed sensor.

Yup, I'm always looking for codes. I did get one when I disconnected the valve but it cleared when I plugged it back in.

My IAC's behaving just like that so I don't think that's my problem. I revved it up this morning and let the throttle go and each time it dipped low enough to stumble(and vibrate, shake at the same time). And one time it died, but started right up. No codes. Strangely my car idles fine but I noticed if I hold it at 1500 to 2000 rpm I get the random bucking or kicking of a cylinder that's missing. Yet it idles fine. So I went back to basics. Did a compression check and there's 160 lbs in all 4 cylinders. New plugs are burning light brown, though the base of all the plugs have a little ring of oil, not enough to foul the plugs. I'm guessing stem seals rather than guides, otherwise I couldn't build that much pressure. New rotor and cap and wires look good. Cleaned the throttle body and tested the pressure and temp sensors on the Regina system. New FPR. Tightened alternator belt and battery shows 13.5 volts at idle. Used propane to find any vacuum leaks and all seemed good. Ran a couple of bottles of Tectron injector cleaner. Any ideas what else it could be?

13.5V is actually on the low side, at least if your battery is fully charged and its not freezing cold out.

at 68F/20C, a fully charged battery should be about 12.6V at rest (eg, an hour or two of sitting off), and with the engine running but all accessories off should be 13.8-14.2V

I dunno jack about the Regina system, they were never sold in California.

Regina or not, when you hold it running just above idle and feel that occasional kick or miss, could it be the injectors? If it was spark, wouldn't it affect the idle and high speed running?

in my experience, occasional misses are usually ignition. fuel delivery problems generally materialize as stumbles or power loss at high throttle, upper middle RPM in taller gears when the engine is working its maximum.

it is possible, I suppose, that you have 1 or more injector with a bad spray pattern, thats possibly dribbling fuel too. but that would more likely affect idle smoothness.

you could try the italian tuneup. next time you're near empty, add a medium size can of Techron (even better, use an 11 oz can of BG 44k) to the tank, and only put a few gallons of fuel in, no more than a half tank. then get out on the freeway, leave the car in third gear to keep the RPMs high, and run that tank close to empty, driving like you stole it.... yes, I know, this is 100 or more miles of driving for no purpose. refill the tank, and do an oil change fairly shortly after this as you likely blew a bunch of crud into the oil.

I did do a half-Italian tune up - a bottle of Techron and just a few gals of gas but didn't run it in 3rd gear for that long but I also ran half a bottle of seafoam thru the tank and the other half thru the throttle body. So, if it is ignition, what could it be?

BTW, I also adjusted the TPS so it clicked going to open throttle and closing. But I've read some info that says it should also click at full, open throttle and mine does not. To test it you're suppose to ground either side connector to the center one and the idle should return to normal. Grounding one side connector does that but grounding the other does not? What kind of signal to the ecu does the TPS send? Also, I noticed this morning that my cold idle is about 800 to 850 rpm and drops to 750 when warm. And when it is in that slightly higher rpm I can release the throttle from 2500 rpm and it doesn't do the low dip and stall. Does this mean my mixture when warm is too lean?

UPDATE: Problem solved. Turns out the vacuum line to the manifold pressure sensor was not holding vacuum. The line itself is a small diameter hard plastic. But at each end is a soft rubber connector that looked ok visually but with engine vibration and heat a crack on the inside of the elbow shape was expanding and losing the vacuum. Not all the time and not all the vacuum, hence, sometimes it would die and sometimes it would dip down and then recover. On Regina systems that pressure sensor and the temp sensor in the air box function like a MAF so any fluctuation in vacuum will confuse the ECU. I got some new rubber connectors which fit real snug but even then there was a small gradual leak so I cinched up small zip ties around each connection and now it holds vacuum steadily. I can revv it up to any rpm,let go of the throttle and it never dips below 750 rpm. Great! Easy fix but drove me nuts finding it. I guess there's 2 lessons here. First, it your car is behaving erratically whenever there's a change in vacuum pressure, double check all sensors that respond to that. And secondly, don't just visually inspect vacuum hoses and/or check leaks at idle. Pull them, attach a mity vac and see if they hold pressure and test them again while revving the engine. Anyways, thanks for your help Pierce and good to see you visiting now and then.

not all B230's have the full throttle switch. they all have the idle switch.

the 'TPS' is just a normally open on/off switch with one side to ground, other side to the ECU... it grounds the signal when the switch is closed.

At least I'm not missing anything. Good to know. Thanks

Pierce, what's your thoughts on this? My problem has returned but not due to the same root cause. Initially, my manifold pressure sensor wasn't holding vacuum but now it does and I checked the varying voltage signal it puts out depending on vacuum and it's fine. Same with the temp sensor in the airbox. In fact, the car drove well for the last week with just occasional rpm dips but not enough to kill the engine. An hour ago, I went for a test drive and at stop signs, idling in drive it dipped and died 3 times in 2 blocks. I finally got a code 2-2-3, Signal missing to/from idle valve. The valve itself looks fine. I hook it up to the battery and it clicks and holds no problem. I double checked the TPS and it seems it's adjusted properly. With the connector off I got it started and it ran at 500 rpm barely, as it's suppose to. So I hooked my meter to the connector, and it shows 12.5 v with engine off and 13.5 with engine running. I revved it up, off idle, and it still showed 13.5 v. That's what confuses me. I thought it stopped activating the IAC valve off idle? Should it still be sending a signal? Or maybe voltage is constant and the ground disconnects? And here's the strange thing, after dying 3 times, the most it's ever done, I hooked everything back up and now it runs perfect! The tach needle is rock steady with absolutely no surging or dipping. Could I have a loose ground somewhere? Could the ECU be failing? I'm lost.

Funky wiring, maybe ? I dunno, I've never even seen a Regina system, much less worked on one. All California Volvos were Bosch.

re your voltage readings, yeah, most of those systems, one pin is hooked up to power, and the other pin is intermittently grounded to actuate whatever it is. this 'grounding' is via an NPN power transistor in the ECU, a circuit commonly known as 'open collector' in electronics.

So, if it happens again, the obvious thing to do would be to first check the connector for ground. And, secondly, this is a wild guess, but I believe Bosch and Regina IAC's work the same, so on a Bosch ECU is there a pin I can check to see if there's a ground there? If I'm getting ground at the ECU and not the connector then, yes, my wiring is funky. If I'm not getting it at the ECU then I guess it's funky.

on the 1991 Regina 740/940, it looks like pin 1 of the IAC gets power from the fuel system relay pin 87/1, and IAC pin 3 goes to ECU pin 33 to be intermittently grounded (via said NPN open collector inside hte ECU).

the IAC power circuit goes through connector D pin 3 (green or blue/yellow), and the ground circuit goes through connector D pin 1 (red/black)... this connector D is one of two 8 pin connectors through the firewall behind the right wheel housing/strut tower.

note, btw, the IAC is normally not switched full on/open, rather the ground signal to it is pulsed by the ECU with a duty cycle. more ground time == more air, less ground time == less air. 50% pulses would result in it being about half open.