Hey guys,

I'm going to try to include as much information as possible here, I've been scratching my head with this car for the past 3 months. My 1989 (non turbo, m46) 240 started idling high about 3 months ago, so I did the standard stuff, replace flame trap, clean throttle body, clean IAC, etc. I got the idle down for a while, but then it started having this weird problem where the idle surges when I'm in neutral, but only if the car is moving. So for example, if I just roll out of my driveway in neutral, the idle will jump to maybe 1600 or so RPM (my tach only works when it wants to), until I bring the car to a full and complete stop. At that point it goes back down to about 850 or so, which I think is somewhat normal. That happens always though, when the car is cold, warm, no matter what. If the car is moving in neutral, the idle is really high.

The problems don't stop there. If I disconnect the battery and allow the ECU to reset, it takes a lot of doing to get the car to idle again. I'll start it, and it'll hunt for maybe 10 seconds and then it dies. If I start it around 12 more times, it will figure out what it needs to do and then it will start to idle. To my ear, the idle is a little high, but I can't say for sure because of the wonky tach. About half of the time I do this (I keep disconnecting the battery to replace parts I think could be the culprit), it will throw a 1-1-3 code. I tested the o2 sensor by monitoring the voltage, and it was swinging from about 0.2v to about 0.7v, which I think is normal. That makes sense, because the sensor is only 5 months old (Bosch OEM style).

Lastly, the car definitely smooths out when warm. First thing in the morning, it will kind of hiccup at low RPM and buck a little. Once warm though, I notice I don't have to give it as much gas from a start to keep it running smoothly. It has never had any problems running at cruising speed, I don't notice any lack of power or anything like that.

Since this problem started, I have done the following:
-clean throttle body
-replace flame trap
-clean IAC
-check for vacuum leaks with carb spray, none found
-replace in-tank fuel pump
-replace engine coolant temp sensor
-tested with a different ECU, no change

Like I said, this one has really got me stumped. Any help is appreciated.

 

There are some ways to test the IAC noted on this forum. Suggest trying that next and might go to a local wrecking yard and pick one up as a spare. I recall doing this and picking one up from pick-n-pull for less than $15. Dont buy a cheap IAC off Amazon or Ebay (lesson learned).

 

normal idle for a fully warmed up engine is more like 700 rpm

cold engine should idle at an elevated 1000 or 1100 rpm for a minute or so til it start to warm up

 

With regard to the 850 idle speed, I can't say for sure what it is. 850 was just a rough guess, my ear isn't tuned well enough to distinguish within 200ish RPM. I did just check out the IAC and when I unplugged it with the car running, nothing seemed to change. Does that mean it's bad? Is there anything else I should do before heading to Pick-n-pull tomorrow to look for a new IAC?

 

the idle air controller is a shuttle valve, apply power and it opens, disconnect power and it closes. on cars that have an adjustment (this is mainly older lh2.2, up to 1988), you get the engine fully warmed up and disconnect the IAC or pinch its hoses closed, then with all accessories off, adjust the idle air bypass screw on the throttle body for 600 RPM idle (which is on the low side), when you re-enable the IAC, the idle should go to 700rpm and stay there if you turn on headlights and/or air conditioning. if you have a digital volt meter with a 'Hz' (frequency) mode, connect it to coil pin 1 (red-white) and ground, and the Hz you read should be RPM divided by 30 (so 600 RPM would read 20Hz, 700 would read 23.3, etc). 30 because there are two sparks per full revolution and 60 revolutions per minute.

 

Thanks for the tip, turns out I'm worse at estimating than I thought. The car is definitely idling high, around 1000 rpm. Also, this time when I disconnected the IAC the idle jumped higher, to around 1150. I don't think my car as an adjustment screw, but as I've shown so far in this thread, I've been wrong before.

 

sounds like the IAC is sticking open.

if you take it out of the car, and 'hotwire' it with 12V, you should SEE the shuttle inside open/close

btw, when you remove it if the hoses are at all hard or cracked, plan on replacing both with new hoses.

 

I really appreciate the insight. I can't tell you how many Volvo problems I've rectified by referencing your posts. I'll head to the junkyard tomorrow and hopefully return with good news.

 

Thank you both so much, it was as simple as a 20 dollar junkyard IAC. The car started right up after reconnecting the battery, and it was idling normally on the test drive. As far as I can tell, everything is back to normal. Now to tackle that intermittent tach...

EDIT: Spoke too soon. Took it out for a second time just to be sure, and all the problems resurfaced. Still idling around 1000 rpm and still doing the idle surge while the car rolls in neutral. Back to square one I guess.

 

is the throttle cable working smoothly so the throttle always returns to the fully closed position and engages the idle position switch ?

 

To my eye, it is. If I open the throttle by hand and let it return, I can hear the TPS (or at least what I assume to be the TPS) click at the same point every time.

I've since discovered two things about this problem. The first is that after disconnecting the battery and restarting the car, it runs almost perfectly for the first drive. Idle is around 700 or so, doesn't do the weird surge. When I let it cool down and then try to start it say a few hours later, the idle jumps back to 1000-1100 and then surges to 1500 any time the wheels are moving. This seems to suggest to me that the ECU is "learning" to create this condition and adjusts the fuel/air accordingly, but I may be way off.

Secondly, I may have discovered the vacuum leak I missed the first time. It's at the flame trap, if I spray the black plastic piece that fits between the "S" shaped hose that leads to the oil separator and the longer hose that feeds back into the air intake snorkel, I get a short surge in idle. My problem is, that those parts are less than a month old and I don't know how I'd go about repairing that leak. I tried refitting them all together, but still have the same problem. At least in my car, there were no hose clamps or anything to ensure a tight fit. They just fit together with pressure, and diagrams online seem to confirm that.

What's so strange about all of this is that surge when the car is moving. I can't for the life of me figure out why that happens. I assumed it would be some sort of electrical problem, as if some sensor somewhere was telling the ECU that the car was in gear or something like that. If it's mechanical, how does the car "know" that it's moving?

EDIT: I just drove the car around the block with the MAF disconnected, following the advice of a Turbobricks member. The idle dropped down to about 800, but it was much rougher and definitely a much stronger smell of exhaust. It also threw a 1-2-1 and a 1-1-3 code after being started with the MAF disconnected. The interesting takeaway was that the surge while moving in neutral was gone with the MAF disconnected. Does that mean it's the culprit?

 

without the MAF connected, its running in 'failsafe limp home mode'. the ECU has to guess how much fuel to inject based on RPM and the O2 sensor and not much more, it runs rich to be on the safe side, as running lean can fry valves and pistons.

if it runs better without the MAF than with it, thats usually a sign the MAF is bad.

 

I remember reading that, but what confuses me is that it seems to be running both better and worse at the same time. Better because the idle is closer to where it should be and the surge is gone, worse because it's pig rich and rough. I'd hate to just throw parts at the car, especially when they can get pricey like the MAF. Unfortunately my local junkyard didn't have one I could pick up on the cheap.

EDIT: Got my hands on a new MAF, and everything seems to be checking out for now. I've driven the car for about 50 miles since the install and all the problems I was having seem to have gone away. Thanks again for your help guys, hopefully this is a real solution and not the same situation that happened with the IAC.