Okay, so I just did a head gasket job on my 1990 240DL and felt pretty proud of myself, thanks for all your help. Then a week later on a Saturday morning, the good Mrs. was about to go grocery shopping, but the idle was rough and surging low (1 second normal, 2 seconds almost dying, etc) so I looked up the wisdom of the InterWebs, whose oracle told me the surging was the fault of the IAC valve. I pulled the IAC and cleaned up what I could, then tested it with a couple wires to the battery. It wasn't sticking, so I put everything back and did a test drive. After about 10 miles, it evened out a bit, but still wasn't perfect.
Back at home doing some troubleshooting, it occurred to me to pull the plug off the MAF while the car was running to see if the idle got better. It did for a second, then went back to a weak idle, but pluggin the MAF back in didn't help. I read later that I probably shouldn't have done that, and probably triggered the Check Engine Light that I noticed shortly after. At that point I decided to pull the engine codes to see what could be found there.
311-121-221.
Consulting the InterWebs again, I soon learned that the 121 and 221 and a few others were the classic signs of a failed MAF and IAC, along with that 311 code everyone always said to ignore. I bit the bullet and ordered a Crank Position Sensor, new MAF, and new IAC valve. I just put the new parts in, double checked my work, cleared the engine codes and took the car around the block for a spin. It idled kinda rough at first, but I noticed it has a bit of "pep" that it didn't have before. After about 5 or 10 minutes of driving, the idle at stop signs evened out so I took her back home, but the idle at park was high again, and the CEL never went out. I pulled the codes again to see what it was this time:
121-221-231.
What the heck? I just got done putting $300+ worth of the appropriate parts, and it says pretty much the same thing? 121 says the MAF is bad, but this is a brand-new OEM Volvo MAF, do they ever come out of the factory bad? OK, the 221 and 321 both point to running lean, which means vacuum leak, right? The big accordion hose coming off the MAF I checked over while I had it out, but I didn't notice any cracks or pinholes. All the other hoses look good, but I have the Bentley manual so I'll check all the vacuum hoses and go from there.
In the meantime, anybody have any ideas?

 

as you just learned, throwing parts at a problem is expensive. before replacing parts, verify that the part has failed, ie, by doing voltage, resistance, and continuity tests using your multimeter.

a CEL code is a place to start diagnosing, not to condemn the part as failed.

perhaps more importantly, did you clear the codes prior to replacing the parts? and after?

 

Clear the codes by disconnecting the battery for a few minutes, see what happens.

 

on LH 2.4, codes can be cleared by holding the button down for like 10 seconds *after* the code is displayed.

 

which button?

 

the button on the code reader block under the hood, same one you push for 2 or 3 seconds to advance to the next code.

 

Sadly, that feature hardly ever works, the battery disconnect is much better!

 

I justified it by convincing myself that those parts would have failed eventually anyway, but yup, lesson learned.

When I read this comment, I asked myself "But why?" because it seemed to me that the diagnostic thingummy was an output, not an input; it should only report what was happening at the time. Turns out, I was kinda wrong. Read on...

 

"Hmmmm..." I said to myself, "The last time I had the battery disconnected, the engine computer had to re-learn my engine's running conditions,"

Eureka!

 

So before I left work, after reading responses here on my cellphone, I cleared the codes by holding down the diagnostic button for 6 seconds. Started up the car, and the idle was atrocious, surging at half-second intervals. It accelerated OK, so I drove it home (20 minute drive on the freeway) hoping the bugs would work out on the way home. No dice. Still the funny surging as I sat in my driveway, and the CEL was bright as ever. I shut off the car, told the wife to keep dinner warm and grabbed a 10mm wrench. I disconnected the battery and started the long wait. Ate dinner, talked and watched a video with my wife, brewed and drank a nice after-dinner coffee, pet the dog, and re-connected the battery.
Vrooooooommmmmmmmmmmm.... Purring like a kitten and no Check Engine Light taunting me. I coaxed my wife to go for a test drive, and it was smooth sailing. No rough idling at stop signs, no hesitation when accelerating... THIS was a ride.

My humble thanks to all you knowledgeable folks who've contributed your expertise and experience so generously.

OK, so what I learned is, the diagnostic shows the state of the ECU and what it is doing for present conditions as it sees things, which means it saw my new parts as interfering with what it had previously learned. Unplugging the battery re-set it's silly ideas of reality under the hood, so it just ran with what it sensed, which was new parts working together properly. I can't say it would have done the same if I had simply re-set it with the old parts, but I don't care at the moment. It works, and works well, and that is what matters.

Thank you all, again.

 

the learning range in LH2.4 is quite narrow, it should not under any conditions create drivability issues, it simply optimizes how clean the engine burns by better anticipating the lambda (oxygen sensor) cycle.

 

I doubt your troubles are solved. Drive it for a while and most likely the codes will come back.

I don't know who told you to ignore the 311 (Signal missing from speedometer) code as it will affect idling - but mainly while decelerating and coming to a stop. Sometimes it will even stall when coming to a stop. The other two codes do sound related to the AMM/MAF and will likely come back over time unless it only appeared after hot unplugging/plugging the AMM/MAF.

 

Understood. That's probably how I got back & forth to work while I was waiting for my parts. However, disconnecting the battery seems to have cured most of the ills I was experiencing. No more rough idle, no more surging, no hesitation accelerating from a stop, etc.

 

Aha, party pooper, eh? Well, just to make sure everything was kosher, I did pull the engine codes, and I got 311 and 322. It's a brand new Crank Position Sensor, so I don't understand the 311 code unless there's a break in the wire somewhere between the junction at the firewall and the ECU (the insulation had failed badly on the old CPS).
The 322 seems a bit more mysterious. "Air Mass Meter hot wire burn-off function not working". Hmm. I'll double-check the wires and see what's up. I also noticed it idling high when I put it in Park in my driveway today, maybe that's related.

P.S. I have cleaned the Throttle Body, and cleaned it good. I saw daylight through all the pinholes, and got off all the brown gunk that was on the walls.

 

I wouldn't go crazy relying on the OBD2 codes! They are often unreliable or intertwine in mysterious ways, I am not saying to ignore them since there is something wrong when there is a code always popping up, but don't consider them 100% true... I'd say you do have an issue all things considered.
Did you have these issues before the head gasket job?

 

I remember the biggest issues were some minor hesitation at acceleration that it has had since we owned the car (2-1/2 years ago) and a high idle in Park and Neutral that started a few weeks before I did the Head Gasket. At that time, I cleaned the Throttle Body, which stopped the high idle in Park for a week or two. It was a week after the HG job that my wife experienced the surge-sputter-surge-sputter that got me started on this dilemma. I never cleared codes in the Self-Diagnostic system all through that (except for the HG job, because I had to disconnect the battery for that); I never knew it was a thing I should do. I think I remember having the 3-1-1 code here and there before the HG job, but I never experienced the symptoms usually associated with a bad Crank Position Sensor.

Right now, the only observable issue is high idle in Park and Neutral, which started a few weeks before the Head Gasket job. The hesitation at acceleration is completely gone.
I just ran through the Self-Diagnostic routine, and all codes came up as normal.

 

After going through the Self-Diagnostic steps in the Bentley manual, I noticed the code-clearing process is:
1 - Hold the button down for 5+ seconds and release
2 - Wait 3 seconds for the LED to come on
3 - While the LED is on, hold the button down for another 5+ seconds and release; the LED should go off
4 - Hold the button down for 2 seconds and confirm the only code that comes up is 1-1-1

Either way seems to be effective, but I'll do the battery disconnect from now on, just to start with a clean slate after doing repairs. If only it didn't clear the Radio Station Memory every time

 

The 311 code is not related to the CPS. It is thrown when the vehicle speed signal to the ECU is missing. This signal comes from the speedometer. The same signal is used by the cruise control module but through a different wire. Does your vehicle have cruise control and if so, is it functioning? If the cruise control works but you still have this code, then more than likely it is a break in the wire carrying this signal between the speedometer and ECU. If you have the 311 code your cruise control is not working but your speedometer and odometer are working then more than likely you have a fried ITT2115 chip in your speedometer. This chip is easily damaged when people hook up the red/white tachometer wire to the wrong post on back of the instrument cluster.

 

I swear I saw that the Bentley book says a 3-1-1 code is related to the Crank Position Sensor, but I'm not sure now, I can't find where it says that. I know the Bentley manual is not the be-all end-all, but it's never steered me wrong so far. I have seen a couple other people around the internet saying the same thing you are, that a 3-1-1 is from the speedo, so it's good to hear another confirmation. My speedometer does work, but it's always ~5 mph behind (i.e. In order to go 60 mph, I have to go 65 per the speedo), and the cruise control does not work. You may be onto something, I'll order a replacement chip and see how it goes.

 

Engine and OBD Diagnostic Codes says speedo sensor.


are you running nonstandard tires? 5 MPH is well within the acceptible error range. going slower than the speedo indicates is a sign your tire diameter is less than stock.