Hi, I've got a 1990 240 wagon with a 1993 powertrain.

When I start the car it stumbles and stalls on the first attempt (hot/cold, doesn't seem to matter). If I try to feather the gas pedal or even floor it the engine just stalls right out, doesn't even attempt to rev up. So I simply let it die and then start it again, the second time it stays running 9 times out of 10.

While driving down the road if I'm not giving it any gas or just lightly pressing the pedal, I can feel the car surging forward and then losing power and slowing and then surging forward again. If I put the pedal down the car performs just fine.

I pulled a code yesterday, 2-3-1 "Fuel system compensating for rich or lean air/fuel mixture at cruise".

I’ve changed the Idle Air Controller and the Control Pressure Regulator simply because I had spares. I removed and cleaned the throttle body. I checked for leaks in the clean air tube and checked all the connections. I haven’t found any leaks and nothing I’ve done has made any improvements.

Unfortunately, I'm a bit stumped where to go from here. If the MAFS, temp sensor, or oxygen sensor was faulty I should get a code for that. What else is there that controls the air/fuel mixture?

I noticed that my gas mileage has dropped significantly from about 320 miles per tank down to 260 so my guess is that it’s running rich.

Any ideas?

Well, it seems I'm onto something.

I unplugged the oxygen sensor, reset codes, drove the car for a bit with the sensor unplugged and it's driving good. No codes, no stumble, no stall after start.

Now the next question is this; I've also got a "Knock sensor signal missing" code that's been there for awhile. Is it possible that this caused the oxygen sensor to burn out?

I think you've pushed the ECU into a default mode which masks the original problem.

I suggest re-connecting the O2 sensor and re-setting the OBD system. The code it threw is common to a vacuum leak, as are the symptoms you reported in your first post. Take a slow and careful look at all your vacuum lines, and completely remove the inlet snorkel between AMM and throttle body to inspect it for leaks. The most common place for leaks are on the underside (of course - you can't see them easily) in the cannelures. If you do find a leak, duct tape is the band-aid solution but you will need to replace it.

When you re-assemble the inlet tract, make sure all the slip joints are correctly connected and their clamps tightened. Then take a test drive (or a few) and see if the fault code reappears.

The important thing is to get back the the original baseline in order to get a correct diagnosis.

Take 2:

I re-read your initial post, and the dramatically reduced fuel economy suggests running too rich, but there will be obvious signs - lots of soft sooty deposits on your tailpipe, and sooty tips on your spark plugs. Pull the latter to check.

There is a relatively rare but documented situation where an exhaust leak in the downpipe upstream of the oxygen sensor resulted in fresh air being drawn INTO the exhaust stream (counter-intuitive, as the exhaust stream is higher than ambient pressure). The resulting dilution of the exhaust gas resulted in the O2 sensor indicating a lean mixture and the ECU drove the mixture richer to compensate.

So among your basic checks, do pull the plugs to see if they indicate lean or rich running, and that should give you a direction for further diagnostics.

Thanks Smitty, I'll check for leaks again.

I've checked very closely but just to ensure it, I'll do the soap bubble test.

My guess is that the missing knock sensor signal caused the engine to run rich enough to burn the oxygen sensor.

The knock sensor is new and the mounting surface was cleaned to a mirror finish and grease was used to install it and it was torqued down. Still getting a code so according to Bentley I need a new ICU.

I will be curious to hear what you find....I have the same codes. I have an exhaust leak in my head pipe that must be messing with the o2 sensor.... I will be fixing it up today to see if that is the source of my vacuum leak(231), but I also have the knock sensor code. Let us know of your progress.
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92 245, 1965 122 Wagon, past:95 850 Turbo, 92 245, 88 245, 82 245 Turbo, 71 164e

Alright, it looks like I have some kind of electrical gremlins going on.

As it turns out, my codes are gone and the car is running great. All I did was clean the mounting surface for the knock sensor and disconnect the battery for a few minutes.

Not only are the codes gone but the car is running good, almost perfect. There is still some strange idling going on. I checked the codes and found 1-3-3, throttle switch shorted to ground. Bizarre.

Lean/rich mixture code, O2 sensor code, knock sensor code, now the throttle switch?

I checked and cleaned the engine ground. Didn't seem to have any effect.

The throttle switch is adjusted just right; a click just off idle and resets on it's own. I have a spare so I could throw it on but I'm having a hard time trusting what the OBD is telling me at this point.

The funny thing is though, the car is actually dispalying different symptoms than it was previously. It now idles high as if it is cold, it will drop the idle if I blip the throttle but then I put it in gear, the idle drops and then the engine rev's up a little to compensate. Then when I put it back in park or neutral, it goes back to idling too high and I blip the throttle again. Sometimes the idle is erradic, moving up and down a few hundred rpm. Maybe something really is going on with the throttle switch.

I'll keep you posted.

And BTW, I didn't find any holes in the exhaust.

I better get down there and check my exhaust pipe then.

I've just been driving around with it as it is.

It's not great but it runs. I'm a bit confused by the knock sensor. Supposedly if the computer says it's bad, then it's bad. But I replaced it and the computer still says it's bad. When I first replaced it the car ran good for a few minutes, even above the 3000rpm knock sensor threshold. Then the check engine light tripped, timing retarded, and back to the way it was. Strange.

Did you have a check engine light on for awhile before getting the 2-3-1 code? The CEL is tripped by a 1-4-3 but not a 2-3-1.

This is a bizarre ending to this story.

So I put an IAC in and it fixed my issues (stalling @ start and hesitation at cruise).

And then a week later I went on vacation and all of a sudden, the stalling and hesitation are back.

I shut the car off at one point and it would not start again. I was able to start it on starter fluid but it died immediately. I jumpered the fuel pump under the dash (red to red/yellow) and couldn't hear anything coming from the main fuel pump.

I ordered the part from a local shop for $169.61 and received a Delphi (great:P) fuel pump the next day. I installed the pump and a fuel filter (couldn't get the check valve) and voila, runs like a champ.

I've never seen a fuel pump fail so slowly like that, it took weeks of nearly daily driving. So I wanted to share my story; hot start problems, hesitation at cruising speed, and stalling right after starting were the symptoms.

I hope that helps someone.