Oxygen sensor? --or what?

Hello all--

Today I was driving my 535i on the highway, about 70 mph (with my foot VERY light on the gas) when I noticed the "check engine" light come on briefly--it went off when I lifted my right foot. I'd never seen my car do this in normal driving before. I also felt a subtle loss in power about the same time, although I was not firewalling the car, so it was hard to be sure.

I exited the freeway shortly thereafter and, as I continued the 15 or so minutes to my house, the check engine light would come on at will--first only when my foot was on the gas, and eventually, whenever. The car appeared to drive normally during this time, accelerating fine under part throttle and cruising briefly at 65 mph. I did not have the occasion (or the will) to floor the throttle though.

As I got close to my house, the check engine light stayed on consistently, the car idled a bit lower than usual (though still smoothly), and felt like it had about half the power it normally does. In other words--when I'm normally accelerating from a stop, I only press the gas halfway down--and if I do press the gas harder than that at any time, I can feel a lot of extra power come on. This past night however, when I pressed the gas halfway, I got the normal power for that increment, but if I pressed it farther, there was no EXTRA power.

Perhaps superflously, I ALWAYS put 93 octane gas (Shell or Chevron) in my car, and I just recently had the fuel injection system cleaned, so I know the issue is not bad gas or clogged fuel injectors, etc.

I'm thinking O2 sensors, or something else? I'd be appreciative of any insight you all have.

hey there 'csieminski' ! !, long time no hear ! ! I'd check the intake boot between the air flow meter and the throttle housing first, your problem 'sounds' like an air leak, which could account for the light throttle 'ENGINE' lamp symptom.

best regards.............................jeff b.

Thanks for your reply, jb--yes, it has been a while, but you can count on a problem to...

I drove the car about an hour to my mechanic, who is pretty knowledgeable about BMWs. The check engine light was coming on from time to time, and the car was noticeably lacking in power with the gas pressed hard, but I was able to cruise smoothly at 75-80 with the engine running fine. The following day I received a call from him, saying that, yes, the O2 sensor is bad, and had been replaced. However, I was told that the car was now running very poorly and he had already replaced the spark plugs to see if that would remedy the issue. I got another call about 3 hours later, saying that no, the car was still acting up and it probably needed a complete tune-up--I think that meant replacing the rotor and some wires, if that makes sense. I might add that this was just as I was about to take an exam, and I told his secretary if that would fix the alleged problem, go ahead and do it. I got a call today saying that the car is finished and now it runs great.

It's hard to resist feeling that the whole scenario was a little shady. I trust this mechanic (bought my 1986 BMW from him in 2000), but know the car was running great until the sensor went bad last week, and I think if it had needed a tune-up or new spark plugs, I would have noticed long before that. Now I have a bill for $700--haven't picked up the car yet.

Ouch..............hummmmmm, I'd be suprised if you'd allow the sparkplugs to go so long that they caused a running problem, on your model they rarely last 30K, I replace them every 15K for that reason. The cap & rotor are a likely suspect, tracking and cracking can be hard to diagnose, carbon tracking especially but in all my years, I've yet to see a set of factory secondary wires fail unless due to an engine fire [750iL excuded]. The O2 sensor may have 'fouled' due to the raw fuel caused by the mis-firing, obvisously wasn't causing the problem and may have cleaned-up after the correct repair..........CODES don't mean replace the noted component, only that there's some problem related to it and possibly it's failed.

best to you and stay in touch.......................jeff b.;

I actually feel much better now--both after seeing the repair invoice, and after reading your post. Here goes:

Oxygen sensor: $137.08
Spark plugs (6): $25.32
Spark plug wire set: $182.82
Ignition rotor: $30.33
Distributor cap: $104.39

Labor for O2 sensor was $117, and $62 for the tune up.

I bought my car with 129,700 miles on it last April--now it has 160,700. So it went AT LEAST 30K with the same plugs--I did not know they had to be replaced so often, and will remember that in the future :rolleyes: . I have to stress that the car was running GREAT before the check engine light came on, still ran smoothly (though with a lot less power) after it illuminated, and runs fantastically now, though not noticeably better than at any previous time. At any rate (even though I'm out all this money), I'm extremely happy that I've taken care of a major service that was apparently overdue. I also don't feel ripped off any more, since my mechanic replaced most all the components you just listed.

As an aside, my radiator has been leaking and my mechanic urged me to get a new one as soon as possible (yes, I keep a constant eye on the coolant level). I'm liable to buy an aftermarket radiator (with lifetime warranty) and save myself some $$ compared to the BMW Behr radiators which I hear are cheaply made anyway--what do you think about this?

Anyway, thanks SO MUCH for your comments--it's good to get a second opinion from someone who is so knowledgable.

I'm sorry to say that I to now avoid Behr or the other plastic tank radiators. Behr has had a corporate shake-up or something, and now appears driven to take profit to the max. there was a time when the upper hose connection was sleeved with a metal bushing that prevented the beakage that's now so common. many of their radiators are now made in South Africa and appear to be cheap copies poorly molding and finished.

Go AFTERMARKET for your radiator with metal tanks or a Lifetime Warranty........................jeff b.

Thanks, that's what I'll do. The car is running great--in the meantime I'll have to keep an eye on the coolant level to keep things that way.

Only thing is, today it was 76 degrees so I turned on the A/C that worked so well last summer (I used it once like 1 month ago and it was fine). Now--nothing. Compressor can be felt to come on like normal, but I just get dogbreath air out of the vents, no matter how long I drive the car. My Acura Legend apparently had a leak just before I got rid of it, and the [different] mechanic said I'd need to do an R12 to R134a conversion, plus he'd need to find the leak, etc. Does no coldness always indicate a leak, or could my A/C possibly just need to be recharged (I assume with R12 if that's still available)? I have almost no money now anyway, so it might just be a hot summer :grinno: .

Thanks again for your earlier comments, as always.