Real Fiero Tech

Here's the situation. 1991 BMW with about 260,000 miles, 2.5l I6, 5-spd manual. Runs good, feels like it still makes every bit of 190hp.

When I first got the car, it stumbled all the time, pretty badly. I replaced the O2 sensor with the cheapest one I could buy, and it helped tremendously, but the car still has a problem. When it is cold, the car runs flawlessly. However after 2-3 minutes of driving following cold start, the car runs like shit at low rpm, stumbles really badly and makes barely enough power to move. Feels like it's on 2, maybe 3 cylinders. Then at 2.5-3,000rpm, it's like a switch is thrown and it runs perfect until I shift again. It's very consistent. Cold start, runs perfect for a couple minutes. Once warm, it runs terrible at low rpm all the time. It's extremely consistent. Always perfect when cold, always bad when warm.

My theory is that in open loop it runs perfect. As soon as the heated O2 sensor causes the ECU to go into closed loop, a sensor that is used then, but not in open loop, is failing. It's a MAF based system.

AFAIK, the only sensor used solely in closed loop is the O2 sensor, am I wrong? I'd replace the O2 sensor, but I did a couple years ago and it it helmed the motor run a whole lot better. So if it fixed some of the engine problems I was having, that indicates it's working. That and unlike typical Fiero owners I don't replace what isn't broken trying to find a problem. And the O2 sensor is expensive and a royal pain in the ass to replace.

Worth noting, the car has the factory exhaust and no exhaust leaks.

Maybe I'll try the Lumina Z34 way of diagnosing it. Unplug one sensor at a time, starting with the easiest to get to, and see if any make no difference once unplugged.

Did you already pull the battery to reset any "learning" the ECU has done?

Negative, I don't want to reset my radio presets. I'll try that this afternoon though. I'll also try unplugging the O2 sensor.

Aaron wrote:Negative, I don't want to reset my radio presets. I'll try that this afternoon though. I'll also try unplugging the O2 sensor.

Unplugging the O2 sensor will just throw it back into open loop..

Which is looking like it'd be a good thing about now since it runs better than way.

I would unplug the O2 sensor and drive it from a cold startup to normal temp to see if it behaves the same or not. It could be the car likes the engine "cold" vs. hot not necessarily open loop vs. closed loop.

Cold start, open loop and Power Enrichment all add more fuel, so you could have a fuel issue as well.

Temperature sensor (DME believes engine is still cold when it's warm)
Exhaust leak
Post-MAF intake/vacuum leak

-could all cause that.

Unplugging the O2 sensor and allowing it to warm up may also be instructive.

I've replaced the intake boot between the MAF and the throttle body on more E30s and E36s than I can count.

Make sure that is totally crack free.

I pulled the coolant temp sensor today, and drove from cold start to operatig temp. It threw a check engine light (Not previously illuminated), and seemed to be down a lot of power, as if it was pulling timing. Once warm it was still bucking/hesitating, as well as being down more power. Next I'm going to plug it back in and unplug the O2 sensor, and check the intake hose.

Thanks so far!

I plugged the CTS back in, SES lamp went off which is good. And I'm not sure how I missed this before, but took a look at the intake elbow that connects the MAF to the TB. I'm thinking Cincinnati wins. The first pic is how much electrical tape was wound around the elbow. I went to the junkyard, and wouldn't you know neither of the 5 series had the elbow still. I'm going to see if I can't find one online, as I think it's beyond repair. Unless someone has some electrical tape they can send me.

Rockauto has a brand new one for $22 shipped, can't complain about that, it's on the way. I think I'll duct tape it for additional longevity.

Just found this on ebay: http://www.ebay.com/itm/13541726634-BMW ... 0787375004

Oop, just saw your post. That's cheaper. Nice!

Hopefully that makes a big difference, we tried electrical taping one up for a friend who was especially broke and while it helped, it never fixed it, until he got a brand new boot.

May not be your only issue, but eliminating a huge post-MAF air leak is probably a good start.

I think it'll help a lot. I drove it after removing a strand or two of tape and it seemed worse, so that's a good sign. And with no SES lamp, I'm hopeful. Thanks again for the idea!

Great suggestion!