Help! It won't accelerate!

albalone · 01-27-2005, 11:43 AM

My 1989 325i idles fine, but it won't accelerate if you use more than about 1/2 throttle. If you drive it very gently, it acts fine. But a little more throttle and it acts like it's running on 3 cylinders.
Using Bentley the following items have been tested and appear OK: Speed sensor, temp sensor, throttle switches and all grounds. It has a brand new Bosch air flow sensor and oxygen sensor. I've looked long and hard for a vacuum leak, especially in the boot between the air flow sensor and the throttle but can't find anything wrong. The computer ran fine in another 325i as did the new air flow sensor.
It's nearing time to tow it to the local BMW specialist. I've never been defeated by this car before and my mechanic's ego is taking a serious hit here. Does anyone have any ideas on this one?

makaio · 01-27-2005, 12:37 PM

I'm just shooting from the hip, but have you checked the spark plugs, cap, rotor, plug wires, etc...?

albalone · 01-27-2005, 02:24 PM

Yes, I looked at the some of the ignition. The plugs are pretty new and looked OK except a little sooty from lots of idling during testing. They did all look the same though. I looked at the spark off the coil and it was fine. (Be careful when you do this. I'm an aircraft magneto specialist and applied a standard magneto test - put a lead in the coil and run it to a 1/2 inch gap on a good ground, like the valve cover.) I didn't look at the cap/rotor yet because it's too hard to get at them. I did just look at them about 6 months ago though. Thanks for the ideas.

makaio · 01-28-2005, 04:55 AM

How about the catylitic (sp) converter? Is it the original? If it's plugged up that would cause it to not run good at all.

albalone · 01-28-2005, 10:28 AM

The cat is an interesting possibility. The car has 130 kmi on it and it could be the original. Other mechanics have suggested that too. One interesting thing is the car doesn't run too bad when it's cold. The problem really comes on once it's warmed up. The only time the car ran good warm during all these tests was after I swapped out the O2 sensor for a different used one. So I put in a brand new one and it wouldn't go past 4000rpm or take more than 1/2 throttle. So I put the used one back in and then that one wouldn't run right either. I got to thinking intermittent electrical connector and opened all I could find and blew them out. No luck. If the cat were plugged, would it show up more when warm than cold? How do you test the cat?

makaio · 01-28-2005, 04:49 PM

Yeah it will show up more when warm.

Justr for reference, I had a 85 Ford Ranger once, that while driving on the freeway, would do the same thing. I would give it gas to acelerate past 50 mph, and it was like it was running on 3 or 4 cylinders.

The only way I know to check the cat is to have it removed (or cut it off with sazall) and visually inspect it. If it is defective you will see it plain as day. You should be able to see through the webbing if it is in good shape. If it is in bad shape, the view will be blocked, thus making it hard for the engine to breathe.

jjskibinski · 01-30-2005, 02:13 PM

When was the fuel filter last changed? Went through this problem about 6 months ago with my ' 84 318i. Vacume hoses, ignition system, timing, etc all got a 'makeover'. Nothing, ran (limited acceleration) the same. Changed the fuel filter, now accelerated fine.
Hope this helps.
Jan