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Reviving 81 528, runs, but no power, Any ideas?
Posted: Sat Aug 01, 2026 11:42 am
by SP250rich
Hi folks, I have a 81 528i manual, CA car (Completely stock) that ran great a year ago. It was reclaimed with the help of 2 other 528i donors and G-son's help. Now with next G-son, and here is the problem for you "detectives". It starts, runs and idles smoothly, but not much power. When we drive off, it has about 20% of normal power, but still runs smoothly. Data: Timing set to spec (mark at 2200 rpm), good compression (160, 159, 154, 160, 157, 142), good fuel rail press (31 to 39 psi), new fuel filter. Eng vacuum (idle =15 inHg), but goes to zero with full throttle (and very little power up a hill) Free revs to 4500 and still smooth. Cat replaced 3 yrs ago; muffler stock. Running out of ideas. Help!!
Rich in Moraga, CA. Near San Francisco
Re: Reviving 81 528, runs, but no power, Any ideas?
Posted: Mon Aug 03, 2026 2:09 am
by tlake
SP250rich wrote: Sat Aug 01, 2026 11:42 am
Hi folks, I have a 81 528i manual, CA car (Completely stock) that ran great a year ago. It was reclaimed with the help of 2 other 528i donors and G-son's help. Now with next G-son, and here is the problem for you "detectives". It starts, runs and idles smoothly, but not much power. When we drive off, it has about 20% of normal power, but still runs smoothly. Data: Timing set to spec (mark at 2200 rpm), good compression (160, 159, 154, 160, 157, 142), good fuel rail press (31 to 39 psi), new fuel filter. Eng vacuum (idle =15 inHg), but goes to zero with full throttle (and very little power up a hill) Free revs to 4500 and still smooth. Cat replaced 3 yrs ago; muffler stock. Running out of ideas. Help!!
Rich in Moraga, CA. Near San Francisco
Perhaps the AFM flap (vanemeter) is sticky
Re: Reviving 81 528, runs, but no power, Any ideas?
Posted: Mon Aug 03, 2026 10:43 am
by Mike W.
Could be the cat. They can plug up and get restricted causing a lack of power.
Re: Reviving 81 528, runs, but no power, Any ideas?
Posted: Tue Aug 04, 2026 3:27 pm
by SP250rich
Thanks for the hints, so far.
Have checked the AFM. Flap moves smoothly. Smooth/even driving response, just no power.
Cat. Is there a way to check this? What should the back pressure be? Assuming I can use the O2 sensor port to add a pressure gauge. Or is there a simpler approach?
Rich
Re: Reviving 81 528, runs, but no power, Any ideas?
Posted: Sat Aug 08, 2026 11:21 am
by Mike W.
Actually just removing the O2 sensor is supposed to bring a noticeable improvement. When mine was plugged, it was restricted with low power, but fully driveable, I could even tow my little trailer, so it was hard to get a conclusive back pressure reading. Search the 'net, there's plenty on it.
Re: Reviving 81 528, runs, but no power, Any ideas?
Posted: Wed Aug 19, 2026 11:57 am
by tlake
Mike W. wrote: Sat Aug 08, 2026 11:21 am
Actually just removing the O2 sensor is supposed to bring a noticeable improvement. When mine was plugged, it was restricted with low power, but fully driveable, I could even tow my little trailer, so it was hard to get a conclusive back pressure reading. Search the 'net, there's plenty on it.
This reminds me of people back in the day putting in a hollow insert into the exhaust system, to see if the cat was bad, and just leaving it like that
The thermal reactor system brought its own problems, but given how early cats and the subsequent oxygen sensors could give trouble, heck, someone told me they have to change theirs on an e83 X3, you can understand why BMW went thermal reactor, until something better came along. Any pollution control equipment, eventually, brings its own problems.
