Posted by Walt Barie on August 25, 1998 at 21:13:30:
In Reply to: rough idle... posted by Chandler Ogden on August 24, 1998 at 18:05:10:
: I am looking for a little help in troubleshooting my '81 528i. I gather from some of the historical posts on the E12 that they are prone to idle rough, but I am looking for a order-of-priority list of what some of you folks with extensive motor tuning experience would do to try and smooth things down....
Ahhhhhh, the joys of the 528i rough idle returning to the internet. Some may remember my year long battle with my 528i a few months ago. I will summarize what I learned from my car:
1. Check for vacuum leaks. This means really checking carefully and this means everywhere. Bad places for my car were the little oil breather hose from the valve cover to the throttle body. Also some of my cars vacuum hoses looked fine but they didn't fit tight anymore since they had hardened up. Also check things like the hoses to the char. canister, hoses back to the gas tank etc. Also check all the air intake bellows. One last place to check for leaks are the injector seals.
2. Make sure your car is perfectly tuned up. This means normal (i.e. not Bosch Platinum plugs which are just band-aids for other problems and are of questionable quality) gapped at 0.030. A matched cap and rotor (i.e all one brand preferably Bosch, don't ask me why I know this can be a problem) The valves properly set. This also means the timing set to spec (or a degree or so advanced:). The base idle mixture set correctly and a working O2 sensor.
Note: I have personally had problems with non-Bosch wires on 6 cylinder BMWs because the runs are pretty long and for some reason BMW has them running tightly together in parallel. In particular most of my idle problem was due to the 9 month old Beru wires that I bought from a Roundel advertiser. The other thing is that some people try to gap their plugs wider for so called more power. This does work in some cars but I found that in my 528i that it seemed to raise the firing voltage alot (looked at it on a Sun engine analyzer) so that with the long plug wires in parallel and big gap that the car actually ran like crap.
3. Make sure normal items like the air filter and fuel filter are replaced regularly.
Now my car still idles a little rough but it is a decent rough idle that doesn't bother me. I really don't think that anyone will ever get a stock 528i to idle perfectly smooth. I have heard that the infamous idle results from the 2.8L using the same cam as bigger 3.0 and 3.3 L engines (although I always thought that the part numbers were different??) I think that normal wear on the distributor shaft and the cam chain (wear that I'm sure most of our cars have) also help cause some of the rough idle. One E12 owner on the list said that he fixed his rough idle by replacing all the injectors and seals. I wouldn't doubt that it would help. I have also played around with my airflow meter because the resistor track was very worn. I very carefully moved the sliding contact onto an unworn part of the resistor (do this at your own risk since it is very easy to screw up the meter or mess up the spring force on the slider thereby wearing a deep gouge on the resistor). It didn't help my idle but it seemed to help the midrange dip that I had. Anyway I'm sure other people will also have their ideas.
Walt Barie
'80 528i