Questions and Answers
BMW Knocked Up
Q. I own a 1989 BMW 525. I noticed a knocking noise coming from the engine about two months ago. I have brought it to the shop a few times but both times they were not able to fix it and had a hard time identifying the problem.
I can hear the knocking sound when I accelerate/idle between 2500 to 4500 rpm first to third gear. Fourth and fifth gear the car runs great, very smooth and quiet like it should be.
Since the shop was having a problem finding the problem and I was running low on $$$, I decided to investigate myself.
I disconnected each spark plug wires one at a time and found that when wire #2 was disconnected the knocking noise disappears and the car runs very smooth even at high rpm. Now what confuses me is when all six wires are back in place and I disconnect the airflow meter wire the noise also disappears.
I took it to a friend mechanic and had a tune-up done hoping that might fix the problem. wires, cap & rotor, plugs, oxygen sensor were replaced but problem still exists. Compression ratio checked out OK. He thinks that it could be a bad injector or airflow meter. He told me that the knocking sound its making is not caused by a rod knock, but by fuel exploding rather than burning.
I took it back to the shop and they told me it was a rod knock and would cost me lots of $$$$. I'm afraid that the shop is just trying to milk money out of me because this is the third time I have been there for the same problem and they keep charging me for something else that they fixed other than the problem I went there for.
Should I go back to my friend and have him check the injectors and the airflow meter or do you think this is a rod knock and I should take it back to the shop????? What do you think is causing this knock?
Please help me
JimmyA. I can give you some places to look for the problem. The first would be the ignition coils.
BMW issued a TSB on a problem with the same symptoms as you are experiencing. I quote from the bulletin:
Situation:
A complaint of rough idle and/or a misfire under heavy engine load may be received on E36 and E34 (M50 equipped) vehicles due to a break on the secondary side of the ignition coils. These coils were produced by the vendor named "May und Christ".Correction:
If a vehicle is encountered with the above complaint proceed as follows:
- With the cylinder head cover removed (exposing the tops of the ignition coils) identify the vendor:
Vendor Identification
"May und Christ" ZUNDSPULE
Bosch Bosch- Identify the production year and month of the "May und Christ" coils located directly above the word ZUNDSPULE, i.e., 91 M 06 (year = 91, month = 06 (June)). Coils with a production date code prior to 91 M 01 (January 1991) may have a break on the secondary side which can be diagnosed by removing the ignition coil from the engine.
- With the coil removed, twist and pull the spark plug connector off the coil and examine the connection.
A poor connection may be seen as green corrosion or (a) piece(s) of plastic found in the ignition coil socket which has broken off the spark plug connector. In this case the coil must be replaced.
The other is a bad rod bearing. When you unplug the spark plug wire you take the load of that cylinder. if the knocking noise goes away, then it's a pretty good indication that the bearing is bad. At this point the next thing to do is to remove the oil pan and check them. If the damage is not too severe, you can replace the connecting rod bearings without too much difficulty.
Additional Information provided courtesy of ALLDATA
