Hi, forum peeps.
I picked up an 85 318i automatic about 6 months ago as a DD/second car/project. I'm trying to figure out a couple mechanical issues before spending money on more fun things like suspension and turbo.
The main issues are intermittent cutting out and a low pitched irregular rumbling sound, (from the rear I think).
For the cutting out issue, it can happen at idle, low speed, or high speed. It can go weeks without doing it or it may happen several times in a day. When it cuts out, the idle acts like it wants to die, but something stops it from dying. In gear, the tach bounces between 400-900. Occasionally it will die, but usually not. So far, it's only died maybe 6-7 times and only when stopped. If it's cutting out and I put it in P or N, the revs go up to around 1500-1600, (up from normally around 1000). Also, when it's cutting out the MPG/fuel economy gauge goes to infinity, (for the duration of the cutout). The longest cutout was about 6-8 seconds on the highway, but it's normally very short and random. Obviously the car is very jerky when driving, especially at highway speeds. The cutouts usually stop on their own. The previous owner was told it was the throttle body. He bought a junkyard throttle body with attached TPS, but only installed the new TPS on the original throttle body. He thought that fixed the issue, but didn't drive it much afterwards and the problem remains.
My research shows it could be almost anything except maybe the steering wheel.
- Fuel tank (rust or water)
- Fuel pumps (or bad electrical connection to in-tank pump)
- Fuel filter (replaced by PO)
- Fuel pressure regulator
- Fuel pump relay or relay socket
- One of the other 2 relays next to the fuel pump relay
- Wiring harness/connection from the 3 relays to ICU/ECU
- Fuel injectors
- ICV (replaced by PO)
- ICU (replaced by PO with a blue colored unit)
- ECU
- Distributor (cap/rotor/plugs/wires freshly replaced)
- Bad ground or electrical gremlin somewhere
- Vacuum leak (most hoses replaced by PO, recently spray tested and doesn't seem like there's a vacuum leak)
- TPS adjustment
- AFM
- RTABs (just kidding)
When changing the distributor cap, I retarded the timing a touch to bring the idle down. After doing that the car was very sluggish. I tried this for a day before changing it back, but I noticed that the cutouts were less severe/drastic. Maybe that's an important clue.
For the rumbling sound, it's hard to describe. It's noticeable to me at 30-40mph and gets a little louder as I go faster. The sound isn't rhythmic and doesn't go faster as the car does. I took a mechanic friend for a drive and it took him a while to hear it and he didn't think it sounded like anything bad, but I can't imagine test driving it in 1985 and thinking it's normal.
Possible issues:
- Bad diff (checked/changed fluid recently)
- Wheel bearings (doesn't appear likely, but I haven't ruled it out yet)
- Bad muffler hanger (one of them is loose and causing some play, but I don't know if the sound would be a low rumble)
- Broken cat internals (don't know if that would make a low rumble)
- Rear suspension or bushings, (PO had some of the front end worked on, but not the rear)
- Center support bearing and/or driveshaft (don't know if that would make a low rumble)
- Something else I haven't thought of
Thanks for reading this far, (unless you skipped to the end). Open to ideas to hopefully narrow this down. I've been a backyard mechanic for a couple years, which means I can follow a DIY and replace parts, but the hard part for me is figuring out what to replace.
I picked up an 85 318i automatic about 6 months ago as a DD/second car/project. I'm trying to figure out a couple mechanical issues before spending money on more fun things like suspension and turbo.
The main issues are intermittent cutting out and a low pitched irregular rumbling sound, (from the rear I think).
For the cutting out issue, it can happen at idle, low speed, or high speed. It can go weeks without doing it or it may happen several times in a day. When it cuts out, the idle acts like it wants to die, but something stops it from dying. In gear, the tach bounces between 400-900. Occasionally it will die, but usually not. So far, it's only died maybe 6-7 times and only when stopped. If it's cutting out and I put it in P or N, the revs go up to around 1500-1600, (up from normally around 1000). Also, when it's cutting out the MPG/fuel economy gauge goes to infinity, (for the duration of the cutout). The longest cutout was about 6-8 seconds on the highway, but it's normally very short and random. Obviously the car is very jerky when driving, especially at highway speeds. The cutouts usually stop on their own. The previous owner was told it was the throttle body. He bought a junkyard throttle body with attached TPS, but only installed the new TPS on the original throttle body. He thought that fixed the issue, but didn't drive it much afterwards and the problem remains.
My research shows it could be almost anything except maybe the steering wheel.
- Fuel tank (rust or water)
- Fuel pumps (or bad electrical connection to in-tank pump)
- Fuel filter (replaced by PO)
- Fuel pressure regulator
- Fuel pump relay or relay socket
- One of the other 2 relays next to the fuel pump relay
- Wiring harness/connection from the 3 relays to ICU/ECU
- Fuel injectors
- ICV (replaced by PO)
- ICU (replaced by PO with a blue colored unit)
- ECU
- Distributor (cap/rotor/plugs/wires freshly replaced)
- Bad ground or electrical gremlin somewhere
- Vacuum leak (most hoses replaced by PO, recently spray tested and doesn't seem like there's a vacuum leak)
- TPS adjustment
- AFM
- RTABs (just kidding)
When changing the distributor cap, I retarded the timing a touch to bring the idle down. After doing that the car was very sluggish. I tried this for a day before changing it back, but I noticed that the cutouts were less severe/drastic. Maybe that's an important clue.
For the rumbling sound, it's hard to describe. It's noticeable to me at 30-40mph and gets a little louder as I go faster. The sound isn't rhythmic and doesn't go faster as the car does. I took a mechanic friend for a drive and it took him a while to hear it and he didn't think it sounded like anything bad, but I can't imagine test driving it in 1985 and thinking it's normal.
Possible issues:
- Bad diff (checked/changed fluid recently)
- Wheel bearings (doesn't appear likely, but I haven't ruled it out yet)
- Bad muffler hanger (one of them is loose and causing some play, but I don't know if the sound would be a low rumble)
- Broken cat internals (don't know if that would make a low rumble)
- Rear suspension or bushings, (PO had some of the front end worked on, but not the rear)
- Center support bearing and/or driveshaft (don't know if that would make a low rumble)
- Something else I haven't thought of
Thanks for reading this far, (unless you skipped to the end). Open to ideas to hopefully narrow this down. I've been a backyard mechanic for a couple years, which means I can follow a DIY and replace parts, but the hard part for me is figuring out what to replace.
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