W123 280E - Too good to part out

[TonyFromWestOz]

I have just bought a W123 280E as a parts car. It has a little cosmetic rust, good A/C, paint, seats, sheepskins, rubber and all are fine.

Here is my problem:
The 280E engine starts easily, but will not run below around 1500 RPM (by ear) and it misses badly at that speed. I haven't tried it on the road yet (slip into D at 1500 rpm??)
I have searched on the Tech Forum and although I found it has the M110 engine, I haven't found a diagnosis of my problem.
I also read that they have valve guide and head issues and require regular valve adjustments.

If any of you have this engine fitted to any of your cars, or have experience with this model, and can assist me, I am open to suggestions.
I will be towing it home tomorrow night and will be able to test compression, vacuum etc then.
Thanks in Advance.
ps, Photos to come.
pps, also posted in Diesel forum, where I know many members.

[TonyFromWestOz]

I have the car at home. I drove it all of the way and only stalled it once, just checking to see if it would idle at all. Obviously not.
I was able to keep the engine running with judicious throttle application and slipping into neutral while rolling to a halt at traffic lights (not many I know the back roads here well).
While driving at 50 MPH, the engine pulls well (I am used to a non-turbo 300D remember) and tranny kicks down to 3rd on full throttle.
Below 40 MPH, there is an intermittent miss and backfire thru the aircleaner at neutral throttle.
From a standing start, I can have the engine at 1500 RPM (irregular firing) and slip it into D. The engine RPMs drop, but with slightly more throttle application it pulls away well without any obvious miss. I am not talking WOT acceleration.

I tried WOT acceleration as well and the car moved out well, although with that engine I thought I would have some loss of traction.
Tranny shifted well, up and down the gears, although I didn't check 3-2-1 while stopping, as I was in neutral nursing the throttle.

Possibilities:
1. Burnt valve (s) or other compression issues
2. Fuel injection issues - pump, injectors, etc
3. Spark issues - dizzy cap replaced by mobile mechanic for PO with no changes to symptoms. Do these engines have electronic ignition?
4. Anything else??

I will pull the plugs to determine which cylinder is not firing well, and do compression test on each pot while plugs are out.

Can anyone help me with diagnostic tips for the fuel injection?

Thanks,
Tony

[Greg in Oz]

Tony,

A reluctance to idle without working the throttle and backfiring through the intake are classic symptoms of very lean running. This may be because someone has fiddled with the air/fuel adjustment in the top of the fuel distributor although I would perhaps be more inclined to suggest a vacuum leak. A vacuum leak will show up worst under light load and at low speed, especially at small throttle openings. Remember that any such leaks are not metered by the air flow meter (flap) at the fuel distributor. At high speeds and high loads a vacuum leak will be insignificant compared to the large air flow through the air flow meter and the lack of vacuum under high load conditions will also reduce the leak.

There may be a problem with fuel pressure or a fuel blockage however I would expect this to show up worse under load and at speed and you suggest that it seems to perform well under load and at speed. There could also be issues such as burnt valves or low compression however again your description of how it drives would tend to rule this out. I would suggest having a look for vacuum leaks.

Greg

[TonyFromWestOz]

Greg, I have posted this question on the MB NSW Forum too.
Thanks for your reply.

This afternoon, I pulled the aircleaner and checked for vacuum leaks. All vacuum hoses are secure and not showing signs of deterioration. Fuel hoses look good too.
Pulled the spark plugs - all black - indicating excessively rich running. Couldn't find my Compression Gauge - last used in 1990 when I had a SI engine.
Looked ar dizzy. It is set fairly most of the way anticlockwise. Tried 5° clockwise, caused engine to slow down and lose power (retarded?). Reset to original position.
Still missing and backfiring thru inlet manifold at neutral throttle.
Car has a silver box marked Bosch, with wiring going to the engine near the FI system & Dizzy/coil. Is it spark control &/or FI control?
Would it cause this issue?

[TonyFromWestOz]

Anyone have an idea to fix this engine?