560 SEL Suspension / Engine

[Columbo]

Hi there I bought a Merc 560 SEL about 6 months ago, it's a great car, I had a few issues with the self levelling suspension (needed new spheres due to a very harsh ride). Since the spheres were replaced the car feels good over bumps, but is quite noisy over potholes at the front and rear of the car, I am not sure what is causing this, could it be rubber bushes?
Also when idling the car seems to almost not fire on one cylinder, my mechanic checked compression on the valves and said everything was fine, I replaced the HT leads but it only seems to be a problem when idling, not when the revs start to rise. Could this be fuel injectors?

 

[guydewdney]

could be fuel injectors - could be bad spark plug,

try idlign the engine and listening for the missfire - pull off one lead (either with very insulated pliers or stop engine first - does the note change? yes? then its not that one - no - then it is that one... first chage the plug (cheap and easy) then pull the injector and check it / swap it for another one and see if the missfire moves...

just an idea...

 

[Columbo]

Thanks for the reply, the mechanic I go to did change the spark plugs and funny enough he said it was running smooth, but after running for a few minutes the car resorted back to running a bit rough at idle (i.e. the car seems to shake a lot and the revs fluctuate a bit).
He did loosen off each injector while the engine was running in turn, but it was very difficult to notice that any one injector was causing the fault - would anything else cause this problem? Or is it most likely still the injectors?

 

[guydewdney]

bad dizzy / rotor arm? weak spark? bad coil? bad idle valve (if it has one)?

new plugs can cover up a weak ign system..

 

[Columbo]

Thanks for the reply, are there any other things I could do to test these myself? The distributor looks ok as theres no corrosion on the contacts or the arm, I did spray a bit of WD40 in there just to make sure theres no moisture but it didnt make a difference. Also a while back I changed the OVP Relay thinking it could be a voltage problem, but again it didnt make a difference. Would it be an air leak? Or perhaps not as it only seems at idle to be a problem.

 

[guydewdney]

low or high fuel pressure? but you need a tester for that...

 

[Columbo]

I can get that tested, the fuel pump is quite noisy (I gather thats what buzzing noise from the rear end of the car is) but I thought it may be normal on these as my old BMW M535i was the same, I did have the fuel filter changed just in case.

 

[guydewdney]

im more thinking the fuel pressure regulator - but a noisy 20 yr old pump wont help...

 

[Mikesmerc]

Does it accelerate smoothly from start when accelerator is pressed quickly ? I have a slightly lumpy idle at the moment and a hessitant acceleration, amongst other problems. Idles fine though.
Just curious, sorry I cant help.
560SEC.
Regards
Mike

 

[Columbo]

Yes the accelleration seems fine actually, which is making me think perhaps it is fuel injectors, as they have both an idle jet and running jet (not sure if this is the correct term!) I have tried this injector cleaner additive, but never had much luck with it really - seems to be a bit of a placebo - makes you feel better but doesnt affect the car. It wasnt leads as my mechanic tested each in turn by removing them when the engine was running and there was no difference. He also loosened each injector in turn but was very hard to notice any difference. The distributor cap I would have thought is ok if the car is running fine at driving revs (anything above idle) also had a look and it seems to be fine, no moisture or corrosion. I haven't yet tried tearing my hair out, perhaps that will have a positive effect!