1987 420 SEC idling/revving problem
 

My 1987 420 SEC has an intermittent revving/idling problem. It starts first time, but immediately revs to between 1500 rpm and above, before falling fairly slowly and erratically to about 500 RPM. (My old 300SE would go to about 800 RPM then drop smoothly and quickly to 500 RPM). Also, when doing about 30-40 mph in town, the revs can suddenly drop slightly.
The car has only done about 300 miles in the last year as it has been on a garage forecourt before I bought it recently. The dealer also allowed the fuel tank to run dry.
The fuel consumption does not appear to be too bad, and this problem does not seem to occur at high revs on the motorway(Freeway). The car has done 84,000 miles and was last serviced about 18 months ago at 82,000 miles.
Can anyone suggest any possible solutions to this problem?
Thanks in advance for any assistance.

Hi johnnyhughes,

If you do a search on rough or high idle, you will find more than you want to know about it.
My first thought would be to clean the idle control valve. Also worth a search.
Second would be a fuel injection cleaner in a tank of gas and give it a good work-out.
The slight drop when driving could be the AC kicking in at that time. The system is supposed to correct for that, but when a car sits idle (no pun intended), rubber parts dry out and thinks get stuck.

I would change all fluids also and thus make sure all the bases are covered.

Enjoy the car.

Reinhard Kreutzer

The car has new plugs fitted, and the leads, distributor cap, and rotor arm have been checked and are all ok. The fuel tank has had two cans of injector cleaner added to it as well. Also the fuel filter has been replaced. The idle control valve has been cleaned and I have replaced the OVP.
The car runs very well, apart from difficult hot starts. After the car has warmed up, it is difficult to restart the car after between 15 and 45 minutes. The Car cranks for about 15 seconds before starting, and then idles at about 1500 RPM, until it falls to about 600 RPM. During the time that the RPM takes to fall to 600, the car hesitates slightly from time to time.
The car nearly allways starts first time from cold, whatever the weather or temperature. Any suggestions regarding which part(s) need checking/replacing?
Thanks in advance,

Remove your air cleaner assembly. Walk over to the passenger side of the engine compartment. Look down at the valve cover on the passenger side. There is a small plastic tube that bridges two rubber connectors. Make sure that it is connected between the two boots. I had the same problem on my 86 420 SEL. Hopefully this is your solution.

John I feel your pain. My 88 sel does the same exact thing. I have read and read and get many different solutions to this problem. Fuel accumulator--fuel pump check valve--Ovp--idle control valve(which I have cleaned)--air flow meter($635)--fuel relay--fuel injectors---these parts are too expensive to guess the problem. Let me know if you find a solution. Mine will stall sometimes when slowing to a stop. Dennis