I have a 84 380SL that idles rough. It was sitting for 3 years, I got it out changed the battery and oil, filled it up and have been driving it every weekend. It feels like it has a mis in it when in drive. I have replaced the spark plugs, wires, cap rotor. The right side camshaft and timing chain have been replaced. The idle is 500RPMs in drive and just doesnt seem as smooth as it used to be. Also my mechanic said the frequency valve is seeping fuel, is this related? I cant find any vacuum leaks. I dont know what to check next.
Look closely at the injectors and you will see the little cup that holds them. Get the engine hot, and let it idle in D, block the wheels for safety. Get a can of carb cleaner with the tube that goes in the nozzle (like Valvoline). Spray some cleaner around the inside of each of the cups and see if the idle smoothes out. If it does, you have found a bad injector seal.
If that doesn't show anything, get some clean shop towels and, with the car idling in gear, go around the fuel distributor, bleeding off each cylinder. If the idle gets rougher, that cylinder is OK. If it does not change, replace that fuel injector. If that does not fix it, you probably (yuck) need a fuel distributor.
Personally, I would suggest replacing all the injector seals on principal. They probably dried out while the car was sitting. They are cheap and a very satisfying DIY repair. Since you have the injectors out while doing this, you may be able to find someone to clean and test them very inexpensively.
Before I popped for a fuel dis, I would run two tanks of gas with 2 bottles of Techron in each tank.
Good luck!
Chuck in Falls Church 2 1984 280SL's 1 1987 560SL 1 1973 280C
The Fuel Distributor was replaced a few years back. Also I sprayed carb cleaner all over the engine to try and locate vacuum leaks, I had a few, and this helped. While I did not spray the injectors good, I will go do that tomorrow. I drove the car again, and it was a bit smoother after an hour, it did not miss. Also should I change the fuel filter, it was changed in 99, but only 10,000 miles on it, and imagine its fine, the car has alot of power thru the RPM range, it picks up better then any other 380 I have driven. I ordered a new freqency valve because it is leaking, what exactly does this thing do? Any idea where I can get the hose for it? Or will a high pressure fuel hose cut to fit work? I have used 2 tanks of gas, but only 2 bottles of techron, I remember in the past this stuff really helped, I will put more in.
I sprayed carb cleaner all over the injectors. The idle did not change at all. I did this several times.
I bled out the fuel distributor and this made the idle get rough on each cylinder.
I also noticed the frequency valve changes in its buzzing, it would slow down and the car would drop a hundred RPMs, then it would buzz faster and the idle would speed up.
Nice wrench! Lots of people would have gone to a shop and spent big bucks on a set of injectors and maybe a FD.
To answer your earlier question - the frequency valve is "trimming" your fuel mixture and the change in buzzing is occurring as it either leans or enriches the mixture. There is a computer in the pass footwell monitoring the voltage from the O2 sensor that controls the valve.
What is probably happening is that your mix is on the lean side, and this causes the idle to drop a bit. Then the computer picks this up from the o2 sensor and richens the mix. RPMs pick up, and the o2 sensor sends a "rich" reading, computer goes lean, idle drops and so on.
The fix is to install a new o2 sensor. Get a generic Bosch S001 for $20 and splice the wire. Very easy. Then get the mixture right by following my instructions in this thread:
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