560SL a little rough cold starting
 

Subject line tells the story. What could be causing the the rough idle upon a cold start? Not only what could it be, but what tests can I run to make sure I get the right results the first try. I really hate to throw parts at a problem hoping that something will eventually fix it. That could get pretty expensive.

Len:

Many things could contribute to this, but one of simplest driveway checks would be to remove a plug wire and see if you can isolate the miss. It may be as simple as a plug fouled from the cold-start enrichment. My SL does this every now and then but clears up right away after it warms up and the plug cleans up.

Have you replaced the plugs recently? Check the plugs and check out your ignition wires too. A bad wire with high resistance may be fine after warm up when the engine runs easiest, but during cold starts that extra resistance can cause a misfire. Also, for whatever reason, a lot of folks have found the NGK plugs to perform better than the Bosch. I suspect they may have a wider temperature operating range.

230/8

Len,

Mine sometimes stumbles when I first start the car for one-two seconds and then it runs very smooth. But it always starts right up...instantly.

I have wondered what it would be like if I just held the key on start until I was sure it passed through the stumbling period....this just goes against my grain, but I should try it.

I just changed the plugs a couple of days ago and used NGK plugs. Can't be sure, but the stumble may be a little worse after the plug change. I would say my stumble lasts for a lot longer than a couple of seconds. I'll start it up this weekend and pay close attention.

Thanks for the replies guys.

BTW, here's a note I got from NHDOC - "I replaced all of my fuel injectors and idle air piping since all of the rubber parts had deteriorated over time and after that the car idles like glass at doesn't race at all like it did when I bought it." Not exactly my issue, but something else to consider.

Unfortunately there is no code to be pulled on these cars, so all you can do is go after the likely culprits. If it is a simple stumble for 1-2 seconds after the engine starts, then it is PROBABLY one or more injectors that have either leaked down or don't spray quite right. The big bucks fix is new injectors and seals. The budget fix is seals and and a couple of cans of BG-44 or Techron in the tank. In the middle is having the injectors cleaned.

I have had better success with the Bosch copper plugs.

BTW - I have an injector tester/cleaner and will do your injectors if you pay the postage and for a can of BG-44 to pump through them. Send me an email if you're interested.

Thanks for the offer Chuck. I ran one bottle of Techron through the system a couple of weeks ago. This is a car that has been used very little over the past fifteen years, so I figured that there was a good chance some things might be gummed up. However, I've been pleasantly surprised to see that so far there haven't been any real issues due to lack of use.

When you do the injectors, do you set the pressures too? I think I recall reading somewhere that the injector pressures are supposed to be within 5psi of each other.

Just started the car. It always starts fine and keeps running, but remains a little rougher than I think it should be. Seems like the rpm's are higher than they should be too, although the tach shows it at 700rpm's.

On another note, when I installed the new plugs, I set them at 0.038" rather than the book number of 0.032".

There is no adjustment to the injectors. There is a fairly complicated procedure to adjust the fuel distributor for each cylinder, though. But that won't help if an injector is leaking down.

My story and sticking to it is Bosch copper at .032.

Mine does the same thing...runs a little rough when first started - but runs GREAT after.....new EHA, Rotor, Cap, Wires, Plugs....... step on the gas...perfect acceleration.......could it be a leaky injector??? 97000 miles......87 560SL.
Car starts right up every time.

Any ideas?