91 300SL running rough under certain conditions

[waybomb]

Hello

Recently bought this 91 300SL. It runs great and gets great gas mileage too!

But it has a few issues. In this case, if I drive the car for a while, park it and turn it off, then in a few minutes, start it back up, it stumbles quite a bit for a few minutes.

If I start it from cold, it runs fine, though the idle seems a bit high for the first minute or so. But then it goes down to a normal RPM, about 6-700.

The problem is only after a warm start, and only for a minute or so. All other times it runs grand.

Any ideas on what to check for?

[dkveuro]

Originally Posted by waybomb

Hello


The problem is only after a warm start, and only for a minute or so. All other times it runs grand.

Any ideas on what to check for?

Low fuel pressure too soon after warm shut down.
Fuel pressure accumulator leaking or other leaks in fuel system such as injectors.

[waybomb]

How do I determine if it is the accumulator or the injectors?

[dkveuro]

Originally Posted by waybomb

How do I determine if it is the accumulator or the injectors?

The shop proceedure is to hook up a test gauge to the fuel distributor supply line. Should maintain 30psi down to 15 psi for at least 20 minutes after engine stopped..

With the engine warm, low fuel pressure allows the heat soak to boil off the fuel in the injector lines so that when you try a warm restart, the engine only gets vapor until liquid fuel reaches the injectors, causing a rough/lumpy post start run quality.

Injector weep usually fouls out a spark plug on warm start.
If the accumalator pressure holds during tests then most likely injector(s) leak.
Most EFI injectors of the CIS type open at 56 psi so, when the engine stops the fuel is under pressure prior the injectors, as the line pressure drops to 40 or less immediatly.

If an injector weeps after shut down it floods the cylinder and boils off fuel too. Restart is lumpy until line pressured with liquid fuel again.

[NDavies4]

Did you get anywhere with resolving this? Would be keen to know what the solution was.

The only thing that's different with mine is that I've had the emissions tested both when running smoothly and when idling rough (it almost feels like a series of 'hiccups'). The difference was un-burnt fuel (an increase in the hydro-carbons reading?) noted when running rough.

As I've checked out/replaced everything on the spark side (plugs; leads; rotor arm; dizzy cap; and coil) with no joy, couldn't this point to suffering from a leaking injector or faulty fuel distributor also?

Regards,
Neil

[stevecora46]

I have a 92 300SL, 89K miles that did the same thing. Cold start no problem, drive for a few miles, then it starts cutting out and trying to die. Even left me stranded one morning and had to tow to shop, where they found no problems! Solution was new distributor cap and rotor. I think they were the original equipment. The o-ring was dry and hard, and I think it was allowing moisture in, when the engine got warm, the moisture condensed, and it ran terrible until the moisture was gone. Runs perfect today. Try ********AZ for the parts as they were lowest price by far. Steve

[sldan]

Had the same problem on 91 300SL 50K miles. Found cracked rotor and distributor cap. It sure seemed like a fuel system problem. Replaced rotor, cap, and spark plugs. Runs great.

sldan

[waybomb]

I have been incredibly busy and haven't had a chance to work on the SL. But I have been driving her. The problem is still there. I bought new injectors, an acumulator, plugs, cap, rotor, etc, and should have changed them all already, but no time. I haven't even changed the oil since I bought her. Fortuntely, the oil was fresh when I bought it. Has about 1500 on the clock since I bought it. Maybe in a week or two, I'll change one item at a time until I find what actually was causing the problem. I think with the miles and age on her, she probably needs it all done anyway.

[NDavies4]

I originally had the poor idle when warm plus the occassional stall. The full saga is covered within this other post:

"http://www.peachparts.com/shopforum/showthread.php?t=118349"

Did do the rotar and cap a few weeks back which indeed appears to have solved the stalling problem, but unfortunately I'm still left with my original 'hiccupy' idle - hence the interest in this post.

waybomb,
Please post your progress as your symptoms seem a close match to what I'm experiencing right now. I'm rather hoping you find it's the injectors as that's next on my list to try and I'm told not too expensive/difficult to do.

Regards,
Neil

[Captain Scott]

I am still battling a similar problem that has been happening since I got her in Feb of this year. I have a 1974 with 212K miles and only drive her about 20 miles a day to/from work. From this thread I am wondering about possible injector weep, but don't want to go off half-cocked.

When I start her up maybe 5% of the time (maybe once or twice every other week), she sputters and has terrible acceleration and pushes out gas-smelling exhaust smoke almost until I get to work. Letting her warm up for 10 minutes or so before getting on the road makes no diff. Once I get up over 2000 RPMs though, it smooths out and runs fine until I slow back down. Then it runs like crap until I suffer up past 2000. No, it does not improve with operating temp or change with outside air temps. She may run perfect getting to work and then when I split for lunch she runs horribly. Then when I go home at the end of the day she may run fine again.

I always run Chevron mid-grade gas and checked and cleaned the contact points on the cap/rotor (appear new) and trigger points and the plugs and wires are new. I found the wire that goes from the distributor to the trigger points had chaffed and was possibly shorting on the valve cover, but replacing it didn't change things... Oh, I get rotten mileage (9-10 MPG!) but guess that is about par.

Most of the time she runs like a champ, but when it doesn't I just want to junk her! But because it happens so infrequntly, I just don't feel like taking it to a mechanic cuz I don't think it will be mis-behaving when I get her there.

Ideas?? What else can I check before I drop her on a shop?

Frustrated In Seattle
(Scott)