Basic Tests if you have poor/ rough idle, or misfire - PeachParts Mercedes-Benz Forum

Benz Mondi · #1 12-06-2010, 01:11 PM

(I posted this inthe SL Forum, but it's experience I've gained from maintaining by 124's...)

I've only had my 1993 300SL for about 6 months and it's been fantastic. For the first time though, it left me stranded on the way to pick up a Subway sandwich. Coincidentally while I was in the middle of doing an oil and filter change on my 1992 300E. Thinking back, my SL started to have an irratic idle at times... should have been my clue to take a good look before... it stranded me!

Here are my tests, in the sequence that I follow, for these symptoms with the 3.0 liter:

1. Make sure that you have enough gas
2. Make sure you have enough voltage at the battery
3. Open the hood and make sure that you have the breather hose from the side of the valve cover connected to the air cleaner
4. Make sure that the sensor at the side of the air cleaner is connected
5. Check the hood liner above you to see if there are any fresh tell-tale splash marks
6. If you can turn the car on and get it to idle,. even if rough (for a short time to see if anything odd is happening that you can see.
7. With the engine off, disconnect what is attached to the air cleaner and lift the air cleaner off. Cover the throttle body with a clean cloth to make sure nothing falls in there.
8. Look at the EHA valve (match box sized black box behind the fuel distributor) Note if it's wet with gasoline. Then while it's still attached, clean it well enough with some carb cleaner and a paper towel. You want to be able to observe if it gets wet again after cleaning.
9. Disconnect the electrical clips attached to the EHA and the Idle Control Valve checking for any breaks in the wiring. Reconnect the clips.
10. Check the attachment and condition of the thick black vacuum hoses that are at the sides of the ICV. Are there any cracks in the hoses?
11. Making sure that there's nothing in the way of the fan and the, now exposed throttle body....

Take the cloth off of the throttle body and Turn the engine on.

12. Look at the EHA and check to see if you see a wet spot forming on the upper edge and sides. If it's failing, it will usually seep gas out of the tiny weep holes. Now reach down and pull on the throttle linkage and increase the idle speed to 2000+ rpm's and watch the EHA again. Now is there a wet spot?
13. Now look at the thin vacuum line that extends from the Fuel Pressure Regulator and is usually plugged into the side of the breather hose at the valve cover. Disconnect it from the breather hose and note if you see any gas coming from it (it's actually just a drain hose in case the diaphram in the FPR has failed which will then drain the raw gas into the valve cover instead of the hot engine causing a fire.
14. Now disconnect the lead from the ICV and notice if the idle jumps around, especially while under loan from the A/C unit. If it does then it's working properly.

That's my list. When you encounter a failure, resolve it and continue with the rest of the tests. If your car passes these tests then you'll be checking for: cracks in the distributor cap, faulty spark plug wires, spark plugs, coil, and even new injectors. It's a good idea to change these anyway if you are behind in the maintenance schedule.

In my case, the culprit was the EHA valve that was leaking through one of the weep holes. I make it a point to have a tested spare in reserve along with the little green donut O rings on the fuel distributor side. I carefully take them out with a hooked pick. Everything else passed and now my SL sings it's praises once again.

In my experience, with the 7 124's I've had, if the leaking EHA has not been the sole cause of my rough idle/ stalling problems, it was involved with it. Sometimes the two Torx bolts were just loose or the O rings just needed refreshing. If gas is coming out a weep hole though, you need another working EHA. As far as I know, there's no way to refurbish these.

Tips on EHA R/R, place a cloth directly under the EHA to catch the two bolts that you'll likely drop into a deep recess of the engine. Place another cloth directly over the EHA to catch the gasoline that will come squirting out of the EHA as you loosen the bolts... at least wear some glasses. Gas really stings when it get's in your eyes- HA!

Good luck.

Al

stevenstevensteven · #2 11-27-2015, 03:14 PM

Thank you.

MTI · #3 12-01-2015, 02:04 PM

For a newly misfiring engine, I would have started with the plugs, especially if they were of unknown vintage, along with the wires and, on later models, the igniters.