Car wants to stall at idle, more details included - PeachParts Mercedes-Benz Forum

Marrs · #1 03-24-2012, 04:19 PM

My car idles fine most of the time, starts and runs, the idle in "D" at rest when warm is about 700, and I also just had the timing re-checked and the vacuum line to the throttle body checked, the butterfly adjusted. Basically what happens is the car will occasionally start to drop idle when I'm at a stop light and in "D", it tends to suddenly become more frequent and prominent over the next 10-minutes until I get to a point where I have to apply more throttle, fighting the brakes at stop, to keep the engine revved enough to not die on me. This happened a couple of weeks ago before I took it to the shop and it did die in traffic, so I learned to add more throttle to get me to a safe place to stop.

If I put it in "P" it will idle higher of course, but still not enough to prevent the stalling out once it gets more severe. My car has the MFI, has had new fuel filters and injector cleaner ran through (BG 44k), and the work done recently as mentioned above. The funny thing is when it was at the shop they didn't notice the idle drop and when I got it back I didn't either until about 45-minutes into a drive, but it was very slight so I assumed the idle adjustments they made corrected the issue. Temperatures outside yesterday and today are similar, but today the problem reappeared and was so bad I feared it would die again, so adding throttle at every stop until I got to a highway got me home. The car will go back in Monday, what should I have them look for or what can I do over the weekend?

I am thinking maybe points/cap/rotor, someone else suggested coil, read that the distributor could be worn and have a wobble on another post with idle issues tho not the same as mine, maybe fuel pump (tho to me it just doesn't "feel" like a fuel delivery issue, but I have no idea).

Thanks to all, I feel it's a simple issue but can't quite figure it out. I'm hoping it will do it while the shop has the car this time.

Marrs · #2 03-24-2012, 09:51 PM

Update: I pulled dist cap off and there is evidence of arching, the copper isn't smooth but "corroded" looking.

Also, I realized the car already has a Pertronix system fitted under the distributor! I had considered this but had no idea there was one in the car already. This may shed some more light on the problem.

mercmad6.3 · #3 03-25-2012, 04:16 AM

Because it a MFI engine and you say the throttle butterfly was adjusted ,I suggest you go back to the beghinning and start with a correct tune up. Firstly, the injection is the LAST thing to be touched.
Start with a cold engine.
1/ remove the plugs inspect them and take a note of the color of each one.
2/ do a leak down test .NOT a compression check.
3/ adjust the tappets and check the timing chain condition..Replace the chain if the wear equates to more than 5 degrees of crank rotation.
4/ run a test on each plug lead to check for high resistance. Replace the terminals NOT the leads and if it has silicone carbon string leads ,throw them in the garbage bin and use Correct mercedes /Bosch parts.
5/ Check the distributor .Overhaul as per necessary. Replace the points with a pertronix.
6/ check the rotor arm resistance.
Check the fuel pump pressure (14PSi) and flow rate at the cold start valve (1 liter per 30Secs)
7/ Set the distributor timing to specs at cranking speed.
8/ replace all Worn throttle linkage ball ends.
9/ clse the throttle butterfly and set it so that it closed tight and can be felt to "bite" when Opened.
10/ The rod that goes across the cam cover must be set so that with the throttle closed,the ball end for the link rod must in line with the holwe in the raised casting on the manifold ( normally a tool is passed through this hole to fit against the linkage.
11/ make sure the oil level in the MFI pump is at the correct level (too much and the car will be sluggish) .
12/ make sure the lever is against the idle stop and fity the up rod to the lever and the cross rod. it must fit without moving either the lever or the cross rod,adjust it's length to tget it right.
13/ Move the linkage through it's arc, both the injection pump and the throttle lever must go from dead idle to full throttle and rest against the stops together.
IF NOT, YOU GET EITHER TOO LEAN OR TOO RICH RUNNING. IT IS VITALLY IMPORTANT THAT STEPS 9 THROUGH 13 ARE FOLLOWED RELIGIOUSLY.
14/ Remove the air filter on the MFI and start the engine. leave it to idle. It must be idling faster than normal .
As the enginer warms ,the rubber hoses running to the MFI must be hot.
If not check for blockages.
As the engine idles and it reaches 80C on the gauge the idle speed should slow down . ZFeel the port where the MFI aircleaner fits, if there is suction and the engine speed is still fast,the thermo is fualty.
(Note, the engine thermostat needs to be perfect)
15/ do a CO check on the exhaust at hot idle. it must be 2-5%. Any less and it's too lean.
To enrich or lean out the engine at idle, STOP the engine and turn the thumb screw on the end of the pump clockwise to rich ,anticlockwise to lean but never more than three clicks in either direction and ALWAYS on a hot engine.
16/ Check the two way solenoid on the trans .
17/ Adjust idle speed by turning the airbypass screw. open the screw more and you will need to enrich the mixture at the pump by one click.and to slow down viceversa. If turning the screw makes no difference ,the intake has either a faulty thermo on the MFI or an air leak (check power booster on brakes)
18/ turn off the engine, turn on the ignition .
open the throttle slowly, a click must be heard from the transmission as the throttle is moved off the closed position (about 1mm off the stop) .
Adjust the throttle switch until it works correctly.
18/ touch the acelerator pedal, as it moved slightly,a click must be heard from the trans, there is a switch on the rear of the pedal and this MUST work.
Finally , push the acelerator to the kick down and you should hear click as the trans solenoid click in the other direction .
If you cannot get the trans solenoid to clicj=k,it must be rectified otherwise the trans will never work correctly.

When driving, you must take your foot right off the acelrater when stopped in gear. If you leave your foot on the pedal , the trans effectly stays in gear and will be trying to drive the car. The throttle switch activates a oil pressure valve in the trans and releases the pressure from the fluid flywheel (it's not a torque convertor,thus no stall speed and it's constantly driving) when the acelreator is released.
If the engine is slowing when in drive and all of the above is OK,then set the idle speed solenoid on the intake manifold . When in gear ,the solenoid is activated when the pedal is released ,and it opens the throttle slightly . By holding your foot on the pedal you making the trans drive the car.

Marrs · #4 03-25-2012, 12:52 PM

Terrific and thorough info, thank you. My shop did just complete a thorough adjustment of the throttle, found some issues that were rectified as you described, even as far as replacing a stop screw in order to get it perfectly aligned. The gentleman who does this is an oldtimer master with the MFI cars and is the only person allowed at the shop to touch my car's setup. In fact, it was there for three weeks just for this adjustment because he was out for an illness and they would not let anyone else make any adjustment other than him. They set timing, etc. Because of a failure for the butterfly to close all the way and the distributor air line to throttle body being clogged, and mixture adjusted to proper after setting and clearing, he took the car out and it worked fine so they didn't check the cap/rotor assuming it was fixed. I checked the oil level in FI pump, ball ends are tight on linkages. Will check the rubber lines for heat as you described as well.
The only other parts of your write-up that have not been tested are the transmission/pedal travel suggestions so I will do those as a matter of completeness, I have never heard of those checks so again, thanks. I love learning about this car.

By saying the cap is "corroded" I should clarify there is no patinae or age wear, more like arching to the metal as you would see in an electrical short between two wires. If the Pertronix was left "on" but not with car running could this cause the arching? Or perhaps it's just time for a new cap, I'm not sure. Also, and I may be wrong, but it seems this happens once the car is hot, but it's only happened twice so I cannot be certain. Changing the cap and rotor today and will report back.

EurekaBenz · #5 03-25-2012, 12:58 PM

Awesome write up, should be a 'sticky' or added to the DIY list......
dws

Marrs · #6 03-25-2012, 04:55 PM

Next step is to replace coil and ballast resistor as some suggested early on, my mechanic was hesitant and I listened since coil tested ok, but the symptoms are just too similar to failing coil so after I get them tomorrow I will report with my results.

Marrs · #7 03-29-2012, 06:35 PM

After installing Bosch coil # 0 221 119 030 and Bosch ballast resistor # 0 227 901 014 an driving the car for a couple of days with no more problems evident, it seems safe to say it was coil failure. Thanks to all and especially my MB Guardian Angel.

Marrs · #8 04-08-2012, 08:07 PM

Updating this thread, today I had the same idle drop but it has not happened since I did the coil swap over a week ago and before I did the swap it was missing constantly so I assumed that fixed it. I'm wondering if either (A) the Pertronix system in my car, or something else, is possibly causing coil failures, or (B) if it could be a simple matter of injectors need cleaned?

Further symptoms, it bogs down when I give it more than about 3/4 throttle such as going up an incline or merging with traffic. It died twice while moving at neighborhood speeds.

Mercmad6.3... everything seems in order based on your comprehensive tutorial but I did notice the fuel injector pump has too much oil on the dipstick, considerably. That's the one thing I overlooked last time. Is this a sealed unit or supplied by oil from the engine, basically I am asking is there any possibility oil would seep in causing it to overfill or if I extract some oil to get it to the proper level it will remain there (assuming there are no leaks)? Also can you explain if my symptoms sound like a possible result of this? How exactly does the oil level cause the car to become sluggish? Be as technical as you like, I enjoy learning.
Also, what oil should I use in the pump? My engine oil is 20w/50 which would seem too thick, can I use 30w synthetic I have on hand or buy a specific conventional oil? I just drained the current oil, it was fuel smelling and dirty. I also had no filter under the cap as some have suggested. Guess I need that.

mercmad6.3 · #9 04-08-2012, 09:23 PM

The pumps use oil pressure from the engine to feed a "seal" which prevents fuel leaking intro the engine via the pump. Eventually the neoprene seals under the pump elements fail and oil finds it's way to the governor via the rear bearing. Once the governor gets over full you will experience problems with sluggish acceleration as the governor weights cannot swing out correctly . The oil acts as a brake . because the governor wont work you don't get enough fuel under load. Hence your missing going up a hill etc.
drain the pump by sucking out the old oil with a turkey baster through the dip stick ,or if you want too, remove the large nut on the rear of the pump and drain it out through there. In your case I would use ATF, then go for a drive and see how the car behaves. If that fixes your running problem but the oil then rises over a few days of driving ,you might consider getting a pump core and getting it rebuilt,then swap it in later .

Marrs · #10 04-08-2012, 11:08 PM

Thanks, I refilled and drove it, but not really to full warm, just 15-minutes since it was getting dark. Will check oil level over next few days as stated. The oil will only overfill under pressure correct, ie: I need to drive it daily to compare?
I noticed it immediately idled differently, so I leaned it out (3-clicks leaner) and this seemed to help. My logic was that when it was overfull with oil then fuel pressure/volume was lower and my mixture was adjusted for that specific scenario, now that it's refilled with new oil it would return to proper volume/pressure which would be richer than previously so leaning it out was the right thing to do. Sound correct?

Also, when the car died at low speeds earlier today, is that also evidence of the oil level issue or should I just go ahead and get injectors cleaned as a safety? I will likely do that anyway since I don't know history of the car.

mercmad6.3 · #11 04-09-2012, 01:09 AM

When you adjust the screw on the rear of the pump,you must also adjust the air bypass screw . It becomes a real juggling act as you screw the pump adjustment in or out and compensate the air by pass.
And yes, you will have to return the adjustment where it was before you checked the oil level. it's nice and easy to do these jobs on a 6 but on a V8 (6.3 and Aston martin DBSV8 ) it is a real chore. You really should do these when the car is full running temp (minimum 80C) and you can be assured the engine is at the correct temp with the cold start thermo air bypass closed off.

Palolo · #12 04-09-2012, 01:36 AM

Fun stuff. Keep this thread going... I'm looking on from the sidelines.

Cheers,

Pat

Marrs · #13 04-17-2012, 02:31 AM

Apparently there was a massive buildup of sediment in the screen in the fuel pump, this was suggested by someone but this was the first trip tot he shop again since, so I'm hoping this solves the riddle. I may eventually need to have the tank etched and sealed if it happens again, but my mechanic did not seem to think this is likely.

mercmad6.3 · #14 04-17-2012, 02:47 AM

Thats pretty common after 40 years, and it's the reason the screen was put there. Put 1 liter of auto trans fluid in the tank for every 100 liters of oil and this will keep everything from the tank to the engines valves clean . it's also why you should do a flow and pump pressure test on a SE car every so often to make sure the correct flow is happening.

Marrs · #15 04-18-2012, 03:36 AM

Quote:

Originally Posted by mercmad6.3

Put 1 liter of auto trans fluid in the tank for every 100 liters of oil and this will keep everything from the tank to the engines valves clean.

Can you clarify this, do you mean 1-liter of trans fluid in the fuel tank per every 100-liters of fuel? Same type as I already use on the car as there are different types? I'd come across similar suggestions in the past on the forums but assumed that was for diesel cars. Is this a substitute for something like BG 44k or does it work on an entirely different premise?