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#1
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1984 W126 500SEL bad idle
I am having a hard time to get this engine to idle properly when warm.
The engine is shaking and surging a bit as if ignition is not clean. Engine almost stalls at a traffic light. It actually did once. I bough the car a few years ago and it was not drivable because of the many issued this neglected car had. I am pretty new to this car. I have now changed the rubber and fuel injectors, replaced the spark plugs but still the engine does not want to idle properly. I set timing at 27 degrees before TDC (at 3000rpm without vacuum) [the book says 25 degrees]. Idle is at 20 degrees [16 + - 3 per book] Does anyone have experience with this engine and bad idle? Thanks for your help, Martin |
#2
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My experience isn't specific to your car; but, on any older car I like to start with a compression test when chasing down engine operation problems. Watch and listen for pinging/detonation due to advanced timing @27 BTDC. I'd go to spec or 25 BTDC if anything.
Triple check the firing order...'ya never know. Use the timing light on every plug wire pointed at something dark in color while looking for any intermittent flashes indicating an ignition misfire. Good luck!!!
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"Rudeness is a weak man's imitation of strength" - Eric Hoffer |
#3
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Thanks Sugar Bear,
firing order is checked and confirmed. The engine has great power above idle. The problem is specific to a very shaky and unstable hot idle. I can adjust and reduce the advance from 27 to 25, no problem. However I doubt a bit that this makes a big difference. Any misfire at idle would show in the strobe light? If misfire this would be the spark plug cables...correct? Martin |
#4
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An accompanying skip in the timing light flash should occur with the misfire on the cylinder the light is on. A misfire can be anywhere in the secondary/high voltage side of the ignition from the coil to spark plug. Move a spark plug or wire one at a time to see if the misfire follows the part. Inspect the cap and rotor and remove the coil wire to inspect for corrosion especially in the coil tower.
Plugs or wires usually break up under heavy acceleration though and this problem is at idle. Still worth checking. Good luck!!!
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"Rudeness is a weak man's imitation of strength" - Eric Hoffer |
#5
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Thanks Sugar Bear,
I'll check for intermittent spark at one the 8 wires. I did install new plugs (Bosch #7905) but this did not improve the idle. Corrosion at coil? Shall I look at the exterior for corrosion? Distributor cap and rotor did not look suspicious. I had to change the green cable from the el. box left side (close to the fender) and the distributor..... but this also did not change anything to the idle problem. The green cable is a URO part....evil. I have ordered a new set of spark plug wires from Bremi. I hope I am getting better results finally. Is there a possibility to adjust idle speed manually? Martin |
#6
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Corrosion at the coil... examine clean and tighten the primary leads (small exterior) and remove the secondary/high voltage coil wire to inspect down inside the coil tower. I've seen the tower range from bright and clean to heavy green corrosion.
Adjust the idle manually? I don't know. Good luck!!!
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"Rudeness is a weak man's imitation of strength" - Eric Hoffer |
#7
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I had an 84 500sec which would refuse to start if shut off warm and tried to restart five or ten minutes later. Leave it running at the 7 11 and you will be golden.
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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC] ..I also have a 427 Cobra replica with an aluminum chassis. |
#8
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Hi Sugar Bear,
I'll inspect the coil for corrosion. Adjusting idle: I read that there might be a small short cut line in the rubber connector at the idle air valve that might be able to increase idle speed. That bypass line is plugged by default and if opened up that line could potentially improve my too low idle speed when hot. However that would not solve the idle misfire. Missing vacuum line: I also saw that one vacuum line was missing at the engine: this particular line is starting at the rubber connector/formed hose from right side valve cover into the air filter. I don't know where this vacuum line ends up. Is there a vacuum diagram for the 117963 (my 1984 Euro 500SEL)? Are there specific vacuum tests in order to test if the engine is not pulling too much false air? I guess I can spray brake cleaner with the hot engine and see if the idle is increasing? I hope that I can eliminate the idle misfire with the new spark plug wires that I ordered. Also I'll set the ignition timing to 25 degrees before TDC (it is at about 27/28 degrees now) Best, Martin Quote:
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#9
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Vacuum leaks can be located by sound, misting carb cleaner toward a suspected leak (this can be a fire hazard) or by injecting smoke into the intake system to see where it exits. It may be worth checking for an independent shop that would do a smoke test.
Good luck!!!
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"Rudeness is a weak man's imitation of strength" - Eric Hoffer |
#10
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Here is some progress from last week.
I was throwing parts at the engine but so far I have not made any progress Spark Plug wire change: I bought Beri Spark Plug set for the 500 SEL and installed it. It did not do much if anything at all. I have the same issue with misfire at hot idle Ignition Timing: I adjusted the timing according to the book (Euro Engine 500SEL 117.963) 25 deg before TDC at 3000 (w/o vac) 16 deg before TDC at idle (w/vac) This did not change anything with regards to the idle issue. Idle Air valve regulator: There is an aux device that is regulating the idle air, a solenoid is moving a sleeve that is regulating the amount of idle air ... I think. Sorry that I am not firm with the terms, it is #23 in the diagram. There is a thinner plastic hose that was plugged internally (no connection) But I read that the plug can be cut off in case the idle is too low. So I cut this hose open and reinstalled it (see diagram with the arrow). I am not very positive that this changed much. maybe the idle speed went up by 20rmp but the main problem is still the misfire and shaking of the engine. Engine Vacuum: I checked vacuum at the colder engine and found 17 -18 inHg at idle. I don't think this is bad. Still I need to check vacuum at warmer engine. Then I was starting to get into the K-Jet system: I hooked up my fuel pressure gauge into the line from top/center of Mengenregler (Fuel Distributor #20 in diagram) to the Warmlaufregler (Warm-up Regulator #70) - I am not very firm with the English terms. I added a picture of the pressure gauge hook up. I have the data of the system pressure (5.0 - 5.6bar) and regulator pressure (3.2- 3.5bar) [this is for regulator #101 of the Euro Engine] I started the engine and waited until the engine was warm 10 minutes and I the gauge was reading 2.2bar regulated pressure (way too low). And when I shut off the line in order to measure the system pressure the engine stalled right away. Something is wrong here. Any help is very much appreciated. Best, Martin |
#11
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w:
Potential causes of misfire: 1) Low compression due to a leaking valve. 2) A defective spark plug and/or secondary wire. 3) Defective distributor cap. 4) Injection nozzle check valve leaking or sticking. 5) A mis-adjusted diaphragm spring in the fuel distributor. |
#12
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Thanks Frank,
I'll try to go step by step. 1) Low compression due to a leaking valve. I guess I should do a compression test with the engine. 2) A defective spark plug and/or secondary wire. I have replaced the spark plug wires 3) Defective distributor cap. I have not replaced this one 4) Injection nozzle check valve leaking or sticking. I have replaced all injectors with new ones. 5) A mis-adjusted diaphragm spring in the fuel distributor. This is K- Jet related. Yes I have found some potential problems with my first pressure tests. The system pressure was at the lower end at 5.0 bar and the control pressure was way too low with 2.2 bar with the engine warm. I need to dive into some education regarding the K-Jet system. When I switched the control pressure off in order to measure system pressure the engine stalled. Martin |
#13
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The Idle problem is solved.
I found the culprit after throwing a lot of parts onto the car. All ignition related items were not the problem (I changed plugs, plug wires, set ignition timing to spec). It was not: the idle air valve, not the injectors, not the rubber air manifold -false air. (I installed all new parts here) The issue was the added on Lamda system in my Euro Engine... took me a long while to figure that out. That was the reason for the low control pressure of the WUR (2.2 bar) and the shaking engine at hot idle. I disconnected the Frequenc valve and reset the idle mixture and voila! ( i have to say that in my aftermarket system the o2 sensor was missing and thus the system could not work properly) Finally idle : no shaking, no intermittent ignition. All seems to be fine now. I decided to remove the entire aftermarket system all together in future. For now it is just disabled. Thanks everyone for your help. Martin Last edited by werminghausen; 06-10-2022 at 09:24 AM. |
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