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Discussion Starter #1
Hello everyone;

Recently acquired a 1988 300 SEL with 93K miles from an auction (the car was not running). The car cranks and I even got it to run after long cranking. She would run as long as I kept the RPM around 2000, but as soon as I let go of the gas she would begin to stall at about 400 RPM and when she stalled, it was impossible to restart until maybe (and I say maybe) the next day. I hope that someone could help or guide me in resolving the problem(s) to get this thing running. Here is what I have done so far: Fuel: I drained the tank and put new fuel (93 octane), changed the fuel filter, changed the oil/filter, have good pressure going to the injectors, but still no fire. So I checked the spark plugs to see if I had fire and when I pulled them, they were wet and were fouled with black soot. I checked for spark at the plugs and had spark (orange in color, not strong blue). The car had Bosch Iridium plugs, but took these our and replaced with Bosch Super Plugs (Copper electrode). Next I checked the wires and they felt hard and one had 53K ohms resistance, 2 had 1.53K ohm, and the other 3 1.3K ohms so I decided to replace them with a new HGK set because of the bad wire and the way they felt. Next I checked the distributor cap & rotor, which I changed both because the contacts on the cap were really worn. Next I checked the coil and even though it was within spec, it was weak with a primary winding reading of .4 ohms and secondary of 8.37K ohms, so I changed it since it was on the low side & I thought with iridium plugs and some bad wires, the spark at the plugs was weak and may possibly be being blown out during compression. Tried to start the car, but nothing. She will not fire up. I then tried to check the timing, but could not find a reference pointed and the markings on the harmonic balancer did not make sense to (0/T, 1/T, 2/T, 3/T, 4/T, & 5/T. Further around the balancer it also had 12/T & 24/T) and I don't know what these mean. I then tried to turn the engine by hand to find TDC on the compression stroke and when I took the cap off to verify, the rotor was pointing in the direction of the #1, so I am assuming timing is OK, but can't verify 100%, but with no pointer? A friend told me that on these engines, you can't set the timing so this may be true, thus the no reference pointer, but not sure. Can the timing be checked & adjusted and if so how?. I also checked the reference resistor and was OK right at 750 ohms. I checked the O2 sensor and the outside was really crusted around the pipe and when I took it off, it was heavily carbonated so this was also replaced.

When I crank the car, there is a real heavy smell of gas (and again, when I pull the plugs, they are wet) and on the couple of times that I had it running, the smell of gas is heavy as if she was running really rich. On the fuel side, I took out the injectors and tested them. 2 were leaking badly so I assumed more could be bad so all 6 were replaced. I've also checked the coolant sensor and it checks OK (2.8 ohms). I've visually checked the Overvoltage relay and the fuse is OK and looks in very good condition (no ABS light on) so I'm assuming its OK. I've also taken off the Cold Start Valve and while cranking the car, she spray fuel. I've also removed the Idle Control Valve and she looks OK. SO I am not really sure where to go next. I think I've eliminated most of the ignition system (less the control module - EZL but don't know what or how to check it) so I don't think my no start problem is ignition related. I am by no means a "Mechanic", but do have a very basic understanding. However, with this being my first Mercedes and not knowing much about these cars or their components I need all the help I can get. I bought the car because she looked very nice, my wife fell in love with the car when she saw it, and I thought I could get it running, but I an obvious over my head and need HELP!!!! Hopefully somebody can jump in and guide me further. THANKS.
 

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1990 420 SEC
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So does the car have a strong blue spark when it is not starting? If not here is how to troubleshoot ignition system:
That is V8 engine but ignition system is similar. Ignition timing is built in the ignition module. Crank shaft position is detected by the crank position sensor from the flywheel/ flexplate. Timing is not adjustable.

One possible reason for no spark same as above but M103:

Cold start valve should not be spraying constantly; just a short priming squirt.

Have you noticed this on the upper part of front page of W126 section?
 

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1989 W124 260E
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Dint even think about making any adjustments untill all ht side of the electronis are replaced . Rotor arm distributor cap , plug leads ,and spark plugs. .All of the parts to be Bosch or Beru dont even think about any other maker ,If you do the parts will fail in a short time of use .Check the CPS with a meter . Check all fuses and earth points -like clean them a put a dab of grease on them and bolt the lead back on to the body .There could be a quick fix for your car .. Pull a spark plug lead off then put an old plug in to the end of the lead ,,hold it with insulated pliers close to the plug in the engine so you dont get a shock ,a shock from these will give you a pair of angels wings ha ha . See how strong the spark is . If its a weak spark checkout the coils. Look for fuel leak from the fuel distributor .
 

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Discussion Starter #4
Heikkif & Plinker17722, thanks for the response. To answer your questions, after replacing the ignition parts (I.e. plugs, coil, wires, cap/rotor) the spark is now mostly bright white to slightly blue so I think its pretty hot. . All grounds have been checked for corrosion & contact points cleaned and all fuses removed from the fuse block/box with any kind of corrosion (which there was hardly any) removed and reinstalled with some dielectric grease at all contact points. The resistance on the CPS was checked and was OK at 799 ohms. However, when I first checked the Cold Start Valve, I just cranked and when I saw the fuel spray, I stopped. Tomorrow I'll check again and crank it longer to see if it sprays continuously or just an initial puff. I hope it sprays continually and this would be my problem with all the raw fuel going into the cylinders. Heikkif, what do you think about trying to start the car with the Cold Start Valve out? Does the hole where it goes needs to be blocked with something solid (I can fabricate a metal piece to block it). The ambient temp at home is about 80 deg(F) so maybe the Cold Start Valve is not needed. What do you think? But if the Cold Start Valve just puffs, what would be the next step to check? Thoughts...….
 

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1990 420 SEC
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Did replacing the coil change the spark quality? If so then it was the original fault. I understand you never seen the car in working condition. So called duty cycle adjustment may have been messed in order to make the car work. My car was like that when i bought it. A new coil and readjusting the lamda screw made it run so it could pass the inspection. To check if cold start valve sprays you do not have to plug the the hole but the car wont run right with the hole open. If it sprays all time try leaving the cable disconnected and see if it still sprays. You should be able to start the car with the cold start valve cable disconnected.

Some reading on duty cycle adjustment:

 

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1990 420 SEC
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By the way when replacing ignition high tension parts pay attention to correct resistance values on those.
 

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Discussion Starter #7
As far as I can tell, the Lamda screw has never been touched. And you are right about checking resistance on the new ignition parts (one never knows if a new part is bad). Sometime today, I will pull the valve and check it for spray and try to start. I'll let you know.
20200223_073844_HDR.jpg 20200223_073904_HDR.jpg
 

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1990 420 SEC
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The question of HT ignition part resistances rises because of modern cars have resistors in spark plugs and high tension leads. It was different in the old days. Resistorless spark plugs and leads are hard to find. Actually modern cars have coil packs by spark plugs but anyway.
 

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Discussion Starter #9
OK, so I took a look at the Cold Start Valve (CSV) and here is what I got: Took off valve and checked resistance at the valve plug terminals and got 4.7 ohms. Not sure if this is good or not, but I wanted to know and now I have for reference. Fabricated a "Blank-off plate" and installed on engine to block off CSV hole. My thought was to try and start car without CSV, but it did not work, car did not start. Anyway, tested CSV spray (see attached video at
) and it seems OK but not sure for how long the CSV should stay energized. The spray pattern looks good too. When I crank the car, the valve energizes and sprays for about 1.5 seconds then stops so I guess its working OK. What do you think? With the CSV off the car and after cranking, I pulled the #1 plug and was wet, not as much as when the CSV is installed, but wet never the less. Re-installed everything, tried to start, but nothing. No start.
 

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1991 560sec. 1969 280SL
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Could be the control plunger of the fuel distributor is sticking. Just a thought.
 

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Discussion Starter #11
Hey Gogi, not really sure, but I think the plunger is OK. When I push it down, it moves easily and freely and when it comes back up, I can hear when it hits the stop with a small "thump". However, at my level of knowledge, not sure what else I can check on it. This thing is got me baffled. I got good compression, fuel, air, and spark, but she just does not want to start. But not knowing Mercedes and having just a basic knowledge makes things kind of hard. Do you think it could be timing? Could the distributor have jumped a tooth, thus the hard to start? I've had the car running a couple of times, but only after severe cranking and when I let go of the gas, she stalls and can't start anymore. Now she just won't start period.
 

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1990 420 SEC
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Distributor does not have to do with timing. It just directs the spark to correct plug. As long as distributor arm points to cylinder at top dead center of compression stroke it is timed ok. Wet plugs do not make spark. Try to clean them with brake cleaner or replace with new.

You could check free movement of measuring plate, when the fuel distributor starts spraying when measuring plate is pushed down and check fuel pressures. There is system pressure and control pressure. Check the values of free movement and correct pressures at service manual which is online. For these checks you have to make fuel pumps run by jumping the fuel pump relay.
 

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Discussion Starter #13
Heikkif; Thanks for all the help. The Bosch plugs are brand new. I'll try to measure the fuel pressures this weekend. I'll keep you posted.
 

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68 250S, 91 300se, 98 SL500, 450SEL 6.9, 14 CLS550 Past MB's; 300SD, 300E, 300TE, 190E, ML420
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This is classic symptom of fuel distributor since the car has sat so long. Rebuild FD and WUR - send to CIS FLowtech for rebuild and calibration. You car will run great when you get it back from them.
 

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300sel W109, 280S 70 , 220s 62. 230 66. Lancia Flavia 63.Pininfarina
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300SE with 103 KE engines should be without a WUR.
 

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1991 560sec. 1969 280SL
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There are detailed youtube video’s on FD rebuilds. The kits are less than a $100. Watch the vide several times to get a full understanding of procedure and if you think you can handle it, go for it. You’ll save hundreds of dollars.checkout Michael Pillay’s
 

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Discussion Starter #18
Yesterday evening I started looking into the fuel distributor and bit closer and one of the ports looks like it has a small leak on port going to the #4 injector. All other 5 appeared to be OK, but the FD is definitely leaking (see video below). I reached out to CIS Flowtech to see what the process is for having the FD rebuilt & tested (I.e. price, turn around time, etc.), but no response yet. I would try to rebuild myself as looking at the videos it does not look that hard or complex, but I am hesitant because I have no way of testing until it is back on the car and if I make a mistake, then its another $100 for another kit. I have to look at this closely as I don't really have the money to be making $100 mistakes. Any thoughts on the rebuilding myself and the lack of flow checking the unit???

 

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68 250S, 91 300se, 98 SL500, 450SEL 6.9, 14 CLS550 Past MB's; 300SD, 300E, 300TE, 190E, ML420
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I highly recommend CIS. They will test and adjust to your particular engine. IIR @$600
 

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1990 420 SEC
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That kind of leak is a problem but it does not explain why the rest of cylinders are not running. There could be a leak also at control plunger seal at the bottom of the fuel distributor.
Also it would be worth checking the free movement of measuring plate before control plunger movement is felt. I think the basic setting explained in the manual.
 
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