Hello,
I recently purchased a '73 280sel 4.5 and need a little help diagnosing a problem. The engine has a miss at idle and at lower RPMs. I have narrowed the problem down to what I believe is a fuel delivery problem. With the car running, if I disconnect the spark plug wires from 4 cylinders, (#2,#4,#7,#8) there is no change in engine performance. Spark is good to these cylinders so I am guessing my next step is to verify fuel is getting to them. Can anyone please tell me how to check that the injectors are working properly? I do not think they are getting voltage from the control unit. -Mike

Suggest check the EFI trigger points in the tray in the distributor beneath the ignition control. There are four sets of points, each controlling a pair of fuel injectors. Injectors for cyl. pairs 4-8 and 7-2 are each controlled by different points, so this fits your symptoms.

Over time the EFI points are fouled by oil vapors from the driveshaft at the bushing at the base of the distributor. The points will likely work just fine again if you clean them (costly to replace, worth the effort to try cleaning). Each of the points has a plastic cover that must be removed to clean them. I put the covers back after cleaning, but I believe others have simply discarded them.

Other possibilities would be a bad connection between the trigger points and the ECU, bad fuel pressure adjustment (should be 2 bar?), mixture too lean (does the miss disappear at higher speed?), bad/clogged injectors.

good luck!

I pulled out the trigger points and am in the process of cleaning them. It's cold here in Wisconsin, so when I put them back in (probably this weekend when it gets above 30 degrees), I will let you know if there is any improvement. Thanks -Mike

I cleaned the trigger points and put them back in. No change in performance. There was some wear on one of the two metal prongs on the underside of the trigger points. At least 1/32 " is worn away. Could this be the problem? I do not want to spend $200 on new trigger points and then find out they were not the problem. Also, my spark plugs do not have that top metal cylindrical piece screwed on, they just have the threaded top when you take the plug connector off. I never saw a car before that was like this. Shouldn't that screw-on top be attached? Thanks -Mike

micheal-look into the plug boots. there should be a small clip that pushes onto the plug tip you see. this is what it is supposed to be. take a plug and experiment. you'll feel the threads going into and out of the clip. if no clips, those are the wrong wires.

on the injection system, that signal at the injector is tough to see with a test light. it's pretty low voltage. try an led on 1 that works and see how it triggers. if that works, then go to the dead ones. then go to the intake and make sure all the ends are on the ground wires and the connections are clean and tight. if ok, then go the ecu with the led and check pins 13,14,21 and 22. inputs from the trigger pts. if ok, check pins 4(4 and 8) and 6(2 and 7) outputs. if no, check pins 3 and 5-other injectors-to verify. all this is done with the car running, if possible, or cranking. if the ecu has power and ground-it does, 1/2 the car runs, all 4 inputs from the trigger pts are there and the grounds from the dead cylinder injectors are ok, this means either the harness is broken or the ecu is. keep us posted, please. good luck, chuck.

If the injectors are working, you should hear them "clicking" as the motor runs. If they "click" but no fuel is coming out, you have clogged injectors or a pressure delivery problem. With the engine not running but the ignition "on", you should hear a pair of injectors "click" 20 times as you move the throttle linkage from idle to full, rotate the engine to activate the next EFI switch point and pair of injectors. If the sliding blocks on the points are too worn, the points still may not work even after cleaning (might be able to bend it to get it working again.)

Is it missing at all load conditions or just at idle?

OE plug wire set for this MB is threaded post type, this is normal.

good luck,
Alan

OK, here is the update. Using an LED: (1) All injectors are getting the signal from the ECU. (2) Connections 3,4,5,6 (injectors) & 13,14,21,22 (trigger points) in the ECU are OK.

When I checked the signal at the injector connections with the ignition on and moved the throttle linkage this is what happened: (1) Two pairs of injectors were always getting a signal, not just one pair, is this correct? (2) Also,there are six possible combinations when two pair of injectors are getting a signal at the same time, but only four occurred.

These combinations occurred: 1,5 8,4
1,5 7,2
3,6 7,2
3,6 8,4

These did not: 1,5 3,6
7,2 8,4


Should I pull the injectors?

Please help with the next step. -Mike

You are starting to get in over MY head here but fwiw, here is my take. The firing order on that motor is 15486372. So injector pairs 1/5, 4/8, 6/3, 7/2 fire in their respective order. If there is a bit of overlap in the EFI switch point, then 1/5-4/8, 4/8-6/3, 7/2-1/5, could overlap in your static test. 1/5-6/3 and 4/8-7/2 should never occur, as you observe. The actual pulse to the injector is quite short compared to the interval between triggers when the motor is running.

It's not hard to unbolt the injectors and pull up the ring line and injectors out together. You can then observe what the injectors are really doing with the fuel as you crank (collect the fuel using some kind of containers, preferably clear so you can see the spray. Careful, pull the coil wire off and far away to avoid ignition! If the rubber hoses on the ring line are old and cracked (common), good time to replace them now.

You can also use a 1.5 volt battery to manually test each injector (don't use 12V, these are low-impedance solenoids!) The injector should "click" and the pintle should open and you can spray fuel/solvent thru the injector.

Could try swapping injectors 1,3,5,6 with 2,4,7,8 & see if problem also moves.

Another problem that can occur is when the distributer shaft bushings get worn. When this happens the ignition points drive the shaft off center which causes the trigger points to open different amounts. It could cause more than one set to be open at the same time and could also make it so that certain trigger points never open, especially at slow speed.

Try removing the ignition points and see how the trigger point activity changes in your cranking tests. In cases where this happened switching to an optical trigger for the ignition can extend the life of the distributer.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

OK, here is the good news: All injectors are spraying properly and in the correct order. I pulled out the ring line with the injectors attached and they all spray nicely. So now that I know each cylinder is getting fuel and spark, what would be the next step to find why cylinders 2,4,7 & 8 are missing? Compression test the cylinders?, maybe stuck/bad valves? I should also mention when the engine is at higher RPMs (> 2500), you do not feel the missing hardly at all. Thanks -Mike

Does the miss improve if you richen the mixture? The idle mixture adjustment is a plastic knob on the ECU with about 20 detents, so you can count clicks and get back to your base setting. You could have a vacuum leak that causes a lean miss at idle at the offending cylinders at the injector o-ring seals, or in the seals in the intake manifold.

Poor idle could also be caused by insufficient valve clearances, especially the exhaust valves.

Are you sure the ignition system is working well? Might recheck plugs, wires, cap, rotor....