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1972 350sl 240k miles silver with blue interior. 1973 450sl
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Discussion Starter #1
hi all, I have a 72 350 with 240k miles on it. the car has been properly maintained as per the maintenance book since day one. car has had a problem for the last year or so and I have been chasing the cause. car will run absolutely perfect then start sputtering and losing power. turn car off and restart it it might be perfect again...or not. car will do this cold or hot, wet or dry, uphill or down, bumpy or smooth road. there is no known common situation that triggers the sputtering.
things I have done: plugs wires cap rotor, points, coil, took distributor apart and thoroughly cleaned it. remove gas tank [it was like new inside] new fuel filter and fuel pump, new fuel lines front to back and all the injector hoses. have swopped around all relays, cleaned any wire/connections I thought might be involved with distributor or fuel injection. swopped computer with my other car. wiggled, jiggled, pulled and poked the fuel injection wiring when it was running good, or when it was running rough. it started as a short, infrequent sputter, to happening more often and for a longer time.
what's my next step?
Thanks
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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10,459 Posts
I have same car. Seems you have done many of the things that first come to mind. Fuel system & Ignition.

I didn't see anything about the trigger points. The rubbing blocks on them do wear out and they then malfunction. My originals lasted about 150k miles. If you haven't addressed those, let us know. (they are expensive to replace, but can perhaps be repaired)

Injection harness connections can be a problem. With help from Cush, I rebuilt my harness and that eliminated slight miss that I had at times.

The other intermittent problem I had, was with the green wire that runs from the distributor to the connectors on driver side near coil. That wire can short to the distributor casing or to any other metal. Mine somehow touched the manifold and melted the insulation!

It is possible that the ignition module/switchgear is failing. Can be bypassed if you switch to a Pertronix (which might be an idea anyway, even if you keep switchgear!) Or try swapping in a spare.

One other thing - What type and model of fuel pump did you install?
 

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1972 350sl 240k miles silver with blue interior. 1973 450sl
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15 Posts
Discussion Starter #3
I have same car. Seems you have done many of the things that first come to mind. Fuel system & Ignition.

I didn't see anything about the trigger points. The rubbing blocks on them do wear out and they then malfunction. My originals lasted about 150k miles. If you haven't addressed those, let us know. (they are expensive to replace, but can perhaps be repaired)

Injection harness connections can be a problem. With help from Cush, I rebuilt my harness and that eliminated slight miss that I had at times.

The other intermittent problem I had, was with the green wire that runs from the distributor to the connectors on driver side near coil. That wire can short to the distributor casing or to any other metal. Mine somehow touched the manifold and melted the insulation!

It is possible that the ignition module/switchgear is failing. Can be bypassed if you switch to a Pertronix (which might be an idea anyway, even if you keep switchgear!) Or try swapping in a spare.

One other thing - What type and model of fuel pump did you install?
I did take the distributor apart and cleaned and inspected the trigger points. they had been changed I believe before I bought the car because they looked new. I had found a problem with a wire to the coil, it had been hanging on by one strand. after I fixed it the car ran great which led me to believe I had fixed it...it was a coincidence. same thing happened when I, [fill in other repairs made ]. most annoying and hard to diagnose because it just won't die, stop, blow up make smoke etc....that would indicate where the problem is. I did swop the ignition module with my 73 and it made no difference. I swopped in a known good Bosch fuel pump, later style with the banjo fittings, that was the last thing I changed and the car was good for a week then went back to it's intermittent cycle. I guess the next step is to cut the harness open and check wiring?
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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10,459 Posts
Those later style pumps are mostly not correct for the Djet. They are able to put out too much pressure and may have a lower flow rating. Internal relief setting is also too high. Can cause a safety problem with fuel rail hoses. Post model number and we could check. If you have the original, I would put it back. There is a new Bosch/MB replacement pump and there is also a Nissan pump that is exactly the same as the original. Been discussed here and on Peachparts several times)

You should check that the fuel pressure in the fuel rails is steady at about 30psig. Or even a little higher. Adjust if need be. Sometimes the regulator can be faulty. And check flow from exit of the fuel damper (that is after the fuel pressure regulator) is close to 2L/min (1L/30sec) with a fully charged battery.

This may not be the cause of your running problem, but I would check it anyway.
 

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1972 350sl 240k miles silver with blue interior. 1973 450sl
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Discussion Starter #6
Those later style pumps are mostly not correct for the Djet. They are able to put out too much pressure and may have a lower flow rating. Internal relief setting is also too high. Can cause a safety problem with fuel rail hoses. Post model number and we could check. If you have the original, I would put it back. There is a new Bosch/MB replacement pump and there is also a Nissan pump that is exactly the same as the original. Been discussed here and on Peachparts several times)

You should check that the fuel pressure in the fuel rails is steady at about 30psig. Or even a little higher. Adjust if need be. Sometimes the regulator can be faulty. And check flow from exit of the fuel damper (that is after the fuel pressure regulator) is close to 2L/min (1L/30sec) with a fully charged battery.

This may not be the cause of your running problem, but I would check it anyway.
thanks I will do that.
Only because I wrote it down for quick reference. It's the Nissan 17011-P7211 fuel pump.
thanks. does it mount the same?
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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10,459 Posts
thanks I will do that.

thanks. does it mount the same?
The Nissan pump is exactly the same as the original Djet pump. I have one here, still new in box as my spare.

By the way, because the trigger points "look" good doesn't mean they are good. Have a look at the Comprehensive guide under Djets in EGV107 where wear on the rubbing blocks might be documented. I am sure I posted elsewhere on how to check the angle that each set stays closed - it is a measure of the wear. Thread here. In theory it shouldn't matter , but in practice it seems that when they stay closed for more than about 145-150deg, the ECU gets spurious signals and injectors fire incorrectly
 

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1972 350sl 240k miles silver with blue interior. 1973 450sl
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Discussion Starter #10
so I ordered a couple of the fuel pumps, one for my 72 and one for my 73 they both had a couple used pumps from later cars. I installed the new pump in the 72. here are the photos of the old original pump, the new Nissan pump, and the temporary Bosch pump
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IMG_0301.JPG
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IMG_0303.JPG
 

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1972 350sl 240k miles silver with blue interior. 1973 450sl
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15 Posts
Discussion Starter #11 (Edited)
car barely made it into the barn. After I switched the pump it ran good and drove fine....but this has done this before. fixed? we will see
IMG_0304.JPG
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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10,459 Posts
Check the fuel pressure in the rails and adjust if need be. (You can measure using the cold start injector connection as a temporary measure.

Glad to see those Nissan pumps are still available.

The pump that was removed, was designed for a 5bar system - 72psig at about 150 lph. Djet needs about 30psig at 120lph. I couldn't find exact specs, but that pump would likely try and flow something like 200lph at 30psig. Fuel pressure regulator must have been adjusted to do that or the fuel pressure could have been higher causing rich mixture. It could also be that the high flow would cause the internal pump relief to operate and bypass fuel back to suction. This could cause overheating and possible vaporization of fuel in pump or at engine.

Whatever, the new pumps should be good. Just set the rail pressure with the regulator to spec or slightly higher 30-32psig.

Good Luck!
 

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1973 450 SL
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2,062 Posts
Those old Bosch D-Jet pumps can be cleaned up, flushed out and repaired with new seals. If they sat inoperative for a long time, they will be full of varnish to a point where the motor can not turn. I use a channel lock and a shop towel to grab the rotating assembly and free them up. I also install new M5 screws with hex heads replacing the Phillips head. Take pictures if you decide to open them up so you can reassemble correctly - note the notch in the metal collar relative to the inlet and electrical connections. There are 5 small cylindrical rollers that sit in the rotating assembly. Those little rollers want to get onto your garage floor and find a hiding place.

The O-Ring Store - 2 x 33 BUNA
 

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1972 350sl 240k miles silver with blue interior. 1973 450sl
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Discussion Starter #14
thanks for all the help. I will take that original pump apart so it can be a back up for either car. I put a new pump on the 73 when I got it a year or so ago, car has been sitting in a climate controlled building and only was driven a mile[ ?] maybe when that pump died. a friend gave me a box of Mercedes pumps/filter/fittings and I swopped them into both cars. the 73 has a miss that I need to diagnose that started not long after I cleaned the tank and changed the pump and filter.
 

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1972 350sl 240k miles silver with blue interior. 1973 450sl
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15 Posts
Discussion Starter #15
ok, car ran great for a couple days, then started to sputter and miss. turn car off let it sit a while and it starts and ran fine, moved car a few feet shut it off, let hot sit, started running bad again. got it home and into the barn installed a fuel gage. steady at 30 psi. when it was running bad. turned it off started to run better gage still at 30 psi. when it was running bad I sprayed some either into the air horn and it bogged down. not sure if what that means but I guessed it wasn't looking for fuel. I checked the point gap, again, dwell right at 30 degrees. decided to clean the three terminal junction block near the battery. made no difference .
IMG_0331.JPG
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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10,459 Posts
Maybe already mentioned, but set points gap to 0.014" (Dwell may be inaccurate?) Check the green wire from distributor is not shorting to ground at distributor or on the way the the junction block. (Points not likely the problem, but a Pertronix 1885 as points replacement would eliminate both of those possibilities). Also check wiring harness to injectors and sensors. You already looked at trigger points. Presumably no oil coating?
 

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1972 350sl 240k miles silver with blue interior. 1973 450sl
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15 Posts
Discussion Starter #18
when reading dwell, the trigger lead on my dwell meter is hooked to the positive side of the coil. it reads exactly half [not clear as to why] and when switched to tach, it reads exactly double when compared to the cars tach I thought that might be normal for this type ignition?
I am not a fan of the petronics set ups.
 

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1972 350sl 240k miles silver with blue interior. 1973 450sl
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Discussion Starter #19
saga continues: started car and it is barely running, wouldn't move under it's own power without stalling. had my wife keep it running while I hooked my timing light up and move the inductive clamp from one spark plug wire to another. tested all 8 cylinders and had a consistent flashing light. pulled injector plugs out one at a a time. 6... six...yes SIX did not change the way it would run. the two front cylinders would stall it when disconnected.......checked wiring harness and connections again. decided to change the throttle body because I had one from a lower mileage car. I could feel a lot of slop in the one I took off compared to the "new" one. swopped it out and the vehicle ran the same, 2 cylinders and maybe a third would kick in at some random point.
IMG_0373.JPG
 

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1972 350sl 240k miles silver with blue interior. 1973 450sl
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Discussion Starter #20
I then pulled the distributor out and removed the lower set of points and looked at them, they looked no different than the other three times I had them apart. I wanted to check the dwell of them and came up with this solution. I hooked a continuity tester to the center terminal on the plug. I then one at a time hooked the other lead to each wire to the points and rotated the distributor. I made a mark on the body at the start and stop of the points being closed, then measured the degrees of closure using a clear plastic protractor.
is this the way it is done? and now that I know they are off how do I set them? there seams to be no visible way to get to the lower sets of points. trial and error?
decided to put the distributor back in and put the car back into the garage till I found my answers.
of course the car ran absolutely perfect and the throttle response was noticeably better. I put a foot and a half of rubber down in the driveway. fixed? I doubt it.
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