Author Topic: Fuel Injection Harness  (Read 5577 times)

brettj

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Fuel Injection Harness
« on: 09 August 2011, 05:07 PM »
After replacing the cam shafts in my 1975 450SE D-Jet,  the motor seems to be running very quiet and smooth. One thing I did notice was a very slight periodic misfire shaking the motor. I've replaced just about all of the fuel injection system and was thinking that perhaps I have a short in the fuel injection harness which may be the cause of the misfire. Before I had the cams replaced I had a warm start stall probelm that I couldn't shake and now am thinking that the harness may be the source of that problem too. Has anyone out there replaced their fuel injection harness? If so did you notice a large improvement in the way the motor ran? How hard of a job is it to change the harness itself? I can't seem to find anything on harness replacement in the service manuals.

jbrasile

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Re: Fuel Injection Harness
« Reply #1 on: 09 August 2011, 10:36 PM »
brettj,

Did you replace your distributor cap, rotor, ignition wires, points, spark plugs and coil?

I would make sure the ignition system is 100% sorted before blaming the fuel injection harness.

Replacing the harness is not very difficult however, the part itself is quite pricey and NLA for certain models.

If after checking your ignition components you still feel you need a new harness,pls advise your VIN number so we can try and quote a new one from MB.

Tks

Joe

brettj

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Re: Fuel Injection Harness
« Reply #2 on: 10 August 2011, 05:08 AM »
Hi Joe,

I replaced the distributor cap, rotors, spark plug wires, battery, coil, points and even the trigger points located in the lower section of the distributor. I've also replaced all of the fuel injectors, throttle switch, map sensor, fuel pressure regulator and all of the electrical relay cubes. So what it comes down to is the harness. I'll be picking up the car tomorrow from the shop. Hopefully my mechanic was able to get rid of the misfire and the warm start stall by adjusting everything properly. I plan on driving the car a while to see what happens. If no change, I think I'll move on to the harness. Do you feel I'm on the right track?

koan

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Re: Fuel Injection Harness
« Reply #3 on: 10 August 2011, 06:28 AM »
When everything ignition and injection have been replaced and there's still a slight misfire at low idle it's time to start checking compression.

koan
 
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brettj

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Re: Fuel Injection Harness
« Reply #4 on: 10 August 2011, 06:33 AM »
Koan,

If you have compression issues, does that mean valve job?

koan

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Re: Fuel Injection Harness
« Reply #5 on: 10 August 2011, 06:47 AM »
If you have compression issues, does that mean valve job?

Yes or worse.

A compression check should be done dry and wet. Dry is done cranking a warm engine, wet is the same with a several ml of oil squirted in spark plug oil.

A significant difference in the dry and wet readings suggests leakage past piston rings. Similar readings on a down cylinder indicate a valve job is needed.

The manual gives expected figures and acceptable variation between cylinders.

koan
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brettj

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Re: Fuel Injection Harness
« Reply #6 on: 10 August 2011, 06:25 PM »
I went to test drive my car at the shop today after having the cam shafts replaced. The motor definitely has more power and is so much quieter. The bad news is that the cam job did not solve the rough idle at low idle speeds issue. With the motor running in park, it still periodically shakes. Sometimes when you put the car in reverse the rpm's drop quickly and the motor tries to stall. At this point I'm ready to give up on this car. What could cause this condition? I've had this probelm since I pulled the car out of the garage 4 years ago. It just won't go away! My mechanic feels that there may be leaks in one or more of the cylinders. I am still thinking that its the fuel injection system harness as the motor does not display the normal symptoms of a motor with cylinder leaks. The motor has plenty of power, doesn't get poor gas mileage, I average between 14-15  mpg, and it doesn't use  oil. You never see blue smoke coming from the exhaust.

Koan you said that the car may need a valve job or worse. What is the worse?

koan

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Re: Fuel Injection Harness
« Reply #7 on: 10 August 2011, 09:18 PM »
Koan you said that the car may need a valve job or worse. What is the worse?

Worn piston rings is a far worse job to tackle than heads needing a bit of work.

But you say you get a near stall when you engage a gear, maybe your idle RPM is just a bit low. Not at all familiar with D-Jet (where's oscar when we need him) but there will be an idle speed air (not mixture) screw somewhere to lift the idle RPMS to avoid the near stall, also might help with the rough idle.

But a compression check is worth doing anyway.

koan
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gavin116

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Re: Fuel Injection Harness
« Reply #8 on: 10 August 2011, 11:17 PM »
Hi Brett

I had similar problems to you, the car shaking violently especially when at rest in "D" at a traffic light or in a traffic jam.  When I did my rev-counter update, it was then that I realised that my idle RPM was too low!  A quick adjustment of the idle screw brought the revs up, and the engine was now wonderfully smooth!  To get this spot on, the car must be at operating temperature.  With the handbrake on, and the car in "D" adjust the screw till the RPM reads near enough 700 to 750 RPM, and that should do it.  See my picture below for the screw to adjust (you can actually turn the 'plastic' screw by hand).

I recently changed the coil, HT leads, rotor and dizzy cap, and this made things a little worse, but bearing in mind that the CO and ignition timing was set up for the old HT system.  My car is at the shop too, and I think once it has had a 'professional' tune she should be running really sweet!

(It is also worth checking that the vacuum lines are not un-done or broken or that the rubber connections to these are not perished and leaky.)

Good luck, and here's to your smooth running!

Gavin

The idle adjust screw is circled in red.  I can't remember, I think you turn it anti-clockwise to up the RPM, but you will soon see which way to turn it.
« Last Edit: 10 August 2011, 11:31 PM by gavin116 »

koan

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Re: Fuel Injection Harness
« Reply #9 on: 11 August 2011, 01:27 AM »
Good advice Gavin but don't know if the pic is any use, Brett's is D-jet, yours is K-jet.

koan
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gavin116

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Re: Fuel Injection Harness
« Reply #10 on: 11 August 2011, 03:22 AM »
Whoops!  Well spotted Koan.
 :-[

oscar

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Re: Fuel Injection Harness
« Reply #11 on: 11 August 2011, 09:03 AM »
Howdy all,  

- Gav's suggestion applies to d-jet too but if adjusted, I reckon Brett's idle rpm will runaway well over 1000 as soon as he puts it back into Neutral or Park.  I believe the vacuum switch on the firewall that adds some advance to ignition when the A/C is on, is also supposed to add advance when the engine is under load at idle. My car still does the same thing as Brett's here but I've yet to sort it out or discover what is suppose to make this advance happen to avoid these stalls.  

- The harness.  I'd be checking for continuity through all the wires with a multimeter before I'd contemplate replacing a harness.  Though a new one would look beaut, I can't imagine it would be cheap if available.

- This misfire.  I'd point finger at the distributor and guess vertical shaft play as the issue.  To check, you or your mechanic should attach a timing light and check how the timing marks appear.  Do they jump back and forth by small amounts or are they stationary?  Your timing marks should appear stationary regardless of other issues that might cause a misfire.

I finally found an old thread where I first heard about the effects of vertical freeplay so I'll quote parts of oldmercs3's posts rather than attempt an explanation,

Pulled another distributor from a junker this weekend, my original dizzy had excessive vertical movement in the shaft which caused erratic timing.  Disassembled the second distributor, lubed everthing up, put in new points and installed it; timing is now solid as a rock.  With the original distributor I felt performance was sluggish even with the timing set to 8 deg BTDC, I was also experiencing what felt like a high speed misfire.  Advanced the timing to 10 deg BTDC and what a big difference!  Can't really say how much of the improvement can be attributed to simply having the tighter distributor vs the advanced timing though, at any rate I liked it so much I cranked in another 2 degrees for a total of 12 deg BTDC; car feels 1000 lbs lighter, no pinging and the running temp seems to remain the same......  
And...
If you have a noticeable amount of vertical movement then that usually translates into sloppy gear lash which will have a tendancy to alter your timing depending on the load placed on the distributor drive.  Didn't think much of gear slop until I compared my dizzy w/another, if you notice how the rotor will change position when you install and remove a distributor then you have an idea what mine was doing to a lesser extent when bolted down....... If your timing is jumping around then I'd definitely make a sloppy distributor the #1 suspect.
1973 350SE, my first & fave

KenM

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Re: Fuel Injection Harness
« Reply #12 on: 11 August 2011, 03:11 PM »
That's interesting info Oscar, thanks for posting that. Brett if this is your problem, here'e the answer,

http://cgi.ebay.de/0231403008-Zundverteiler-Mercedes-450-W116-W107-Bosch-/110631409403?pt=DE_Autoteile&hash=item19c22556fb

Seems like you've come this far with the car, might have to grab one of these while they are available.
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Chevy Chase: 'On the lips will be fine'

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jbrasile

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Re: Fuel Injection Harness
« Reply #13 on: 11 August 2011, 08:32 PM »
brettj and Gavin,

Idle speed should be adjusted to 750-850 rpm  with transmission in P or N.

Is the miss at idle only and does it feel like a "stumble" or is it present at all engine speeds like a missing cylinder?

Would you describe it as a constant shake at idle? Have the valves been adjusted after the camshaft replacement?

I think you are on the right track but we need to exhaust all possibilities before looking at a harness replacement.

Tks,

Joe


brettj

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Re: Fuel Injection Harness
« Reply #14 on: 12 August 2011, 06:05 AM »
Thanks for all the replies! I drove the car home from the shop yesterday $1100.00 dollars lighter. What did I get for my money?  A horrid rough standing idle! The car shakes while in drive and are stopped at a light or in traffic. In the past to correct the idle I have done what Gavin suggested and raised the idle. That usually smoothed it out but doing so did just what Oscar said raised the RPM's up over 1000.

So, last night I took out my handy exhaust gas analyzer to see if the fuel mixture was set correctly. I told my mechanic to balance the fuel mixture and its obvious that he didn't because the tester shot right over to too rich. I then proceeded to drop the idle which was set too high anyway and adjusted the fuel mixture at the CPU. I got the mix  just about perfect but am concerned that the analyzer indicator does not stay at balanced but moves a couple of notches to the rich side and then a couple of notches to the lean side and then back to the middle (balanced) stays there a while  then once again fluctuates. I assume this means the fuel is flowing to the motor and is being consumed properly. The result of all of this is that the motor smoothed out for a while, the engine speed seems good, it doesn't want to stall in reverse but the rough idle still exists in drive and park.

Since we are talking distributor issues, I thought I would test the dwell. It got dark out last night before I could get my dwell meter hooked up so that test will have to happen today. So from what happend last night, do you feel that a bad distributor could cause this fluctuation in idle?

Joe, the rough idle is strongest in drive and exists in park as well but is not as strong in that gear. The misfire feels like a stumble and is not present at all engine speeds. While driving, the roughness goes away and only comes back when you slow down to stop. The car has plenty of power and accelerates very well.