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1975 450sl
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Discussion Starter #1
Hi everyone, coming here hoping to get some more of the infinite wisdom (other account got locked out).

I've got a 1975 450sl, been restoring it, did not run when I got it. One of the timing chain guides was broken (missing), replaced guides and timing chain, used a woodruff key to get both sides lined up correct. Cranked right up. I should add that it has a new fuel pump on it but pressure was checked and set at 28 psi. My problem is that it runs really rich and after only a few miles or idling for 15-20 minutes all of the plugs are completely black. Im running an electronic ignition instead of points with correct coil. Resistor wires with non resistor plugs (NGK BP5ES). Computer under dash has knob set in the middle (I think 13 clicks).

Any ideas? It sometimes will idle a little erratic like the computer is trying to regulate something, but pretty consistent when warm (until plugs start getting more fouled then it starts to die). Timing is set at about 5-10 BTDC, but sometimes sounds like it skips a cylinder and will pop out of the back and sometimes out the front when cold? Double checked the plug wires to ensure nothing got swapped. Any other smoking guns? I've checked for vacuum leaks and what not.
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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Seems like we have an epidemic of rich running Djets lately!

A few questions to get us all on same page:

  • Type of new pump - is it a Djet type pump or one for a later model or ???
  • Type of electronic ignition?
  • Resistor wires? Carbon core or ????
-Coil changed?
-Resistors?
- Original switchgear (Electronic ignition module) still in circuit?

Moving on:
1. Plugs are OK.
2. Timing should be at somewhere in TDC-5BTDC at idle with vacuum connected. Where you are may be Ok if it is with vacuum disconnected. Vacuum retard timing by about 7deg. Sounds like your timing is off, for whatever reason.
3. ECU knob only controls idle mixture.
4. If idle surges up and down, that is usually because AAV is sticking and not fully closing as engine warms up.

5a. For rich running, I would first look at the coolant temperature sensor (right next to AAV) and check it vs the specs in the shop manual. Also make sure it IS properly connected. Air temp sensor not so important, but check it too.
5b For Rich running, check the angle that each pair of trigger points stays open. As the fiber blocks wear this angle gets larger and ECU seems to get spurious signals that cause injectors to fire too often or too much.
5c For rich running, check the adjustment screw on the MPS and see if it looks like it has been tampered with. Test the MPS as outlined in the shop manual.

6 Check that fuel rails maintain pressure for at least 20 min after engine is turned off. 28psig is a bit low. I would use 29-30 for a start (adjust with Fuel Pressure Regulator)

Read The Comprehensive Guide in the stickies (EGV107) by Proctologist under Djetronics. It has a lot of good stuff from a guy going through similar process with newly acquired Djet. It may answer some of the questions you may have.

For what it's worth, when I had a similar problem, I had to adjust the trigger points so they all stayed closed for 120deg. But I had not changed ignition, as you have.
 

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1975 450sl
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Discussion Starter #3
  • I will have to go out later and check pump brand/model (swapped it a while back).
  • Ignition system is pertronix, followed the guides here.
  • Wires, will also check, is carbon core good/bad? (sorry for my ignorance here)
  • Vacuum is connected, around the 5-10 mark it seemed to be the happiest so I left it there, can move back though.
  • Idle surges only while cold, when warm it typically acts better until the plugs turn black.
  • Blows black smoke out of the back when I floor it
  • I dont think the fuel lines hold pressure, could that mean my injectors are sticking open?
  • Will check temp sensor as well as mps
  • Been through the comprehensive guide frequently, answer is probably there but I havent found it yet..
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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If your fuel lines don't hold pressure, look at section of engine manual starting at 07.4-120. It covers testing fuel rails for leak-down. It is possible for example, that Cold start valve leaks and puts un-needed fuel into manifold. You need to tee into fuel line so that the CSV gets tested. Pressure should not go down to less than about 22psig 5min after engine is tuned off.
 

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1975 450sl
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Discussion Starter #5
Okay ill give it a look tomorrow, double check the pressure, fuel pump model, and spark plug wires. Anything else anybody would recommend?
 

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1975 280SLC
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If it's only black soot? thats easily cleaned off sounds like too rich. If it's oily sticky harder to clean worst case could be oil passing rings or valve stems.
 

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1975 450sl
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Discussion Starter #7
That's a fair point to clarify Andyslc, its definitely just black soot, runs very rich, no oil. I'm going to look at the cold start injector and make sure it is up to code and not leaking.
 

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'72 450SL, 107.044-12-000422
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As Graham pointed out, the leak down check is a quick way to determine if any injectors, including the CSV, are stuck open. Adjust pressure to 30 then check if it holds with the pump off. If that’s good you could try running it with the CSV unplugged. It’s possible that ecu or temp sensor is not giving it the signal to close after warm up.

Your idle mixture pot on the ecu is in about the middle position (the full range is 22 clicks so middle is 11). Does adjusting that to the full lean range have any effect? I can’t remember if CW or CCW is lean but you could try both ends of the range unless someone else chimes in.


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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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As Graham pointed out, the leak down check is a quick way to determine if any injectors, including the CSV, are stuck open. Adjust pressure to 30 then check if it holds with the pump off. If that’s good you could try running it with the CSV unplugged. It’s possible that ecu or temp sensor is not giving it the signal to close after warm up.

Your idle mixture pot on the ecu is in about the middle position (the full range is 22 clicks so middle is 11). Does adjusting that to the full lean range have any effect? I can’t remember if CW or CCW is lean but you could try both ends of the range unless someone else chimes in.
CCW is leaner.

This knob only controls idle mixture. If throttle is only a hair open, this adjustment is not active.
 

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1975 450sl
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Discussion Starter #10
Updates after looking at it today
  • Fuel line held at 22/23 psi after engine off for 20 minutes
  • Checked the resistance of the temp switch, it was fine both cold and hot
  • Turned the ECU all the way CCW, didnt really make a difference
  • Vacuum hoses from to distributor were backwards (misinterpreted the diagram) swapped them and retimed to 5 deg BTDC
    • Thinking this probably wouldnt cause the plugs to foul but would explain odd misfires if was retarding with higher rpm's?
  • Unplugged the MPS and car wouldn't start? I think I had read where some people leave it unplugged? What would this do to the djet system? Plugged it back up and it cranked right up.
 

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1973 450SL w/ 65,000 miles
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To your last bullet point - my '73 will not start without MPS plugged in, so I don't think there is anything peculiar with your's not starting with it unplugged (MPS is only taken into consideration when throttle position sensor is not at idle). Have you checked whether your MPS can hold vacuum? Is the black cover on the adjustment screw still on the MPS or has it been removed?
 

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1975 450sl
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Discussion Starter #12
I have not messed with it, I think the black cover is still present. I haven't checked if it can hold vacuum, to be honest I haven't done much with the MPS.
 

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Black cover still being there is good because it means no one has adjusted the screw, which should really not be adjusted without exhaust analysis equipment. Great overview of MPS with testing info here at 4.4 4. MAP sensor (MPS) on how to test vacuum and resistance of two windings.

If it does not hold vacuum. There is a thread by Cush on this forum where he identified o-rings that will likely solve vacuum leak issue.
 

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1975 450sl
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Discussion Starter #14
Thanks for the info, ill take a look into it tomorrow hopefully. If it doesnt hold vacuum could that cause it to run rich?
 

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I think so because the purpose of the MPS is to provide load enrichment (full load = vacuum near 0; idle = vacuum near 15in hg) if MPS cannot hold vacuum doesn't it think it needs full load-enrichment at all times because it thinks engine is at full load... I think that's how it works... check out Anders for tons more info.
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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  • Unplugged the MPS and car wouldn't start? I think I had read where some people leave it unplugged? What would this do to the djet system? Plugged it back up and it cranked right up.
You might have been thinking about the TPS (Throttle Position Sensor) . MPS and ECU work in tandem and are both needed!

Sounds like your fuel pressure is in order.
You said you tested the temperature "switch". Hopefully that was the coolant temperature sensor TS2.
Vacuum test of MPS is important
Timing is now at 5BTDC at idle with vacuum attached. Does timing advance to about 27BTDC at 3000rpm? (Centrifugal advance should do that)
Check the closing angle of each set of trigger points. You can do it in place or more easily if you pull the distributor. You will need Norbert's tool to adjust, but do the test at least. (numbers over about 140-150deg can cause the richness you see. We don't know exactly why, but probably bounce in points causing incorrect signals to ECU.
 

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1975 450sl
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Discussion Starter #17
Must have been thinking about the TPS.

Yes TS2 was tested, ill double check the air temp sender (had it running with the air intake taken off/air temp unplugged and it seemed like it ran better).

Timing advances, and engine sounds good, not sure if it goes to 27 as I didnt have a second person to look while advancing.

Are you referring to the points in the bottom of the distributor? I have pertronix ignition on the top. Im following the other D-jet thread as well, I know that was mentioned but he still has a point system.
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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Are you referring to the points in the bottom of the distributor? I have pertronix ignition on the top. Im following the other D-jet thread as well, I know that was mentioned but he still has a point system.
Yes - the lower points are the trigger points. aka pulse generator. They are a key part of the Djet system. As they open and close, they send a signal to the ECU to start an injection. They fire two injectors at a time. Injectors fire into the intake manifold, just upstream of the intake valves. These point have fiber rubbing blocks, and much like ignition points, these blocks wear. Bosch said they had a life of about 100k miles. There is no adjustment, but with the tool I mentioned, they can be adjusted by physically bending the point arms. Send a PM to Nordfisch and he will send you a drawing and instructions. He used to mail the tool for about $20, but at present is not doing that, it seems.

To measure closing angle, use an ohmmeter between the middle contact 12 and each other one in turn, while rotating rotor (or engine if distr in place) Here is a recent pic from an owner treozen on Peachparts. That is just a cap off a coffee or similar can. Some used an old CD!
 

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Discussion Starter #19
MPS holds good vacuum and resistance is right on spec based on Anders guide. Looks like im gonna pull the distributor and get to work on the bottom points.
 

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Discussion Starter #20
Looks like I found a problem, points measured 160, 165, ~180, 150. Measured several times, the 180 surprised me.
 
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