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450 SL 1975 US Version
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5 Posts
Discussion Starter #1
Hej!

I have had this car just 2 months and it has just failed its MOT due to too high CO 8 (should below 4) whilst idling.

Actions so far:
New thermostat/oil/temp sensor plus checked plugs/distributor/trigger points/vacuum hose/cold start valve. Today I tried lowering CO by turning the knob on the ECU whilst at a garage - the CO did not change a bit despite turning the knob.

Anyone had similar issue? Could it be something with the throttle switch? Any ideas appreciated.

Regards
Patrik
 

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Outstanding Contributor
450slc5.0cab 280sl5sp 280se4.5 500se+500slAMG +250seStkW108 350sl4spdX3 500secEuro
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22,672 Posts
Where is your timing set? I believe CO goes up with more advance.

Are you sure all your cylinders are firing? If one cylinder is dumping unbury fuel into the exhaust, you will have more incomplete combustion.

Ideally, you want CO2 and H2O coming out with like 1% CO I think. It should be right on the emissions tag where the timing should be.
 

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450 SL 1975 US Version
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5 Posts
Discussion Starter #3
Hej!

Yes all cylinders are firing. Measurement done whilst idling. Thanks for input.
 

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Outstanding Contributor
450slc5.0cab 280sl5sp 280se4.5 500se+500slAMG +250seStkW108 350sl4spdX3 500secEuro
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I remember my exhaust being MUCH cleaner when set to spec like 5 ATDC at idle with no vacuum on my 1973, but the 1975 is different I think.

Do you see the emissions plate I’m talking about in your radiator support?
 

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'72 450SL, 107.044-12-000422
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282 Posts
The knob on the ecu only controls mixture at idle so it it possible that throttle position sensor(TPS) is out of adjustment. If the throttle plate is only slightly open, mixture is controlled with manifold pressure sensor (MPS).

Were you able to check CO at ranges other than idle? If high at all ranges, assuming ignition is good, may be a problem with too much fuel from the FI system. What do the spark plugs look like?


A few things to check:

1. Fuel pressure at 30 psi.
2. Fuel pressure should not drop off from 30 quickly after the pump stops running. If pressure does not hold (it shouldn’t drop below about 22 psi after 20 minutes), could be one or more injectors stuck open.
3. trigger points worn/out of adjustment.

Heres a couple of recent threads with a similar issue.

73 Running Rough & Rich


Plugs Keep Fouling...


Sent from my iPhone using Tapatalk
 

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Premium Member
'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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10,563 Posts
If idle CO is the only problem, have a look at a few things:

1. Do you have vacuum at the distributor? There should be and it should retard the timing. I am not familiar with the 75. It is more complex that earlier versions (covered in section 14 of shop manual under 1975 model), but I think the vacuum connection is the lower one on the distributor vacuum pod. If no vacuum there, see if the vacuum hose is disconnected at the throttle body or at the switch over valve that is part way down the vacuum line. Theck if you have vacuum at that valve. Tubes may be reversed if no vacuum on distributor side.
2, A/C should be turned off when doing the idle CO test because it disables the vacuum retard
3. If you have a timing light, check the timing at idle. For the 75, it should be at TDC with vacuum connected. It is probably at about 8deg BTDC without vacuum (don't test for CO if that is what it is!)
4. You could reduce fuel pressure to say 27psig, that will make mixture leaner, but maybe not by enough (don't leave it like that!). If it is higher than say 30psig, that would cause higher CO.
5, Measure resistance between pins 12 and 20 of the throttle position switch - it should be zero. (You can read my kind of rudimentary DIY here: https://www.benzworld.org/attachments/adjusting-and-cleaning-throttle-position-switch-on-r107-me…-pdf.257408/
 

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450 SL 1975 US Version
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Discussion Starter #7
Hej!

Sorry for the delay. Been on a mini vacation with family.

HUGE appreciation for your responses - very high quality and really helpful. I have adjusted the TPS as outlined in your DIY guide and now the knob on the ECU actually has an impact. On my way to the garage yesterday to check CO I ended up in a very long queue and was idling for quite some time (15 mins) - eventually the car just died. Half hour later we managed to get it going with starter cables but no idle. Went back home. Battery was kind of dead. Recharged battery and tried starting but no luck. Used started cables again and after 2-3 tries the engine fired up. Seems like battery is gone.

Kind regards
Patrik
 
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