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  #1  
Old 04-15-2002, 01:41 PM
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380SLC rough idle and running hot and cold

My 1981 380SLC has started idling and running very roughly when either hot or cold.

Starts immediately at all times.

Has twin row chain. 125k on engine. Fuel pump and filter changed 3k ago.

Any ideas would be welcomed - couldn't quite find something similar in the archives.

I'm wondering if it's a bad spark plug - is there a safe way of pulling these with the engine running to see if there's any change? Would it be OK to pull them at the distributor end? (The engine end is tough on a hot engine).
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  #2  
Old 04-15-2002, 04:15 PM
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Engine Running Rough

Thomaspin,

This thread might be of some help:-

http://forums.mbnz.org/forums/R107/vthread.asp?messid=881

Remove one of the spark plugs to check whether it is running too rich to eliminate one of many possible causes.
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  #3  
Old 04-16-2002, 12:12 PM
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Diagnosis so far

I pulled the spark plugs one by one and the engine continued to run very poorly, with no change noted.

Checked the OVP relay (the 1981 still uses an internal fuse - this migrated to an external fuse on 1982 and later) and both the diode and fuse in the relay checked out OK. It's in a red metal can cube above the fuses in the passenger footwell and has a pin-out diagram stamped on top - very helpful.

Checked the vaccum hose routing and noted that the hose (white/blue/purple) from the ignition advance/retard unit (on the ignition distributor) to the air pump (pumps air into a/r unit) was fractured. This runs across the engine and must be subjected to a lot of heat - it is so brittle that parts simply break off if you try to flex it.

I will replace this weekend - could that be the source of my problem?
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  #4  
Old 04-27-2002, 04:15 PM
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More diagnosis

I replaced 2 fractured vacuum hoses - one (white) that runs to the warm-up compensator from the green side of the white-green valve, the other (white/blue/violet) from the air pump to the advance unit. No change.

Replaced the OVP relay. all 8 plugs gapped at the recommended 0.8mm (Bosch W8DCO copper, no resistor), checked resistance of all plug wires from boot to cap (all 5kohm), replaced distributor cap and rotor (overdue anyway) and nothing changed.

Checked resistance between terminals 1 and 15 of the coil - came in at 0.2 ohm, slightly below factory recommended 0.3 ohm minimum.

Followed the diagnosis in the MB 116.96 factory engine manual at 15.7 III-562/7. This states that pin 3 of the diagnosis socket (LH front wheel well) should return battery voltage against ground with the ignition switched on (I asume that means engine not running). I got only 3.80 volts. Next step is to test voltage between diagnosis socket pins 5 and 4 which should return 0 volts - I got 2.81 volts. Book says this means the ignition switching unit s/b replaced. Before I replace this expensive part, does any one have any input which might invalidate my diagnosis?

Once again, the engine runs rough at all speeds and temperatures.
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  #5  
Old 04-28-2002, 11:37 AM
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Rough iddle

Thomaspin,

Up to sept 81 this vehicles had a TSZ4 transistorised ignition switching unit and two separate ceramic ballast resistors with a resistance of 0.4 and 0.6 Ohms respectively (imprimmed on the color clamps). Check these and verify resistance.
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  #6  
Old 04-28-2002, 09:53 PM
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Resistor check

Thanks for the input. The car only has 120k miles on it so doesn't that rule out the fuel distributor?

I checked the two resistors and they measured 0.2 and 0.5 ohm - close to spec.

Any other ideas would be welcomed.
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  #7  
Old 04-29-2002, 11:34 AM
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Diagnosis

Bribenz - you may wish to read my earlier posts. Plugs are new, so is the ovp, distributor and rotor.

I confirm no arcing at any of the plug wires.

I have sprayed engine starter on vacuum connections when the engine is running and no change noted, suggesting no (obvious) vacuum leaks.

Is it the age of the distributor or the number of miles on it which points the diagnostic finger - wear being a function of aggregate fuel flow volume, no? 125k miles is a very low mileage for a car run on gas purchased from high volume, crud free CA pumps.

Or are you saying you have specific experience of this symptom suddenly appearing because of fuel distributor problems? If so, your diagnostic process would be appreciated. Also, why would the engine suddenly exhibit rough running symptoms rather than gradual deterioration, which I would expect of fuel distributor problems, or could check their 1981 380 motor for comparable readings?

Anyone have a specific answer or specific experience of my earlier detailed technical reference to the voltage readout at pins 3/ground and pins 5 & 4, please?
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  #8  
Old 05-01-2002, 06:03 PM
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It's not the warm up compensator

The car runs rough hot and cold; also, I have checked the warm up compensator using Probst and the appropriate gauges, and it's fine, so I doubt it's the warm-up compensator.

How about my diagnostic socket readings - anyone out there with a 1981 380 motor?
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  #9  
Old 07-25-2002, 01:50 PM
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Follow-up

I thought it might be useful to share the resolution of this problem, given that no one could pinpoint it.

In frustration I took it to a local independent who used an interesting technique of pumping a visible gas into the engine (having agreed with me that it was not an electrical problem) to test for leaks. I was informed that historically this technique used propane (invisible) and a sniffer had to be used to detect leak locations - obviously imprecise. The colored smoke is much easier to 'see'.

This diagnosis disclosed leaks at the fuel injector seals (#7 was especially bad) and smaller leaks at the intake manifold. Having just replaced all the injectors and seals on my 560SEL it was frustrating to have to pay someone to do a job I can do competently, but I paid my $473.88 for the injector seal job and the problem is (largely) gone. SF Bay area pricing.

The cost of fixing the intake manifold leaks was quoted to me at $1,400 - evidently lots of labor hours are involved. Does this sound fair?

A related point. Earlier in this thread I wrote: "Followed the diagnosis in the MB 116.96 factory engine manual at 15.7 III-562/7. This states that pin 3 of the diagnosis socket (LH front wheel well) should return battery voltage against ground with the ignition switched on (I asume that means engine not running). I got only 3.80 volts. Next step is to test voltage between diagnosis socket pins 5 and 4 which should return 0 volts - I got 2.81 volts. Book says this means the ignition switching unit s/b replaced. Before I replace this expensive part, does any one have any input which might invalidate my diagnosis? " I checked my findings with Barry Stark (a 380SL owner and a good diagnostician) and he confirmed the MB manual appears to be incorrect. His car runs perfectly and returns similar voltages to mine.
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