1972 350SL -- no start - Page 2 - Mercedes-Benz Forum

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post #11 of 21 (permalink) Old 03-26-2008, 01:09 PM Thread Starter
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Thanks to all responents for your thoughts.
About ready to throw in the towel!
Have decided that the efi sytem is ok. Checked all the inputs to the ECU and measured good. Checked power and ground and OK.
Have reset the timing. Replaced coil with factory replacement.
When trying to start, get back fires and flames from air intake. Seems like timing is incorrect, have rechecked timing but it appears to be correct per timing marks on flywheel and location of rotor in distributer (tried rotating distributor 180 degrees).
Also replaced distributor points with PERTRONIC 1885 with same results and then replaced original MB points and ignition system.
Anyone have any thoughts ?

Thanks in advance.

Last edited by halkamp; 03-26-2008 at 01:19 PM.
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post #12 of 21 (permalink) Old 03-26-2008, 04:28 PM
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I guess the next thing to check would be timing chain, for valve timing, etc. Chains can be loose. Or, may have jumped a couple of teeth. Good luck!
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post #13 of 21 (permalink) Old 03-27-2008, 06:26 AM
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recheck ignition ,always start with the engine on #1.Line the mark up on the damper with pointer.
the distributor rotor points at a small line in the housing .Check the rotor for cracks. It also has a resistor in it,check it to make sure it has continuity.
The distributor rotates clockwise.Double check the firing order.it's on the cam cover.
How old are the plug leads?
never use silicon carbon core plug leads because they have too much resistance for the Mercedes ignition. Use only genuine plug leads and ends.
Check the ballast resistor(s) for continuity.
The Green lead from the distributor is crucial .Make sure both ends are in perfect condition. Don't be tempted to change it for a another piece of regular wire because even the length of the green lead is important.
you have pertronix so points are not an issue.
Check the trigger points under the distributor.have they been replaced lately?
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post #14 of 21 (permalink) Old 03-28-2008, 01:19 AM
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No Start problems

OK, I am assuming you have a D-jet system with electric injectors that look very much like a modern EFI system but the brain is triggered by the extra sets of points in the distributor.
You mention to start with that noid lights aren't triggered with engine cranking. Before you go too much farther you need to see if the ecu is getting power and if so, then you need to verify the connections to the trigger points sets (not the ignition points SET) and then trip them manually with the system powered and see if you get noil light trips. Temp sensor 1, 2, warm up valve, won't prevent a start, the MPS (the fist sized manifold pressure sensor) can prevent running well but shouldn't prevent injector triggering. The TPS (throttle position sensor) should be also checked for connections but this won't prevent the injectors from firing. There is not a lot more to that system as for fuel delivery

Your ignition system is in a good way I assume?
If the car will fire (bump) with some starting spray shot down the throttle body then your ignition system is working, if not, then you need to see that you are getting fire...

all the best

900 for an ECU sounds high honestly, I'd check inside the unit and see if there are any burned connections at the board/component solder joints. couldn't hurt
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post #15 of 21 (permalink) Old 03-28-2008, 12:37 PM
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Quote:
Originally Posted by rabbit188 View Post
I guess the next thing to check would be timing chain, for valve timing, etc. Chains can be loose. Or, may have jumped a couple of teeth. Good luck!
Did anyone suggest a compression test?
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post #16 of 21 (permalink) Old 03-28-2008, 01:52 PM Thread Starter
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Thanks,

Checked compression and read 135 lbs.
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post #17 of 21 (permalink) Old 03-28-2008, 02:00 PM Thread Starter
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The vehicle does have a D JETronic system.
Checked the trigger points at bottom distributor....OK
Noid light does flash when cranking engine.
Checked power and ground at ECU connector and is OK.
Car does occasionally backfire and flame from air intake when cranking.
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post #18 of 21 (permalink) Old 03-28-2008, 02:01 PM Thread Starter
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Any easy way to check timing chain ?
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post #19 of 21 (permalink) Old 03-28-2008, 10:32 PM
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timing check

Timing check is simple. You are going to have to clean up around your valve covers then remove them.
Rotate the crank to TDC making SURE you turn the crank only the correct way (you can confirm this by watching the radiator fan and making sure it is rotating in a direction to pull air BACK towards the engine.
(crank can be turned by a large socket on the hub retainer bolt inside the belts pulley)
look at the marks on the cams and the front cam bearing assembly, if they are close then you have the crank at the right position, if they are not there then turn the crank through another 360 degrees and they should come close to lining up
the marks will probably not line up perfectly but may have up to 3-4 degrees off with the crank at TDC, this is due to the timing chain wearing into the timing chain guides. (you will also want to look at the upper plastic guides and replace them) this can actually be corrected by offset woodruff keys MB sells but what you are looking for is a lot more than 3 degrees, if you see something like 10 degrees off then one of the sprockets has jumped a tooth
if this is the case, run a compression check on the car, all 8 cylinders and if they are all within 10-15% of each other then you are ok and can retime te engine, replace your upper timing chain guides and replace the tensioner assembly
if you have some or all cylinders with no compresson then you are looking at cylinder head removal
just remember, keep everything as CLEAN as possible

a lot of problems have been anecdotally reported for V8s with single row timing chains and best practice is to change sprockets and the chain for a double row setup but this is a true PITA procedure
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post #20 of 21 (permalink) Old 03-28-2008, 10:45 PM
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follow up

one last thing, you should remove the distributor cap when you do this procedure (take off all the plug wires and remove the plugs also, makes turning the engine so much easier) when you get the cam marks lined up look at the distributor rotor and see if it is pointing near the slash mark on the distributor (lined up more or less with the #1 plug wire (cap is marked), if the cam marks are lined up and the distributor is 180 degrees out then you have your problem
provided everything is ok and you can get it to run well you really should replace the oil pipe plastic pieces and the chain guides as previously mentioned

those old 107s are truly neat cars
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