Hello,
Now that the weather is getting better here in Wisconsin, I am again trying to find the cause of an engine miss in a '73 280SEL 4.5 I purchased in December.

Cylinders 2,4,7 & 8 are not firing. I have replaced the rotor, cap, wires & plugs. Ignition points are on order. All cylinders getting spark. Dwell and timing are OK. Compression test OK. I pulled out the trigger points and cleaned them. I pulled out the injectors, cranked the engine and they spray OK. Valve clearances are OK. What else should I check? Please help. Thanks. -Mike

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1973 280SEL 4.5

I would go with a trigger point problem since since 4/8 and 7/2 are groupings on two of the points. Check also the connector.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

I though it might be the trigger points for the reason you mentioned plus also one of the metal prongs is worn which may allow 2 sets of the points to not work properly if at all. But if they need replacing, why would the injectors work when I pull them out with the ring line and crank the engine?

Here is a picture of where the metal prong on the trigger points is worn.

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1973 280SEL 4.5

I suppose it is possible that they work at cranking speed but not at running speed. I have seen some where the pivots were so tight that the points would take a length of time to close.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

It's deja-vu all over again!

http://www.peachparts.com/shopforum/showthread.php?threadid=83788

Please describe more thoroughly the nature of this "missing" and how you isolated it to these cyls.

I isolated it to these cylinders in two ways. (1) I disconnected each plug wire one by one at the distributor cap. There is no change in engine performance when cylinders 2,4,7 & 8 are disconnected. (2) When I changed plugs, cylinders 1,3,5 & 6 had plenty of black carbon residue (too much) on the plugs and cylinders 2,4,7 & 8 plugs were so clean they looked like they hadn't ignited fuel for a long time. Shouldn't they be wet with fuel if it's being delivered properly? -Mike

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1973 280SEL 4.5

I should also note: Not all 8 of the injectors are exactly the same. They are all BOSCH, but not all the same part number. I assume that at one time this car ran well without missing with the current configuration, but maybe that assumption is wrong. -Mike

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1973 280SEL 4.5

switch to Pertronix electronic ignition. worked great on a 73 450 sl 4.5 liter. 89.00 bucks. it run great and if the distributor is a little old it doesn't matter.

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Thanks, Juan

Since you've already pulled out the ring line and injectors, it would be good to see how well matched the injected volumes are, especially if you think you have different injectors mixed together. I don't know what the spec is (maybe +/- 10%?), but it will be hard to get the engine running right if they are so mis-balanced so that the mixture setting for the leanest cylinders makes others rich and fouling.

got the part numbers of the injectors?

I did not copy down the numbers when I could have done so easily, DUHH! Tomorrow, if weather permits, I will do so. -Mike

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1973 280SEL 4.5

Bosch part number ending in 036 should be the right one. As noted it is very unlikely that having a different injector number will cause your problem; they were probably working somewhat before the current situation.

In flow testing of injectors, a 10% difference is considered a failure. This is the case for a modern car expected to run at lambda (.5% CO basically). Your car would never run at lambda and at the 2-3% CO they are usually set up to they probably would never notice a 10% flow difference.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Cylinders 1,3,5,6 & 7 have the correct Bosch injector (0 280 150 036). Cylinder 4 has Bosch (0 280 150 015). Cylinders 2 & 8 are blue injectors with no part number and are made by BRET, not Bosch! -Mike

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1973 280SEL 4.5

If it were me, I'd swap positions of the injectors for working and non-working cylinders and see if the problem moves with the injectors.

Failing that, I think we're back to trigger point issues, perhaps sticking pivots, or a worn distributor shaft bushing. I don't understand the worn "prong", I don't think anything is spinning or sliding there. Perhaps the prong sets the position of the points tray properly relative to the cam, and poor alignment is causing triggering problems...

By this weekend I will do exactlty that. If moving the injectors does not change anything, it's time to buy new trigger points. -Mike

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1973 280SEL 4.5

I switched the 4 injectors in the working cylinders with the injectors in the non-working cylinders. No effect. At least I know the injectors are not the problem. -Mike

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1973 280SEL 4.5