W108 280SE 3.5 - injection (?) problems - Mercedes-Benz Forum

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post #1 of 7 (permalink) Old 04-20-2011, 07:55 AM Thread Starter
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Post W108 280SE 3.5 - injection (?) problems

Hi folks,

I am the happy (and proud) owner of a 1971 W108 280SE 3.5 sedan which I've been driving to work everyday for the past 12 years, but recently it's started 'misfiring' and behaving weirdly at unpredictable times (no link to temperature or humidity that I could detect), spurting on less than its 8 cylinders --sounds like at least two or three cylinders suddenly stop firing and just guzzle pure gas, so that when I rev the engine up it roars but there are flames coming out the exhaust, and reversely at low levels the engine just stalls completely.

My specialist mechanic changed the MAP sensor and cleaned completely the injectors (says there are no new spares to be bought here in Switzerland), but to no avail, one day the car will be running fine and smooth, the next it's hopeless again, and dangerous as I need to jump to and from Neutral - Drive in order to keep the engine on higher revs when stopped at red lights...

I received advice from "alabbasi" to check for idle and cold start valves, but it appears this is not the cause, and clearly I've got some cylinders running fine while others are just guzzling raw gas = not a 'central' problem but one specific to only a few of the eight cylinders.

Any advice on what could be the cause of my troubles will be greatly welcome

Geneva, Switzerland
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post #2 of 7 (permalink) Old 04-20-2011, 08:26 AM
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Hi Philip,

welcome to the forum.

Before looking into the injection you should make sure that ignition is flawless (no cracked dizzy cap etc. and all up to specs).

Then try to see if it is always the same cylinders that are misfiring, by taking the plugs off and looking at their color. If some are always firing OK and some not, I would look into the trigger points. They are in the lower part of the ignition distributor and they tell the calculator when to fire the injectors.

You have some people here and on Shopforum who are quite experienced with the trigger points, how to test them and how to "repair" them. I'm sure they will kick in the discussion.
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post #3 of 7 (permalink) Old 04-20-2011, 08:37 AM
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Try this link to proper trouble-shooting information


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post #4 of 7 (permalink) Old 04-20-2011, 08:48 AM
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Used to have problems with trigger points in the past, they have a firing order IE. 2 and 4. Although that's just an example I can't remember it was a long time ago,but if you determine which cylinders are missing you can compare it to the trigger point order which you will find on here somewhere.
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post #5 of 7 (permalink) Old 04-23-2011, 04:00 AM
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I had a similar issue with my 4.5. The fuel delivery system is very very similar.

Chasing down a different issue, I discovered I had some bad wiring (original wiring harness). I replaced it, and the car ran on all 8 cylinders, it always starts now, etc etc. I woud suspect you have an electrical issue. Trigger points might be the culprit, but I'd look more at wiring.

Once you pull the plugs, I suspect you find they are fouled in increments of two. The ignition system fires two cylinders at a time, so something common to both cylinders could be it (trigger points, wiring harness, cap or rotor). The raw gas out the tail pipe is a clear indication something's not firing correctly, as is the stalling at low speeds.

Keep a close watch on your oil. With so much unburnt fuel being pumped out the exhaust, it's highly likely you've ingested fuel into the oil, greatly thinning it out. Too much thinning out, and your engine siezes.

See the "online resources" portion of this forum - there are a lot of articles posted about the fuel delivery system that, if nothing else, would give you a greater understanding of how the system works.
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post #6 of 7 (permalink) Old 04-23-2011, 04:05 PM
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I would suspect the ECU. Faulty trigger points will cause problems but not the ones you are describing. The trigger points initiate the injector pulse but the ECU calculates the duration. If a injector is staying open it is because the ECU is telling it to. if the problem is only on certain cyls it could not be the pressure sensor. You might want to consider Megasquirt before you burn up your car. I used to be a D-Jetronic purist but when I discovered how sophisticated Megasquirt is I changed my mind. You don't have to use all the expensive, proprietary Bosch stuff.

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post #7 of 7 (permalink) Old 04-28-2011, 04:54 PM
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If you end up needing an ECU, don't buy a used one on Ebay, etc. Send yours to Programma in Boca Raton and have it rebuilt for around $450...it will be good as new!

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