Hello, I came upon this forum, and it looks like a good place with a lot of knowledgeable folks.

I have a W108 280se 4.5 V8. It had been running fine for quite a while, but then began to lose power and I found out to my dismay that only 6 of the 8 cylinders were firing. Specifically valves 4 and 8, in the rear of the engine when looking at it from the front are no longer firing. The front six are working fine without problems.

We noticed weak spark across the board, so in hopes of repair we replaced all spark plugs, the distributor cap and rotor, plus ignition wires with Bosch. The spark was much stronger, so this replacement didn't feel like a waste of time and money, but the two cylinders are still not firing. Next thing we are thinking to do is replace the ignition points as well since they appear worn - and of unknown age - but we're not sure if that will fix it.

Has anyone else experienced this issue and could provide some feedback? It does feel like more than a coincidence that it's the rear two cylinder sequence that are not firing. Perhaps there is something further up the chain that we need to check/replace?

Thank you for any suggestions.

EFI trigger points. They are probably oil-fouled. Clean with non-abrasive stuff like a soft cloth, and use an index or business card after to ensure they're lint-free. You can also use CRC electronic cleaner. They fire the pairs 1+5, 4+8, 6+3 and 7+2 in that order.

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Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

I was thinking trigger points myself. I had this same problem on a Porsche and the trigger points were the problem.

Be careful with the cleaning and any repair as these are NLA. I am sure if you looked hard enough you might be able to find some NOS but they would be expensive.

When you say that the rear two cylinders are not firing, do you mean that the plugs on those cylinders are not firing?

If so, that would not have anything to do with the trigger point. They fire the injectors. More likely a problem with the distributor cap, the rotor or perhaps the distributor shaft is worn and the points do not fire evenly.

I would first check the points gap. Set to 0.014". If problem persists, buy a Pertronix points replacement. That will overcome the shaft wobble problem. Cost about $70 and very simple install.

http://www.benzworld.org/forums/r-c107-sl-slc-class/1714668-pertronix-install-without-switchgear.html (use one that retains switchgear, at least for a start)

You might also try switching the plug leads and see if same plugs still dont fire.

If the plugs are firing, then ignore above and check if the injectors are firing. Trigger points do get fouled by oil leaking through distributor seal. The fibre followers also wear out - mine did after about 140k miles. There are threads here about that and how to check.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Hi, I don't know if you've managed to sort your problem, but I have the same model, 1972 4.5, with the same problem, but I ended up on 2 cylinders at one point, driving across spain! I spent LOADS in various "specialist" garages (just like I saw everyone else doing on here before I picked up MY nightmare, sorry I mean dream car )

The problem started with a bit of hesitation on the motorway, then when I slowed down, it would shake and stall, and have no power what so ever, sometimes it would barely make it up a hill.

I took it to a couple of garages,

"It's definitely your injectors" clean them
"No it's the trigger points" change them
"Ok maybe it's your distributor" ok get me a new one
"It's your plugs and leads" ok change them
"It's actually your fuel filter" change it
And of course there was the obligatory "ohh and we took the liberty of welding this, and changed that" so even more £££. Which was nice.
I was desperate to get it running properly, All I wanted was my beautiful baby to drive normally.. So forked out for this work, all for me to get five minutes down the road, and it starts chugging like a steam train again, just like it did before I dropped it in.

"Welcome to the world of old cars" was the explanation I got from the garage..

So I had no choice but to limp it here via ferry and a bit of driving.

Took it to a local garage here, and for €70, he very confidently diagnosed it, (he better be right), and did a clever little wiring thingy connecting one cylinder to another cylinder to allow me to drive it for the time being. Which was nice.

Aaanyway, he says it's a faulty ECU, and that it's not telling a few of the cylinders to fire.. It sounds about right and he seems very sure, so I ordered a replacement ECU from the US, that's been stuck at spanish customs for nearly a month now, but that's another story!

I'm pretty confident it's going to sort it as I have read quite a few other peoples stories on here that sound exactly the same as this, they tried everything, then finally swapped the ECU, and it now drives like the day it was made! So I'm praying I'm in that club, and that happens to me too.. Cuz if it doesn't, I don't actually know what else there is to try...!! I'll keep you posted, and I didn't check the date on your post, have you managed to sort yours yet?

Speak soon

Calum

Sorry for the delay in responding to these comments.

I just checked back in and was surprised to see the post from Calum was just posted yesterday! The problem still isn't repaired, not due to any complexity in diagnosis, but unfortunately my other projects have taken precedence.

As the other posters mentioned, it's either the EFI trigger points that need cleaning, or possibly the ECU as Calum said. Wiring appears fine, spark is strong on all 8 cylinders. I'd assume injectors are also controlled by the ECU, so that might be the culprit. Hopefully it's those EFI trigger points as that would be a cheap fix. Cleaning the EFI trigger points will be the first thing I will do. I'll try to get to that in the next week or so.

Thanks again for all of the replies, they are appreciated and I feel have put me on the right track. I will post back as soon as I've progressed a bit further.

Mine had an intermittent ECU, although the symptom was that it would run so rich that it would flood and die.
Both the old ECU and the one I got at a salvage yard are rebuilt, so that tells me that these have always had problems.

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Csaba
1972 280SEL 4.5, silver

An extremely belated update. I had not had a chance to work on the car further due to having been working outside of the country for many months and other projects. With January I made a resolution to not go anywhere or start anything new until the Benz was working reliably. As of 2/4/2015 the car is back in running order and firing on all cylinders. Bliss.

Sorry for dredging up an old thread, but I thought it would be useful for anyone else searching these issues as I’ve seen this thread appear on Google in searches. This might be a roundabout way of doing things, but I did learn a lot, replaced a lot of old parts that would have failed me later, and performed a lot of maintenance that I might not have otherwise gotten too until another failure.

My diagnosis steps to reach a final conclusion to this problem:
- Initial diagnosis appeared that either cylinder 8 or the rear two cylinders, 4 and 8 were not firing. In troubleshooting noticed weak spark, old wiring. Parts are of unknown date and origin. Replaced spark plugs, distributor cap and rotor, and ignition wires. Spark is strong things look and sound better, but still failing to fire in rear.
- Followed Tomguy and Pooka’s advice regarding EFI trigger points. I had no idea about the location or existence of this EFI trigger point, nor how to get to it. Purchased Mercedes Benz shop manual off eBay, read a lot of forums. Distributor shaft wobble did not appear to be an issue. Carefully removed distributor, removed EFI trigger point, everything was indeed dirty and crusty. Carefully cleaned the electronics with cleaner, returned everything to its original place. With each step there’s a felt difference, a perception of improvement, but still not firing on all cylinders.
- Noticed fuel injector harness electrical connectors on FI5 and 8 are reduced to pieces and dust, disintegrated. Replaced with Standard SK21 Fuel Injector Harness Electric Connector. Still not firing.
- At this point I entered my 6-month W108 hiatus, I left the country and car is driven back and forth between my place and my buddies a few times on 6-cylinders. No further troubleshooting takes place.
- Upon return, December 23rd troubleshooting begins in earnest. I had noticed a gasoline smell in the cabin, had attributed this to “old car” phenomenon and not thought about it further. Took air filter off and ran the car for a considerable amount of time while looking into engine bay (Watch for those spinning fan blades!). Tested unplugging various Fuel Injectors to see how engine sound changes, seems erratic, hard to tell which one is not firing. I begin to fear the ECU is haywire as vandor suggested. I noticed around the injectors a lot of grime and gas residue. FI8 appears to be leaking, but from where?
- Disassembled fuel injector rails, removed fuel injectors. FI securing collars are grimy, old and cracked. FI nozzle tip seals are completely disintegrated on 7, 8, and torn or paper thin on 1 2 3 5 6. Is this my leak? Only seal that appears sound is 4, but it’s hard as a rock. Cleaned and replaced all seals and collars. Cleaned everything, replaced all bolts and washers.
- The injectors appear to be Bosch(?), shiny metal body, but are unlabeled and inconsistent, some have dual barb fuel line fittings, others single barb. They show signs of rust/corrosion. Are these things original or from the 80’s? Pictures online of these injectors show completely different color scheme and body type plus part part number moldings on the plastic. These lack any of those features.
- All fuel lines are dated from 1997 or older, I replaced all rubber fuel lines and hose clamps in engine bay, because why not at this point. Fuel rails appear clean and free of rust.
- Reassembled the whole mess, turn it over and it feels even better, but damn it, STILL not firing on all cylinders. I purchase a FI test light (Yes, I should have done this much earlier). Test all FI electrical connectors and verify that they are flashing consistently, ECU does not appear to be the culprit, thank goodness.
- I thought about replacing the leaking fuel injector and getting the entire mess flow tested and cleaned. Price for a new injector plus a cleaning would be nearly $400. For $740, I can just replace all the injectors new and hopefully forego future leaks and failures. The old injectors are mismatched and of unknown origin/date. I bit the bullet, did the right thing, and bought 8 new Bosch fuel injectors.
- Disassembled again, new Bosch injectors, reassembled. Installed new Bosch fuel filter at rear-left of car by gas tank. Fuel pump is good. The one record I do have is that this was replaced as recently as 2011.

Moment of truth. I turn the key, it fires right up and I’m running strong and smooth on all cylinders. What a relief. Thanks to everyone who read this and contributed their suggestions and time!

For anyone that finds these useful here are the part numbers and total prices of what I used since these are always so hard to come across. For the cheapskates or purists; Yes, perhaps parts are available cheaper, or I could have spent quadruple and used Mercedes fuel line, seals, etc. I thought it was valuable to include the parts and prices so that other people can get a ballpark range of DIY costs as of January, 2015 in the USA.

Bosch Distributor Cap 03015 ($37.69)
Bosch Rotor 04018 ($11.38)
Bosch Ignition Wire Set 09027 ($61.54)
8x Bosch Spark Plug 7900 ($12.16)
MTC Fuel Injector Collars 1160780073 ($23.84)
MTC Nozzle Tip Seals 1160780473 ($22.84)
2 meters CRP-Contitech Fuel Hose Line 91603000052 13mm OD 7.5mm ID ($28.00)
16x ABA Fuel Hose Clamps ABA0019976990 13mm OD 7.5mm ID ($26.72)
6x Standard SK21 Fuel Injector Harness Electric Connectors ($13.00)
8x New Bosch Fuel Injectors 0280150036 ($741.20)
Bosch 71001 Mercedes Fuel Filter 0000927601 ($16.51)