Running on 4 of 8 cylinders - PeachParts Mercedes-Benz Forum

#1 02-23-2004, 09:21 AM

Hi Everybody. Well, I busted my tail this weekend on my project car--1975 450 SEL. Spent hours upon hours trying to solve the problems of very rough and sluggish idle, and black smoke/soot from exhaust.

Here's what I have noticed:

1. Sometimes if the car sits for an extended period of time (several days) then I start it up, it's ok for a while. If I shut it off and restart, the problems seem to start immediately. (Since sometimes it runs/idles OK, I assume the injectors are working.)

2. She's only firing on 4 cylinders: 1,4,5,8 (The 4 corners if you will). I pulled the plug wires from the other cylinders and there was no diff in running.

3. Pulled the plugs and yes, getting spark to bad cylinders.

4. Car runs ridiculously rich. Black smoke, some soot on ground under exhaust, very poor fuel economy.

Heres what I did:

1. Pulled/Cleaned trigger point with business card, electronics cleaner etc.
2. Re-timed everything.
3. Pulled fuel injectors, soaked in carb cleaner. Most of the O Rings were ok.

My mechanic looked at the car a week or so ago and said needed new triggewr points and at least 4 new injectors. Quoted me $1500-$2000 which is WAAAY more than I will spend on the car.

What is your advise to do now? I am thinking of replacing the ignition with a Pertronix system. If I do, can I avoid trigger points altogether? Does anyone know which trigger points control which cylinders?

I don't mind buying some new injectors, but unclear whether my problem is ignition or ful injection (or both).

Advise would be GREATLY appreciated. On the positive side, I learned a GREAT deal this weekend and truly enjoyed working on this old classic car...

Thanks
-Bud

Tomguy · #2 02-23-2004, 12:20 PM

Really sounds like you need new trigger points - only the left or right pair (not sure which) is working, by the sounds of it... 1 and 5 are controlled by 1 contact, 4 and 8 by a 2nd, 6 and 3, then 7 and 2 are not working on your car... suspect either bad points or a bad plug at the trigger points. Mixture's been adjusted overly rich to compensate for this, as a result, 4 cyls run super-lean (if they run at all, but they actually might under load when manifold pressure permits some crossflow of the air/fuel mixture) and the other 4 are over rich.

It COULD be bad plugs or wires or something electrical, but it isn't likely. It also is equally unlikely that the ones that aren't firing are just bad injectors.

It's more than a coincidence, in my opinion, that 2 pairs are firing and 2 are not. Trigger points are costly, make sure that the wires in the unit are all connected (still soldered), and the plug is good. Check for resistance while they're out, you should have 0 resistance (or very minimal) between all the pairs!

oh, and about Petronix: It eliminates ignition, not trigger points. To drop the trigger point system you'd need an expensive aftermarket engine management to replace the standard D-jet setup.

psfred · #3 02-23-2004, 12:43 PM

Pull the distributor and check the condition of the trigger points. Also check for play in the distributor shaft -- if the bushing at the bottom is worn, you will get excess oil on the trigger points AND the pressure from the igintion points can cause the left side trigger points to fail to open, especially if they are worn.

Trigger points are in the bottom half of the dizzy, you must pull them to check. Inspect the plug, too -- if the wires are broken or unsoldered at the plug, you won't get injection. You must pull the dizzy to get them out. Check with a VOM for contact. Spray them out with brake parts cleaner to remove all the old oil and crap. DO NOT attempt to remove the plastic covers, they will break if you do, and you need them.

I'd not worry about injectors until you get the trigger points working -- no signal and they won't inject.

finally, there is a big rat's nest of grounds up on the firewall (behind the battery on the W108, don't know where on the W116). Take this loose and clean all the wires, grease, and re-install. All of the injectors ground here individually, so if part of the bunch isn't connected due to corrosion, the injectors won't work. YOu can test with a test lamp or VOM at the injectors with the engine running -- you will get fluctuating voltage at the injector "hot" wire if the ECU is firing the injector.

My W108 cold starts on 4, then 6, then runs on 8 when warm -- I believe this is because the ignition points push the dizzy shaft to the right, and when the trigger points are cold, they wont' fire some of the injectors. A Pertronix or Crane breakerless ignition will help a lot here, as removing the points takes the pressure off the dizzy shaft and it tends to run much more vertical, opening the trigger points properly.

Peter

Tomguy · #4 02-23-2004, 12:55 PM

FYI: The ground for the injectors is right on the intake manifold for the 116.

j9fd3s · #5 02-23-2004, 02:16 PM

if i'm understanding my bosch injection book, it could also be one of the trigger wires. i understand they "batch" fire it, in 2 banks of 4 cyls.

#6 02-23-2004, 02:31 PM

Ok, here's what I saw and what happened. We resolderd the trigger point wires. They had been hanging by a thread and finally just broke apart when we pulled out the distributor and trigger point plug. I am 90% sure we resoldered them back correctly where they were originally, but the wires were sort of cris-crossed. i guess this could make a diff, BUT we had the problems BEFORE the soldered contacts broke.

The previous owner replaced cap, rotor, coil, spark plugs, wires, and ignition module (rebuilt). he just ran out of patience/money with this car.

The plugs are new, but have already blackened because of rich running.

I am concerned that perhaps the dist. shaft is bad (opening/closing only 50% of trigger points) and by spending the $300 to replace trig points I wont be any better off!

Should I think about buying a used dist? complete with trigger points?

Thanks

Bud

Tomguy · #7 02-23-2004, 05:42 PM

Bud: If you want to take a chance, go for it - a new dizzy can have even more wobble or worse bearings than yours! The trigger points could be worse!

I'll post a pic of the points in a bit, I want to clean mine!

psfred · #8 02-23-2004, 07:50 PM

Bud:

the wiring to the trigger points isn't exactly obvious. I seem to remember a single ground, but I do know it's somewhat difficult to test them, as several paths will give you continuity.

Best thing to do is to pop them out and trace the connections, and test them in place once you get the wires sorted. This will tell you if you have a trigger point problem or a wiring problem.

I will look later an see if I have a schematic with wire colors -- if you got a pair of wires crossed, its very likely you are simply not triggering injection. The problems before was a bad connection -- the ECU probably cannot detect a closed point with only a single strand holding, voltage drop too big across the connection.

Peter

#9 02-23-2004, 07:55 PM

Peter--thanks a lot on this. I will check at home and post the wiring colors on the wires I soldered, in the order I soldered them. Maybe somebody will know if I soldered them correctly or not.

As I said, they were hanging by a thread when they broke. It was just an good guess as to which wire went where.

psfred · #10 02-24-2004, 12:19 AM

Here goes:

Terminal 14 yellow with green stripe

Terminal 21 yellow with red stripe

Terminal 12 yellow with black (5 V, I think)

Terminal 22 yellow with blue stripe

Terminal 13 yellow with white stripe

this is the order, left to right, on the diagram, but I don't know if that is the order on the plug or not, you will have to check.

Numbers should be on the plug, but if not, the hot (yellow with black) should be connected to all the points, with the individual points connected to the other wires. I don't suppose it really makes all that much difference which point set is connected to which injector set -- the injection timing is far from critical on this engine. However, if you got the hot lead swapped with one of the returns, chances are that only two of the switches would work.

Peter

#11 03-05-2004, 10:29 AM

OK--back from my ski trip vacation and took the plunge. Ordered new trigger points and 4 new injectors. Couldn't seem to locate any used. Spent about $500. Will install this weekend and hope she starts, idles, and runs ok. Then have to fix the exhaust leak!

If nothing changes after the new trigger points and new injectors, she'll be for quick sale.

Wish me luck!

#12 03-08-2004, 10:25 AM

Replaced the trigger points this weekend, and WOW what a difference. Before repalcement, she was running rich on only 4 of 8 cylinders and didn't have the juice to get up the driveway.

Much better now, thanks to Peter and Tom in PA for all their advice.

I didn't yet replace the fuel injectors because as soon as she was running with the new trigger points, I took her on a "spirited drive" until got dark.

Now my next projects are:

1. fix leak of exhaust manifold.
2. replace front and rear seats.
3. fix windows.

More threads to come.

Thanks to all. What a feeling of satisfaction!

Bud