350SL No power

[Graham]

This is a 1972 350SL 4.5L with about 145k miles on clock.

Car was running OK, just a small stumble once in a while and perhaps slightly less power than I recalled, so as a start, I decided to replace some of the original and older stuff.

Changed spark plugs, plug wires, distributor cap, rotor.

When I went to start, it started OK, but on road test had no power - would not go over about 40mph.

Tried switching back to old rotor and cap and slight improvement - Still awful performance.

Checked points gap and it was small, so opened it up a bit - Tried 0.35mm and 0.45mm. No improvement and now I hear slight popping or backfiring sound when driving. PLugs should be OK - they were the proper Bosch plugs and I checked gaps - all 0.032".

I have checked that wires go to proper plugs.
I have removed points (no pitting) and cleaned everything with contact cleaner. I don't have a dwell meter, so can't check dwell.

At my wits end! Any suggestions for things to check before I take into shop?

Questions:
- Bosch pamphlet that came with rest of stuff said something about need to change coil when changing ignition wires because of possible damage to ecu. Not sure what they meant.
- Old (original) plug wires were different - they had hard plastic inserts that seemed to have resistors in them at distr. end. New Bosch wires don't seem to have those.

Graham

[meltedpanda]

Just some thoughts here
knocked out of time when you replaced rotor ? ( my bet)
the rotor on in correct oriention?
Points should be at .014in
clean the trigger points
Make sure the plug wires are secure over the ends of the plugs. On mine you have to remove the small end cap
make sure your wires are going to the right plugs off the distributor (this is easy to mess up) (second bet) I have a diagram if you need it

[Graham]

Thanks very much for your comments melted panda- Seems like a good check list!

Originally Posted by meltedpanda

Just some thoughts here
knocked out of time when you replaced rotor ? ( my bet)

This could be it, but how does it get knocked out of time? Isn't it a direct drive? I did hear a sound like belt slipping when I first started her up, but could not figure why.

the rotor on in correct oriention?

I think it only goes on way - Small slot has to engage.

Points should be at .014in

My .35mm is about right then.

clean the trigger points

Will do this, but those are for injection system, are they not? If so, should not have changed with changes I made.

Make sure the plug wires are secure over the ends of the plugs. On mine you have to remove the small end cap

I had to take caps off new plugs and threaded ends push firmly into plug wire end, it seems.

make sure your wires are going to the right plugs off the distributor (this is easy to mess up) (second bet) I have a diagram if you need it

I marked old distributor cap then copied it. This is what I have:

Lets say:
#1 is at N or 12 o'clock
then
#5 is at NE or between 1 & 2 oclock
#4 is at E or 3 oclock
#8 is at SE or between 4 & 5 oclock
#6 is at S or 6 oclock
#3 is at SW or between 7 & 8 oclock
#7 is at W or 9 0clock
# 2 is at NW or between 10 and 11 oclock

Does this match your diagram?

Regards,
Graham

[meltedpanda]

yes pretty close, firing order is 1-5-4-8-6-3-7-2, and looking at the engine on the left bank 1-2-3-4 ane the right 5-6-7-8 cyliders.
So if everything is going the right way and hooked up properly make sure the plug wires are not wrapped around each other.
Also make sure the wires are pushed all the way home, especially in the cap and coil. Sometimes the boots hold them up in a way that they do not conduct properly
There is a small hex nut to rotate entire rotor, I am just thinking if it was loose, taking cap off may have moved it , just a guess. I think it is 5MM hex, you need a long extension to reach it. Clockwise retards, CCW advances.
Trigger points - you are correct
Did you change out condenser with the points?
Let me know what you find, we can figure this out, It has to be something to do with your service so retrace your steps?

[Graham]

Originally Posted by meltedpanda

yes pretty close, firing order is 1-5-4-8-6-3-7-2, and looking at the engine on the left bank 1-2-3-4 ane the right 5-6-7-8 cyliders.

I am not where the car is at the moment, but I have a horrible feeling that although I was careful to mark the distributor end properly and I know that the right side wires are correct, I may have the wires on the left side reversed - in other words 4-3-2-1. I guess that WOULD do it!

So if everything is going the right way and hooked up properly make sure the plug wires are not wrapped around each other.

The wires are in contact with each other. Originals went through a rubber tube). But, maybe that's not what you meant?

Also make sure the wires are pushed all the way home, especially in the cap and coil. Sometimes the boots hold them up in a way that they do not conduct properly

Initially, some were not, but it's possible to hear them securing properly when they are fully pushed in.

Did you change out condenser with the points?

I have not changed points yet. I just took them out and found they looked OK. I did clean them but did nothing with condenser. Manual says to exchange condenser for a shielded cable. Have not tried to source this.

Let me know what you find, we can figure this out, It has to be something to do with your service so retrace your steps?

I will - Gut feeling tells me I screwed up on the right side wire attachments!

BTW - Thanks a lot for your help!

[Gurunutkins]

I would recheck the plug leads, I have been doing it for years and still been known to get it wrong. also you say you have your points at .035 (I think you mean your spark plugs but you said it under the points question from melted panda) that would also kill your power (if it started at all). also check you didnt break the carbon rod in the centre of the dist cap when you reinstalled it, it might then only make intermittent contact and you could run but have little power.
cheers
len

[Graham]

Originally Posted by Gurunutkins

I would recheck the plug leads, I have been doing it for years and still been known to get it wrong. also you say you have your points at .035 (I think you mean your spark plugs but you said it under the points question from melted panda) that would also kill your power (if it started at all). also check you didnt break the carbon rod in the centre of the dist cap when you reinstalled it, it might then only make intermittent contact and you could run but have little power.
cheers
len

I think messed up on right side plug leads - still have to go to car to check. Put them on right, then took them off and installed looms to tidy them up, then put them back in reverse order on left side

Points were set at 0.35mm not 0.035in. ,0.35mm is about 0.014in. Plugs were set at 0.032in.

Carbon rod is fine. In fact I switched caps and rotors several times to see what problem is, but now I am convinced it is left side plug leads going to 4-3-2-1 instead of 1-2-3-4!

Will update later today!

[ScottinSoCal]

Originally Posted by Graham

Will update later today!

I changed the plugs on my 75 a few weeks ago - just got the Bosch units that the auto parts place said were appropriate - and since then I've been dealing with a stumble at idle and a slight hesitation on acceleration. I've eliminated timing, vacuum, wires, cap and rotor as the cause (all new, but swapped out with old to make sure). I found a looooong thread talking about resistor vs. non-resistor plugs and found that the new plugs were resistor plugs. I've ordered the non-resistor and will post when they're installed to see if that cleared up the last bit of problem.

[Graham]

Originally Posted by ScottinSoCal

I changed the plugs on my 75 a few weeks ago - just got the Bosch units that the auto parts place said were appropriate - and since then I've been dealing with a stumble at idle and a slight hesitation on acceleration. I've eliminated timing, vacuum, wires, cap and rotor as the cause (all new, but swapped out with old to make sure). I found a looooong thread talking about resistor vs. non-resistor plugs and found that the new plugs were resistor plugs. I've ordered the non-resistor and will post when they're installed to see if that cleared up the last bit of problem.

The plugs that ******** Arizona sent me were Bosch WR7DC+ . I think the R designates that they are Resistor plugs. When I search on-line, various sellers have both W7DC and WR7DC+ listed for the '72 model 350/450SL.

Would be interested to know which plug is right and whether it really makes a difference or not.

[ScottinSoCal]

Originally Posted by Graham

Would be interested to know which plug is right and whether it really makes a difference or not.

The thread I read indicated that the MB plug wires have the resistor built in. Going with resistor plugs doubles the resistance and can cause stumbling. A more controversial theory (some people argued for, some against) said resistor plugs could fry the ECU. I've exhausted everything else, so I'm willing to pop for the $4 per plug and see if this smooths things out.

[230/8]

W7DC or NGK BP6ES. No resistor plugs, the wires provide all the resistance needed.

230/8

[Graham]

Originally Posted by meltedpanda

Let me know what you find, we can figure this out, It has to be something to do with your service so retrace your steps?

Just wanted to update you!

In retracing what I did, I realised that I probably messed up the wires when I put the plastic covers on to tidy them up.

With new plug wires properly directed to new plugs, new points, rotor
and distributor cap in place and injector trigger points cleaned, car is running
MUCH better than before I started all of this! I had local mechanic who
owns an SL remove distr for trigger point cleaning and then reset the timing
and dwell .

I am a bit concerned in that ******** sent me WR7DC+ resistor plugs while the old ones were W7DC,s. My mechanic says it should make no difference.

New Bosch wires are different from old ones too:

- New ones have brass connectors at each end that plug directly into the distributor cap. They are Bosch Premium Wires - Opti-Layer Mag wire. Box says Fits Mercedes 8-cyl Engine.

- Original wires have a hard plastic end that plugs into the distributor. I took one of these apart. It has a small resistor inside!! It's about 5/8" long x 1/8" diam. I measured it on my old analogue meter and it reads 16 on the 100X scale, 160 on the 10X scale - 1600 Ohms?

Would it then not seem that if we don't have these old plug wires, we should have resistor plugs?

[meltedpanda]

Glad to hear things are working out, I use the NGK BP6ES, and have no problems. My wires are the old style. Not sure about the resistor wire idea, but you points are logical. Did you measure the resistance of an old wire VS a new one? That may help to see if in fact there is a big difference

My original manual had Bosch W175T30 for the plug ( we have the same car)
these are what it crosses with
W175T30 NGKBP6ES Champion N12Y, UN12Y,

[Graham]

Originally Posted by meltedpanda

Glad to hear things are working out, I use the NGK BP6ES, and have no problems. My wires are the old style. Not sure about the resistor wire idea, but you points are logical. Did you measure the resistance of an old wire VS a new one? That may help to see if in fact there is a big difference

Yes, I did check the resistances. The old original wires with the internal resistor measured 8 kohms. The new wires measured 1.6 kohms (Bosch Premium opti-layer mag, part 09027). Old plugs were W7DC and had low resistance - about 5 ohms. New plugs are in car, so were not measured, but I guess should be in 5-10 kohm range, so total resistance should be about same. Resistor plugs with old wires would probably have too high a resistance. I read somewhere it should not be over 15 kohms.

My original manual had Bosch W175T30 for the plug ( we have the same car)
these are what it crosses with
W175T30 NGKBP6ES Champion N12Y, UN12Y,

Yes, my manual says Bosch W 175 T 30 or Beru D 175/14/3 A or Champion UN 12 Y.

But, I wonder if they would recommend same plugs today given changes in fuel and condition of these aging engines!

And especially if the wires have a lower resistance, as do the aftermarket Bosch wires I just bought!