Starting troubleshooting, 175CDT carb fueled W123

[tsa]

Hi,

my first post here. I am looking for a trouble-shooting guide for starting problems with my 175CDT equipped 1985 W123.

Current problem: Engine started on first attempt from cold (after standing more than three weeks). Was running a bit lumpy, hesitant on moderate throttle input, but could be made to run by keeping the revs up. Then, after parking after three miles, and trying to start after 20 minutes; wouldn't catch. Wet plugs, dried off. Feeble attempts to catch, but didn't.

Looking for a step by step guide to check the integrity of the 175CDT, with its added complexity; the fuel cut-out valve, the "Startautomatic", "thermo-delay-valve" (translated from the German designation in my service manual: Thermoverzoegerungsventil).

Or does is sound like the ignition system may be responsible for problems with re-starting a luke-warm engine?

Thanks for any advice!

 

[grober]

http://www.mbclub.co.uk/forums/part...015-stromberg-cd175-mixture-adjustment-2.html

 

[grober]

Zenith_Stromberg_Ersatzteilliste Vergaser

 

[tsa]

Graeme,

many thanks for your replies with interesting and useful links for parts.

"www.ruddies.business.t-online.de" seem to be up to date with info on the 175CDT's.

If adjustment of the carb may be a common problem/fault, as opposed to failure of any of the ancillary add-ons, I am wondering if I should look more at the ignition side first? After all, my car recently started immediately on the first attempt. Not sure of how to check if the added "extras" as thermo-delay valve and/or start-automatik are working as intended.

Lots of experience with rebuilding manual carbs on old boxer MCs, but then it is much easier to check the working status of e.g. a choke on a fully manual carb

 

[grober]

Cars that go dead after a few miles which have been laid up for several weeks may be suffering from internal condensation in the distributor. Initially dry--- as the engine warms up moisture in the lower part of the distributor body evaporates into the top HT area and shorts out the HT to earth. Solution is a thorough drying out of the entire distributor

 

[RichardF]

In my experience carbs just don't suddenly go out of tune, most "carb" problems are actually ignition. Get the ignition side of things spot on before touching anything else.

 

[E55BOF]

+1 ^ What he said...

 

[tsa]

Thanks for further great advice. I actually had a look under the distributor cover, but couldn't see any signs of moisture.

After getting the car towed home, I've had better opportunity to check some functions (fuel pump, quality of fuel, sparks to plugs by means of inductive pick-up timing light) and noticed that one of the spark plugs had a broken porcelain insulator. I couldn't get the top of the plug out of the plug cap, so need to replace that HT lead. Silly that they are routed under the cyl head cover :-( I also noticed there was surface corrosion on the distributor rotor tips, indicative of previous dampness. I see that a new rotor can be had inexpensively, so will renew that. Otherwise I am not a great fan of replacing expensive components without signs of failing, just as a troubleshooting process.

The carburetor ancillaries has given grief previously, in that the fuel cut-off valve function hasn't worked, leaving the engine stumbling on up to five seconds after I've turned off the ignition.

 

[grober]

Some further carb data
http://ftp.hamsk.ru/auto/W124/MB_200_1988/Stromberg/m115_175cd/072-090.pdf

 

[tsa]

Some further carb data
http://ftp.hamsk.ru/auto/W124/MB_200_1988/Stromberg/m115_175cd/072-090.pdf

Many thanks for this link!

Very useful data.

 

[toolman1954]

Night time or in the garage in darkness is a good time to look under the bonnet for stray sparks from the HT Leads or shorting on the Distributor cap. If there is a capacitor in the Distributor change that also. Are you getting a big fat spark from the coil?? I spent ages long time ago, fault finding on an Avenger that was all down to a weak coil.

Steve

 

[tsa]

Night time or in the garage in darkness is a good time to look under the bonnet for stray sparks from the HT Leads or shorting on the Distributor cap.

Yes, I've been using that trick for my MCs. But first I need to get the car engine to run

If there is a capacitor in the Distributor change that also. Are you getting a big fat spark from the coil?? I spent ages long time ago, fault finding on an Avenger that was all down to a weak coil.

No, my W123 was built in autumn 1985 - so it came with the electronic ignition module version. I think the capacitor only was used with the contact breaker ignition systems?

I measured the resistance values of the coil, 0.9 primary and 9k secondary, and the connection posts are clean with no signs of corrosion. Only sign of corrosion was as mentioned on the distributor rotor.
_ _ _ _ _

Wrt the plug cap/HT lead that I mentioned I will replace, I saw that my DIY guide states that the HT leads are screwed into the plug and distributor caps - can anyone here confirm if this is true? Currently I have BERU 1 kOhm resistor caps fitted, which seem very tight on the HT leads, so I am not sure if those are the same the DIY manual described.

 

[grober]

You might find more detail here:-
https://www.startekinfo.com/StarTek/outside/12265/?requestedDocId=12265

or here

https://www.startekinfo.com/StarTek/outside/11832/?requestedDocId=11832

other possibility is an electrical fault with the ignition/starter switch meaning no power is getting to the ignition circuits.

 

[tsa]

You might find more detail here:-
https://www.startekinfo.com/StarTek/outside/12265/?requestedDocId=12265
or here
https://www.startekinfo.com/StarTek/outside/11832/?requestedDocId=11832

Thanks again. In the chapter on engine electrics in the latter I found this:

"An interference suppression resistor of 1 kOhm is installed in the spark plug connectors. These can be unscrewed (M3 thread)."

I understood this to mean that the plug caps (and not the resistors) can be unscrewed, so I am looking for a new 1kOhm plug cap. The BERU ones I have show the part number 0300152101, and searching indicated some vendors listing these e.g. on eBay. But I wasn't surprised to see that they cannot be bothered to provide info about the resistance of the caps they're selling . . . .

Any of you got some spare HT leads with those caps lying around?

. . . other possibility is an electrical fault with the ignition/starter switch meaning no power is getting to the ignition circuits.

Since my inductive strobe timing test lamp flashed when hooked up to the HT leads during a start attempt, I hope I can take that as proof the ignition switch is working, at least to some extent!

 

[grober]

The pukka ignition leads are copper cored with a 1kohm resistor in the connector cap. [ needless to say this set up should use non resistor copper core spark plugs] The connector caps are normally screwed but the lead is usually crimped also. Best to renew the complete lead+connector cap. There is a ceramic insulator in the cap and these can crack when trying to remove them. Mercedes used to provide a special pliers for the job to avoid this.

[YOUTUBE]iv46dIo7WcE[/YOUTUBE]

 

[toolman1954]

Hi.
Sorry I didnt read your original post correctly but I thought you had it running intermittently. So at the moment it is a non runner/ non starter.
Have you whipped out the 4 plugs, connected up the leads to the plugs, making sure the plugs on on the engine for earthing purposes and got a friend to turn over the engine so you can see 4 big fat sparks from the plugs?
What happens next depends if you have sparks. Weak sparks may be leads, coil or even old tired plugs. As you get old and tired you lose a bit of the spark-----sigh.
If you have big fat sparks, then I would just have a quick looksee at the carb. Unscrew the dashpot damper, is their some oil inside it. When did you last inspect the rubber diaphragm? 4 Phillips screws and the top of the carb comes off. Have you ever has this off before as the screws can get notoriously tight and if you dont use the right phillips screwdriver you will bugger up the screws.Try giving the screwdriver a sharp wack while you turn it just to free it off.
Check that the carb slider moves up and down and check that the diaphragm is not holed or ripped. There still exists the possibility of some crud making it over fuel, but after putting all the plugs back in I would get your friend to turn it over while giving it a squirt of engine start.
If still no start then time to get out the compression tester . Fuel + sparks + compression = engine running. Keep us posted. Good luck.

Steve.

 

[tsa]

SORTED: Starting troubleshooting, 175CDT carb fueled W123

Sorry I didnt read your original post correctly but I thought you had it running intermittently. So at the moment it is a non runner/ non starter. Have you whipped out the 4 plugs, connected up the leads to the plugs, making sure the plugs on on the engine for earthing purposes . . . . . . . . . . If still no start then time to get out the compression tester . Fuel + sparks + compression = engine running. Keep us posted. Good luck.

Steve, many thanks for this advice. Others who have read this thread should please note that what Steve said is GOSPEL when it comes to trouble-shooting the problems I described!

Yes, I had checked the carb diaphragm a couple of months ago, in very good condition. Luckily I had some experience from my BING carbs, so managed to get the four screws off without destroying the screw tops. Gently applied heat from a hot air heat gun helps expand the alloy body of the carb to release the grip on the steel screw theads. Cleaned out some soot, but no obvious wear compromising the movement of the piston. Plenty of damping oil, I'd been topping it up with ATF oil regularly. The potential problem with the electrically controlled choke etc on the 175CDT is that it is much more complicated to check if the temp sensor etc works to engage the choke mechanism when needed.

After I had verified I had signals to the plugs, I had a look at the distributor and found corrosion/wear at the rotor. Resistance measuring resulted in a very erratic reading. Today I could pick up a new rotor I ordered, installed it, and . . . . . drum roll . . . . . - the engine started. It was running a bit lumpy (as if on 3.5 cylinders), which I attributed to the broken copper core of the spark plug in cyl 2. I then visited my local motor factor, and armed myself with some odd HT lead & spark plug caps. One 5kOhm NGK straight cap grafted onto a copper core HT lead, cramped onto a cannibalised distributor connector, attached to an intact spark plug; and the engine started again, and runs beautifully. For now.

Again many thanks for the great info and advice you guys provided!

PS. If any of you have a left-over original 1 kOhm spark plug cap / HT lead, I certainly would be interested in it, to replace my over resisted Heath-Robinson DIY substitute!

 

[tsa]

The pukka ignition leads are copper cored with a 1kohm resistor in the connector cap. [ needless to say this set up should use non resistor copper core spark plugs] The connector caps are normally screwed but the lead is usually crimped also. Best to renew the complete lead+connector cap.

Yes, agree to all the above. Except that it is difficult to fit a new original complete HT lead, since it is routed under the cyl head cover on the W123 102 engine.

There is a ceramic insulator in the cap and these can crack when trying to remove them. Mercedes used to provide a special pliers for the job to avoid this.

[YOUTUBE]iv46dIo7WcE[/YOUTUBE]

Interesting. That's obviously why I found out that one of my spark plug insulators was broken, leaving the top end of the spark plug jammed in the plug cap.

To avoid further problems with difficult-to-get-off plug caps, I simply smeared a little high temp silicone grease on the top of the spark plugs. Worked brilliantly, for now, will have to check if it still does after e.g. a year.

 

[toolman1954]

Congratulations ....I am pleased you now have it running.It is without doubt such a satisfying feeling when an engine that had refused to start finally bursts into life. Eyes to the skies and " Oh Joy " who so ever Joy is.

Steve.