Hot start issues 1970 280SL Auto

[Bshaunessy]

Help Pls!
  Recent hot start issues have left me stranded a few times and one tow job home! 
Car starts , idles and runs fine from a full cold start, however, after 10-50 mile run and a shut off engine ( eg at a gas station to refuel….egads) car refuses to start or even cough.  Turns over fine.
   I have verified spark, timing is correct (123 distributor) and can smell fuel when cranking but will not start……..until waiting for cool down ( less than 100f).   See tow job pic below a few days ago.  10 mile tow back home…….car unloaded and then started up and ran just fine as it had cooled down during tow job!
  OTHER:
   Car has run fine for last year , both cold starting fine and hot starting fine.  No hot start is a very recent issue.
   Have read ( tech manual , this site….what a valuable resource ) and understand
-Cold Start Valve….tested functionality ( injector in a jar / confirmed spray pattern ) .  Confirmed proper operation of circuit and Thermo Time Switch functionality.
- have taken "LINKAGE TOUR “ ( twice!!….a nice tour!)
-have read every "HOT START" post I can find but can not find similar issue post.

My conclusion so far is the hot start issue is engine temperature related.   However, there are only two temperature driven components of the fuel system :(1)  Cold Start Valve ( which only operates when engine cold…..and I have confirmed that CSV a is not injecting when hot) (2) I am wondering if the Warm Running Device ( part of fuel,injection pump….supplies additional air and fuel during warm up) is a contributor to my problem.  It should not be operational when engine is hot.  I have not yet tested the WRD.

Looking for guidance and advice as to which rabbit hole I should go down next ??
   

[rwmastel]

What about the relay that is supposed to add fuel for 1 second regardless of temp?  You say the cold start valve is not working when engine is hot.  If I understand starting aids, and maybe I don't(?), it should operate and add fuel for 1 second under any circumstance.

Here's the two best starting aid links I can provide.  A very long discussion thread and the Tech Manual link.

https://www.sl113.org/forums/index.php?topic=27927.0

https://www.sl113.org/wiki/Restricted/Engine-starting-aid-tour

[teahead]

When it's hot, check the cold start injector w/a test light while cranking if it lights up.

I would think that it should NOT light up when engine is warm/hot.

Wondering if it's dumping fuel when it shouldn't.

So plenty of spark when cranking?  Odd.

[Bshaunessy]

RWMASTEL: thx for both links.  I’ve gone through STARTING AID TOUR front to back to front.   The second link on Thermo Time Switch and alternates is also useful, thx.
    Not sure about the relay you are referring to that adds fuel ( via the CSV Injector system I presume?) for 1 second regardless of engine temp?  The CSV has only one relay connected to it and that is controlled by the TTS ( Thermo time switch).   Engine Starting Aid nor elsewhere in tech manual don’t refer to a 1 second injection system??

TEAHEAD: Also thx for reply.  When car is hot,as you suspected,  Cold Start Valve does NOT add fuel ( I think this is as it was designed?).  I’m pretty sure CSV is operating as designed.

Both RWMASTEL and TEAHEAD: thx for the leads. The pursuit of the gremlin continues!  I cold started car today just fine ( 3 rd day since dreaded "tow job") .  Again , it failed to start once warmed up!
There is definitely a temperature related issue.

Q? Do you think it might be related to the mysterious WRD?  ( warm running device)??

[ja17]

Fuel must be continuously circulating back to the tank. The circulating fuel keeps the fuel and the system components cool to prevent vapor lock especially in hot weather. Too much fuel during hot starts can also prevent starting. Try disconnecting one or both starting devices to rule out "over rich" mixture.

[rwmastel]

Hi Joe,
Above he states "I have confirmed that CSV is not injecting when hot."  Not sure if he's disconnecting and watching for fuel, or using a test light.  He could still have a return circulation issue, leaving warmer fuel in the line.  Is it possible for his WRD to affect hot start?

Bshaunessy,
In the long thread I linked above, there was just one reference: "As an added bonus it has circuity that gives the cold start injector a 1 second shot even when warm which Mercedes tried to do in a later retrofit kit for the 280SLs."

I did find this thread about the 1 second fuel supply: https://www.sl113.org/forums/index.php?topic=10422

I don't know if using the gas pedal would effectively replicate the use of this fuel supply.  Probably best to get things working as the smart company engineers designed.

[Bshaunessy]

RWMASTEL
(1) I confirmed no power to CSV when cranking when hot.   I’ll remove CSV and test to a jar ( when hot and when cold) per your questions…thx
(2) I read the 2009 post RE 1 second fuel added via CSV , both hot and cold.  I conclude this was a post manufacture add by MB to aid in hot starts.  My car does not have this.  I am going to try a manual 1 second power up of CSV during a hot start attempt.   It seems several folks have installed a dedicated switches 12v supply to CSV for both hot and cold starts, as a manual alternative to a working TTS.

[mda]

the valve WRD ( warm running device) may not completely overlap due to wear. Try to drown it out. There will be a bit of a difficult cold start, but the rest of the problems should go away. sorry for eng)

[Bshaunessy]

MDA:
  Thank you for suggestion.  I per some other suggestions, I am going through the process of disconnecting the WRD and then, separately and in another test, the CSV.
  I’ll report back on conclusions once next round of testing done.

[Bshaunessy]

UPDATE JUNE 1,2023…..all is well….no more tow jobs!  Car reliably starts both cold start and hot start.
    ISSUE RECAP: car reliably cold starts ( ambient 10c-25c) but refuses to hot start.
   SOLUTION: faulty Thermo Time Switch.  ( very common fault).  Cold Start Valve (CSV) working and injects fuel BUT only for 1 second when cold (20c) and NO SECONDS when hot ( normal operating temp 190f / 95 c ish).  ( CSV a should deliver fuel for at least a second when engine hot). By following the testing procedure in ENGINE STARTING AID / THERMO TIME SWITCH / specifically measured resistance in TTS vs temperature, I determined my TTS is faulty.   
   Per instructions, ( as a work around if your TTS is not functional)I wired the W terminal direct to ground and now the CSV delivers fuel whenever engine is cranking , regardless of coolant temperature and regardless of ambient temperature. ( always on when cranking).  RESULT: engine fires up immediately every time …..no more tow trucks…yippee!! And back on the road again!
  Pic of the culprit attached…..only $800 usd Authentic Classics!
 

[teahead]

Nice find.  I would have thought that a faulty TTS would still be activating the cold start valve.  Hence it lighting up while cranking while hot.

$800 used?

Didn't want to try the Porsche route at 1/3 the price new?

[Bshaunessy]

TEAHEAD
   When bench ( and in situ with G and W wires disconnected) testing the TTS resistance measured to ground at G and W terminals was all wrong….per TECH MANUAL / STARTING TOUR/TTS …..indicating a failed TTS.   TTS and CSV system delivered fuel for 1 sec only at 24c ( should be approx 6 seconds I think….up to 12 seconds if much colder).  No fuel delivery when engine at running temp 95c / 190f.   I can’t find the spec for duration of CSV for hot starts but it must be at least 1 second I think.   

  Found a BOSCH TTS at BUDS for $259 usd ( US Dollars…..I’m Canadian….$cdn) ( vs $800 Authentic) so it’s likely the Porsche substitute.   I need to phone Buds for thread details and connection details….think I have to solder new connections.
https://www.budsbenz.com/catalog/230-250-280-sl/g-exhaust-parts-intake-system/G207-013

[teahead]

Do a search here, but I ordered mine I think for Pelican and no soldering needed.  I just had to remove the top connection piece.

I used:  91161711700

https://www.pelicanparts.com/More_Info/91161711700.htm?pn=911-617-117-00-INT

[BobH]

Hello Bshaunessy, if you have no cold start problems, your TTS should be fine and delivering extra fuel via the CSV, if the TTS wasn't working, ie the contacts were open permanently, then your car would have problems starting when cold.  I may be wrong, but i can't see that a faulty TTS would switch a -ve (ground) to the cold start relay for only 1 second, it's a simple device, the contacts remain closed until either the coolant temperature causes them to open, or the heater circuit does the same after cranking.  Unless of course you have the later electronic TTS which i guess could have an internal timer that has failed, but this still wouldn't account for you having no cold start issues

The TTS doesn't supply the 1 second trigger to the CSV, there were various additional devices for this over the years, including a 1 second timed relay, which as far as i know was available as an add on to later cars, you say you don't have this, but i would check again, this relay doesn't feed the CSV directly, it feeds the cold start relay, which in turn feeds the CSV.  Have a look at the 230 wiring diagram, which i assume would be similar to a modified late 280

Apologies if i've read your answers incorrectly, just didn't want you to waste money on a TTS without double checking

[Bshaunessy]

Bob H
  Thx for the thoughts and knowledge.  Sorting out this hot start problem has been like peeling an onion.  The more layers you peel, the more complex the issue becomes.
  I have ( temporarily) rectified my persistent hot start issues by circumventing the CSV relay and logic by wiring a ground to the "W" terminal on the TTS.  Net effect: CSV delivers fuel to intake whenever starter cranking, regardless of ambient or engine temperature.  This has solved my hot start issues , as car reliably starts every time hot or cold, BUT……I am baffled as to the sudden cause of the hot start issues  after two years of flawless starting ( something changed / failed…but I don’t know what.  I am able to add a workaround as above but don’t think I’ve solved the problem.)
  Onion peeling continues and I’m glad I can drive the car…..but I’m not confident it won’t leave me stranded somewhere!

[BobH]

Agreed, Mercedes seems to have made it overly complicated by these changes over the years, and very little documentation to record it

Pagoda Notes have details of the system changes and test procedures, most of which are also in the tech starting aid tour, but there may be some added information that is useful, have a look at the issue below, and also the other two issues in volume 2, search "pagodanotes" in the tech manual if you've never seen these before

https://www.sl113.org/wiki/Restricted/PagodaNotes?action=download&upname=PNv2i3.pdf

It would help you if you identified exactly what starting system you have in your car, if you check your chassis number, and then search again in pagodanotes, these are some publications detailing what changes happened at what chassis/year, have a look at the pagodanotes index, this may help you confirm what system you have, what relays etc are fitted, and what fuel injection pump is fitted, as the FIP also included a mixture control valve, that may be involved in hot starts.  I think this valve was removed on later pumps, check the 280 wiring diagram out that shows the relay that feeds this valve

You should also try and find if you do have the 1 second add on relay, by tracing and noting what relays are actually fitted, if you are getting a 1 second pulse at the CSV with the TTS disconnected, it would indicate that you do have this relay fitted, and that could be your problem if it only works intermittently, perhaps the contacts may need burnishing

I would check all of the above out and make a sketch applicable to your car, so you have a record of what is actually fitted, and then test each item out methodically

In the meantime it looks like the temporary earth to the TTS is working, so hopefully this will help the hot start issue, just got to be careful it doesn't flood the engine when cranking for too long

[BobH]

Cold Start Valve (CSV) working and injects fuel BUT only for 1 second when cold (20c)

Just read your earlier post, looks like you definitely have the 1 second relay fitted, i would locate this and check the contacts and connections, perhaps that's your culprit

[Bshaunessy]

Bob H
  Interesting article and a good system description thx.  This article is also in TECH MANUAL / STARTING AID.
  Good advice RE making my own wiring and device diagram and documenting just what I have.  Will do.  I have already identified and labelled most ( but not all) relays…..so far haven’t found the one second relay but the sleuthing continues.