1987 300sdl-- 1stcold start /day --starts/dies
 

Looking for help

1st start of day this car turns over than--dies (temp around 65 degrees)

Idle--seems low- if I simply give more throttle for 20 seconds or so--car will stay running/driveable. ONLY happens-so far On 1st start/day.

Tach- has been wonky for 2 years--works when it wants to-- now NOT working at all.

I attempted to increase Idle w/ ELR knob-- there seems to be ZERO response
In either increase/decrease.

I have read thru some OLD posts on same symptoms --people seem to point to EDS bad...

How do I resolve that? Or are there other places to check/go for increasing idle- if that indeed is the solution.

I thought it was fuel issue/first... But no smoke/no shake...
There is a "knocking exhaust" type sound -as if car is choking or exhaust is blocked when it is 1st started- and idle seems to low... but quickly goes away when i give it more throttle to increase idle

Check on the OVP (over voltage protection) relay and the circuits that provide the ECU power. When functional the ECU adjusts fuel trim on the injection pump and also operates the tach. The ECU will idle up the car upon cold start. Without it the car will want to stall. At least this is the case with my car.

Local Indy mechanic suggested I pull the OVP - and If Idle changes-- It is a faulty OVP?

I started car... idled low... pulled OVP-- 1 second Surge in idle- then quickly returns to low-ish idle... does this prove OVP is bad or good??

Anyone know?

If the tach isn't working, chances are the ovp relay needs to be replaced anyhow. Is the ABS light on? When mine went bad the non op tach coincided with an ABS light.

ABS lite is NOT on

Does it run/drive out any after first start of day and it has been run some?

Look at the injection pump. Is there a trace of fuel on the body?

How is the car parked? Level, nose up, or nose down?

Car is parked level
in garage- around 65 degrees

It just starts w weak idle-- and turns over a little slower than it used to.
If I don't give a little throttle on start... it might even die - until I give assistance.... after 5-10 seconds of lite throttle-- it idles w a little more confidence...and stays running... ONLY happens 1st start day-- I can go park for 2-3 hours and starts no issues.

Lastly- I read last nite- a Crude test for bad OVP-- pull the OVP and cold start... IF Starts exactly the same-- It is an OVP issue. I don't see how that proves anything... but that nugget came from MB world website.

Regarding IP and trace fuel... Inspected that today-- NO FUEL--However very back of IP where the Manual stop lever is connected-- there is some OIL RESIDUE around that area... does NOT appear to be leaking--yet there is a 4-5 inche radius of Oil residue in the surroundings- once again--NOT heavy residue- but you can tell something is going on due to build up of dirt into that greasy area.

Any thoughts on how/if this is part of my start/no TACH/AC issue? Or is it a separate issue form Tach/AC ( which if Tach doesn't work- AC can stop-cuz it supposedly relies On reading RPM to engage?)

Yes it drives perfectly fine once started. and restarts w/o issue after that 1st start... although idle does seem a tad low still... by ear/feel.

The strange thing-- is that ELR knob that trims the idle up/down -is doing NOTHING now... I don't know if that is related to OVP being bad --or completely different issue.

Nobody has that OVP in stock--MB discontinued... and Knockoffs are out of stock...

You have a source for New OVP?

^^ THIS - If the OVP is not functioning the idle will be low and the car will want to stall like the symptoms you describe.

Also, when my OVP relay was malfunctioning the tach and AC did not work. I've never had an ABS light. As for sourcing one, junk yard or ebay is probably your best bet.


There is a test procedure posted on benzworld https://www.benzworld.org/threads/ovp-basic-function-and-test.1572015/

Given your temp related issue, I would hazard a guess that you have a weak solder joint in either or both the OVP and or ELR relay. They are both crucial to idle management.

If you have a soldering iron, you can reflow the primary contacts between the pins and the boards.

Lots of how to vids and posts here.

I received the Backup (working according to Ah-Kay) OVP--plugged in... and same symptoms... Low idle/No Tach/ No AC

Where exactly are these "weak solder joints-to be repaired"??

Anyone else with an opinion/guess as to what to inspect next as the cause of this situation? Thanks!

Here is a link to another post that had similar situation. In that post are various links to soldering the OVP and ELR relays as well as youtube video tutorial on the how to. Read the input from DSVW and myself as there are good text explanations of what is to be done in those plus the aforementioned links.

It's not a hard process, and it really does not hurt anything. In fact it can at least rule out the relays if their contacts after reflowing are now known to be good.

https://www.peachparts.com/shopforum/diesel-discussion/415931-1987-300d-bad-ovp-symptoms-but-ovp-good.html

Good luck and let us know how it works out for you. This is becoming a far more frequent solution to a more frequent problem given the age of these relays.

The OVP was removed from my working SDL so it is a working unit. NO tach and NO AC has nothing to do with OVP. OVP is a very simple device, it has a zenor ( 12 or 14v ) diode inside to reference the voltage. The diode can go wrong or unit has bad solder joints. No other active components.

Shaky idle may be OVP. You have now eliminated one possibilities.