WARNING! : DO NOT START WORKING ON YOUR FIP BEFORE YOU ACHIEVE THE BASIC SETUP FOR YOUR ENGINE : CORRECT VALVE CLEARANCE AND IGNITION TIMING AND LINKAGE SETUP !! THIS CANNOT BE STRESSED ENOUGH , OTHERWISE YOU ARE TRYING TO SOLVE ONE PROBLEM BY CREATING ANOTHER ,WHICH WILL DRIVE YOU CRAZY IN THE END ! !.
230SL bad engine running/starting mini tour Part 1 : the basics (
https://www.sl113.org/forums/index.php?topic=38391.msg280468#msg280468 )
230SL bad engine running/starting mini tour Part 2 : Air and Fuel supply (
https://www.sl113.org/forums/index.php?topic=38392.0 )
230SL bad engine running /starting mini tour Part 3 : CSV , injectors and checkvalves (
https://www.sl113.org/forums/index.php?topic=38428.0 )
230SL bad engine running /starting mini tour Part 4 : FIP basic mixture control (
https://www.sl113.org/forums/index.php?topic=38454.0 )
As mentioned in Part 4 two other inputs to control mixture are the WRD and starting magnet (FIP enrichment solenoid).
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WRDBesides the WRD air filter you can distinguish 3 sections , starting from top to bottom :
a) a thermostat also called heat feeler
b) an air regulating sliding valve , the air valve
c) a fuel (mixture) controlling pin, the plunger
Most common cold start problems related to the WRD are a thermostat failure,a stuck air valve or incorrect mixture due to a wrong number of shims.
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WRD Air filter It filters the supplemental air being sucked in during the warmup phase. Air should be able to flow unrestricted, so check for clogging and replace if necessary.
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Thermostat:
On the 230SL the early "long" model thermostat is used . It is an extremely expensive part and can NOT be replaced by the later "short" version .
It is a wax filled unit. When warming up the pin at its bottom extends just a few mm , pushing down on the air valve /plunger assembly.
WARNING : NEVER EVER try to pull at the thermostat pin , you might ruin it beyond repair ! ! Submerge it in a pan with hot water instead to check for movement and operation .
Check the thermostat housing for clogging and unrestricted flow of coolant fluid. Clogging prevents proper operation of the thermostat and thus incorrect mixture after cold starts.
TIP 11 : Use a high temperature silicone sealant between housing and thermostat to prevent coolant leaking resulting in a stuck plunger due oxidation.
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Air valve assemblyIt has a dual funtion : it regulates EXTRA air AND fuel (and as a result raising the rpm) during the warm up phase at a cold start
Below the thermostat is the air valve. On top is a tiny black adjustment bolt with locknut .
Supplemental air flow should be completely cut off somewhere between 65-70 ° C (coolant ! ) temperature. It is the sum of thermostat pin lenght + the total lenght of the black bolt /airvalve combination that determines the size of the remaining air opening at initial cold start AND the airflow cut off point versus temperature.
This tiny black bolt is the only way to adjust the WRD airflow (cut off point) versus temperature. Making it longer reduces the initial air opening and lowers the airflow cutoff temperature ( the thermostat pin needs to travel less,thus the airflow closes at a lower temperature ) and vice versa. Tampering ( in the past ?) with this tiny but very important bolt may result in poor cold start behavior and may drive you crazy, so better check it.
Note : Adjusting this black nut requires to disassemble the WRD by removing the rollerpin and removing the thermostat override bolt, marked with "O" first !.
Underneat the air valve a so called plunger is attached. As you may remember from the drawing in Part 4 :This pin/spring combination of the WRD pushes on an internal lever of the FIP which controls the rack position and so the supplemental amount of fuel as a function of ( coolant) temperature. As earlier explained it is just one of the inputs controlling mixture. Simplified expanation :the further it pushes in the FIP the leaner the mixture gets during warmup . This means adding or removing oval shims underneat the WRD housing either richens or leans the mixture or AFR .
Note : WRD airflow vs. temperature is not influenced by adding/removing shims !. Use the split linkage test or Colortune during warmup to determine if you need to adjust the number of shims.
In the same drawing a "stop pin" is shown which seems to prevent extreme leaning of the fuel mixture by the WRD.
Reading the above you may have the impression that WRD airfow cutoff ,at say 70° C, coincides with the maximum travel of the thermostat pin. It is not. With (coolant) temperature rising above 70 ° C the thermostat pin extends even a little bit more and thus gradually leaning the mixture even more until a fully warmed up engine. This is also called by some as the "lean dip" and may explain running problems - or even stalling- with a partly warmed up engine due to a (too) lean mixture in this last phase. An extra shim may be required to solve the problem.
https://www.sl113.org/forums/index.php?topic=35463.msg259012#msg259012 (WRD fuel leanout stall at end of warmup, just after WRD air shutoff )
https://www.sl113.org/forums/index.php?topic=33088.msg241321#msg241321 ( engine stalls after 4 min in lean dip period , 280 SL )
Summerizing there are 3 WRD phases when starting a cold engine ,see attached drawing :
Phase 1 (Initial coldest phase) : thermostat pin , airvalve and plunger are in their most upper position, maximum supplemental airflow possible*, richest WRD mixture input (plunger) position.
Phase 2 ( air cut-off phase) : at appr. 70° C , thermostat pin,airvalve and plunger are in an intermediate position , airvalve just closing ,so no additional airflow,WRD mixture input position leaner than phase 1
Phase 3 (final phase) : at a fully warmed up engine: the thermostat pin,airvalve and plunger are at their lowest position.Airvalve is closed, WRD mixture input is at its leanest position
* Even with a cold engine the air opening from the air filter adjacent to the air valve is only partly open !
The combination of airvalve and plunger must be able to slide up and down smoothly within the housing. A very common problem is a stuck valve due to corrosion from a leaking thermostat(housing).
TIP 12 : I used coppergrease (=high temp. resistant grease ) for lubricating the valve. Works perfect.
You can imagine that ,when stuck, you may end up with a random "fixed" position of the valve and its plungerlenght causing starting and running problems . This due a mixture being incorrectly "fixed" as well during the warmup phase or fully warmed up engine. A quick check : With the WRD air filter removed and a finger over the opening you should feel the suction of air at coldstart , decreasing till air fully shut-off with rising temperature indicating the valve is moving.
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Starting Magnet ,also called the (FIP) enrichment solenoidIt is located at the back of the FIP and is electrically activated by the electric starting aids ( see Part 6 : Starting Aids)
When activated it pushes a rod forward which in turn pushes the rack forward to a richer position as well . Tweaking the rod lenght can be done but is not recommended.
A quick function check : remove the round "rack cap" at the front of the FIP and screw a M5 bolt in the rack. Then apply 12 V to this solenoid and you should hear a rather loud click and see a forward movement of the M5 rack bolt when the solenoid activates. The rack should move back slightly slower when power is removed.
If you observe anything incorrect in my text please PM me so I can adjust it.
To be continued with Part 6 : electrical starting aids
