Date registered: Jun 2004
Mentioned: 1 Post(s)
Quoted: 17 Post(s)
Before you go touching the MFI ,let me tell you what is happening. The Cold start,cold running device is sticking. What is happening is the cold engine needs more fuel (rich mixture) and more air (fast running) .The problem must be diagnosed correctly BEFORE touching the MFI. if the engine is hunting on cold warm up cycle,rev the engine,is it coughing out the intake? If yes,it's lean too much air and not enough fuel.
Is it backfiring in the exhaust and leaving a lot of smoke?
Too rich and not enough air.
Find this out before proceeding.
Never start diagnosing a running problem by messing with the injection. Start with the basics by ensuring the ignition is PERFECT.
Check the fuel pressure and flow. You need 14PSI and 1 liter per 15 seconds.
Next,remove the rod connecting the throttle to pump. Move the arm on the pump to full throttle. Then very slowly let the arm move back to idle position. it must move freely with out any catching or feeling of binding. Any binding indicates wear in the space cam linkage in the pump ( rebuild time) .
next,look at the forward end of the pump,there is a round steel cap on the front face of the pump. Remove this and under it is the end of the fuel rack. This should be very loose and move freely . to check this ,screw a 5 mm bolt into the end of the rack and with the engine running rev it up and down,the bolt should be seen to move in an out as the governor works. if not a rebuild in necessary.
Cold engine speed is 1000 RPM.
If that is OK ,the pump should be able to deliver fuel correctly. On the rear of the pump is the governor. There is bolt with little T handle. that is the governor oil level ,check it by removing and check the oil level. If oil pours from the dip stick hole, the governor is over full . That could be bad maintenance or the injector pump element oil seals have failed. Again only a pump rebuild will sort this out.
The MFI is connected to the engines water system via those two rubber hoses . Start by checking that the coolant is flowing to the MFI on warm up cycle by removing the engines thermostat and placing it in water and boiling it on the stove. it should be fully open on a USA car by 75 * C. Once that is checked and verified ,disconnect the two water hoses on the MFI and blow compressed air through them to make the water will flow freely .
next,the device the two hoses are fitted too is the MFI's thermo. On the 250SE it will be adjustable. Remove it carefully by undoing the two screws on the base of the metal tube with the two pipe fittings.
Inside is a brass thing. it's your wax thermo. it works by extending the pin down
wards the hotter it gets making the overall length longer. In doing that the cold running device starts off by moving the fuel rack to a position where more fuel is pumped. At the same time a valve under the thermo is opened allowing more air into the engine.As itr warms up the vale is allowed to rise closing off a port in the valve body and moving the rack to a position where less fuel is delivered.
Now Hunting is caused by two different causes,lean or rich.
if your engine is lean...
What is happening is that the cold air is opening but insufficient fuel is being delivered.
Check the thermo for operation by first placing it the freezer for a few minutes then hang it in the water from a wire for a few minutes and measure the length. If the length hasn't changed it needs replacing.
if it has changed take a note of the change.**
next,that valve under the thermo needs to be able to move freely ,there is a spring under it and if you push it down it should go down and come back easily.
If not,check on the side of the valve body for a bolt,this bolt is an eccentric adjustor,very carefully give a nudge with a wrench. if it moves carefully remove it .If not soak it with Kroil or similar. if you break it ,you are in trouble. Be aware though,not all cold running valves have this feature.
If the engine is running too rich on cold start ..
The air cleaner canister on the MFI is blocked. Check it by slowly unscrewing it while the engine is idling and if the idle speed increase it is blocked it can only be replaced,not poked at and cleaned.
Next step is to fit your new (or checked OK ) thermo and ensure that the hoses are secure. The thermo is fitted with a gasket compound to ensure NO water can leak into the MFI.
Start the engine,From dead cold and check when the water temp reaches the temp when the MFI shuts off completely.
How do you check this?
When You removed the small can shaped canister from the REAR of the MFI,it's air cleaner, it happens to be where the engine sucks air when warming up.
You simply place your finger over the hole and check the vacuum. Keep checking until the vacuum stops and make a note of the coolant temperature at this point.
As i mentioned,USA cars shut off earlier and the MFI should shut off at at 50-55 *C, all others shut off at engine running temp of 80* C.
So ideally you need to set the cold start to shut off completely at 50-55* plus ,no earlier and no later than 80* C.
How do you adjust the shut off point?
On the top off the valve under the thermo is a 6mm bolt and lock nut. under the bolt are washers.
To shut off earlier you add washers under the bolt.
to make it shut off later,remove washers.
On top of the washer stack is black washer,do not swap,lose or other mess with this washer as it supposed to bear against the end of the thermo plunger.
Now the air /fuel might not feel right,so to adjust the mixture you need a rev counter and a vacuum guage.
The engine needs to be dead cold,either leave it over night or use a large cooling fan to get the temp down as far as possible.
Dis connect the throttle link on the valve body.
Connect your rev counter.
On the throttle body is an M4 grub screw, this is where you connect the vacuum gauge to get absolute engine vacuum.
You need a tee fitting to enable the distributor vacuum connection to be drawn from the same point.
You need to do this to prevent the ignition form advancing to quickly and giving the wrong reading while increasing engine speed by 300 RPM.
Block the distributors vacuum fitting on the throttle body.
Start the engine and let it idle.
Take a note of the RPM's.
Open the throttle valve on the throttle body VERY slowly .
Keep moving it slowly open until the engine starts to stumble ,ie max speed.
From the idle speed it should increase by 300 -400 RPM.
If you open it too far it will stall the engine.
If the revs are not increasing to 300 RPM or not increasing at all,Stop the engine immediately ,you need to keep it cold.
IF the engine goes faster than 400 RPM above idle speed ,undo the thermo valve and remove 1 or 2 washers.
If the speed is under 300 RPM ADD washers.
Good luck and remember the Old Bosch engineers refrain CHECK..MEASURE ADJUST but most important of ALL, THINK before you touch!.