Hot start problems 1988 300TE, M013 Gas engine

[rdw]

Hi Everyone.

I'm hoping someone can help me with a persistant hot start problem on my 1988 300E, W124 with the M103 gas engine.

I’ve had my car for about 10 years. It has 120,000 Km on it. It is a German model with the adjustable EZL-KAT resistor. The car was built in late 1987. Since I’ve had it it has been difficult to start if I leave it in the sun or park in underground heated parking. Initially it had to be really hot outside or underground for about 2-3 hours. Lately it is getting worse. Starts fine when hot for 15 or 20 minutes but longer than that up to several hours is hard to start.

Car starts well when it is cold, runs fine, never hesitates and gets acceptable fuel economy. It has a very slight random misfire when idling. When warm it idles at about 600 rpm in gear. When starting the car and not touching the gas it runs quickly for a couple of seconds and then slows down to about 1000 rpm (in neutral) until it warms up.

Over the years I’ve replaced and tested several items, not all because of the difficulty starting when hot, some just out of regular maintenance. About the only difference was that when I replaced the injectors, the problem actually got worse but the engine seemed to have better low rpm power.

To try to diagnose the problem I put a light and a switch on the cold start valve. I thought possibly it is engaging when the engine is hot and flooding it. The light comes on at cold temperatures but not hot. These days, about the only way I can get the car to start when hot is to apply power to the cold start valve. This seems quite counter-intuitive, it would be like choking a carbureted car when hot. This will get it to fire and often because I’ve opened the gas pedal a bit, the engine will jump up to 2500 rpm, quite smoothly but then die if I take my foot off the gas. Without engaging the cold start valve, often times the car will fire a bit while cranking but not enough to actually start.


This is a list of what has been done so far
Fuel:

• Replaced Oxygen sensor.
• Set mixture using duty cycle measurement on an oscilloscope.
• Replaced Fuel filter.
• Replaced Change-over valve.
• Replaced Cold start valve.
• Replaced Idle speed control valve hose.
• Tested Idle speed valve for valve’s smooth operation and not sticking using a variable power supply.
• New Bosch Fuel injectors, o-rings and holders.
• Measured the profile of the temperature sensor at various engine temperatures, measures almost exactly the resistances quoted in the Mercedes manual. My temp sensor is the two pin variety.
• Replace fuel pressure regulator.
• Both fuel pumps are working and quite quiet.
• Tried swapping Jetronic engine computers, although the one I swapped out was a slightly different number. No difference in how the car ran or started except it idled too fast with other computer.

Ignition / electrics:

• New Coil.
• New Sparkplugs (Denso non-resistor).
• New Distributor cap and rotor.
• Replaced OVP relay.
• Swapped EZL ignition control module.
• Measured Crank position sensor as 800 ohms cold.
• Measured resistances of the EZL-KAT plug, all are on spec.
• Used spark tester to measure spark voltage. It was higher than what the meter could measure

Years ago a local shop had checked the fuel pressure and did a smoke test looking for vacuum leaks. Both were okay.


I don’t know where else to look. Any help would be appreciated.

An additional question I have, does anyone know the difference between a couple of the jetronic engine computer models?
My car came with a Bosch “0 280 800 206” , aka Mercedes “005 545 57 32”
The cars here in Canada all seem to have a “0 280 800 256”

When I swap in the 0 280 800 256 the car runs about the same except it idles too fast, at about 1200 rpm or 900 rpm in gear, instead of about 600 rpm with my computer. There is no hint of misfire when idling at 900 rpm though.

Thanks very much.

 

[guydewdney]

wiring loom degradation?

 

[optimusprime]

Start it up in the dark and look for electrical flashing in and around the loom.May not be that ,but its a way to see if it is.What make is the rotor..

 

[rdw]

The distributor cap and rotor are both Bosch brand. I didn't mention that I also checked for corosion on the coil and spark plug wires. All are very clean.

Where would I look for flashing, what part of the loon? I never hear any arcing when under the hood. I can look after it gets dark, which is quite late up north now.

 

[coalville126]

Throttle valve or Throttle position switches?

Within our family, we've had M103 engined Mercs from when they were new until now. When you say yours is hard to start from hot, how many cranks does it take to fire and run evenly?

I've never experienced this issue even in high ambient temperatures when I've toured Southern Europe in the past.

 

[stwat]

The only thing left to replace is the EHA valve on the side of the fuel metering head.

 

[rdw]

He Everyone thanks for the replies and suggestions. I had a really good look & listen under the hood for any arcing this evening and all seems well.

To answer the how long does it crank before starting: I would say maybe 30 seconds. I usually only crank it for no more than 10 seconds then wait a few seconds and try again. After about 5 seconds of cranking one cylinder will start to feebly fire, but it won't start. Moving the throttle makes little or no difference.

I tested the Throtle position switch and it is closed (continuity) when the throtle is closed and open when the throttle is moved slightly. However I did notice that one of the two little female connectors inside of the plug was not in its slot. Who knows if it was making contact or not. It has gotten cold here again so it may be a while before I can tell if fixing this made any difference.

 

[Wackeldackel]

The only thing left to replace is the EHA valve on the side of the fuel metering head.

Maybe not replacement but adjustment...the EHA has a little slider track inside that is the electrical contact (hope that makes sense), over time it can wear slightly where it rests when the engine is idling so it may need adjustment (loosen the 2 tiny torx screws slightly and then lever it up or down slightly until desired voltmeter reading is achieved and then checking with a voltmeter, from memory it should be 0.70V across it at hot idle...this cured my lumpy idle and poor hot starts on my 300E.

I personally would say wiring loom degradation is unlikely with this engine, the m103 is such a tough old beast and compared to the later DOHC engines theres very little wiring near anywhere hot. When you replaced the injectors did you replace the clip on collars as well as the o-rings that seal ariund the white plastic injector holder tubes?