M103 hot start and uneven idle

[mbzman]

When I start my 300CE when the engine is at operating temperature the car will start and then die less than 1 second later. On the second try it will start but the idle is quite low and uneven. If I put the car into drive the car will idle down to around 500 rpm and feels like it wants to stall. Once I get under way it is fine.

It has only started to do this when the weather got hot up here. It has always started and run easily enough during the rest of the year. Of note is that the fuel accumulator, FPR, and all six injectors and seals have been replaced within the last year. The EHA valve is also new. The IACV is also operating smoothly and I couldn't find any vacuum leaks. I'm thinking that the car is running too lean but I already checked the duty cycle and it was within specs (55%).

[rayhennig]

Originally Posted by mbzman

I'm thinking that the car is running too lean but I already checked the duty cycle and it was within specs (55%).

If you have the M104 engine with KE-Jetronic fuel injection, check the air intake sensor. To do this, unplug it and measure the resistance across the terminals. Mine measured open circuit.

I changed it (€12) and the duty cycle had gone from 50% to 90+%. Re-adjusted to around 50% and things have been much better during hot starts.

RayH

Damn! Just seen that you state M103 engine. Pretty sure there's no air intake sensor. Oh well, I'll leave this here in case it's useful for others.

[remotemark]

Originally Posted by mbzman

When I start my 300CE when the engine is at operating temperature the car will start and then die less than 1 second later. On the second try it will start but the idle is quite low and uneven. If I put the car into drive the car will idle down to around 500 rpm and feels like it wants to stall. Once I get under way it is fine.

It has only started to do this when the weather got hot up here. It has always started and run easily enough during the rest of the year. Of note is that the fuel accumulator, FPR, and all six injectors and seals have been replaced within the last year. The EHA valve is also new. The IACV is also operating smoothly and I couldn't find any vacuum leaks. I'm thinking that the car is running too lean but I already checked the duty cycle and it was within specs (55%).

My 88 300CE often acts similarly. But if I make sure that the a/c is switched off when I turn the key, it doesn't happen. Of course, it's been around a hundred here lately, so I've been forgetting . . .

[lkchris]

Engine's possibly flooded.

Try starting with about half throttle.

If better, then injectors are leaking and you need new ones. Very common.

[mbzman]

Originally Posted by lkchris

Engine's possibly flooded.

Try starting with about half throttle.

If better, then injectors are leaking and you need new ones. Very common.

All six injectors and seals were replaced a year ago. How can they possibly already be bad?

[S-Class Guru]

My '91 SE has done exactly that for 15 years. Tried everything to fix it over the years, except replacing the whole fuel distributor. Mine is definitely due to insufficient fuel during hot start after heat soaking awhile. When engine is cool, or when it only sits less than 15 minutes when hot, it starts and idles perfectly. After heat soaking it starts on the second try, and idles rough for a few seconds, then smoothes right out.
If anyone has this fix, I'd definitely appreciate it; I've posted a hundred times over the years each time I tried something new, but still no fix.
Bottom line, you may have to live with it.

DG

[samiam44]

Try putting a fuel gauge and check pressures! Hot soak problems to me are ussually:
1) pressure dampener by the fuel pump
2) bad check valve in the fuel pump

That's based on lots of 80's 911's.

[mbzman]

The fuel pressure regulator was also replaced when the injectors were done. Where exactly is this fuel pressure check valve that you speak of?

[spit64]

Cold start valve faulty or thermo switch some system don't have thermo switch, Fuel pressure incorrect, airflow sensorplate rest position wrong.
If you have a car with lambdacontrol this could also be faulty. The first thing to do is check the residual pressure to see if it is in specification or out of and go from there. Also check the engine temp sensor.

[Cal Learner]

The fuel pressure (vent) valve is part of the evaporative emissions system, fitted to the return line coming out of the charcoal filter and located about 2 inches below the fuel tank.

[mak]

Originally Posted by S-Class Guru

My '91 SE has done exactly that for 15 years. Tried everything to fix it over the years, except replacing the whole fuel distributor. Mine is definitely due to insufficient fuel during hot start after heat soaking awhile. When engine is cool, or when it only sits less than 15 minutes when hot, it starts and idles perfectly. After heat soaking it starts on the second try, and idles rough for a few seconds, then smoothes right out.
If anyone has this fix, I'd definitely appreciate it; I've posted a hundred times over the years each time I tried something new, but still no fix.
Bottom line, you may have to live with it.

DG

I have had this problem in summer time and eventually sorted it by installing a Heat wrap on the exhaust manifold . A Amazing drop in under-hood temperature and starting.
mak

[spit64]

Is it possible that you had the car parked since winter and still have gasoline från that time in the tank?

[mak]

unlikely .as there were may refills of techron on a regular basis .