380SL Hard Start & Poor Mileage

[GregZ]

I am struggling with some similar problems except my car runs waaay to rich (black smoke rich). No vacuum leaks (fixed all of them, and inspected the air housing seal as well (I had it off to replace the TPS). My WUR is good my injectors are good, my cold start valve is not leaking, so I am going to replace (and overhaul) my fuel distributor next.

My question is though: after you fixed the brain problem (and before you fixed the air guide housing rubber), was your car running much better?

Oh, yes, I am gathering all components for a megasquirt conversion. I really dislike this CIS...

Thanks

[ctaylor738]

What is your system pressure?

What are you seeing for control pressure hot and cold?

Have you used the duty cycle to set your mixture? What are you seeing there?

Is your oxygen sensor OK? A low reading will cause the brain to enrich.

Usually IME, a FD problem causes a miss or roughness rather than a rich mix.

[Strife]

This was a very interesting post. Can anyone with experience say if these cracks are typical at this age, or did something unusual happen to this engine?

Operating at 70MPH, where is the vacuum needle on an 85 380SL? I would say mine is at 5/6th left on flat level road.

[GregZ]

I cannot run (do not want to run) the car long enough for it to reach operating temperature. It runs way too rich (a lot of black smoke). The control pressure is about 3.2 bar, and I get the fuel pressure to raise to the right level within about 10 seconds. The car is idling so the oxygen sensor will not effect the 60-40 preset duty cycle.

[ctaylor738]

Then the two things I would check if you haven't already are a sticking plunger in the FD, and the condition of the o-ring on the plunger.

Actually, the car should go into closed-loop at idle and start reading the sensor, once the oil temperature switch opens.

[GregZ]

plunger is free as a bird I have not checked the o-ring yet.

I remember reading in the manual that the TPS in idle will switch off the closed loop frequency valve control and it will be set to 60-40. Are you sure that it goes into closed loop after the oil switch turns off? Even if it did, could a malfunctioning brain cause such richness?

Thanks for your help!

[GregZ]

Strife: based on my experience they do tend to crack. What I do not understand is why Tim's car was running rich though, I would expect it to run lean (more air) unless the fq valve is overcompensating.

[ctaylor738]

I don't have my technical stuff with me right now, but I am quite sure that at idle stop with a warm engine you should be in closed loop. I don't think the brain can drive the mixture to the point of black smoke - at least I have never seen it. That takes low control pressure, high system pressure, leaky CSV, a leaky injector, or a FD problem like the o-ring.

[timm9]

gregz,

Sorry for the delay. I have been dealing with a daughter getting married, and another kid that is facing surgery...

The replacement of the "brain" did not have as big an "effect" on the car as the cracked air housing. The biggest difference was the huge amount of air entering the system through the cracks in the air housing that was telling the system to add more fuel. Prior to the air housing replacement, the exhaust at idle was black and staining my garage floor, not to mention my mileage was in the toilet. While I'm still having an issue getting the duty cycle perfect, the power has drastically increased and I suspect my mileage has increased. I have been too swamped to get an accurate reading considering that much of the fuel was consumed while at idle and working on the car.

I can tell you this, Ctaylor is the "Master"! Do exactly as he suggests and you can't go wrong. Thanks again Chuck!

[GregZ]

I took all the injectors out and the fuel distributor (again). I checked the injectors again and they all open at around 3.9-4 bar and they do not leak under that pressure (they do not leak at 3.8 bar). I removed the plunger (still not stuck at all), I do not see an o-ring on the plunger (I do not see a place where it could go to either) nor in the plunger cylinder.

Chuck: any chance you have a picture of a plunger with an o-ring?
Also, I reread the manual, and I believe you are right when the oil switch turns off (mine is always off) then the lambda control takes over except for full open throttle.

Thanks,

Greg

[Euro Driver]

Good day all,

I just bought a 1984 280SL that has the exact same problem.
- Starts fine when cold.
- Once the engine is warm you need to pump the accelerator to start the car.
- I can watch the fuel gauge drop as i drive.

The one unique issue I have is the car also starts to idle heavy intermittently - almost like the engine is misfiring. This doesn't happen all the time so i haven't found a pattern yet but enough that its a major concern.

I have also noticed that once the engine is warm, if i turn the wheel all the way to one side while idling the battery light will come on and the car will stall if i don't apply the accelerator. I have particularly noticed this problem when parking the car in reverse. I am suspecting this is due to low idle rpm's but am not sure and not sure what to do about it.

As a note the car is equipped with an aftermarket emissions control system. It was imported to California from Germany in 1985.

Does this sound like a Air Guide Housing issue as mentioned above, a different problem, or a combination of problems?

I really am looking for some guidance so i can point my mechanic in the right direction. I have a really good and honest mechanic but he's not a Mercedes specialist.

[ctaylor738]

Brain fart or senior moment, your choice. K-Jet doesn't have the seal on the piston, it was added on KE-Jet in the 560's, 300E's etc. Sorry. But you need to be sure the o-ring that seals the FD to the mixture control unit is in good shape.

However, if your oil temp switch is open all the time, then the system is going to be in closed loop at startup. You will have very low voltage from a cold O2 sensor so the brain will think the engine is very lean and try to enrich. In addition the WUR is going to enrich the cold engine as well.

Suggest you replace the switch, but in the meantime, you can ground the wire to the oil temp sensor. That should kick the brain to 60/40. Then see if you can lean the mixture screw (go ccw, 1/8 turn at a time, at least 10 seconds between turns) and get it to run better.

[GregZ]

I am working on replacing the switch, however it is not that easy. The old engine I had in the car the PO killed badly, so I have a replacement engine now. This new engine did not have a oil temperature switch (it did not even have a 2-switch TPS but a 1 switch). Where the oil switch was on the old engine there was a plug in the new engine, however the plug has an M14 thread compared to the M16 on the switch. So I used my lathe to put an M14 thread on the switch but I am not done yet. I am not even sure I really want to do that as I think I am pretty much set on a Megasquirt system. However I will take your advice and ground the cable to see if I can get the freq valve to work more reliably. I thought that in the Texas heat that switch would not be on anyway.

BTW: were the 380SE engines in Europe any different from the 380 engines in the US (compression and stuff)?


Euro driver: The 380-s and 560-s are V8 engine with all the injection relate components inside the V, your engine is a straight 6 (if I recall it correctly) with all the intake and injection parts on the left side. I am not sure you even have a rubber "air guide housing" seal above your throttle. However the fuel accumulator problem may still be the same.

[timm9]

I had intended to update this post quite some time ago but earning a ‘living’ has been interfering with life…

I wanted to give everyone a heads up on something I experienced after the replacement of the Air Housing Chamber. After everything had been reassembled, and the duty cycle properly adjusted, the car ran like a champion (21+ MPG) and had much more power. Approximately 300 miles after the fix, I noticed that the idle was slowly creeping up until it eventually was at 1300+- RPMs while in Park. I again searched for a vacuum leak that I could not find, so I had another smoke test which revealed that the OEM strap (acting like a hose clamp) was not tight enough to seal off the connection at the throat of the Air Guide assembly where it connects to the engine. I again dismantled and this time replaced the OEM strap with a hose clamp. I reassembled everything and all was good for a few weeks, then the idle started creeping up again. I figured it must be associated with the Air Guide replacement, and eventually discovered that while bolting down the FD and mixture chamber, the Air Guide Housing was being pulled up away from the connection with the hose clamp at the throat. In all I disassembled and reassembled this three times. If you are faced with this repair, I might suggest the following; 1) Don’t waste your time with the OEM strap, use a hose clamp. 2) After reassembly, drive the car and then check the hose clamp again. You might need to do this several times over a period of a few hundred miles. There is just enough room to retighten the hose clamp without any disassembly. I hope this helps someone in the future!

[timm9]

After the discovery and replacement of the Air Guide Housing I was still having issues with inconsistent idle and a slight miss. I assumed it was a vacuum issue but could not find it so I went back to the smoke test.

If you find yourself doing the Air Guide replacement make sure to note that without a newly design 'flange' you will still have a vacuum leak. Unfortunately, I was not aware (nor was the parts supplier) that with the new Air Guide Housing you must also install the new flange.

Hopefully this will prevent someone from having to do it twice!