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  #1  
Old 06-18-2013, 10:38 PM
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Flooded on 'cold' start w108 4.5

This evening it was ~85F (~30C) and for the first time the Benz would not start. First I checked for spark, and it was ok and strong. Then, assuming it had no fuel, I sprayed carb cleaner in the intake while cranking. This will fire up nearly anything as long as there is spark, but the engine still did not start. I removed the plugs and they were super wet with gas! I let it air out for 15 minutes, put the plugs back and pulled the fuel pump fuse.
Car started and ran for ~10 seconds on the leftover fuel. I reinstalled the fuel pump fuse and it started up fine.

Any idea why did this happen? I did not think it could flood so fast that it does not even start up. What controls the cold start injector, and what is the temperature threshold where it stops working? On L-jet over ~65F the cold start injector does not fire.
Thanks,

Csaba
72 280SEL 4.5
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  #2  
Old 06-18-2013, 11:23 PM
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Quote:
Originally Posted by vandor View Post

Any idea why did this happen? I did not think it could flood so fast that it does not even start up. What controls the cold start injector, and what is the temperature threshold where it stops working? On L-jet over ~65F the cold start injector does not fire.
Thanks,

Csaba
72 280SEL 4.5
There are two ways mixture is enriched when engine is cold. One is the cold start valve, and the other is the ECU which enriches mixture by pulsing injectors more than when warm.

The cold start injector only activates while the engine is being cranked, and only if the temperature is below 95F. There is a thermo-time switch that keeps the valve open after starting. At 95F for just 1 sec, and progressively longer up to 12sec at -4F.

It could be that the thermo-time switch is not working properly. You could pull the cold start plug at summer temperatures and see if it makes a difference.

It could also be that one or more injectors and/or the cold start injector are leaking by. The manual describes how to do a test.

Old worn trigger point fibre cam followers can also cause rich mixtures. I had that happen on my own car. But that would happen all the time.
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  #3  
Old 06-21-2013, 11:32 PM
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It happened again

Today on the way home it happened again. Grrrr. I played with it for half and hour and could not get it to go, then waited an hour for the tow truck, and by the time it got there the car would run.

A bit before it died it seemed like it missed a bit, like someone turned the ignition off. Then a few seconds later the car quit and I coasted to the side of the highway.
It had the same symptoms as last time: no start, but if I pull the fuel pump fuse and crank it a bit then it starts for a few seconds. Put the fuse back and it will crank, but not start. I cranked the idle mixture all the way to lean and then it would idle, but as soon as I touched the gas pedal, it died. Tried this several times, same result.
After the tow truck got there it would run and idle, but the idle is still at full lean, and rpm drops if I richen it at all.

I have checked the electrical connections to all the sensors and switches and all looks OK. Of course there could be internal breaks somewhere.

I think these things would cause an instant rich mixture:
-Engine temp sensor bad or disconnected
-MAP sensor hose leak or disconnected
-MAP sensor bad or disconnected

I don't think anything else could make the mixture soooo rich that the car dies. (right?)
I plan on checking the MAP vacuum hose and check the sensor wires for continuity to the ECU. Is there a wiring diagram for D-jet? I don't remember seeing one in the guides.
Thanks,

Csaba
72 280SEL 4.5
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  #4  
Old 06-22-2013, 02:34 AM
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Are you using an OE fuel pump?
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  #5  
Old 06-22-2013, 01:47 PM
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If I understood correctly the car was running and it went off and only starts after it cools down, right?

Its the coil! That could also explain the wet plugs.
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  #6  
Old 06-22-2013, 11:20 PM
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>Are you using an OE fuel pump?

It looks OE, but it was on there when I bought the car.

>only starts after it cools down, right?

Not really. Since I was able to get it to idle I let it idle for ~1 hour while I waited for the tow truck, as it was very hot and I had the AC on.

The car would die if I just barely touched the accelerator pedal, but after idling for an hour it was fine. The wrecker driver drove up onto the truck.

The coil is new. However I have a replacement one in the trunk, they are easy enough to swap out to try it next time.

I really think it's a mixture problem, tho, as it starts once I disconnect the fuel pump.

BTW, the cold start injector is unplugged and I checked it for leakage and it had none.

Csaba
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  #7  
Old 06-23-2013, 03:19 AM
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It may be worthwhile to put a fuel pressure gauge on the fuel rail as the regulator may have gone bad, or is out of adjustment. At idle, the injectors fire 2 at a time but at higher RPM, all injectors are open which could be why it's loading up under throttle.
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Al

Check out the W114, W115 enthusiast website.
http://www.stroke8.org

http://www.w108.org

Join the Mercedes W108 group
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  #8  
Old 06-23-2013, 11:43 AM
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could also be failing switch gear... those suckers short out and stop working, or the ignition resistors could also be going bad.
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  #9  
Old 06-24-2013, 02:53 AM
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Well, it's running well again, so I'll have to wait until it dies the next time. Unfortunately this means it will get little use this summer, as I am not willing to risk breaking down with my kid in the car in the summer heat...

In the meanwhile I will redo the coolant and air temp sensors' connectors, as on both the insulation is peeling off the wires by the connectors.

Csaba
72 280SEL 4.5
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  #10  
Old 06-25-2013, 11:25 PM
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Slow progress

Today I fixed the coolant temp sensor's connector wires. The insulation was broken and the bare wire was visible. I cut both wires, reconnected them with crimp connectors, put heatshrink tubing over each wire and then a larger one over both wires.

I drove it for a bit and the idle was very low. I checked a plug and it's black from running rich. Hm, the idle mixture adjustment is all the way to lean (CCW) and turning it CW slows the idle.

I checked fuel pressure and it's 29-30psi. Not sure why it's running rich. Does the MAP sensor have any effect on idle mixture?

One thing I noticed was that the coil HT wire was not fitting well, and every now and then a spark jumped from the HT connector to the +12V terminal! I pulled the boot down and secured it with a ziptie for now. I will have to find a HT coil wire with a better fitting boot.

Csaba
72 280SEL 4.5
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  #11  
Old 06-26-2013, 02:30 AM
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I had a similar experience with my 280SEL 4.5 when I owned it. It would run perfectly for a while and then all of a sudden load up. The issue was resolved with a replacement computer.
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With best regards

Al

Check out the W114, W115 enthusiast website.
http://www.stroke8.org

http://www.w108.org

Join the Mercedes W108 group
http://autos.groups.yahoo.com/group/mercedesw108/

My 280SL restoration

http://www.w108.org/gallery/albums/a...0959.thumb.jpg
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  #12  
Old 06-26-2013, 02:46 AM
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I have a spare computer if you'd like to borrow it to see if it makes a difference. Also, the computer only controls mixture at idle. Off idle the map sensor controls the mixture, thus why these cars are so sensitive to vacuum leaks.

Best Regards,

David
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  #13  
Old 06-26-2013, 01:04 PM
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The MAP controls overall mixture too, including idle, from my understanding of the primitive system - which is why there's an idle mix wheel, once you get load mix right, you then need to set idle mix.

I second Al's opinion, your ECU might be toast. It's not common but it's quite possible. Other causes could be leaking injectors (dribbling or pouring gas into the engine when they should be closed), major vacuum leaks, or other issues.

Does your door lock system work? If not, plug the vacuum source line. Brake booster should hold a good vac for a while after shutting off the engine. Wait like 5-10 mins and crack the line open, it should release a fair vacuum. The transmission sources its vacuum from the engine for the shift points & pressure, that line may be disconnected or rotted under the car. This is all in addition to the MAP hose itself, of course, and the rubber bits for the idle air circuit.
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  #14  
Old 06-26-2013, 01:51 PM
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Hi David,

Yes, that'd be great, I'd like to borrow your computer for testing.

Tom, the door locks work, and will work for ~12 hours after the car is shut off, so the system is fairly leak-free. I'll check the booster/brake pedal, but I think I would have noticed if it leaked down too fast (I've been know to dead-stick it into to driveway :-)

Csaba
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  #15  
Old 06-26-2013, 07:33 PM
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I get back into town this Sunday afternoon. I'll shoot you a text. Fortunately the ECU's are easy to swap.

Thanks,

David
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