w126 300SE Air Fuel Mixture Problem. Won't start. Flooded?

[RossCormack]

Hi all, been searching the web for someone who's seen this problem before. I was adjusting the air fuel mixture on my 300SE and the engine cut out when pushing the allen key down on the spring (adjusting screw on top of metering head). Now the engine will not start. I think I might have made the mix too rich and flooded the system with too much fuel and now something is clogged up and unable to clear. I left it overnight for any fuel to evaporate. I have also turned the key with the fuel pump relay removed in an attempt to clear it out. At the moment, it seems to fire and then die right away. I've tried adjusting the screw again and still no start. Any advice on this would be fantastic, thank you.

 

[John Jones Jr]

All the signs that you have upset the mixture by excessive/incorrect adjustment. Anti clockwise to lean it out but even if it's set a fair bit too rich the engine should start but may need to depress the throttle in order to do so. Re-adjust by turning the screw 1/8 of a turn at a time and you should manage to get it to start without depressing the throttle. Why were you adjusting it and how are you testing the mixture?

 

[stwat]

It only takes miniature adjustments of the mixture screw to make big differences. If you are not using a meter whilst making adjustments, you are taking a lucky dip. If you push the mixture screw down too far the engine will cut out, that is perfectly normal. If you have adjusted it too far either way then good luck getting it back in the correct position again without the correct machine to read the gasses.

I recently had a similar issue after mine had been laid up for some time. It would crank over but not start. I had a good spark and the fuel pump was running fine. I could smell fuel strongly but it simply would not start. I took the air filter off and looked under the AFM plate. It was soaked in petrol under there ! With the ignition on I pressed down on the AFM plate and heard a clunk from inside the metering head. Hmm, I thought and then went to start the car. It fired up instantly. So something had become stuck in the metering head

 

[RossCormack]

Thanks for the replies

I'm adjusting the mix because I know it was too rich. There is meant to be a way to measure if it is right using a multimeter. You plug into a point on the diagnostic port and read the cycle it outputs. The duty cycle setting on the meter should be about 50% for the correct mixture. I am aware that this method might not quite work as it is meant to so I was doing a bit of a lucky dip. The mechanic nearby also said he would just turn it towards lean until it almost stalled and then turn it back one and a half turns.

I might know the click you mention, Stwat. When you adjust the screw, sometimes it sticks and it will click back when you push the plate.

I tried some more this afternoon but still no joy. We got it going when I got some help from a local guy and reved it quite high but it cut out when I took my foot off the gas. Then he had to go and look after a customer at his shop. We got it started twice and I think he was pushing the metering plate down with a spanner to get more air in (but I'm not sure). Couldn't get it started again after that.

Got very little help from the nearby mechanic as he was so busy. He said I needed to clean the plugs so I did that. The theory was that running it too rich had sooted up the plugs and they were pretty black. Then I tried it some more and checked the plugs were still ok and dry. Lastly, I turned the screw four turns anti-clockwise to be pretty sure it was in a lean position then tried to start it over and over, turning clockwise a quarter turn between each try. Still no joy at all. If anything, it is worse than yesterday. When I left it, it did not sound like it was going to fire at all.

Grrrrrrrr

 

[grober]

Engine and exhaust needs to be up to temperature. Adjust via a T handled allen key through the adjustment port in the air cleaner . It should only needs the slightest of quarter? turns to get it right. Only accurate way to do this right is using an exhaust gas analyser probe up the exhaust pipe.

BOSCH KE3-JETRONIC MIXTURE ADJUSTMENT

 

[John Jones Jr]

Thanks for the replies

I'm adjusting the mix because I know it was too rich. There is meant to be a way to measure if it is right using a multimeter. You plug into a point on the diagnostic port and read the cycle it outputs. The duty cycle setting on the meter should be about 50% for the correct mixture. I am aware that this method might not quite work as it is meant to so I was doing a bit of a lucky dip. The mechanic nearby also said he would just turn it towards lean until it almost stalled and then turn it back one and a half turns.

I might know the click you mention, Stwat. When you adjust the screw, sometimes it sticks and it will click back when you push the plate.

I tried some more this afternoon but still no joy. We got it going when I got some help from a local guy and reved it quite high but it cut out when I took my foot off the gas. Then he had to go and look after a customer at his shop. We got it started twice and I think he was pushing the metering plate down with a spanner to get more air in (but I'm not sure). Couldn't get it started again after that.

Got very little help from the nearby mechanic as he was so busy. He said I needed to clean the plugs so I did that. The theory was that running it too rich had sooted up the plugs and they were pretty black. Then I tried it some more and checked the plugs were still ok and dry. Lastly, I turned the screw four turns anti-clockwise to be pretty sure it was in a lean position then tried to start it over and over, turning clockwise a quarter turn between each try. Still no joy at all. If anything, it is worse than yesterday. When I left it, it did not sound like it was going to fire at all.

Grrrrrrrr

I think you may have damaged the metering head in some way (hopefully not) if you can't get it to start & idle, even to idle roughly. Could be a wise move at this stage to get car inspected by someone that knows K/KE injection systems. Bear in mind too it might not just be a metering head fault either. Original injectors certainly won't help nor a vacuum leak (commonly affects engine idle) also a faulty OVP replay or weak fuel pump are several items that could be past their best.

 

[RossCormack]

Hi, that was a good shout about the metering head. I got a mate to help and we got it started again. I pushed on the metering head plate with a spanner and he was holding the pedal to try to keep it turning over. We found that we could get it to idle constantly by holding the plate down in a static position (and I think his foot on the gas a bit). I had a spare metering head from a 260E so I replaced that (just the mass airflow part but not the fuel distributor). After a bit of cranking, it fired up and I slowly took my foot off the gas, which left it idling at about 750. It was good but I've made some tiny adjustments to the air/fuel adjuster screw on the new part and smoothed out the idle even more. I have to say that it is now running like a Swiss watch.

Before this starting problem, I was not that happy about how the car was running. It sounded rough and it would sometimes stall. This problem had recently gotten a lot worse, maybe due to running rich. I am still not certain if the old MAF part was broken in some way or if it was just set up so badly. The Air/Fuel mixture was set in a garage last year using the CO emissions tester but I was never happy with it after that, even though it had passed the MOT. Anyway, the replacement has done the trick for sure.

For anyone with a similar problem but no spare parts lying around, I would say you could get that part pretty cheap off ebay. However, you can't buy a fuel distributor for cheap, they cost about £100 at least. If you can't get a hold of a MAF Sensor and you have a similar problem to me, I would suggest that the part is not actually faulty but the air fuel setting is just way out. Now that I look at mine and see how open the plate is when it idles, I think it looks way more rich than the old one. The plate is barely open and the air going in is just hissing through the smallest crack. It's odd that this should be the case when I was so sure the thing had started off set too rich. Somehow, it ended up far too lean and would not start for lack of fuel (possibly). I'm sure I could get the old part back on the car and eventually make it work but I think that would be a bit of a waste of time. I would suggest anyone else in this position to try to get a look at another Merc to see where the plate sits. With the engine off, push it down until the plunger on the other end of the lever is fully up and see how wide the air gap is.

So thanks for all the useful replies, the problem is solved