w203 Why are there two fuel level sensors?
01-23-2017, 12:44 AM
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'01 C240 POS, BMW 740il (e38) '98 ..KAR (Kick Ass Ride)
w203 Why are there two fuel level sensors?
I'm just curious because I just wasted (well not a total waste as I now have new cps and kps sensors) a few days on an '01 C240 that cranks but no start, and no codes issue.
I had no fuel pump activation, so I bypassed the relay and could hear the fuel pump, tested the relay and then the coil voltage, which indicated the SAM was not turning it on for some reason. I learned here that often one of the position sensors can cause this. Once those were ruled out, I again bypassed the relay, and got no fuel pressure at the rail. Then I checked the fuel line out of the tank and got no gas. Pulled the fuel pump, and even though the gauge read 1/2 tank, the tank was empty. Problem solved.
My question is, just out of curiosity, how did the ECU know there was no fuel and the gauge did not? Is the fuel pump float sensor hooked up to the ECU and the gauge to the one on the other side? And if so, how lame is it that the ECU didn't inform me the tank was empty and that it had shut down the pump? I know, I know, those were dark times for software / firmware, but still..
Last question, I swear, how would the savvy IRL auto tech figure out the tank was empty without wasting a lot of time like I did?
I had no fuel pump activation, so I bypassed the relay and could hear the fuel pump, tested the relay and then the coil voltage, which indicated the SAM was not turning it on for some reason. I learned here that often one of the position sensors can cause this. Once those were ruled out, I again bypassed the relay, and got no fuel pressure at the rail. Then I checked the fuel line out of the tank and got no gas. Pulled the fuel pump, and even though the gauge read 1/2 tank, the tank was empty. Problem solved.
My question is, just out of curiosity, how did the ECU know there was no fuel and the gauge did not? Is the fuel pump float sensor hooked up to the ECU and the gauge to the one on the other side? And if so, how lame is it that the ECU didn't inform me the tank was empty and that it had shut down the pump? I know, I know, those were dark times for software / firmware, but still..
Last question, I swear, how would the savvy IRL auto tech figure out the tank was empty without wasting a lot of time like I did?
Last edited by Benzlemon; 01-23-2017 at 12:46 AM.
01-23-2017, 01:59 AM
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Split tank, it has a sensor on each side.
If one sensor is stuck full, it will show a 1/2 tank as the gauge averages out the numbers. The suction jet pump on the other side is supposed to pull fuel over to the side with the actual pump when it's running, so if you check the other side and it's actually empty, it's a bad sender. If you check and find fuel, you've got a transfer problem, the lines can come off inside the tank.
No real magic to it, takes all of 5 min to pull the back seat and both fuel senders and look. I'd always start with fuel pressure, which you had none. If the pump is heard running, the next suspect is actual fuel level, add some fuel. In your case it would have fired up, then you know you have a gauge issue.
If one sensor is stuck full, it will show a 1/2 tank as the gauge averages out the numbers. The suction jet pump on the other side is supposed to pull fuel over to the side with the actual pump when it's running, so if you check the other side and it's actually empty, it's a bad sender. If you check and find fuel, you've got a transfer problem, the lines can come off inside the tank.
No real magic to it, takes all of 5 min to pull the back seat and both fuel senders and look. I'd always start with fuel pressure, which you had none. If the pump is heard running, the next suspect is actual fuel level, add some fuel. In your case it would have fired up, then you know you have a gauge issue.
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Benzlemon (01-23-2017)
01-23-2017, 12:20 PM
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'01 C240 POS, BMW 740il (e38) '98 ..KAR (Kick Ass Ride)
Thank you, I really appreciate the response. I guess I had an aversion to pulling those due to battling the seal leak problem a few years back. The thing that sent me down the electronic failure path (other than I'm a tech, so like the old saying about the hammer, everything looks like a nail to me) was that the pump relay was never engaging. Isn't it supposed to run till the fuel pressure comes up? And if it errored out, I would expect an error code or malfunction alert. But like I said those days were plagued by limited memory and poor programming.
It's nice to know even the pros have to look in the tank though, I couldn't help but wonder if there was an easier way. Wish I could get those out and back in in five minutes! That seal ring on the left side did not want to go back on straight, what a PITA. /gripe
Not your fault though, and thanks again!
It's nice to know even the pros have to look in the tank though, I couldn't help but wonder if there was an easier way. Wish I could get those out and back in in five minutes! That seal ring on the left side did not want to go back on straight, what a PITA. /gripe
Not your fault though, and thanks again!
01-23-2017, 02:32 PM
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'01 C240 POS, BMW 740il (e38) '98 ..KAR (Kick Ass Ride)
Follow up:
After driving it in to work today, (I usually drive our beemer) I now know marginal cps / kps sensors can make this car seem totally gutless and sluggish. What a difference!
I always wished I knew someone else with a C240 so I could compare the performance because this one sucked major you-know-what. But I still probably wouldn't suspect the sensors because the car ran OK, passed DEQ and threw no codes. Anyone know if a star diagnostic would catch this?
Change those sensors W203 people!
After driving it in to work today, (I usually drive our beemer) I now know marginal cps / kps sensors can make this car seem totally gutless and sluggish. What a difference!
I always wished I knew someone else with a C240 so I could compare the performance because this one sucked major you-know-what. But I still probably wouldn't suspect the sensors because the car ran OK, passed DEQ and threw no codes. Anyone know if a star diagnostic would catch this?
Change those sensors W203 people!
01-23-2017, 07:04 PM
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Thank you, I really appreciate the response. I guess I had an aversion to pulling those due to battling the seal leak problem a few years back. The thing that sent me down the electronic failure path (other than I'm a tech, so like the old saying about the hammer, everything looks like a nail to me) was that the pump relay was never engaging. Isn't it supposed to run till the fuel pressure comes up? And if it errored out, I would expect an error code or malfunction alert. But like I said those days were plagued by limited memory and poor programming.
It's nice to know even the pros have to look in the tank though, I couldn't help but wonder if there was an easier way. Wish I could get those out and back in in five minutes! That seal ring on the left side did not want to go back on straight, what a PITA. /gripe
Not your fault though, and thanks again!
It's nice to know even the pros have to look in the tank though, I couldn't help but wonder if there was an easier way. Wish I could get those out and back in in five minutes! That seal ring on the left side did not want to go back on straight, what a PITA. /gripe
Not your fault though, and thanks again!
The SAM commands the relay on for a couple seconds the first time to build pressure, but it doesn't actually wait until it sees pressure, just shoots the pump on for a bit. If the pump is dead, the relay will still turn back off. I just put a fuel pump in a 203 a couple days ago so I went through the same troubleshooting, to ensure the pump was out before I handed the customer a big estimate that wasn't the issue.
I don't recall 100%, but I don't believe there was any fuel pressure fault stored in the car. I know the 204 will do it, especially the post-FL cars with the DI motors, they log everything fuel pressure related.
Rear seat bottom is 4 bolts, the covers are 6 bolts, then turn the big plastic nut off. Not too bad compared to some cars.
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Benzlemon (01-23-2017)
01-23-2017, 07:46 PM
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'01 C240 POS, BMW 740il (e38) '98 ..KAR (Kick Ass Ride)
That's good to know, I didn't have anyone to turn the key and check the pump relay and that dang beeping when you first turn the key didn't help either!
Actually, the C240 has quick release tabs and hinge up seats so access is a snap. I'm getting old and I forgot I bought some channel locks for that when it was leaking.. yikes. Plus I left them outside and they are all rusted.
But my main excuse is since I don't drive that car I never would have believed the gas gauge because I would have noticed it didn't move below 1/2 tank for like a week! But then I never would have learned about the sensor thing and it would still be driving like an old VW bus, so still a net positive.
Cheers!
Actually, the C240 has quick release tabs and hinge up seats so access is a snap. I'm getting old and I forgot I bought some channel locks for that when it was leaking.. yikes. Plus I left them outside and they are all rusted.
But my main excuse is since I don't drive that car I never would have believed the gas gauge because I would have noticed it didn't move below 1/2 tank for like a week! But then I never would have learned about the sensor thing and it would still be driving like an old VW bus, so still a net positive.
Cheers!
Last edited by Benzlemon; 01-23-2017 at 07:52 PM.
