98 E300 what does the fuel temp sensor do?
 

Vehicle: 1998 E300D, 95,000 miles

Problem 1: Check engine light came on yesterday, showing code P0180 "Fuel Temperature Sensor A Circuit Malfunction"

Problem 2: Delivery valves on injection pump are leaking. I have crush washers and a full set of Viton o-rings for the fuel system and am ready to do the repair, but want to be prepared to tackle all problems once the intake manifold is off.

Problem 3: Engine runs rough when vehicle is not under load, but not at idle. For example, when coasting towards a red light, but before the engine cuts back to idle speed. Engine is silky smooth at idle and under load.

Question: Are all 3 problems related?

Important details:

About a thousand miles ago I installed an Elsbett veggie oil system on the vehicle. This is a one tank system with a coolant/fuel heat exchanger located in the engine compartment immediately before the OEM fuel heat exchanger, fuel filter, fuel shut off valve, lift pump, injection pump, and so on. The conversion also utilizes special injector nozzles and longer glow plugs.

Regarding problem 1: Since the Elsbett system supplies hot fuel (veggie) to the fuel system, is this confusing the fuel temp sensor and causing it to think it's malfunctioning? Does a constant supply of hot fuel make the car think something is wrong? What does the car do with the fuel temp info anyhow?

Thanks,
Paul


Here are some threads that I found helpful on this site:

Error P0180:
http://www.peachparts.com/shopforum/tech-help/40962-check-engine-light-98-99-diesels.html

Replacing fuel lines and seals:
http://www.peachparts.com/shopforum/diesel-discussion/105291-99-e300td-injector-pump-leak.html?highlight=fuel+manifold+ring
go to post #8 by Rick Miley

Thanks Marty D., This is what I learned from the thread you pointed me to:

The engine control module continuously checks the voltage value of the fuel temp sensor and compares it with the threshold values.

The P0180 fault is recorded when:
<0.2 V longer than 2 seconds
>4.7 V longer than 2 seconds

Attached is a chart of the specified temp and voltage values (Green line)
I extrapolated the line up to 100 deg C (orange line)
The High and Low limits are shown in Red and Blue

So, even when the fuel is delivered hot by the veggie oil conversion kit, it shouldn’t exceed the high limit. (Veggie is heated by the coolant and my car rarely exceeds 80 deg C)

Therefore my fuel temp sensor is bad.

OK, that’s a relatively easy fix. But why does the engine care what the fuel temperature is anyway? I know the stock engine has a fuel heat exchanger that works when the temp is very cold and the diesel has the potential to gel. I’m not sure how the ’98 works, but my 1986 190D preheater is controlled by a mechanical thermostat. If the ‘98 is the same, it wouldn’t need an electronic fuel temp input signal. So what's it for?

Thanks,
Paul

These cars have electronic fuel management computers. It probably uses the fuel temperature as a way of determining the optimal amount of fuel to inject when all of the parameters are considered such as incoming air temp, throttle position, boost pressure, etc. Your vege conversion may be the cause of the problem after all though...perhaps it is just the sensor reading temps that don't make sense to it as it was not designed to have the fuel heated by means other than through the car's own heat exchanger. In other words it may be comparing the readings from a coolant sensor to the fuel temp sensor and knows they should be within a certain range of each other and your system is providing fuel that is outside of that expected range.

You may try putting it back to stock and seeing if the warning light/codes go away...if so you will know it's the vege conversion causing the CEL and code.

MDISAV runs an elsbett system too. You might want to compare notes with him to see if he has encountered this issue.

I suspect Marty is correct that the IFI is confused by the pre-heated fuel passing the fuel pre-heating thermo. Probably thinks it's aleady 180F outside and is trying to shut the thermo down.

Fix for me when I had the P0180 code, the fix was a new fuel temperature sensor to the tune of about $125. I went through the diagnostic steps in WIS and the results didn't make sense to me, nor did they solve the problem, so after exhausting my options, I did what I hate to do and threw a new part at it and that fixed it. Been a couple of years now and all is well. I'm not running bio-diesel in mine though.

Thanks for everyone's suggestions and explanations.
I took a look at the stock fuel heat exchanger and didn't see any wires going to it, so I'm assuming it is controlled by an integral mechanical thermostat, not the fuel temp sensor. It looks like the temp signal is only used by the engine control module, as proposed by Marty. There's no telling what adjustments the computer makes based on the fuel temp.

Thanks Terry for pointing me to MDISAV. He actually helped me pick out the car and gave me tips as I was installing the veggie conversion. His 99 E300 hasn't shown the fuel temp sensor error with his Elsbett veggie system, so I'm assuming my temp sensor is the culprit.

Len, were you able to replace just the temp sensor, or is it only available with the fuel shutoff?

The CE light has gone off on my car. I'm going to give it a few more days to see if it comes back. If not, I'll address the leaking delivery valves and save the temp sensor for another day.

Thanks,
Paul

Paul - replaced the whole unit for that price. Don't know if it's possible to just replace the sensor?