Dear all

1995 X300 XJ6 4.0 N/A.

Have been suspicious that my fuel pressure regulatior (FPR) was defective and pressure was low (did not have ability to test though). So I changed it out with a brand new one and instantly I could feel more power from the engine and more responsive to throttle.

However, while previously my STFT was hovering appropriately around 0% maxing at maybe -5% or +5% i now have the situation where STFT is varying violently but tending -ve up to -24.9% (Causing engine light) . Positive readings are up to about 15%. At idle the STFT is around 0 to -5%. As I increase RPM this gradually increases up to the max of -25.9%.

In trying to chase down these STFT variations I have run OBDII and identified that my coolent temperature sensor never reports more than 84c unless I run the engine very hard and then will not exceed 86c. Usual temp about 82c-84c.

I am wondering whether the following is possible.

My FPR was providing low pressure and therefore the ECU compensated by increasing the injector duty cycle to allow for more fuel to be injected to reach stoichiometric ratio. This in turn has been compensated for by the LTFT (which by the way I have given up trying to read in OBDII as nother makes any sense +53.9% at present). Therefore, STFT was adjusted back to around 0%. At the same time, the CTS reporting low temp would cause the ECU to add more fuel thinking the engine is still cold. Therefore, cold engine enrichmnet and LTFT were combining and compensating for low fuel pressure at the rail.

Now that I have a new FPR - presumably giving correct fuel pressure, the need for the longer duty cycle has been removed. Thus maybe due to slow LTFT changes the system now is reporting a rich condition and reducing the injector duty cycle. Added to this, the vehicle coolent sensor is still reporting a cold engine thus adding more fuel adding to the rich mixture. i.e with low fuel pressure the effect of the low coolent readings were being masked but now with proper fuel pressure the impact of the low coolent temp is increased.

Anyway, does anyone know what sort of impact the coolent temp sensor reporting say 5c cooler would make. Given these are so cheap I will probably just buy a new collent sender and see if I can get close to the 88-91c operating temp range (is this normal ?).

I'll try to pull some OBDII logs to add to this thread.

Advice appreciated

Matt

 

First target would be a accurate coolant thermostat valve

No O - ring on installation even if it comes with one , the O - ring will break the housing

You may see on your present setup it flipping back and forth out of ECU regulation closed loop mode

 

Red 14

Red 31

Temp sensor ECU sockets , look for missing pinching fingers

 

You will need a gasket for your thermostat....Parker is correct that the o-ring is not wanted here, but a flat thermostat gasket will be needed. Make sure you get that.

I would ignore the STFT for now. I would monitor the LTFT and see if it moves. It'll take a few drives to push that LTFT number, but that will reveal whether you're heading in the right direction or not. If you had a PDU, you could zero it out, unfortunately there isn't any other way to change it, other than driving. Unplugging the car from the battery will not make any difference, nor will crossing battery terminals, etc.

The coolant temp sensor is a required sensor reading to start the car. If you don't know when it was last changed, it is worth the $20 investment. It could cause fueling issues similar to what your experiencing, although I have to admit that I didn't think it could be that significant to trigger a light.

 

Thanks Vee. Ive swapped out the CTS and now at full op temp I’m getting 91c. I’ll monitor the trims however as usual the LtFT readings are hard to gauge as most OBDII devices give very strange results - at present I,m getting about +52.3 % both sides. I guess if I see the STFT shifting closer to 0 then I can assume LTFT is adjusting. Thanks again Matt

 

you may be chasing a bad # on the fuel trims because of your device

the 91 C is good

Tracking MPG on the instrument cluster ? or Km/ liter