Disentangling my previous set of problems I am concentrating on the high fuel pressure. 2 weeks ago I had a P0193 code for high fuel pressure. Logging data with the Torque Lite app I was seeing fuel rail pressure at 54-56 PSI steady with the engine running. Without the engine running I measure about 56 PSI with key on when the fuel pump comes to life for a "prime" of the system. After that I saw a rapid decay in fuel pressure.

I had the intake system apart due to a fuel leak so put in a new fuel pressure sensor. With the new press sensor and (knock on wood) fixed fuel leak at an injector O-ring I see (via Torque App & OBD) 70.1 PSI at the fuel rail pressure sensor. Engine running or not. Key on without engine running. Key on, engine not running for minutes, the pressure does not decay now.

I am beginning to wonder if 70.1 PSI is the limit of the fuel pressure sensor. Unfortunately I do not have a schrader valve to measure pressure. Next step is cutting into the fuel line near the fuel rail and Tee-ing in a pressure sensor.

 

I took the old fuel rail pressure sensor I removed this week and powered it on my variable power supply with 5Vdc then measured signal from pin 2 (ground) to 3 (the pin that reports to the ECU). Connected a pressure gauge, tie wrapped some tubing to the bicycle pump, and lo and behold when the sensor is maxed out, putting out the full input voltage, the pressure is 70 PSI. I think my old sensor works and the new one is bad since the ECU is reporting 70.1 PSI all the time.

 

dh53, your fuel injectors need to see about 50ish pounds above the intake pressure to work right. This is why when you turn the car to RUN and do not start the motor, it jumps up to the 55 psi range and then when you start the car, it drops to around 40-45 psi. Your intake drops to about a -10 psi pressure. This will keep the 50ish psi difference across the fuel injector. So, having the new fuel pressure regulator keeping things at 70+ psi, that is not going to make the engine run all that well. I would do what you are doing and put the old fuel pressure regulator back in and see what you have.

Now, if you notice that the fuel rail pressure remains at 55ish psi al the time, then I would be looking at the line between the regulator and the intake for either a crack in the line or the line being plugged. That may be the cause of your overpressure condition when the engine is running (ie, the fuel rail is up at 55 psi when it should be at around 40 psi).

 

I was wondering why that fuel pressure sensor had a vacuum port...makes perfect sense now, thanks. I haven't dared take the car further than around the block but it idles and runs just fine. I would have gone for a long drive today but the CEL is on for high fuel pressure P0193 so I didn't want to chance it. Next time I start it up I will log the MAP sensor too. I looked at manifold pressure as reported by the OBD scanner briefly today with the engine running and it was around +5 PSI. So is that +5 PSI absolute which is about -10 PSI gauge pressure? Or are OBD units in gauge pressure meaning I am too high so have a major vacuum leak? Pressure scales confuse me because there are so many and none are obvious about the +14.7 PSI we live in near sea level.

 

You can look at fuel trims to see if you have "a major vacuum leak".

 

dh53, I would start with simply turning the car on (engine not running) and see what it says at that point. If it reads 14.7 ish psi, then yes, the +5 would be psia (absolute pressure). But, it could be reading say 29.8, which would be inches of mercury. Which scale did the programmer pick? You never know.

 

Replacing the new fuel pressure sensor that always reads maximum pressure (70 PSI which is "full signal", 5 Vdc) with the old pressure sensor I managed to break the vacuum line the goes from the FPS to the intake manifold into 2 pieces. It may have been leaky before, no way to tell. Luckily had a set of generic vacuum lines in various sizes from vacuum problems on a different car and was able to make a sturdy air-tight patch. I tried the patched line with the new pressure sensor, I still got 70 PSI all the time. Swapped in old fuel pressure sensor and suddenly the OBD reader is reporting 54 PSI at the fuel rail. No CEL. Went out for a 40 mile drive on some twisty roads and a little bit of highway and she purrs again. I am still seeing spikes in fuel pressure up to 70 PSI at the key on priming cycle but while the engine is running it is near 54 PSI all the time.

I plan to remove the in-line check valve I had put in after the fuel pump change to see if that makes a difference. I am thinking now the quick depressurization of the fuel rail and the hard start after sitting for a couple of days were due to fuel leaking out and air leaking in at the bad injector O-ring. Maybe that check valve is overkill? I'll see.

As for pressure units, Toque Lite MAP is in psia (absolute), it reads 14.4 with the engine off. These sensors are designed for ruggedness and inexpensive production, not for accuracy.

 

dh53, good to hear that things are running better for you. Some of these problems can be real pains to figure out what they are caused by. I know it took me awhile to figure out a P0101 code on the XJ (bad MAF sensor). Turned out it wasn't a MAF at all, but a dirty throttlebody. Made for a lot of head scratching and finally some "well, lets give this a try and see what happens".

It is always nice to go out to a car that you can feel confident that is not going to give you troubles in the day.

 

Hello all, I'm embarrassed to ask but in reading this thread realize I'm missing an important aid. I have a 1995 X-300 XJ6 naturally aspirated car and can't find an OBD II port. Where is it? I know where it is on my 1998 XK8 but thought it wasn't manufactured on the 1995 X-300 due to it being an older car..

 

piper888, it is very possible that your 199 car does not have an OBD-II port. This was not made mandatory until 1996. So, some 1995 vehicles came with it and some did not. It all depends on when the manufacturer wanted to implement the new standard. Per law, the OBD port has to be within 3 feet of the steering column. If you know where it is on you 1998, it should be in the same spot on your 1995 (if you have it).

 

That explains it! Thank you for the fast reply.

 

I think it will have one but it may not be within 3 feet. Check the Electrical Guide. If it's shown on there it may also say where it is.