Hi Guys, For a while now I've been thinking of swapping out my FPR to see if my fuel pressure is too high. The car runs fine and I get ok gas mileage on the highway but 17.1L/100k - 13.75/USmpg - 16.5/imperial mpg are the combined city/hwy averages I'm getting. I'm convinced it could be a bit better.
I do mostly in town driving though, so I'm not expecting miracles. The car has been running a bit rich since I bought it a couple of years ago - I have black deposits in the tailpipes and get an occasional FF23 about once every 6 months.

Anyway, I've been through the FF23 diagnostics many times and replaced this that and the other on the list but I haven't checked the "High Fuel Pressure" possibility.

I was at a wrecker last week and scored what appears to be a good FPR off a 94 VDP in the yard. Luckily for me they thought it was some sort of emissions device or a PCV valve and only charged me $2 for it.

Couple of questions, I think I read that you can depressurize the rail by removing the o2 sensor heater relay while the car is running and just let the car die, is that right? Also, do I need any sealant on the threads when I refit the FPR?

Thanks for any tips

Larry

 

Hi Larry,

The easiest way to depressurize the rail is to loosen the fuel filler cap, then remove the fuel pump relay in the right hand side of the trunk/boot behind the trim panel, near the right rear tail lamp assembly. The relay sits up high near the top of the inner fender. With the relay removed, start the engine and let it run till it dies. If the engine won't start, just crank it for 5 seconds or so and the rail should be depressurized. Disconnect the negative battery cable before proceeding.

I've never seen any evidence of sealant on the FPR or other fuel rail fittings, and have had no leaks using no sealant. The brass fittings don't have to be overtightened either. Pay attention as you are loosening the nuts on the fittings and try to replicate the same tightness when you are reconnecting everything. The one nut that was very tight on our '93 was the one that secures the FPR to its bracket. The fittings themselves were just nice and snug but not overtightened.

Before you install the salvaged FPR, you might apply a little vacuum to the fitting to be sure it will hold vacuum. If it won't, it's a paperweight. One of the causes of rich running is failure of the diaphragm in the FPR allowing unmetered fuel to be inhaled into the intake manifold through the FPR vacuum fitting.

Below is a link to the first album of my fuel rail overhaul showing how I disconnected the FPR from its bracket:

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By the way, another cause of rich running is a Coolant Temperature Sensor that is out of spec on the cool side, prompting the ECM to apply cold start fueling enrichment longer than it should (or continuously).

Cheers,

Don

 

Hey Don, thanks for the tips - fortunately the FPR was removed from the donor car complete with bracket, so no problem there!

I'll put the Mitivac on the unit tomorrow and see if the vac holds - good idea, thanks!

I replaced the coolant temp sensor about 18 months ago when I replaced the thermostat, thought it was just a good idea at the time, but still have the original one in the spares box complete with a new sealing washer so maybe I'll swap back to the original and see if that makes any difference too.

Thanks again, will report back!


Larry

 

Checked the replacement FPR and it held a vacuum, but so did the old (still fitted) part. Instead of going ahead and swapping it out, I decided to re and re the vacuum hose from the FPR to the manifold and also the vacuum hoses connected to the t-piece union. They weren't really super firm and there was some slight swelling at the ends. Much more snug now.
Before going ahead with any other replacements I decided to reset the Avg fuel consumption (which has been displaying 17.1L/100k for the last few tankfuls) and drive around for about a week or so to see how she does. I drive pretty much the same course week after week so this should give me an idea if the vac hose re and re makes any difference.

Larry

 

Hi Larry,

Good idea to replace the hoses. Bear in mind that just because an FPR will hold vacuum doesn't mean it is still functioning properly to maintain approximately 40 psi in the fuel rail.

And a heads up on measuring fuel consumption: when I was conducting my own "quasi-scientific" tests to determine any differences made by cleaning the fuel injectors and installing Andy Stodart's timing advance bracket, I learned that the onboard computer calculations of fuel economy are often wildly inaccurate, as in 4-5 mpg off. They are an approximation to begin with, and they can only take into account the sensor inputs to the ECM, TCM, ABS module (vehicle speed), etc. If you have any faults that cannot be sensed or managed by the system, the fuel economy calculations will be even more inaccurate than usual.

And checking fuel economy the old-fashioned way is tricky on late XJ40s because the fuel tank is intentionally designed to leave 10% of its volume empty to allow for the expansion of fuel as it warms, and to allow for proper operation of the Evaporative Emissions Control system (if the tank was ever overfilled, liquid fuel could make its way into the carbon canister and foul the activated charcoal). The method I came up with is to use the same fuel pump at the same station, run the pump till the nozzle clicks off, then run it 10 more clicks. Doing it the same way every time was the best method I could think of for being able to compare apples to apples. Fill the tank, drive enough to use several gallons, fill the tank again and divide the number of miles driven by the number of gallons used. If you want to be even more accurate, use a GPS to track your mileage instead of relying on the odometer, ensure your tire pressures are all spot on before you begin your tests, and always drive with the same equipment on (A/C, radio, headlamps, etc.), all windows closed, etc.

Cheers,

Don

 

Don - sounds precise all right! I'll give it a go and see how she fares over the next little while, keeping all the systems at the same settings. Thanks!

My suspicion is that the fuel pressure may be too high; I realize the vacuum test doesn't figure in that equation if mine is defective - I did the vacuum test you recommended just to make sure I did NOT have a paperweight!

all the best

Larry

 

I don't know if the fuel pressure being too high would lead to your symptoms if the fuel injectors are working properly, but if any of them are sticking/leaking that could cause your rich running. Cleaning our injectors improved our economy - I think it was between 1 and 2 mpg, but I'll have to go back and check my numbers.

Cheers,

Don

 

Quick update on gas consumption -

Performed a "hard reset" on the ECU today to erase the saved fuel trim settings.

Apparently a neg terminal removal doesn't completely reset the ECU so some learned settings calculated when problems existed are retained even if fault code is cleared.

Hard reset sets the ECU back to factory default and new fuel mappings will be based on current engine sensor feedback, TPS position, air flow etc.

In theory, if the sensors are all working properly and there are no vacuum leaks, the ECU will learn a new optimal fuel map after a reset.

Prior to the reset, I was getting 17.1 litres/100k. (16.5 MPG Imp/13.75 US Gal)
Today my avg for a 30 mile mixed city/hwy dropped to 10.4 (27 MPG Imp./22.6 US gal)

Admittedly, I was driving in super-economy style so I can't really expect to keep those figures, but looks like there is definite improvement


all the best

Larry

 

Thanks for the update, Larry! Please keep us informed!

Cheers,

Don