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1987 XJ-6 Fuel Pressure

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Michael J.

Jedi Warrior
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I was having trouble with the car making low power, low RPM and backfiring. Car would idle fine.

I put a fuel pressure guage in the fuel line at the rear of the fuel rail.

30 psi at idle but only 15-20 psi at 2000 RPM. I changed the fuel pump. I now get 30 psi at 850 rpm idle, 30 psi at 1000 rpm, 30 psi at 2000 rpm and 28 psi at 3000 rpm.

I am trying to find out what the correct system pressure is.

Anyone know what pressure the regulator is supposed to be set at?
 
28.5in/lbs to 30.8 in/lbs. to check the regulator clamp line form reg to tank, pressure will increase dramatically. Check your filter and non-return valves.
Low rpm backfire? Thru intake or exhaust?
 
I think the backfiring was in the intake and some in the exhaust as well. Fuel pump made a considerable improvement.

I will check out the weblink and try the pressure test by clamping the line from the regulator at the front of the fuel rail, which should be return.
 
Still having the problem, but only when I make a 90 degree turn either left or right, then RPM drops off to idle and is slow to come back. Car does not do this when going through curves, only when making a full turn from a stop such as at a stop sign or traffic light. In thinking back about the symptoms, that was how it behaved before swapping out the fuel pump.

Fuel pressure remains nearly consistant but does drop off to as low as 25 psi. I put a small c-clamp on the return hose at the front of the fuel rail where it connects the fuel pressure regulator with the return line rigid tubing. This boosted the pressure to about 35 psi. problem remained and the lowest pressure observed was about 32 psi when whatever is happening was happening.

I have a new fuel filter installed.

I have swapped the airflow meter earlier in this problem and I had some kind of bind in the throttle body in which the gas pedal was extremely hard to press. I lubed the throttle shaft on both ends and the bind went away. I replaced the throttle switch/potentiometer which is nothing but a switch inside the housing on US cars.

The pump had to be some of the trouble because the fuel pressure was dropping off,and then comming back to around 30 psi before I changed the pump.

This has me baffled.
 
Maybe.
On constant-flow duel injection systems, the fuel continuously runs from the tank, pump, fuel rail, and back to the tank.
If you have a baffle issue in the tank, and the fuel runs to one side on a curve, you starve the system.
These tanks are fairly narrow, so there may not be a baffle.

How much fuel is in the tank?
Does the level of fuel make a difference?

The suction line in the tank may not be anywhere close to where it should be. Looks like it is in the sender unit.
Before you fill the tank for further testing, drain the tank, remove the sender unit and check the inlet pipe.
 
I see some listings for XJ-6 with twin tanks, some with in-tank pumps.
And, I see a Fuel Sump on some models, with an in-tank filter.

Fuel Sump Tank C41794, filter JA-CBC5649.
JA-CAC5516-1 RH JA-CAC5517-1 LH with in-tank pumps.
C40190 RH and C40191 LH with external pumps, to VIN 2T 58223.
JA-CAC5522-1 RH, JA-CAC5523-1 LH, VIN 2T 58224 onwards.

A tad confusing.

You have one or two tanks?
You have a fuel sump?
Internal or external pumps?
 
The 87 XJ6 has an external fuel pump (same as my 90 XJS), in this case, each fuel tanl has a low fuel line that runs along the floor of the trunk to the rear of the spare tire and each line connects to the electric fuel change over valve. From the valve, a fuel line about 12" long runs to the inlet for the fuel pump. From there the fuelruns to the front of the car.

I will check the sender to see what is going on with the pick-up. I have one tank empty at present.

Power steering pump was empty. Car has been sitting a while while I have been fighting with this problem. Off to the store to get some Type F fluid for the pump. I'll comment about the effect it has.
 
Well...Problem still not solved yet. Power steering reservoir no longer low.

I have about a 1/2 tank fuel in the left tank, which is the tank I have been running on. I'll add some fuel to the left tank, or maybe fill the right tank to see if tank selection factors in.

I plan to start by pulling the right tank sender, since the tank is empty to check the fuel pick up.

I did notice that when it is making low rpm and low power, iff you pump the throttle you can work up the rpm gradually. Does this sound like an Air Meter problem?

There is no fuel sump tank. My V-12 XJS has one, so I wonder if the XJ12 version has the sump mentioned by TOC?
 
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