Fuel pressure drops under load

'98 XJ, 4.0L...I already replaced the fuel pump with a Napa rebuild kit and put in a new fuel filter / regulator from the dealer. The fuel pressure will hold steady when at idle, even reving the engine in Park.

Once it is put in gear and put under a load the pressure drops. It's even driveable if you feather the pedal, but punch it and it dies. TPS, CPS, new plugs wires, cap and rotor were also replaced.

any ideas???

How much does it drop? Normal is 49PSI.

Maybe someone can help here, but on the early Renix based systems, the fuel pressure was relative to the manifold vacuum. As you accelerated by opening the throttle butterfly, the manifold pressure will drop, leading to a drop in fuel pressure. This is normal.

Do the newer engines us the same type of fuel pressure regulator? If so, then a drop is to be expected. You could expect a drop of up to around 13psi. How far does it drop? If it drops significantly more, there are only a few things that would cause it. All are related to a restriction in the fuel line for the most part. You changed the filter, so that SHOULD be good. You may have caught a rock on the fuel line and the line may be pinched. That would allow it to flow enough to support an idle, but not any more.

While not likely, you could have a voltage problem driving the pump. Lots of luck.

In 97 and later, the regulator is located at the fuel pump, above the tank, and has no way of knowing what the manifold pressure is. The pump is of the constant flow type and the regulator recirculates whatever gas is not injected.
If the pressure at the rail drops substantially below 49PSI, the pump, regulator and fuel line have to be checked. The fuel filter is on the pump's pickup and may be clogged. EDIT: (But this is not the case, since it was recently changed.)

This thing drops to the point the engine dies, from 55 straight to nearly 0 under a load.

The two "sock" filters fitted with the pump were replaced and the round silver regulator atop the pump was also replaced. That regulator also contains a filter.

Check the fuel line to see if it's clogged or flattened. Otherwise your new pump/regulator may be bad. Pressure shouldn't drop more than 5PSI.

hmmm, sounds like something with a line then. I did the pump and regulator about 2 weeks apart and the problem has just persisted.

Can you pull the return line back to the tank and run it into a bucket, then turn on the power to the pump. You should get a pretty good flow, if not you have a line problem You could also do a similar thing back at the tank by pulling the hose from the pump and running it into a bucket to check for flow. If you have it at the tank and not at the engine, the problem has to be the line.

Hate to say it but maybe here's another one with an aftermarket fuel pump problem. Could you have crimped a line putting the tank backup ??

Oooops. There's no return line either in a 98. The regulator is at the pump and returns direcly to cool the pump. The flow test must be carried out at the fuel rail's hose: at least half a pint in 7 seconds. Fuel cap off. According to the FSM, you may toast the pump if you keep it on for more than 7 secs with the hose open at the rail. (Probably since there's no fuel returned to cool the pump).

RichP said:

...Could you have crimped a line putting the tank backup ??

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The lines looked good and the problem has been the same, both before and after the new pump and filter. I'm starting to place bets on a clogged line. Talked to someone else with similar problem on a different vehicle and it was a line problem.

...is there any way to test for a clog or blow it out?

Only for flow, disconnect both ends of the line, the best way to blow it out is from the front to the rear, from rear to front might just wedge whatevers in there tighter, shop air will work fine for this. Stick an old pillow case over the end so you can see what comes out.. I'd go over that line inch by inch though and check for a crimp or flat spot first. Then it's just a matter of running a new line...
Also what is that thing hanging down near the axle thats mounted from the body with the hoses in it, have not spent much time back there looking it over, will break out the fsm and see if that could have any bearing on the issue....

Wouldn't a sock be more comfortable to work with than a pillowcase?

jldiaz said:

Wouldn't a sock be more comfortable to work with than a pillowcase?

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I think a sock would go into orbit or hurt somebody if they were standing down range the pillow case would allow more air to leak out, shop air is around 200-250psi usually... I know when I use my blowgun w/shop air to clean out a computer thats full of shop dust I stand about 8 ft back otherwise I'd be blowing components off the mainboard not to mention the fans hitting 20 or 30 thousand rpm in a tenth of a second...