Irritating XJ Fuel/Accel problem

To get my 4.0 89 XJ to accelerate without stumbling down, I have to leave the maifold to Fuel Pressure Regulator vac line undone at either end. As soon as I hook it up, it dogs down like the timing is advancing with throttle movement. I undid the knock sensor, as Ive traced it down to that being the ONLY thing that can adjust timing. While having the KS plugged in, with the FPR VAC undone, it ran great. I recently replaced the IAC and TPS, its a 5spd. Fuel lines and filter are great, pump works well, 40PSI on the rail. My idle is frigged though. Really lopey and sometimes it will shudder down to almost a stall, but the fact Im on a slight angle and have less than 1/8th a tank left may be the idle issue.

What gives? Im out of idea. Havent ran it with the KS undone yet, that comes in the morn when I head to work.

anyone?

what is the fuel pressure at with the vacuum line connected? it should 31psi.

i would say the feul pressure regulator is bad and is dumping too much fuel.

fuel pressure with vacuum line not attached should be around 40psi. you only want the pressure this high is when the engine load is increasing and vacuum is going down. if you have this much fuel at all times than it will dump too much fuel and run extremely rich.

looking back at your post, are you driving with the vacuum line disconnected or is this at idle?

Driving with the line disco'd. Thats the only way I can. With it connected, any throttle will make it shudder like its running out of gas.

if you need to disconnect the vacuum assisted fuel pressre regulator inorder for it to run, i would think the FPR is not working properly.

you just reminded me that I've still got mine disconnected-- been that way for about a year, now. I unplugged it for troubleshooting and liked the way it idled-- I guess I'll go figure it out one of these days, but she runs so well, I may not......

With the FPR VAC line disconnected you are getting too much fuel most of the time including idle, running it rich. You are also letting air bypass the IAC with the VAC line open, disconnected from the FPR thus running it lean at idle. The IAC and ECU can only compensate so much that bypasses its control.

The KS does NOT adjust timing, only the ECU adjusts timing and it uses data from all the sensors (including the KS if it senses knocking) to compute the best timing.

It's obvious that you need to replace the FPR. It uses an internal spring countered by engine vacuum to pass excess fuel back to the fuel tank, thus regulating the fuel pressure at the fuel injectors. It should be 31 psi at idle, not 40 psi. With the VAC hose disconnected from the FPR a good pump will read 40 psi.

It is not unusual for the rubber diaphragm in the FPR to fail and leak gas, especially when under vacuum, thus raw fuel vapor is sucked into the VAC line disturbing the apparent vacuum pressure and thus messing with the FPR regulation, while dumping excess gas into the intake, bypassing the fuel injectors and ECU fuel feed control through the injectors. The result is a bad idle.

Get a new FPR asap.

How would I go about replacing the FPR?

2 torx bolts with nuts secure it to the fuel rail

first, i would test it to be 100% sure.

to replace you will need to get i believe 2 fuel line quick disconnects. i got them from the dealer and they wernt that much money

unbolt the bracket that holds it down and remove from the fuel rail. its a pretty easy job. i would not use the old disconnects, they will most likely leak.
also remove pressure from the system by pressing the shrader valve on the fuel rail.

another thing woul be to pick up a haynes or chiltons manual.

Alrighty. Will check it out

as it stands though, it idles nice, runs nice and when the weather gets nicer, then I'll tackle it. At this point in time atleast I know w.t.f is going on.

It was the EGR.

sorted it all out today.