Please Help!

I just bought a 94 Q45 and Im having an acceleration problem. It has very poor acceleration with serious hesitation and bucking but will cruise fine. I have replaced the fuel filter, air filter, plugs, throttle position sensor, ran injector cleaner through it, cleaned the throttle body and air flow sensor all of which seemed to help but has never eliminated the problem. I have noticed that by pulling the throttle body open manually it performs well with no issues at all however at the pedal it will hesitate and rev up poorly. Any ideas? What am i bypassing when i manually pull back on the throttle body other than just the cable?

I am guessing you are pulling on the throttle body with the hood open and the car parked? With that situation there is very little load on the engine and not much fuel required compared to actual driving. It kinda sounds like the fuel controller is not doing the medium or high setting or the fuel pump is crapping out. You can try running the fuel controller in high mode to see if that helps.

justjuiceit4 wrote:.......You can try running the fuel controller in high mode to see if that helps.

To bypass the Fuel Pump Control Unit.... from the trunk reach in under the rear deck and unplug the FPCU which is shown in the background of the pic below. Run a jumper from the pin shown in the pic to ground. The pump will now run on high speed only but the fuel pressure regulator at the end of the fuel rail will control the pressure. If the engine runs well the FPCU likely has a melted solder joint in the ground circuit.

Thanks guys that good info. I'm going to check it out this weekend I will let you know the results.

Couldnt wait for the weekend took off early from work. Tryed the bypass and WOW what a difference. I have some hesitation still but not bucking and jerking. She is coming back to life. Im going to the junkyard tomorrow think I will get a fuel pump as well. Its making some noise Im not sure what is normal. What do you guys think? I have read enough on the post that if I do replace it then I need a new seal. Any issues with running it with it bypassed last thing I want to do is fry something else.

Since you are in Q-topia (ATL) you might want to pick up another FPCU. I keep a spare FPCU on hand in the event I'll need it someday. Actually, I keep a spare Fuel Pump and seals around for when I need to change it. Q45 are a rarity in junk yards around here.

my94Q45 wrote:......Any issues with running it with it bypassed last thing I want to do is fry something else.

NICO lore says that you shouldn't bypass the controller and run the fuel pump on high speed for very long but we've never really nailed down how long "very long" is. The consequences are that you could prematurely burn up the fuel pump.

my94Q45 wrote:.......I have read enough on the post that if I do replace it then I need a new seal.....

I think you're going to find that the old oring in the top of the gas tank is swelled and you'll have a tough time getting it to fit into the oring groove.

I would not trust a junkyard fuel pump. Running the bypass mode might create a "rich" condition at idle and moderate throttle, as the pressure regulator can not handle that fuel flow. Not sure why Nissan just did not install a better regulator than creating a complex three-speed controller.

justjuiceit4 wrote:I would not trust a junkyard fuel pump. Running the bypass mode might create a "rich" condition at idle and moderate throttle, as the pressure regulator can not handle that fuel flow. Not sure why Nissan just did not install a better regulator than creating a complex three-speed controller.

+!.

The controllers is the item I save from the junk yards.

I have replace the fuel pump and controller both junkyard items paid $48.00 for both didnt have the money for new. Huge improvement no bucking or jerking at all. I dont feel its 100% but at this point I think its just needs to be fine tuned. I thank you all for the help.

goody90q45 wrote: ↑

Thu Dec 11, 2014 6:58 pm

justjuiceit4 wrote:.......You can try running the fuel controller in high mode to see if that helps.

To bypass the Fuel Pump Control Unit.... from the trunk reach in under the rear deck and unplug the FPCU which is shown in the background of the pic below. Run a jumper from the pin shown in the pic to ground. The pump will now run on high speed only but the fuel pressure regulator at the end of the fuel rail will control the pressure. If the engine runs well the FPCU likely has a melted solder joint in the ground circuit.

Could someone repost the image, to help explain how to make the bypass? Thank you, in advance.

I don't have the photo handy, but you can search on posts from Mike Goodwin (goodyQ45). He posted that picture a lot.
The key point (possibly non-intuitive) is that you disconnect the FPCM (FPCU if you prefer) entirely from the circuit, and make a single jump to hard ground on the harness connector side.
Going by recall (dangerous) it's the bottom right as you look into the sockets with the connector thumb tab pointing up.
If you have a discolored pin (heat stress), that's gonna be the one.

Thank you, 3Q Jay.

I'll try and search out goodyQ45's post.