ka24e hesitation

[fullmetalx]

I'm having some issues with a car i recently bought and i'm at the point where i am out of ideas on what else could be wrong with the car.

car: 1989 240sx ka24e stock no mods 5 spd

problem: engine hesitates during the first few seconds of acceleration on low rpm. it feels like if i didn't know how to drive a clutch.

things replaced already

new spark plugs
new spark plug wires
new cap and rotor
new coolant temp sensor
new o2 sensor
new fuel filter
swapped in a working maf
swapped in a working distributor
swapped in 4 working fuel injectors and pressure regulator
removed aiv system and plugged vacuum lines for them
replaced with a used iacv and ficd and cleaned
checked solenoid valves for egr, scv, and prv and all are working
replaced rotting vacuum lines to eliminate vacuum leaks

ran some fuel injector cleaner thru fuel lines already, running with 91 octane gas, fuel tank holds very good pressure. also sprayed throttle body intake cleaner thru the vacuum lines.

ecu throws 55 code everytime.

went thru ecu diagnostics procedure modes 1-5. didn't find anything.

went thru a lot of diagnostics procedure from FSM, didn't lead to anything.

things havent verified if defective:
egr valve and swirl control valves.
maybe needs a new clutch.

i'm stumped. the only thing i can suggest that might give a clue is this.

the engine is hard to start all the time. it takes about 5-7 crank times before and when it does it starts to sputter so i have to to give it gas for a few seconds so it won't die.

in the first few minutes with the engine on, i test drive it, it runs somewhat okay not the best. and then hesitates once its warm

another thing i tried that FSM suggested, i unplugged the CTS sensor plug, acceleration improved and no hesistation. however my check engine light is and my idle went up from 800 to 1100. and when i plug it back in, problem of hesistation when accelerating at low rpm comes back.

i would like to ask for anyones help or advice for anything that i might have missed. i've ran out of ideas. thanks in advance.

[fullmetalx]

some things i wanted to add.

i've swapped in also working tps sensors
swapped working ignition coils.
i've checked continuity on the wirings from sensors back to the ecu. they check out fine.

can a short hidden somewhere in the wiring cause this symptoms?

would a weak ignition coil do this kind of symptoms?

i've unplugged fuel injectors one at a time and they all work and the engine would bog as each plug is pulled.

when i pull spark plugs, it's black and dry, i think they are getting fouled.

would a very dirty intake manifold cause this kinds of symptoms.

i was thinking of the egr runners with the allen head bolts on them. those lines maybe getting clogged.

any help and advice would be greatly appreciated.

[fullmetalx]

bump anyone have any ideas. pls post them. i'm thinking this has something to do with the air fuel mixture. what components in the intake manifold can i take off and plug and still have a decent running engine i just want to eliminate variables that may be playing a part on this hesitation problem.

aiv system already removed.

my engine harness is dry and brittle in some areas but went i checked for continuity from sensors to ecu. they check out fine.

there is still two tests i need to check that may be the problem. i still need to do a fuel pressure check and a compression check. i 'm thinking that that it is a headgasket minor leak that is messing up the air fuel mixture.

[fullmetalx]

bump. still need help.

things done.

did compression check. 130-140-140-150
looks good to me

fuel pressure check.

at idle with fpr vacuum connected: 23psi
at idle with fpr vacuum disconnected: 23 psi

this is where i'm confused. do i have two things that are bad. with the pressure that low, is that an indication of a weak and starting to fail fuel pump. FSM says 33psi to 43 psi respectively on above conditions.

I also connected the fpr vacuum to a vacuum tester and since the pressure didn't change when i applied vacuum pressure to the fpr. is that an indication of a bad fpr also.

i also tried putting the fuel pressure gauge on the supply outlet of the fuel pump in the trunk area. just to eliminate the possibility of clogged fuel lines. but still the pressure on the supply outlet is still at ~23psi. is this pressure to low to be considered a faulty fuel pump unit.

any input would be appreciated. thanks.

[SinGarage]

I would love to give you help but you've actually done everything (and then some) of what I would have done if I were in your situation. I'm also basically stumpped.

[fullmetalx]

problem solved. after all that was done, it was a bad fuel pump. even though it would turn on, it would only achieve up to ~23 psi. probably getting the hesitation/misfire on low rpm due to low fuel pressure.

so i replaced it with a new fuel pump.

also started on the 2nd crank right away. it used to take from 7-10 times of cranking before it would turn over.

[SinGarage]

Glad you got that solved. I will keep this in mind if similar symptoms appear on mine.