jayzsun

I have got a 1992 sc300 na-t and it keeps bucking or hesitating around 3500-4000 rpms. It doesnt go past 4000 rpm. I have replaced the plugs, cap, rotor, 2 used mafs, adjusted tps sensor, and set base fuel pressure to 40psi. The idle is rough and it feels like a small vacuum leak. Now my question is would a vacuum leak cause it to buck or hesitate like that? Also my plugs are gapped at .034. I am only boosting at 5psi and I doubt that would blow out the spark or would it? Any help would be appreciated.

JspecSC3

iTrader: (4)

Are you auto or manual?

Zeal3

iTrader: (1)

i fixed my bucking by changing my distributor cap.
or it may be your torque converter if you have a auto. bcs on our LS it had a hard time getting to speed and didnt pass 4000RPM unless the ECT was on

JspecSC3

iTrader: (4)

Same thing happened to me in the middle of a 3 hour long road trip. I couldnt get past 60mph (or what ever the RPM is at that speed). So I had to limp it home driving 50mph all the way home. I fixed my bucking by simply junking the auto and changing over to 5spd.

jayzsun

yeah it is an auto. Whenever I keep on the gas and it bucks than it feels like there is some sort of fuel cut. So the torque converter could be the cause of this?
Jpsec, did your car have this problem consistantly or intermittent? My car does it every time.

JspecSC3

iTrader: (4)

It would be kind of hard for me to pin point the problem without having to actually drive the car myself. But judging from my experiences with bucking SCs I would definitely say its time to change the tranny out because it is going bad. My SC would do it lightly at low speeds but when you reach highway speeds on a hot summer day you can definitely feel the car bucking. Its a scary feeling if you're far away from home.

ConSynX

iTrader: (1)

do you have Wideband, and how are you controlling fuel? other than pressure. . .

I had a similar problem a while back, but I was real lean, and ended up retuning

jayzsun

No wideband unfortunately. A friend of mine did say that it could be leaning out also because of the harsh bucking and not sputtering like if it was running rich. I thought with only 5lbs that my stock ecu would be okay to run with no problems.

cbrasian

nope,you may be running too lean, it can be running in the wrong fuel cell you really don't know you could try resetting the ecu and see what happens with the relearn but i doubt its that... in reality though
you need something to check it ex wideband o2 and something to tune it with and control it ex. a safc , aem,power fc etc

Turbo?

you need to do some tuning, my car was doing the same thing... because the computer isnt use to pushing that much fuel so it kinda cuts it off abit... so get it tuned see where that goes..

ConSynX

iTrader: (1)

and you said you "raised" the fuel pressure to 40
is that at idle? because at 0pressure 40 is stock.

you can get away with a few psi probably, by sacrificing the low end by making it super rich.
if you knwo your turbo size, and the horsepower rating you can get with it around 5psi(hard to get info)
the 1jz and 2jz seem to use the size injector that there is power(weird coincidence)
so use a fuel pressure/injector flow rate calculator, and see what pressure you can use to have to still be rich up top(and super rich down low, under wot)...normal driving shouldn't be too bad, once the stock o2 sensor is warm

jayzsun

The regulator was set at 35psi no vacuum so I set it to 40psi but I did not want to go higher because I am on stock pump. Would it be okay for stock pump?

So today I took of my charge pipe and ran it like an n/a and revved it in neutral and it starts breaking up around 4000-5000 grand. I pulled the efi fuse for 10 seconds and started it up and it still does the same thing. So I am assuming that it is not fuel related. I am getting no codes and I am wondering if it is my tps or maf. I have searched and could not find which wires to check for the tps. Anyone got the procedure?

ConSynX

iTrader: (1)

i dont know about pump limits, and i dont know how that method rules out fuel delivery completely...but if you are sure, go ahead and start checking your plug gap.
use feeler gauges for the gap, i used one of those cheap coin-tools and was having breakup issues that went away when i regapped with feelers.

i'm unsure how to check TPS the normal way, i used the power fc to check mine. . .sorry, can't help ya there.

when you turn your key to the "on" position does the pump turn on temporarily, then turn off?

yauser

Then stop bucking with it, Lol.. J/k. Do you have those rainbow color vacuum hoses under the hood? If so, put the stock ones back, because the gay hose gets too soft from heat and bends. That was one of my NA-t problems from before. It can also be one or more of your intercooler hoses being lose. Replace all the cheap hose clamps with T-Bolt clamps. Or if you have a T04B Turbo, the exhaust a/r is probably too low, resulting in peak boost too soon. You need at least a .81a/r.