vintagekar

My 91 LS400 after 2-3 minutes of warm up in park/neutral while pushing slowly down on the accelerator upon reaching exactly 1000 rpm the engine cuts down to 850 rpm then immediately back to 1000 over and over, up and down while holding the accelerator in the same position. While driving it cuts on and off, rpm up and down, again at the 1000 rpm point. It is most noticeable in the lower gears. 1st gear very abrupt cut off then back on jerking up and down, 2nd gear same but less abrupt, drive less abrupt with over drive off, and least noticeable in over drive. When cruising above or below 1000 rpm engine runs smoothly, however, when cruising above 1000 rpm or more, lifting off the accelerator the engine cuts off abruptly rather than a smooth de-acceleration as in my 2001 LS. The car idles smoothly at 500, accelerates smoothly, cruises smoothly. Cut off and rpm hunting 850-1000 occurs at 1000 rpm, and engine cuts off abruptly when lifting off the accelerator at higher rpm but no rpm hunting. The symptoms start after 2-3 minute warm up but don't occur when cold. I have new timing belt, rotors, plugs, plug wires, cat convertors, O2 sensors, mass air flow sensor, idle air control valve, water heat sensors, EGR valve, EGR heat sensor, throttle position sensor, blah, blah, blah. The only thing that I haven't messed with is the fuel pump, fuel pump ECM, fuel pump relay, fuel pump resistor. No sure how to check these. Any advice appreciated.

truongrobe

I had Nissan Pathfinder 1999 LE 180k miles .It had the rpm went up and down . Then one day it gott worse , it shut off when i stopped and red light or at stop sign .I took it to the shop, they told me it was throtle position sensor and fuel pump bad .I let them replaced all for 425$ ( part and labor ) because i don't know much about car ( just basic oil change and brake) . Before took to the shop, some people told me should check vacuum hoses ,maybe they were leaked , i did not check them. Maybe the guy at shop try to make more money on me because the cost to repair and test vacuum leak is alot cheaper than replace fuel pump and throtle sensor maybe 1/4 cost , Or maybe he was right, i wrong .By the way my suv is running fine now.I hope this imformation help u .If you firgure out what happening , post up here so people know .Good luck to you

LScowboyLS

I had the hunting problem, I found two issues, bad Idle Air Control Valve (seized bearings) and bad ECU caps

not sure which was the culprit, but fixed both and now no hunting!!