Hey gang,
Got a bit of an odd one here..
I was driving home the other night when, as I was driving along at around 50km/h, the engine rpm started to drop, then rise, then drop, then rise about 500rpm at about 2Hz. The engine was running smoothly at about 1700rpm, when it dropped by about 500, then came back up to 1700, then dropped again. It did this on and off until I removed my foot from the accelerator, at which point it idled normally. I stopped at the stop light, then drove off normally. It ran normally for a minute, then did it again. When the fluctuations were occuring, the gearbox seemed to stay in the same gear, running smoothly. The engine also was running smoothly, no roughness. It felt like the throttle was being modulated, despite my foot keeping the accelerator at exactly the same position.
That being said, I've got a '93 bighorn running a 4JG2T with the Aisin Warner automatic AW30-40LE gearbox. From what I can gather, the fuel system is pretty straight forward:
1. An electrical fuel pump in the tank pumps fuel to a mechanical injection pump.
2. The accelerator pedal is mechanically connected to a lever arm on the injection pump to modulate fuel to the injectors.
3. The fuel pump also takes a pneumatic input. I'm not sure what this input is for.
4. The TPS is attached to the top of the fuel pump, and this provide the transmission with throttle position for gear shifting.
4. The throttle body modulates air flow by way of a pneumatic diaphragm assembly.
What that, i'm thinking:
1. Fuel tank pump is failing
2. Injector pump is failing
3. Something fishy with the throttle body vacuum assembly.
Any thoughts?