4JG2 engine revving/surging

[arthurdent]

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?

[geeves]

First air is not modulated at all except in an overboost situation. A diesel will always accept all the air you can feed it without affecting the burn. Are you sure it is not changing gear or switching the lock up clutch in and out? 50kph is close to where the clutch does its thing. What does it do at a faster speed?

[arthurdent]

Ah, understood. I'm not sure, i'll have to take it for another drive. Will report back.

[arthurdent]

Update! Could not reproduce the issue, engine and transmission both ran smoothly.

I was mistaken in my initial post: the fuel system does not have an in-tank electrical pump - the engine mounted injection pump sucks fuel right out of the tank.

I have a couple follow-up questions:

1. When I turn the key to ON to let the glow plugs do their thing, the A/T TEMP lamp turns on momentarily, then goes out. The AW30-40LE manual I have states that when the key is turned to ON, a CHECK TRANS light should turn on for 3 seconds, then go out. Does a 93' Bighorn with a 4JG2T have this light? Or is it replaced with the A/T TEMP lamp? I also don't see a CEL turn on along with all the other indicators when I turn the key to ON. Does this vehicle even have one?
2. The injection pump is a mechanical pump. Therefore, does this vehicle have an ECM?

Next things to check when the weather is amenable to it:

1. Fuel filter for debris
2. Air filter for cleanliness
3. Test the TPS somehow, since apparently when this sensor wears out it can cause issues..

Any other ideas?

[geeves]

at/temp is the check trans. True. AT and if you had it abs lights are supposed to do a self test of about 3 seconds.
Fuel filter and air filter are disposable parts and should be changed every now and then.
My money is on the tps If it plays up again mark where it is sitting now and adjust very slightly clockwise.
EGR uses a ecm. Over here we normally disable it but that will fall foul of your smog rules

[arthurdent]

Thanks for the extra info. I reached out to jp-carparts to see if they could have a TPS made but they said it's been discontinued. I found another outfit in NZ that has it listed, https://www.autoignite.co.nz/products/8-97065709-1 and also enquired, waiting for a response. TPS' seem to have become unicorns. Do you know where I might be able to find one? Otherwise, install a manual tranny...? Ugh.

I do have ABS but I think the previous owner had issues with it, and I noticed the speed sensors at the wheels were disconnected when I did the bearings. That ABS light has never come on...

The vacuum lines going to the EGR have been plugged with ball bearings by a previous owner. The engine doesn't seem bothered.

I have a K&N reusable air filter, which I clean at regular intervals, last done not too long ago. I replaced the fuel filter 18 months ago (~20,000kms).

[geeves]

They are joking about that price I hope. Zeycel make the pump it might be another way to get one

[arthurdent]

Just heard back, they can't source it anymore.