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1HD-FT Performance Adjustments

The '95 - '98 multi valve factory turbo diesel motor (1HD-FT) is a great motor with gobs of go and gobs of torque. Because however it has a larger turbo housing and is tuned for tight emissions, the off boost response is light. This can be alleviated with simple injector pump adjustment. It's not a difficult task and the results are impressive indeed.

Adjustments are required to three components of the injector pump  and does not affect the factory seal on the maximum injection volume screw. Adjustments are required to the boost compensator characteristic tendency and to the maximum off boost injection volume screw.

Boost compensator characteristic tendency

The stock boost compensator characteristic tendency on the injector pump is set very conservatively on this motor. This can result in lack lustre performance until you get the boost wound up. It varies quite widely from car to car and I've driven a couple that were quite bad down low.

The boost compensator is the diaphragm that sits on top of the fuel pump. On the rear face (towards the rear of the car), just under the diaphragm is a 12 mm bolt (from memory) with a rubber drain cover/tube covering it (Aussie spec only. The European models have that BACS tube in there). Pull the rubber cover off and undo the bolt. Depending on which tools you use, you may have to remove the fuel inlet pipe first. It's tight in there....

Inside the hole is a toothed wheel sitting on the horizontal plane. You can see it if you grow a 3rd hand, shine a light in there and hold an inspection mirror just right. You don't need to see it though.....

Insert a smallish blade screw driver (holding the blade in the vertical plane) and get it on the teeth of the toothed wheel. Now push the end of the screw driver away from the motor (to turn the wheel clockwise when viewed from above). As the wheel turns, you'll hear a faint click as each tooth pushes against a detent spring. You need to turn it 2 or 3 clicks.

Re-install everything being careful not to over tighten the bolts. They screw into soft aluminium.

What this tweak does is to allow the pump to inject more fuel at low to moderate boost pressure. It doesn't affect the maximum injection volume, so you're not over stressing the motor. Another benefit is that it'll pull harder as it approaches red line. There's no noticeable difference in smoke and the net effect is better economy. You'll end up shifting sooner and not having to rev the motor to get going quickly.

Don't be greedy however by screwing it down much further. The gotcha is that it does inject more as the boost drops close to red line and you can get alarmingly high combustion temperatures when revving hard for any length of time.

Maximum off boost injection volume

This adjustment determines how much fuel is injected when there is no boost. Once boost begins increasing, the boost compensator characteristic tendency described above takes over. By increasing the volume of fuel injected, there is more torque available under no boost conditions with the added bonus that with more fuel, more exhaust energy is available to spin the turbo up quicker.

The adjustment screw is found on top of the boost compensator diaphragm and usually has a dab of yellow inspection paint over it (it may however have flaked off with engine washing ;-)    Make a note of its stock position for reference. To adjust, loosen the lock nut and screw the adjustment screw in. Initially, turn it in half a turn and tighten the lock nut.

Now it's a matter of trial and error. Warm the engine up and test the adjustment by looking for a light puff of black smoke as you hit the accelerator from idle. You can repeat the procedure and keep increasing the adjustment half a turn at a time until you do see an increase in smoke.

Try to keep the smoke to a minimum when you do finally find a screw setting that suits your engine.

This will have a noticeable effect in the off idle response and can have the turbo spinning up and boosting right down from 1,000 rpm.

A word of caution however. With all this new-found torque at low boost, don't be tempted to lug it up long hills simply because it now can. Bearings don't like being hammered.

I did this mod to mine 6 or so months ago and jumping into a stock TD now it's noticeable just how different it is.

Boost Compensator Profile

For those a bit more adventurous, there is one more adjustment (other than the main set screw :-) that can have a significant affect on low to mid RPM torque.

The boost compensator is essentially a diaphragm that pushes a pin down as the boost pressure increases. The above two operations alter the pin's off boost position and the spring's preload.

The pin itself is tapered and the taper angle changes depending on the angle the pin is on. This pin is bolted onto the diaphragm itself so rotating the diaphragm also rotates the pin and presents a different profile to the pump.

Access to the diaphragm is fairly simple - remove the 4 allen key bolts on top of the boost compensator.

Mark the position of the diaphragm in relation to the housing it sits in. This gives you a reference point of the original position. Rotating the diaphragm clockwise as you view from the top increases the amount of fuel delivered from idle to close to maximum RPM. Max fuel is determined by the main set screw.

You can rotate the diaphragm past the max point. This now begins to reduce the fuel delivery. There are no markings to show where the maximum point is however you can feel where this point is.

Rotate the diaphragm with your eyes closed (this is important) and feel the point where the tension is the least. Mark that point and perform that operation again to confirm the position. The diaphragm should be in the same position.

I have seen one pump where the diaphragm was set to the maximum position from the factory - leaving no further adjustment. This is uncommon.


The week before last, I visited Dave Webster at Cooma Diesel in Fyshwick (in Canberra) to discuss our vehicle and some rather sluggish behaviour in low-end acceleration. They test drove it for me and said that it was indeed sluggish and that it definitely needed attention in the fuel-pump. They said that whilst in factory-spec, the spec was wide in its tolerance and mine was in the very very lower end of "acceptable".

They arranged for me to contact Toyota (which I did) and review the vehicle for work to be carried out under warranty. This I duly was granted, and did without my truck for three days whilst they (Cooma diesel) worked on the fuel pump on behalf of Toyota.

The results? Unbelievable. There is heaps more power/acceleration from 1,000 rpm to 2,500 rpm and there's more top-end power too. Before nothing so much would happen over 3,500 rpm; now... much more acceleration and power. It's a different vehicle...

What Cooma Diesel did was to re-calibrate the fuel pump and run it up on the dyno to make sure it was at its optimum. They didn't touch the injectors, or spray patterns or anything like that, just re-calibrated the pump. And all done under warranty - the cheapest upgrade I have ever had.

For those who own the 1HD-FT (esp. if it is still in warranty), try what I did - it could give you a huge boost...

BTW, great service from Cooma Diesel - extremely knowledgeable and very helpful.