D-Jet: Can you compensate for Throttle Shaft air leak? Options

[jrmdir]

After months of chasing a rich condition on my '73 1.7 I decided to once again try pressurizing the system and applying soapy water. Glad I did because I found two more leaking hoses and a leaking Cold Start Valve gasket. But the worst culprit turned out to be a leak around the throttle shaft.

There are a number of threads about options for adding bushings and or seals, but am I right in thinking this would mainly impact idle mixture? If so, I'm wondering if other system adjustments, e.g. ECU knob and/or MPS could overcome the effects of this leak - at least until I find a machine shop to fix the TB.

Thanks in advance for any suggestions.

Ron

[r_towle]

It’s unmeasured air.
I agree that you could turn the idle mixture screw (unmeasured air) in all the way and close that hole off if the bushing leaks are large enough.

Depending upon how deep down this rabbit hole you would like to go….the 1.7 typically run rich.
To lean it out, and get it perfect for your engine/elevation combination, here is what I do.

Buy a potentiometer (a volume control switch)
Buy an 02 sensor, with gauge
Buy about 15 feet of double wire.

Stick 02 sensor into tailpipe clamped to a coat hanger.
Run wires from 02 up and over car into window, hook up to gauge.
Wire up the potentiometer inline between the CHT and the ECU
I now put this in the car also, so more long wires taped to car, in window

This is all temporary for tuning and cheaper than a dyno tune
Go drive car
When WARM, start adding resistance to the CHT circuit by very slowly turning up the volume knob
Add resistance until A/F mixture goes down to your desired number
Last test, at this new setting, drive on highway for 15 min at high revs constant
Make sure it’s not too lean
Take ohm meter, measure across the potentiometer
Install a resistor inline to match the potentiometer setting
Enjoy life
Rich

[emerygt350]

It’s unmeasured air.

Which should lead to a lean condition....

[jcd914]

It’s unmeasured air.

Which should lead to a lean condition....

D-jet does not measure air.
It measures manifold vacuum and temperature.
The theory is that under load the vacuum drops, so the lower the vacuum the higher the load, so D-jet adds more fuel.
So vacuum leaks in D-jet cause a rich mixture.

Jim

[JamesM]

It measures manifold vacuum and temperature.
The theory is that under load the vacuum drops, so the lower the vacuum the higher the load, so D-jet adds more fuel.
So vacuum leaks in D-jet cause a rich mixture.

Except the reason it is adding extra fuel is because it is also getting extra air. ie its doing what it is supposed to do, the same thing that happens when you open the throttle.

How the air gets into the plenum is irrelevant (with the exception of the leak being in the MPS line, or as dave said, near the heads) the air is going to do the same thing once it gets there.

Think about how the AAR works, its basically just a controlled intake leak. Doesn't mess with the mixture, just kicks up the engine speed.

HOWEVER
Idle can be especially tricky though as any one operating parameter impacts the others. Idle speed kicked up to high causes the timing to change, timing changes affect the burn which then alters the vacuum which then alters the mixture, and so on.

Bottom line, everything needs to be set to spec or odd things happen.

Leak in the throttle shaft shouldn't be a huge deal though as long as you can bring the idle speed down low enough with the bypass screw.

I suspect your issue may not be due to an intake leak though.