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

[Dave_Darling]

So vacuum leaks in D-jet cause a rich mixture.

You were doing so well until here!!

Leaks in a D-Jet system let air in, absolutely the same as when the throttle is opened more. That lowers the manifold vacuum (raises the pressure) which causes D-Jet to add fuel--the same amount of fuel as if the throttle had let that air in.

There is no mixture change.

That said, there is an exception--if your leaks are out near the heads, such as if the fuel injector seals are bad or gone. Those result in inconsistent running, probably because not all of the air going in there causes a pressure change back in the manifold.


Yeah, I think a replacement throttle body is likely the best answer to the original problem.

--DD