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

[emerygt350]

How much left on the idle adjust screw? If that is your only leak you should just be able to trade idle mix screw turns for bushing leaks.

[JamesM]

Leak wont affect mixture with d-jet, just idle speed.

I think your bigger issue though is that it may wind up giving you a constantly changing idle speed in operation as the severity of the leak varies every time the throttle is closed.

thankfully they made a ton of 1.7s and no one seems to want them, so you should be able to find another TB pretty easily.

[emerygt350]

Little curious about a rich condition and vacuum leaks. What's going on with that?

[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....

[JamesM]

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.

Rich

d-jet measures manifold pressure, not air flow. Intake leaks/idle screw adjustment have the same effect as cracking open the throttle, its all measured.

[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

[914_teener]

Leak wont affect mixture with d-jet, just idle speed.

I think your bigger issue though is that it may wind up giving you a constantly changing idle speed in operation as the severity of the leak varies every time the throttle is closed.

thankfully they made a ton of 1.7s and no one seems to want them, so you should be able to find another TB pretty easily.

For once I might slightly disagree with James.

Part load will be affected/throttle response along with idle speed. I had this exact problem. I'd agree there are tons of 1.7 TB's and I'd find another instead of having it rebushed.

It really depends where the leak is. A leak in the plenum is exposed to high manifold vacuum depending on the position of the throttle plate affecting the mixture across all driving ranges. Most driving is done under part load to idle. So driveablility will be affected the most.

[emerygt350]

I never really connected that with idle. I assumed when the djet knew it was at idle (the TPS tells it so) that it just ignored the mps and determined a mix based on ECU parameters. I sort of doubted that since mps adjustments appear to alter idle richness.

[914_teener]

I never really connected that with idle. I assumed when the djet knew it was at idle (the TPS tells it so) that it just ignored the mps and determined a mix based on ECU parameters. I sort of doubted that since mps adjustments appear to alter idle richness.

The TPS will help with driveability and adds enrichment for acceleration.

You can drive the car without it...but it will "buck" somewhat.

So driveability is different than AFR readings.

Think like this:

Your accountant tells you what youv've spent.....your needs and desires decide what you spend on.

[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

[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.

Rich

d-jet measures manifold pressure, not air flow. Intake leaks/idle screw adjustment have the same effect as cracking open the throttle, its all measured.

Thanks for jumping in to correct terminology
Measured manifold AIR pressure
That pressure is most definitely impacted by a leak in the manifold at the intake.

[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.

[jrmdir]

Thanks to everyone for the great input.

I had a DOH! moment soon after I posted that when I realized what I already knew, that - as has been pointed out - any extra fuel added by the MPS due to vacuum leaks would only raise the RPMs - just like multiple tiny extra throttle openings - not lower the A/F ratio.

I think I was so excited to find the previously unknown leaks that my brain just wanted it to be true that this was going to help fix the rich condition (IMG:style_emoticons/default/smile.gif)

But it will indeed help to take these variables out of the picture so I can hopefully improve the idle situation and keep working on other things that do impact mixture like MPS calibration and CHTS replacement.

Thanks again!

Ron

[emerygt350]

Just out of curiosity, what do your plugs look like?