D-Jet: Can you compensate for Throttle Shaft air leak? |
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D-Jet: Can you compensate for Throttle Shaft air leak? |
jrmdir |
Dec 7 2021, 02:37 PM
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#1
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Member Group: Members Posts: 224 Joined: 13-May 21 From: Central Ohio Member No.: 25,544 Region Association: Upper MidWest |
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 |
Dec 7 2021, 06:26 PM
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#2
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Custom Member Group: Members Posts: 24,624 Joined: 9-January 03 From: Taxachusetts Member No.: 124 Region Association: North East States |
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 |
Dec 7 2021, 06:47 PM
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#3
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Advanced Member Group: Members Posts: 2,410 Joined: 20-July 21 From: Upstate, NY Member No.: 25,740 Region Association: North East States |
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jcd914 |
Dec 7 2021, 07:01 PM
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#4
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Advanced Member Group: Members Posts: 2,081 Joined: 7-February 08 From: Sacramento, CA Member No.: 8,684 Region Association: Northern California |
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 |
Dec 7 2021, 10:24 PM
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#5
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914 Idiot Group: Members Posts: 15,048 Joined: 9-January 03 From: Silicon Valley / Kailua-Kona Member No.: 121 Region Association: Northern California |
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 |
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