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Old 05-22-2019, 03:01 AM   #1
47 dodge
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Default Holley pro jection 2d off idle stumble, poor gas mileage

I have a NOS Holley pro jection system which I have put on a 71 International school bus. 345 engine, 5 speed manual trans. DUI distributor, Intake manifold machined to match TBI. Dual exhaust.

I have installed several of these systems over the years and had great success with them. However this one has been a bit of trouble.

The problem as it is now,
1. a part throttle stumble, not speed sensitive.I have checked the throttle position sensor per Holley's instructions for voltage and do not find dropped or uneven voltage through it's range of motion. It is much worse without the O2 sensor connected. These systems should be tuned without O2 as best as possible then connect O2.

2. The high speed fuel adjustment on the ECU does nothing, no change no matter where it is turned.

3. Fuel mileage is really poor and exhaust smells rich. I have converted International trucks of the same engine and overall weight of the bus, so do have an idea of what to expect. 6-8 mpg typically, and am only getting 4-5 mpg on the bus

I am thinking the ECU may be bad and new ones are not available. How could I check my ECU, or could a GM ECU be used since this is at least based on the GM TBI system. I realize some wiring may need to be changed, what else needs to be done?
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Old 05-22-2019, 10:22 AM   #2
Robert Kenney
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Default Re: Holley pro jection 2d off idle stumble, poor gas mileage

If you are indeed running in closed loop(heated 02) it should self calibrate to stoichiometric AFR.
The stumble you mention is on throttle advance only?
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Old 05-22-2019, 05:38 PM   #3
47 dodge
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Default Re: Holley pro jection 2d off idle stumble, poor gas mileage

The stumble is at light throttle while going steady speed.

Last edited by Robert Kenney; 05-23-2019 at 05:24 AM.. Reason: Edit
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Old 05-22-2019, 05:59 PM   #4
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Default Re: Holley pro jection 2d off idle stumble, poor gas mileage

Quote:
Originally Posted by 47 dodge View Post

The stumble is at light throttle while going steady speed.
I would try to validate the 02 sensor. In closed loop it will be supplying the data that the control uses to dial in the air fuel ratio.
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Old 05-23-2019, 05:28 AM   #5
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Default Re: Holley pro jection 2d off idle stumble, poor gas mileage

I would drive it in the faulty range for a time and pull a couple of plugs. A plug reading should give you an indication of if the unsteady ness you are experiencing is a lean or a rich condition.

When you reference other installations of the same injection system getting 5-6 mpg, what were the engines in those instances?
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Old 05-23-2019, 03:59 PM   #6
47 dodge
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Default Re: Holley pro jection 2d off idle stumble, poor gas mileage

Had not thought of pulling some plugs, but good idea. Can do that tomorrow.

I have ordered an air/fuel meter with it's own o2 sensor, so I can better monitor what it happening.

The O2 sensor in it now( for the holley system) is about a month old, and the previous one did bench test as bad. I did not bench test the new one, so am assuming it is good. Of course assuming at this point does not get answers.

as to the other truck getting better fuel mileage it was a 1600 Loadstar, with a 345 set up with the same pro jection system, and a 4 speed, with two speed rear, and in high, ratio of 5.8, the bus is 5.14 in high on the rear.

The 1600 was grossing 26,000 lbs, and the bus is 18,000lbs normal running weight. By the way the the bus is converted to an RV, so other then water and fuel the weight does not change much.
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