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Holley pro jection 2d off idle stumble, poor gas mileage

47 dodge

New member
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?

Robert Kenney

Super Moderator
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?

Robert Kenney

Super Moderator
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?

47 dodge

New member
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.

47 dodge

New member
Put the air fuel meter on with it own O2 sensor. It shows,
First 1/3 throttle movement very rich about 11:1
Mid range throttle 13:1
Last third 12:1

So rich across all. I disconnect the O2 sensor for the projection system, and started re tuning it to get at least close to 14.7. Then plugged the O2 sensor back in . Works much better now.

A few notes, getting the light throttle to 14.7 is not happening best there is 13. That is with the dial turned as far as it will go to the minimum. I am running it in small block mode, and did verify the throttle position sensor is set correctly.
Mid range does dial in to 14.7, however any change in throttle and it looses track and then takes a second to readjust to 14.7. Must be a rather slow processor.... Have the top end set about 13 now. So clearly only on mid range does the computer read thr O2 sensor on this system. I did not remember that from others of these I have used, but it has been some time since I have used pro jection.

Fuel mileage has improved, and the part throttle stumble has mostly gone away. Might try setting the throttle position sensor a little different to see if I can can bring light throttle in a bit closer.
Last edited:


New member
I see this is an older post so you may have already got this figured out. A friend of mine had the old Holley Pro Jection on a small block Chevy and it to was rich across the board with the control knob backed all the way off. We discovered that the TPS was out of adjustment on the throttle body. The arm on the TPS was pulled back a decent amount at closed throttle. Adjusted that and everything fell into place. Hope you already have it fixed, if not hope this helps.

47 dodge

New member
Yes figured out. I did change the ecu, and was able to tune it for good drivability, and a little better fuel mileage. Light throttle is now close to 14:1. I have checked the throttle position sensor and it is right on.

There is no stumbling at any throttle position at this point, and it is very driveable.

47 dodge

New member
One more note I am now getting just over 6mpg consistently, and that is about what I would expect from a bus of this size.