Fuel Cut RPM test?

[KG Cruiser]

This test is on Page FI-85 of the FSM.
Does anyone know what this does?
What does it mean to "check that there is fluctuation between the fuel cut and fuel return points?​

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Fuel Cut RPM
INSPECTION OF FUEL CUT RPM
1. WARM UP ENGINE
Allow the engine to reach normal operating temperature.
2. INSPECT FUEL CUT RPM
(a) Disconnect the connector from the throttle position
sensor.
(b) Short terminals IDL and E2 of the wiring connector.
(c) Gradually raise the engine rpm and check that there
is fluctuation between the fuel cut and fuel return
points.​

NOTE:

The vehicle should be stopped.

​

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Fuel cut rpm: 1,300 rpm​

Fuel return rpm: 1,000 rpm

​

 

[KG Cruiser]

Has anyone done this?

If so, please help me with a little more detail as to how this is done.
Thanks

 

[Lead Head]

Basically, when the computer receives the "Throttle is at idle" signal, it will turn the fuel injectors off at 1300 RPM and above, and turn them back on at 1000 RPM.

So if you short the pins as it says, then give it a little gas, you should see the tach go to about ~1300 RPM. The engine will then momentarily cut out, and then refire once it gets down to 1000 RPM. It will keep doing that over and over again until you let off the pedal.

I believe the purpose of this is to prevent exhaust popping while slowing down. It effectively turns the engine off until it gets back down to idle RPM.

 

[KG Cruiser]

I ran the test

Lead Head - thanks for the explanation.

So I disconnected and shorted the TPS per FSM.
Then I had my son rev engine to about 1500

No discernible difference.
I was kind of expecting the engine to rev up then cut off then rev up again and continue this.

But I didn't really notice anything at all.

I then OHM tested the TPS per FSM.
Everything checked OK except for the readings between VTA-E2 w the Throttle valve fully open. The digital meter wouldn't stabilize at one reading. It kept fluctuating between 2K-10K OHMs.

All other readings were stable and within spec.

I don't know what to make of that.

 

[TobyB]

Hmm. That indicates a failing tps, but are you sure the VOM wasn't just right at the edge of its range?
That'll make it jump between readings...

A different suggestion, since it MIGHT be the wiring harness or connectors-

stick the VOM into the back of ECU pin 10, and see what the voltage does when you go from
idle to wide open throttle- the helpful service hint says you should see low (1v) at closed throttle,
4+ at wide open, with the key on.

Then meter the idle switch (pin 12) to make sure it closes (goes to ground) as the throttle closes.
The ECU should put some voltage on pin 12 when the throttle's off- idle, so you should see something
(3-5v?) when the the throttle's open, and it should drop to zero (ish) when it's closed.

If the ECU's not seeing the idle switch, it might think the throttle's still a bit open, then not do the
'coasting injector shutoff'.

Maybe an open wire in the harness???

hth

t

 

[KG Cruiser]

Hmm. That indicates a failing tps, but are you sure the VOM wasn't just right at the edge of its range?
That'll make it jump between readings...

A different suggestion, since it MIGHT be the wiring harness or connectors-

stick the VOM into the back of ECU pin 10, and see what the voltage does when you go from
idle to wide open throttle- the helpful service hint says you should see low (1v) at closed throttle,
4+ at wide open, with the key on.

Then meter the idle switch (pin 12) to make sure it closes (goes to ground) as the throttle closes.
The ECU should put some voltage on pin 12 when the throttle's off- idle, so you should see something
(3-5v?) when the the throttle's open, and it should drop to zero (ish) when it's closed.

If the ECU's not seeing the idle switch, it might think the throttle's still a bit open, then not do the
'coasting injector shutoff'.

Maybe an open wire in the harness???

hth

t

Those are excellent suggestions!

I'll retest the TPS again and pay attention to the VOM setting.

I'll also test at the ECU and report back. I'm REALLY INTERESTED I these test results.

Cruiser is now hesitating and sputtering at all speeds when it started just hesitating at takeoff.
So the problem is getting worse but still driveable.

Thanks again!

 

[KG Cruiser]

Measurements at back of ECU - where to put meter leads?

Hmm. That indicates a failing tps, but are you sure the VOM wasn't just right at the edge of its range?
That'll make it jump between readings...

A different suggestion, since it MIGHT be the wiring harness or connectors-

stick the VOM into the back of ECU pin 10, and see what the voltage does when you go from
idle to wide open throttle- the helpful service hint says you should see low (1v) at closed throttle,
4+ at wide open, with the key on.

Then meter the idle switch (pin 12) to make sure it closes (goes to ground) as the throttle closes.
The ECU should put some voltage on pin 12 when the throttle's off- idle, so you should see something
(3-5v?) when the the throttle's open, and it should drop to zero (ish) when it's closed.

If the ECU's not seeing the idle switch, it might think the throttle's still a bit open, then not do the
'coasting injector shutoff'.

Maybe an open wire in the harness???

hth

t

TobyB
I removed the ECU this am to take measurements but then after reviewing the FSM, learned that the measurements must be taken with the ECU PLUGGED in.

But when the ECU is plugged in, there is really no place to insert the multimeter probe.

Should I just force the leads into the back of the connector at the ECU and try to get a reading?
Is that the only way to test at the ECU>

 

[TobyB]

Yup- it's called 'back- probing'- I've used a piece of smaller, single strand wire (like 18 gauge) attached to the probe. That fits into the back of the housing without distorting it, and gives a pretty good connection.

Be a little careful, though- a few of those connections can't be grounded, or the ecu or sensors could be
damaged.

t