Engine loping from 1620 down to 1380 RPM... What would cause this? (1 Viewer)

[gaylon]

Hi, I am currently troubleshooting smog and have possibly made one adjustment too many... Now the engine is loping from 16XX RPM down to 13XX RPM while I steadily hold in the throttle. Attached vacuum gauge video show vacuum dropping from 20" to 15" when the engine decels. What should I adjust or should I not have adjusted

 

[lcolon]

Did you check to see if your EGR is working correctly? Are all of the vacuum lines in the right place? The only "adjustments" are on the carb. Everything else either works or does not work.

 

[OSS]

If it's a 2F, the vacuum switch either no longer works or it's wire has come off the connector.
Either connect the wire to the switch (if its disconnected) or ground that wire to the body.

 

[gaylon]

If it's a 2F, the vacuum switch either no longer works or it's wire has come off the connector.
Either connect the wire to the switch (if its disconnected) or ground that wire to the body.

Which vacuum switch?

 

[lcolon]

Not sure which vacuum switch OSS is talking about, but do you have the emissions manual? All of the tests for the various components are in there. It is kind of a tedious process to work through them all. With what you are describing, I would personally start with the EGR circuit

 

[OSS]

 

[gaylon]

View attachment 2293085

I checked the switch and it has infinite resistance when it is supposed to and none when it is not. I also disconnected the wire and the engine did not start missing, which it is supposed to. I checked the FCS/ICS and it clicks when it is supposed to. I grounded the FCS/ICS and the problem went away. I pulled the Emissions Computer and it looks fine, but I will reflow the solder joints later. Any other thoughts? I wasn't driving, so I am not thinking the speed sensor would be in play, but I am not terribly familiar with this circuit.

 

[OSS]

Try grounding the vacuum switch wire to the body to be double sure. But if the switch passes the resistance test as per the FSM (and the FCS), then you're right to suspect the emissions computer.

The deceleration fuel cut system can be checked with engine idling (at different RPMs) when cruiser is parked.

Normally, when it's working correctly, if the cruiser is parked and engine idling - when the gas pedal is slowly pressed down to raise the RPMs up to about 1800 rpm or so and then the pedal is slightly released, the DFCS will kick in and deenergize the fuel cut solenoid which causes the engine to momentarily "die" until the RPMs drop quickly to about 1380 or so. Then the computer turns the fuel cut solenoid back on- allowing fuel to flow to the carb slow jet --- and then the engine idles normally.

There's normally that dead/dying spot between 1800 & 1380 when the gas pedal is slowly released while idling when the cruiser is stopped and parked

 

[gaylon]

Try grounding the vacuum switch wire to the body to be double sure. But if the switch passes the resistance test as per the FSM (and the FCS), then you're right to suspect the emissions computer.

The deceleration fuel cut system can be checked with engine idling (at different RPMs) when cruiser is parked.

Normally, when it's working correctly, if the cruiser is parked and engine idling - when the gas pedal is slowly pressed down to raise the RPMs up to about 1800 rpm or so and then the pedal is slightly released, the DFCS will kick in and deenergize the fuel cut solenoid which causes the engine to momentarily "die" until the RPMs drop quickly to about 1380 or so. Then the computer turns the fuel cut solenoid back on- allowing fuel to flow to the carb slow jet --- and then the engine idles normally.

There's normally that dead/dying spot between 1800 & 1380 when the gas pedal is slowly released while idling when the cruiser is stopped and parked

I'll do the double sure grounding test . My truck was cycling from 1800 (give or take) down to 1380 or so when holding the accelerator steady - no release. The emissions computer failing could cause the solenoid to kick in without the slight release of the accelerator?

 

[OSS]

Yes it's possible that the circuit in the computer that gets a signal from the vacuum switch is bad. Normally the vacuum switch is the trigger to tell the emissions computer to flip off the FCS.

If you can't determine if the computer is malfunctioning but the cycling remains, either try to find another computer or wire the fuel cut solenoid to an ignition key ON power source. You'll lose deceleration fuel cut (which isn't too big of a deal) but you'll still keep the anti-dieseliing function at shutdown if the FCS is wired to an ignition ON power source.