Fixed Vacuum Issue, now rpm issue?

[MorganMcCann]

About a week or so ago I fixed a vacuum leak that the Ranger had since I bought it. Personally I have never reset the ecu, so I'm wondering if resetting the ECU would help with my issue.

When I'm cruising at ~45 if I let my foot off of the gas completely, the rpm will go down to ~1150 and then jump back up to where it should be. You can feel it when it does it, so I know it isn't just a faulty tach.

Maybe the vacuum leak was larger than I thought so the ecu was used to getting more air, and now it is not getting that air but still compensating for the extra air with more fuel, so when I let off of the gas it runs really rich for a slpit second?

 

[Road Angels]

So is what you are saying is it deaccelerates and then accelerates with your foot of the gas on its own????? or the idle falls down to 1150 then down to normal idle speed??? confused

 

[swynx]

Saying that while cruising at 1800 rpms it'll drop to 1150 then go back to 1800.

Check your 02 sensors. They will give super funky idle problems. Try unplugging them.

I'm not real sure how this could happen being an auto. The torque concerto should compensate

 

[RonD]

Computer controlled fuel injection systems have a fuel savings method that turns off the injectors when RPMs are above 1,100-1,200 RPMs and foot is off the gas(TPS shows throttle closed).
So if you are cruising at 2,000 RPMs and take your foot off the gas the injectors are shut off until engine RPMs get to 1,100 then injectors come back on at idle level, there should be no surge to higher RPMs though.

I would check the OHMs/Voltage at the TPS, also make sure throttle plate return spring is working as it should, when you take your foot off the gas the throttle plate should close all the way, if spring is weak/broken plate will stay partially open from air flow, and TPS will tell computer your foot is still on the gas.

You reset the ECU by unhooking the negative battery cable for 5 minutes or longer, no scanner is needed.

 

[MorganMcCann]

Could fixing a vacuum leak mess with the ECU?
That is pretty much what I'm asking. I never had this until I fixed the leak. When I got the ranger it had manual hubs, but no one ever capped off the 4wd side, so it was just a constant flow of air through the 4wd side of the vacuum side.

 

[RonD]

It wouldn't "mess with" the ECU.

But the ECU does "learn" the parameters of the engine over time, well "learns" the parameters of the sensors over time.
So whenever you replace, clean, or otherwise change something on the engine it is a good idea to unhook the battery cable while doing the work then when you reconnect it it will force the ECU to "relearn" parameters.

The ECU will "relearn" without a reset but it can take a hundred miles of driving for some "relearning".

 

[MorganMcCann]

Just reset the ECU, she idles about ~50-100 lower than she did before the reset.
I think that leak must have been worse than I thought it was.

 

[RonD]

Cold idle should be 1,000 rpm and drop every minute or two as engine warms up
Warm idle 750 rpm

When first starting engine, warm or cold, rpm should go up to 1,500 approx, then drop down to warm or cold idle level.
That's the ECU testing the IAC valve

 

[MorganMcCann]

For a 3.0L?
It was like 1/3rd of the way warmed up and idling around 900.