High idle and misfire

[jklolkik]

Just changed the heads on my 93 ford ranger with the 98tm heads. I know it causes it to run at higher compression.

My problem is the truck is idling high at 2k rpm. The idle set screw on the throttle body is all the way out. It was idling high even with the stock heads.

As i was putting the engine back together I noticed the pass o2 sensor unplugged (unplugged the entire time before head change) so I plugged it back in. Still high idle. Go for a drive really bad misfire and really bad gas mileage 100 miles for half a tank. Unplug it and no misfire. Still high idle.

I know the high idle could be a vacuum leak. All hoses plugged in and no cracks in them. Might be at the intake manifolds.

Main question is would the o2 sensor be causing the misfire and high idle? Going to change it this weekend anyways.

It has motorcraft coilpack unknown age and brand new autozone spark plugs oem equivalent. With brand new wires

 

[jklolkik]

I replaced the fuel filter and the O2 sensor same thing. Runs like a champ without it except the high idle

 

[RonD]

No idle screws on fuel injected engines, anti-diesel screw, and no it doesn't work the same, lol

IAC(idle air control) Valve is what controls idle on fuel injected engines

Yours make be stuck open
Pull it off, 2 bolts
Plug in its 2 wire connector and turn key ON, engine off
Look at the valve inside and Unplug the 2 wire connector
Valve should close with spring pressure when unplugged
Valve only moves 3/8" but will move
Plug wires back in and it should open
Repeat as much as need to decide valve is NOT moving or it IS moving

These can be cleaned, read here: https://www.explorerforum.com/forums/index.php?threads/iac-valve-cleaning-thread-w-pics.84220/

Cleaning doesn't always get them working, but can, and its free to try

 

[jklolkik]

What i mean by the idle screw is the screw thats on the throttle linkage that moves the throttle plate

 

[RonD]

Yes, that is called the anti-diesel screw, prevents "run-on", is also used to adjust throttle position sensor(TPS) voltage as needed.

Never used for idle since there are no "Jets" to set air/fuel ratio with fuel injection, the computer MUST set the idle so it can adjust air/fuel ratios accordingly, thats why all car makers use an IAC Valve for setting idle on fuel injected engines

Once you find the issue with high idle and fix it, you will need to reset the screw
After engine is fully warmed up
Unplug the IAC Valve, it will close and RPMs should drop to about 500RPMs, if not turn the screw out until engine is barely running
And your done
Plug IAC valve back in and RPMs should go up to YOUR YEAR'S engine model's "target idle"
1993 with manual trans should idle warm at about 625-650rpms
Automatic trans, 750-800rpms

Cold idle should be 1,100rpms or so, colder it is the higher the RPMs
Computer does that using IAC Valve and ECT(engine coolant temp) sensor

 

[jklolkik]

Ok thanks. Once check this and correct it should it fix my misfire issue with the o2 sensor?

 

[RonD]

Most likely

When you unplug a sensor, like the O2, the computer runs engine Richer by default since Lean leads to engine damage

 

[jklolkik]

I just cleaned it and it does indeed idle lower now. But at alittle less than a thousand. I have not reset the screw yet. Will have to wait until after work tomorrow. Will also test if it fixed the misfire with the o2 tomorrow as well