Hi
question, 98 Ford ranger 2.5L,250k miles,standard,no cruise control and truck runs great! going down the road and depressing clutch the idle will hold at THAT RPM,until the vehicle stops. and until then the RPMs return to normal. It throws a P1506 code.so I switched out the IAC valve w no effect. I have changed out the PCV and clean the throttle body, MAF sensor and checked throttle cable and vacuum lines,connectors/wires.Now REmembeR it will hold the throttle at (example)3000 RPMs until the truck is stopped (and that could be for minutes providing the truck is rolling )before it returns to a normal rpm. Bizarre
thanks!
kevin

 

No, not bizarre, its an Emissions requirement on manual trans vehicles after 1995 or so

All engines have higher emissions under 1,000rpm
So idle RPMs are bad for emissions, which is why newer vehicles must have Auto OFF/ON when stopped now or they couldn't pass emissions testing

In a manual the RPMs should hold when clutch pedal is pushed down for shifting gears, that is normal
But if you hold clutch pedal down for say 7 seconds, RPMs should drop to about 1,200
RPMs will never drop below 1,000(to idle) until speedometer is under 5MPH


P1506 Idle Air Control Overspeed Error
This could be because RPMs are not dropping after 7 seconds, as you describe
Try this, put trans in Neutral while coasting and let clutch pedal up, see if the RPMs then drop to 1,200 or so
Clutch pedal has 3 switches inside, one tells computer when clutch pedal is UP so driver control RPM, clutch pedal DOWN means Computer controls RPM

Simple test for vacuum leak
After engine is warmed up and idling, unplug the 2 wire connector on IAC Valve
RPMs should drop to 600 or so, or engine may even stall, either is GOOD, it means no vacuum leaks
If idle doesn't drop then IAC Valve/computer can not control low idle, which will set code P1506, you have a vacuum leak or throttle linkage issue


One thing about fuel injection
Its main MPG savings is that it can SHUT OFF fuel injectors when coasting, and it does
So if you are coasting downhill leave trans in gear, clutch pedal up, and foot off the gas pedal
This will turn off the fuel injectors as long as the RPMs are above 1,500 while coasting
0 fuel use
Same when slowing down to stop, coasting in Neutral or with clutch pedal down mean you are wasting fuel

In the old days with carburetors shifting to neutral while coasting saved fuel, just the opposite with fuel injection

 

Thanks for replying Ron, my issue with the high idle just popped up and remember it was throwing me an idle code. I found out through tinkering and YouTube that it was my throttle position was out of whack I adjusted to .94dcv from 1.01dcv and took it for a ride and back to normal, however, you are correct about the ECM holding up the idle just not at 3k rpm’s for extended period of time which was my case just as long as the ranger coasted down the hill or down the highway w the clutch in.
just a note, I did find a few bad vacuum tubings throughout the engine bay, but nothing that contributed to my problem, Iac,pcv and heater control valve above exh manifold where replaced and one of the iac valves I ordered from eBay was defective lol which didn’t help my diagnosis.also all 3 of my iac valves I worked with where NORMALLY OPEN, not normally closed as some say on the forums fyi
Sorry ron just read ur message completely and ur right. Thankyou
kev