Hi,

'89 Ranger w/ 5spd manual and 4X2 with 186K miles

The truck fires up within a few seconds of cranking but seems to be idling around 1500 RPM's with no throttle input and the idle screw on the throttle assembly not even touching the plate.
If you push on the brake it will rev higher but if you hold the brake for around a minute or two (like you would sitting at a light) the engine will idle down and then stall.

The "Check Engine" light is on but its throwing a code "MAF circuit below minimum voltage" but it doesn't have one.

TPS voltage with key on engine off is 1.0-1.1V.

I have sprayed the intake rubber tube, throttle body, intake and brake booster with carb clean to see if there is a vacuum leak but the rpm's don't change (up or down).

I have cleaned the IAC and EGR valves while researching the issue but again no change.

Any ideas?

 

Welcome to the forum

Just FYI, No idle screws on fuel injected engines, it "looks like" an idle screw but it is called anti-diesel adjustment, it can be used to lower or increase TPS voltage and set 500 RPM idle if needed.

Pull off the IAC valve but hook its wires back up
Turn key on
IAC should be open all the way now
Pull off the wires
IAC Valve should close all the way
Plug wires back in and it should open all the way
Repeat a few times to make sure it isn't sticking

If there is no movement check that you have 12v and ground at the two wires(key on), if so replace IAC Valve
IAC valve is just a 12v solenoid, pulls the bar(with valve attached) in with 12v, spring inside will push bar back out with 0v, computer varies the GROUND side(pulses it) to set 4 to 9 volts to set idle RPMs by letting more or less air in
Bad solenoid or bad spring could cause no or limited movement


TPS voltage(center wire) with throttle closed should be under 1volt, .69-.99 is spec
Yours is not that far off so is probably OK but see if you can loosen and turn TPS a bit to lower it
Wide Open Throttle(WOT) should be 4.5v or higher, 4.50-4.99 is spec
TPS doesn't effect idle, it's just a "heads up" to computer so engine has better throttle response, and better MPG.
In the carb days there was an accelerator pump inside carb, when you pressed on gas pedal extra gas was squirted in instantly, giving time for Jets to catch up to demand as air flow increased, so quick throttle response.

With fuel injection the TPS does that job, MAP or MAF sensors will have a slight delay in reporting higher air flow, so delayed throttle response without TPS


Start engine and let it warm up
Once at operating temp unplug IAC Valve
RPMs should drop to 500 or engine may even stall either is OK

If idle doesn't drop then start unplug vacuum hose from intake and plugging port with your finger
Remove, test, and replace 1 hose at a time
In your case start with power brake hose

Check vacuum hose on MAP sensor, they can get hard and brittle, replace if not air tight
MAP sensor is usually on firewall and has 3 wire connector and 1 vacuum hose to intake

 

Thanks RonD.

Checked voltage and ground at IAC. Both are good. Battery voltage on one wire and ground on the other.
Turned ignition key on, connected and disconnected IAC with engine off - It's not moving. Removed IAC from engine, then proceeded to supply voltage and ground to IAC and it tripped the circuit breaker in my powerprobe in both directions without moving. Going to get a new one and hope this solves the issue.

Thanks again.