My son bout his first truck yesterday. 98 ranger sc 4x4 auto 4.0. Beautifull shape 139000 miles. got it cheap as it has issues. its been parked for a year. It has a surging idle when you start it up. rev it to 2500-3000 rpm and it idles fine turn it off and restart it surges again untill you rev it. When we test drove it it ran like it was running out of gas/bad fuel pump, sputter at full throttel but accelorate at part throttel. Gauge said under 1/4 tank of gas. We put 3 gallons of gas in it when we got home and it seems to run fine now other than the idle surge. it had a 174 code(lean bank) but if it was running out of gas it should pull a lean code right? Im thinking IAC or intake leak. Thanks for any help. We have 8 months till my son get his liscence so we thought a project truck would be fun and save him some hard earned cash.. Thanks Eckart

 

My 1997 Ranger had/has a big idle; 2500 RPM at start up drops to 1200 after 5 minutes, hit the gas even slightly and shoots up to 2500 and hovers there for about 3 seconds then drops back.Turned out it was/is a whole slew of problems. Replaced the IAC then the Throttle Sensor. The RPM went nicely down to 950 after the TPS replacement but still idling high (lower intake gaskets are shot proof is on the driveway )

So saying that test the IAC with a multimeter. Put it on the resistance scale (200 or 2K) and with the connector disconnected probe the IAC's prongs. A good read is anywhere from 6 - 10 Ohms.

If the IAC checks out then do the same to the TPS. Put the meter's probes on the 1st and 3rd prongs of the TPS and watch for increased resistance as you move the throttle toward WOT. If the resistance does not change at from 0 throttle to WOT the TPS is most likely bad.

Now onto the 174 code. What is the fuel pressure? The normal rage is 35-45psi. Anything below or above that will cause havoc with the air to fuel ratio. So check the pressure if you can. Also it might be an old code. So do the IAC and TPS tests and repairs (if required) first then clear the code and take it for a short drive to see if it reappears.

 

Thanks I will check the tps. I checked fuel presure this morning and it was low.30 lbs and droped to 28 if reved up a bit. spec is 58-72 for a 98. orderd a new fuel sender unit. be here Wednesday.

 

Yes i don't know why EBSCO is telling me it's 35-45psi but you are correct. The true number you want to shoot for 65psi at all times.

The 98 has a returnless fuel system so it operates a bit differently than a conventional system. Instead of using a spring-loaded vacuum diaphragm in the regulator to change fuel delivery when throttle opening and intake vacuum change, the regulator in a returnless system operates at a constant pressure. The older return-type systems need to vary fuel pressure to maintain the same pressure differential across the injectors when intake vacuum drops. When vacuum drops, the regulator increases pressure to compensate. But in a returnless system, this isn't necessary because the line pressure is always the same. So that being said the pressure should be at a constant 65psi - give or take a couple pounds - during idle AND acceleration. The pressure should never increase or decrease if all is working properly.

Also, the regulator in a return-less fuel system is integrated into the sending unit where as on conventional/return-type systems it is integrated into the fuel rails. So the new sending unit SHOULD resolve your low pressure problem but if it doesn't then we're looking at a bigger problem.

 

What else could it be?

 

The idle surge could be many things:

IAC
TPS
Vacuum Leaks
Intake Manifold Leak
Idle Screw is set wrong
PCV and/or PCV Elbow


Check ALL vacuum hoses and connections. A big one to check is the brake booster line.

Again many things can cause low fuel pressure:

Bad fuel pump
Bad pressure regulator
Clogged fuel filter
Pinched fuel line
Low supply voltage to either the fuel pump or the pressure regulator
Poor grounding

Since you have already changed out the filter check for any kinked supply lines and also check the voltage at KOEO. Make sure the ground is good as well. The voltage should be around 5 volts or so (maybe even 12 depends on the system). If all is fine then i suspect he sending unit to be the culprit.

 

Ive got the surge calmed down alto by replacing the plugs they had almost 90 tho gap. I was woundering what else could the low presure be. swaping out the sender units gotta fix that.

 

Ahh that would definitely help. Not sure why that never crossed my mind... All well having it under control is all that matters.

If the voltage supply to the sending unit is fine as well as the resistance in the wires feeding it and there are no kinked supply lines that would be by far the best best. Can't wait to here the results you get when you install the new unit.

Keep me updated

 

new fuel sender unit is in and runs like a dream. no surge what so ever. Thanks for the help.

 

You're very welcome. Glad you you got things sorted out