I have an '89 F-150 w/ a 300 that has idle surge problems when its cold and warmed up. I have replaced the tank pump, filter, fuel pressure regulator, IAC, O2 sensor, and engine coolant temp sensor. Vacuum is solid at 18. Throttle body is clean. When the engine is warm and you start it, it will surge until you rev it a little and it start to clear up. I don't know what else to try and I don't want to sink any money into unnecessary parts.

 

How fast does the fuel pressure drop when the pump(s) are off? Kind of sounds like one or more injectors are leaking.

 

idk, that is one thing i haven't checked. I'll have to do that, check it while its running and while its off

 

haven't ran a fuel pressure test yet, but i scanned it at work and pulled the following codes:

KOEO: 87-Primary Fuel Pump Circuit Fault, 33-Insufficient EGR Flow, 67-PNP or Clutch Switch Circuit Failure

KOER: 12-Can't Control RPM During High RPM Check, 16-RPM Too Low To Perform O2S Test or ICM Fault/IDM Signal Missing

with this being said, can anyone determine some scenarios? the 33 code with the EGR flow i know is from me because i just disconnected and plugged off the vacuum from the egr.

 

Can't control RPM, RPM too low to perform... I'd be looking in the TPS and IAC areas... probably clean the electrical connections first, I've seen somewhere how to test those components using an ohm meter but I forget what site, I'm sure you could Google it and find out.

That fuel pump circuit failure would concern me, too, but the KOER tests have more significance in my mind.

 

i ran a fuel pressure check and pulled 50lbs solid. when i shut her off she drops to about 20lbs over a period of about 20 seconds and then it really doesn't drop much after that at all. It seems that the cold start idle surge is starting to happen less. Its more just a low idle (500rpms) until it warms up enough to rise to normal idle (750rpms). There's no cold idle circuit. When you first fire it up cold it won't idle high around 12-1300rpms and slowly fall down to normal idle. It just idles right at 500 until it warms up and idles up to normal idle at 750. Guys at work are telling me i might need another ECM for it, idk. I don't want to guess and I don't want to spend money on parts that I don't need. Any thoughts?

 

Well, that's good you have good fuel pressure, but the fast idle isn't working and the computer is telling you it's having a problem controlling the RPMs.

So, again, I'd be paying attention to the IAC and TPS - Idle Air Control (controls the idle speed) and Throttle Position Sensor - tells the computer where the throttle is at.

Could be as simple as the IAC needs to be removed & cleaned, could be corroded electrical connections between those two devices and the harness, could be broken or missing wiring, could be the parts have given up the ghost and need to be replaced.

Go investigate those things, clean the contacts, make sure your wiring is correct, try removing & cleaning the IAC as some people have apparently had luck doing that (I'm not one of those people, mine seem to burn out, they're just electric motors).

None of that will cost you much of anything (maybe a can of electrical contact cleaner, get the kind that's safe for plastics) and you just might end up fixing your idle problem.

 

the IAC is brand new, and even when i replaced it still did the same thing. I haven't replaced the TPS and i think might just do that. its only 25 bucks. i'll inspect the wiring and connections for the IAC as well. one more test i want to do as well is pinch off the return line for the fuel and run a key on engine off fuel pressure check. pinching off the return and checking the pressure will determine better of a injector leak or not

 

turns out my computer is bad. had a mechanic pull the upper intake because i needed to replace the valve cover gasket anyways, and he pulled the injector rails with the injectors in tact. he cycled the key, all injectors fired, sprayed nice, no leaky injectors. flow tested within 5lbs of each other. did another fuel pressure test, 60lbs with KOEO, slowly dropped to 50lbs over a course of 5min. TPS needed to be replaced, it had a chunk of plastic missing where the plug clips onto the TPS. pulled the computer, opened it up, 3-5 resistors were burnt. waiting for the computer to come to plug in and test.

 

computers in and runs perfect, all is well

 

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