Wanted Engine Control Module (ECU) 1991 Ranger 4.0 MT

[Maui-sprint]

Still having an issue with high idle.
I checked for vacuum leaks all around the throttle body (including all hoses) with no luck. Doesn't appear to be any vacuum leaks.

Next, I went ahead and replaced the following with NEW PARTS: IAC, MAF, MAF pigtail, throttle position sensor, PCV valve, intake air temp sensor. Parts were replaced after motor had warmed up. RPM was checked after each component was replaced. AFTER replacing all parts engine speed dropped down to a SMOOTH 1000 RPM during idle. Ok, I can live with that. Mission accomplished. So I thought....

Next day take truck out for drive and it seemed to have a mind of its own. When I get to a stoplight and stop, initially engine speed is about 2000 then settles to about 1500 within 15 seconds, sometimes a little lower. Sometimes it doesn't drop. Has not hit the 1000 RPM mark since I did the replacement. When idling high, I disconnected the IAC harness and there was no change in idle.

Basically, Im not getting a consistent behavior. The idle speed is all over the place and has not reached the 1000 RPM mark since the day I did the replacement.

The only two things I have hooked up to vacuum are the brake booster and fuel pressure regulator. What else can I rule out before replacing the ECU which seems to be the next step?

 

[RonD]

Wouldn't be Computer if unplugging IAC Valve didn't drop the idle, that's the computers only RPM control

Engine off
Unplug and cap/block all vacuum ports on the intake manifold
Brake booster, PCV valve, FP regulator
Start engine, with IAC Valve connected
Unplug IAC Valve

If RPMs are still high then its possible throttle plate is open too far, or there is a vacuum leak
Its also possible computer is over fueling the engine at idle, but spark plugs should be a darker color

 

[Maui-sprint]

Engine off
Unplug and cap/block all vacuum ports on the intake manifold
Brake booster, PCV valve, FP regulator
Start engine, with IAC Valve connected
Unplug IAC Valve

I plugged every vacuum port on intake with no change in idle (1400 rpm) noted when IAC is plugged in or not. Throttle plate is completely closed during idle. Must be a vacuum leak, however, I cannot locate it. Also something to note: Truck has cruise control which I disconnected when I did my Auto Trans to Manual Trans conversion. Maybe this has something to do with my issue?

 

[RonD]

Cruise does use a vacuum line but if all the lines were unplugged then wouldn't matter

Not sure where to go, it has to be a pretty substantial leak for 1,400rpm

 

[Maui-sprint]

This weekend I plan to pull and inspect the plugs. Truck seems to be dieseling when going up hills.
Is it possible vacuum leak could be between upper and lower intakes? Lower intake and heads? Or would a smooth high idle only be caused by leaks upstream of intake gaskets?
I tried spraying some soapy water in these areas but I dont think the spray was able to get in there very well...

 

[RonD]

Pinging under load is usually a Lean mix
Clean MAF sensor
If it goes away with WOT(Wide Open Throttle) then change O2 sensors
If it gets worse at WOT then change fuel filter first, but could be the fuel pump


Computer ignores O2 sensor feedback at idle and WOT

Injectors need 30-40psi of fuel pressure at idle and at high load, if pressure is dropping at higher RPMs and Load then lean mix and pinging/knocking

Long shot is carbon build up in cylinders, it increases compression AND retains heat, so a double whammy that can cause pinging/knocking under a load(higher heat in cylinder)

 

[Maui-sprint]

Replaced MAF, MAF pigtail, O2 sensor, and fuel filter with NEW. Runs much smoother than before. I think the O2 sensor did the trick!
However, its idling even HIGHER now: 2000rpm when stopped at traffic light.

With IAC valve unplugged and idling in neutral, I am able to get it to ideal rpm (600-700) by COMPLETELY PLUGGING the IAC bypass port and slightly covering the little hole in the throttle plate with my fingers. There must be a pretty good vacuum leak getting by the gaskets somewhere (#1/#4 cylinders???). Remember that I already checked all the hoses and PCV.... Spraying soapy water or Gumout all around (especially between fuel rail and intakes) and zero changes to idle speed or overall smoothness of idle. I changed the intake gaskets (lower/upper) twice before when I replaced the heads about 3 years ago but then the truck sat for awhile. Maybe I used wrong or cheap gasket set? Now that I'm looking into replacing the intake gaskets only, I see multiple choices out there. Last time I replaced them I used the gaskets supplied in the head gasket kit. What should I be looking for when selecting intake gaskets to fix this leak?

 

[Maui-sprint]

Question: When the IAC valve is UNPLUGGED, is it supposed to close completely? because mine in not.

 

[RonD]

It should

Its a solenoid which means it has a spring that holds the valve closed when the coil is not magnetized
As power is added to the coil it pulls the metal valve rod back against the spring pressure trying to hold it closed

 

[Maui-sprint]

Ok, well IAC is brand new. It is NOT motorcraft or hitachi brand. Per description above Im getting a vacuum through the bypass when IAC is unplugged. Ive seen on other threads where you recommend a motorcraft or hitachi. I looked online and cannot locate a motorcraft IAC but I do see hitachi available. Im trying to eliminate all potential isssues before changing out the intake gaskets. Speaking of which, do you think my high idle could be caused by leaking LOWER gaskets? Uppers are easy to swap out compared to the lowers. Engine runs smooth. Not rough.

 

[RonD]

Plug the by-pass inlet with finger or ??, that will tell you if there is another unwanted air leak

 

[Northidahotrailblazer]

I fought with high idle issues. I resealed upper and lower intakes, swapped sensors. I got it down to 1000 rpms, decided to put another throttle body on it, and it got worse. i think it was 1400 -1500 rpms. Changed it out with one off a newer OHV i had. and Fixed idles at around 700 now. I fought the high idle for a long time. It was throttle body. I'm guessing it was sucking air some how? Idk I never looked into it more. Id check your throttle body out also.

 

[Maui-sprint]

Its driving me crazy trying to figure this out. Im reluctant to replace the gaskets since I just did them a few years ago when I did the head gasket but that was before I did the AT to MT conversion. Plus the fact that it seems to idle lower when engine is cold. than when hot. If it were the gaskets, I would expect it to idle lower when it gets hot, not higher. Whats the difference in the throttle bodies between the original one you had on and the OHV one? swapping in new upper intake gaskets is simple. I just really don't want to do the lowers only to find out i wasted my time.

 

[Northidahotrailblazer]

Its driving me crazy trying to figure this out. Im reluctant to replace the gaskets since I just did them a few years ago when I did the head gasket but that was before I did the AT to MT conversion. Plus the fact that it seems to idle lower when engine is cold. than when hot. If it were the gaskets, I would expect it to idle lower when it gets hot, not higher. Whats the difference in the throttle bodies between the original one you had on and the OHV one? swapping in new upper intake gaskets is simple. I just really don't want to do the lowers only to find out i wasted my time.

I'm guessing it was sucking air from the throttle shaft. I don't know, for sure. All I know is that was what finally got it. I did a motor swap, put a 96 4.0 in it, swapped upper and lower intakes, new gaskets and all that. It idled high before, but i wanted to try to fix it. Got rid of all the extra vacuum lines. I swapped in the tree from a 2.9 since it had 3 ports on it, less places to leak. I blocked off the brake booster, Ive heard when they fail they do funny things. did nothing, swapped upper intake mainfolds. swapped IAC valves, tps sensor. New intake temp sensor. Nothing. Thought it might be the throttle body gasket, so I changed it with a spare and my idle got worse. Pulled the one of the 96 intake and put it on and bam, Idle fixed.

 

[Maui-sprint]

Ive been focusing my leak tests on the manifold gaskets. Will give the throttle body a good going over and see if I can identify leakage there over the weekend. thanks!