Intermittent Rough Idle

Rustbucket350 · Jan 3, 2017

So the 4.0 was running great til today when I hit a stoplight and the idle started bouncing around. Fuel pressure tested around 35 psi and it has a new ignition coil and injectors that I cleaned and rebuilt myself. I know the internals are good because I rebuilt the entire motor. I disconnected the return line and blew the gas up in the air to clean out anything inside the regulator (new trick I invented) and unhooked the battery for a few minutes to reset the computer. Started it up and it ran awesome for a 3 mile trip and until I got back. What else could cause a low, surging idle?

RonD · Jan 4, 2017

IAC valve may need to be cleaned.
IAC(idle air control) valve is the only idle control for the engine, when engine is cold the computer opens it more for higher idle.
Then closes it more for warm engine idle, where the valve spends most of its time, when engine is running.
So that can be the "sticky" part, after warm up, if valve sticks computer tries to open it or close it more, when it releases/unsticks, idle jumps up or down and computer again tries to adjust it.

BUT..........when you unhook the battery you clear the KAM(keep alive memory) in the computer, IAC valve control setting is one of the things cleared, so computer has to relearn warm engine idle control, this takes 2 or 3 drive cycles, so "hunting idle" is normal for a few days after battery disconnect, but these would be small swings in RPMs, 100rpm or so, not big swings, that could be sticky IAC valve.

Also check TPS(throttle position sensor).
Top wire will have 5 volts from computer
Center wire should have .69 to .99 volts with throttle closed
Use a sewing pin to pierce the wires to test voltage with key on engine off

There is a screw on the throttle linkage, it "looks like" an idle screw, it isn't, it is an anti-diesel screw, but can also be used to adjust TPS closed throttle voltage so it is below 1 volt.
Or it can be used to set "stall idle"
With engine warmed up and idling, unplug the 2 wires on the IAC Valve, RPMs should drop to 500, or engine may stall, either is good it means no vacuum leaks.
But you may want to set anti-diesel screw so it drops to 500 and doesn't stall, which is fine but make sure TPS center wire voltage stays below 1 volt.

Rustbucket350 · Jan 4, 2017

Thanks, I'll check those out. I actually took the actuator off the IAC and tried the one from my 2.9 with no change. The plunger moves freely so I think I can rule that out. Now, I do have a delay between when I shut it off and when it actually stops running so I might need to set the throttle body screw but it hadn't been a problem until yesterday. It was a junkyard throttle body so maybe somebody played with it at one point or the TPS is on its way out.

Rustbucket350 · Jan 4, 2017

Tps reads 5 volts power and 1.035 signal at the lowest setting. The voltage increases smoothly with the throttle. If I back the screw out anymore it no longer touches the stop. I may try another. I should add this is a newer style (95) throttle body and tps and I spliced the newer connector into my 92 harness.

RonD · Jan 4, 2017

TPS often has slotted holes, loosen and turn it to get voltage below 1volt

Rustbucket350 · Jan 4, 2017

I'll try that. Even if it's not adjustable I can certainly make it adjustable by slotting the holes. I need to get a tach for it so I can really see where I'm at.

Rustbucket350 · Jan 6, 2017

Aha solved that one too. It turns out I didn't realize the evap tube on the throttle body was a vacuum line. It was idling at 1700 rpms which explains the gas mileage. Plugged that and the idle dropped to about 1000 but we'll see where it ends up after the computer recalibrates.

RonD · Jan 7, 2017

Warm engine idle should be 625-650 for manual trans
700-750 automatic

After warm up, leave engine idling and unplug IAC valve's wires, it will close all the way and RPMs should drop to 500, engine may even stall, either is OK, it means no other vacuum leaks.
If idle stays above 800 then you do have another leak, or the anti-diesel screw is adjusted incorrectly

4.0l OHV has a known issue with lower intake bolts loosening up, they are only 15ft/lb torque, lock-tite should be used on reassembly, but checking them once in awhile is OK to.

Rustbucket350 · Mar 28, 2017

I figured I'd post a followup to this. It turns out I had a bad brake booster so I did have another vacuum leak. The lower heater hose was also leaking onto the crank sensor wires, which were exposed. For good measure, I replaced the booster, heater hose, crank sensor, coolant temp sensor, MAF sensor, fixed my exhaust leaks, cleaned every sensor connector with electronics cleaner, and retaped the crank sensor wires. Idle speed dropped to 650 warm (it is a manual trans) and it runs way smoother. Gas mileage went from 10 mpg to around 20. The idle still fluctuates a hair so I'm going to replace the IAC and clock the TPS to the proper voltage.

cbxer55 · Mar 28, 2017

On my 3.0, I've gone through more IAC's than I care to think about. Finally bought a Hitachi from Vato Zone, it's their PREMIUM IAC. Costs twice what their normal made in Mejico piece costs, but I've not had another IAC problem in over a year now. Normally their cheap part needs to be replaced almost once a year.

I get an occasional rough idle on start up, not every time though. I've had a lot of problems with plugged injectors with this truck, don't know why that is. I run premium gas due to mods, and since I've started adding E85 to the tank, at a 10% ration, I've had no more problems. Matter of fact, it runs better with the E85 in the tank, than it ever did before. Really hums now. Seems to have an appetite for corn alcohol, just as I do.

Rustbucket350 · Mar 30, 2017

Haha it was the fuel pumps the entire time. More specifically, the in tank pump never worked. This is an 86 Bronco II with the double pump deal. I had put a new frame pump in and it was really loud but I figured that was because it was new. Nope, it was begging for mercy even though the pressure tested fine. Replaced both today (used a forklift for the tank... awesome) and the idle speed went to a rock solid 700. No shimmy, no shake, no complaints.

Intermittent Rough Idle

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