91' 4.0 2WD w/ A.C.


So this thing has been idling high, 1400-1600 rpm, ever since i got it running. I'm not driving it yet cause i plan to get it in good shape before getting it DD capable. There is surely a vacuum leak, ive replace some lines but i have a smoke tester on the way along with intake gaskets, valve cover gaskets, TB gaskets, top end stuff except for the heads (waiting to do a compression test before i plan the heads). The plan was to compression test it today but my buddy backed out on me so i did some electrical snooping.

I've read quite a bit of info on here so far about the IAC, MAF, and TP sensors. The books i have are helpful but they're very lacking in the electrical information so I'll refer back to the wisdom of the wise ones in the forum. I believe my MAF and IAC are good but the TP, I think it is bad. So I checked those three today and here are the resulting numbers (hopefuly I did the tests right), I'll try to lay this out clearly. All the "numbers" following are Key On Engine Off (unless noted otherwise), with the multi-meter grounded at the battery but i did do some running performance tests with the idle varying between 1000 to 1600. There is no CEL until a sensor is disconnected and once it is reconnected the light will go away.

IAC:
R-12.2v
The RPM does change when its unplugged/plugged back in. I cleaned the valve and sensor last night before reinstalling them today, the gasket is still good.

MAF:
Key on / Engine On
R-12.23v / 14.51
B/w-.05v / .11
O/bl-5.90 / .11 (for this wire w/ sensor disconnected but engine on i read 5.98)
Bl/r-.06 / .92 (ditto & read .12)
This sensor looks spotless so i didn't mess with it. In order to fire up the truck i have to disconnect the MAF and let it warm up a bit and then i can reconnect it. It will fire up connected but the idle falls and it shuts off. If the vehicle is warm and i disconnect it the idle does change, up if i remember correctly.

TP:
Key on, sensor off of TB / Key on, on TB
Br/w-5.1 / 5.11
G/w-.88 / .94
G/r-.12 / .07
I removed the TB and cleaned it pretty well last night, the gasket was in decent shape for use until the new one arrives. Whether it is attached to the TB or not it, the voltage does change when it is rotated. With it connected and disconnected from the TB (funny story here for later maybe) i tested its WOT voltage, 4.65. So the range it covers is ~.94 to ~4.65 while attached to the TB and this range seems out of spec from what i have read. I initially tried to adjust it down to ~.70 thinking the AD screw had been messed with in the past, i was wrong. As i was adjusting it down i noticed the voltage stopped moving and then realized the screw wasn't even making contact. The RPM does change when it is unplugged.

The lowest i can get idle is 1000, any combination of the three sensors being attached or unattached, doesn't matter....the lowest it gets is 1000 so i know there is probably a leak in the intake. As far as the diagnostic work goes, does this sound like a faulty TP sensor, i'm happily buy a Motorcraft one but before i do i want to make sure.


Nothing important, just my notes sheet as i was going. That 4.63 on the TP....that was the funny story. WOT on Engine start because i had put the spacer in the wrong way, curiously enough it was revving over 3k before i shut it off

 

TPS voltage is/was correct, under 1 volt throttle closed(0.69 to 0.99), 4.5v or higher throttle at WOT(4.5 to 4.7)

TPS doesn't set idle, or control anything, it just gives computer a "heads up" when throttle is opened quickly so you get instant acceleration

After engine is warmed up, at least 4 minutes running, and idling, then unplug the IAC Valve, RPMs should drop to 500, barely running, or engine may even stall, either is good it means no vacuum leaks
If idle stays high then air is leaking in, or throttle's anti-diesel screw has been messed with

With IAC Valve still unplugged and idle high, unplug each vacuum hose on intake and plug that port with finger, start with the power brake booster and PCV hoses, the bigger ones

IAC Valve stuck partially open, its possible its not able to close all the way, so even unplugged its allowing too much air to pass into the intake, so not a vacuum leak per se, but an air leak none the less
Ford uses a true solenoid IAC Valve, so that's what the computer needs
Motorcraft or Hitachi brands are the only true solenoid IAC Valves
3rd party IAC Valves are solenoid/stepper valves, so can cause issues

 

So I was wrong, the TPS is working properly and within spec. I understand that it doesn't control idle but when it is detached and I turn it the idle goes up. There's no physical change to the throttle plate only the sensor itself. When unplugged while warm the IAC wasn't causing a change in idle when I went back over to check it again yesterday.

The gaskets are on order, all Fel Pro stuff, and I'll be sealing it up in about two weeks when i get the time to do it. The smoke tester will be here also so I'll give that a go before re-sealing the top.

 

The computer will add fuel and reset spark timing for acceleration when you move the TPS, so yes there should be a response with change in TPS voltage to computer

As for the IAC Valve, if you have adjusted the anti-diesel screw, then with IAC Valve still unplug and warm idle, unscrew it a bit to see if RPMs drop a bit, if so keep unscrewing until engine is barely running 500-600rpm
No vacuum leaks, plug back in IAC Valve, RPMs should go up to "target" RPMs set in computer

If adjusting the screw doesn't change idle reset it where it was, not your problem

 

An update:

A little cleaning and the IAC works just fine now, buildup around the seat I guess was keeping it from fully closing. It runs like a top now, didn't bother doing the smoke test. I'll have to do valve cover gaskets but I can't feel or hear any performance issues.

Found my vacuum leak, one boot on the octopus was loose and i couldnt see it from any other angle prior to yesterday when i did my compression test. Got that boot seated right, unfortunately I broke that line in the process of getting to #6 but I can fix that later, it's the small line that runs into the cab near the heater box (I believe) but I know where the break is. Using carb cleaner and brake parts cleaner I can't find any place that's sucking air.

The compression numbers seem pretty good to me but that #6.....I hate that one and so does my hand. 1 and 6 were the first to get tested and the time frame between 6 and 5(the third to be checked (1,6,5,4,2,3 fyi )) was drastic so I assume some of the variance is due to temp changed over the duration of the test cause the last 4 went quick.
​​1: 172
2: 185
3: 180
4: 190
5: 194
6: 174​

After fixing the vacuum leak the IAC unplug test does exactly as you described Ron, thank you for all the helpful input. It still runs rich and I'm betting that is because the o2 sensor needs replacing and maybe the cat. I've got a Flowmaster 50 series delta flow single in and out that i'll get installed before long.


Notes from doing comp. test

 

Good work