I have a 97 ranger that has a vacuum leak. I bought one of those paint can smoke machines off Ebay and the only continuous leak was coming from the top metal circular cap like the one top center in the pic below of the idle control valve.

http://shop.advanceautoparts.com/wcs...4_pri_larg.jpg

Can this be the cause of my leak? The idle doesn't struggle too bad, as the STFT is in the 8-14 range. I am unsure because I don't know if the ICV takes air from the outside through that metal cap.

Any help is much appreciated. I already ordered a new one since they are only $25, just wondering if I should continue to look elsewhere. There is a wisp of smoke coming from some the EGR just to the right of the ICV (though not from the gasket, but instead just underneath the saucer looking thing) and a similar wisps worth from the dip stick. But the ICV is leaking like a sieve and seems the more likely source of the engine lights worth of leaking.

 

Welcome to the forum


The IAC Valve gasket can but not the cap you are referring to

Read here first: How to: IAC valve cleaning thread (w/pics) | Ford Explorer and Ford Ranger Forums - Serious Explorations

That cap and air passage is compensated for by the computer, same as PCV Valve air is.

Best test for vacuum leak is warming up the engine and then let it idle
Unplug the two wire connector on IAC Valve, it will close all the way
RPMs should drop to 500, or engine may even stall, either is good it means no vacuum leak.

+8 to +14 STFT is high for cruising, but not for idling.
Just FYI, idle is always Rich(+ trims) it prevents engine overheating, O2 sensors are ignored at idle

Other causes for high STFT are dirty MAF sensor(under reports air flow), and lower than normal fuel pressure, 1997 computer expect 35psi fuel pressure, it it is 20psi then computer has to increase STFT to get more fuel into the intake.
Air leak from MAF sensor to intake is the same as vacuum leak but won't show up on smoke or above idle test.
Faulty PCV Valve that is not closing enough at idle and lower RPMs sucks in more air than computer plans on

 

Wow, thanks for all the information. I went ahead and changed it (truck has 200k miles) since I doubt it has ever been, and also put in a new PCV valve since it was cheap and easy.

I guess I just don't understand STFT and LTFT then. Everything I have read before your post suggested it should be near zero at idle/after reaching temp. Since I drive a standard, when downshifting while slowing down it might momentarily hit a high STFT due to compression but not to worry about that.

I have an app that I can watch the STFT and LTFT while driving, and just cruising down the road at a steady speed the STFT will bounce between -3 - +3 and the LTFT will be ~ +3-5. When I hit a red light and am stopped the LTFT will almost immediately jump to 25 and the STFT will slowly climb to about 14-18 and then walk itself down to 8-10.

I did another smoke test, and there isn't any visible leaks. Not even from the dip stick (guess the new PCV fixed that). And the new IAC valve shut off the vehicle when I pulled the plug from it (it struggled for about 15-20 secs then shut off).

I have a new fuel pump and filter, and this engine light predates their installation. So, the pressure should be fine, or at least not due to the pump or filter. I have to borrow a fuel pressure kit from Autozone this weekend to be sure. I looked at the MAF sensor, and both were nice and silver/metalic... I guess I am doing the smoke test wrong, as I just saran wrapped the air filter intake part, so any leak between the MAF and rest of the intake should have been visible.

It idles really nice now (it does sound better than yesterday) but the only other sign that might help in diagnosing is, when revved really high, upon releasing the throttle, it will make a popping sound. Not metallic, more like a balloon popping or almost like a backfire, but not... not sure.

Could it be a fuel injector? If so, would sea foam or something similar work, or would I need to remove them (youtube shows it to not be that complicated) and create a vacuum contraption to push carb cleaner through it?

Thanks again for the help, and the welcome. I have had this truck for near a decade, so I have been lurking this forum for years. Finally ran into a question (IAC leaking smoke) that I couldn't readily find the answer to without asking myself.

 

Computer is setup for the engine size, in this case it "knows" it is running a 2.3 LITER engine.
So this engine will suck in 2.3 liters of air at wide open throttle every 2 RPMs(4 strokes)

And it has a air/fuel mix table based on the above.

It also "knows" how much fuel will flow from the stock injectors based on 35psi fuel pressure.

Air/fuel mix is done by WEIGHT, not volume
14.7:1 air:fuel is a WEIGHT ratio
If 14.7 POUNDS of air come in then computer will add 1 POUND of gasoline
14.7 grams of air then 1 gram of gasoline

You can see gr/sec(grams per second) if you select that on your OBD2 reader for MAF sensor display, and at warm idle MAF gr/sec should be engine displacement, so about 2.3 in your case, but it is based on idle RPMs so can be a bit higher

The above is why "pre-vaporizers" or 200MPG carbs are bogus, lol, doesn't matter how well you mix or vaporize gasoline, you still need a gram of gasoline WEIGHT for every 14.7 grams of air, Laws of Physics are not negotiable, as most of us know by our experiences with Gravity..........ouch

But a good mix is important to maximize a good burn in a cylinder, so clean injector tips are a benefit.
I run a can of Seafoam in the gas tank once a year on all my fuel injected engines, this helps clean injector tips.

OK, to fuel trims
Computer uses the factory air/fuel tables until O2 sensors are warmed up
O2 sensors don't work until they get up to 600degF, very hot, this is why they have heaters built in, to get them up above 600degF as soon as possible.

So cold engine is running in what is called Open Loop, open because computer has no feedback from O2 sensors about rich/lean burn.
After O2s get hot enough computer will switch to Closed Loop, closed because it now has feedback and can adjust fuel trims on the fly without factory tables

Factory tables are on the Rich side, because lean side has such a BIG down side, engine damage from pinging and overheating, pistons literally melt.

So O2s are working
Computer uses MAF sensor to get the WEIGHT of the incoming air, it also has the Air Temp sensor(IAT) to fine tune that WEIGHT, "hot air rises", that's because it is lighter than cold air.............a weight difference, so computer has to adjust for that.
Altitude above sea level is also a factor, less oxygen in the air, so lighter air, MAF compensates for that in the way it works(look up hot wire MAF).

Computer uses the above to get the weight of the incoming air, and then it can calculate the weight of the gasoline it needs to add.
That calculation = 0 on the STFT display

STFT + or - numbers are TIME numbers, the computer calculates the 0 fuel trim and then opens the injectors for, say 10 milliseconds, it then looks at O2 sensor voltage to see if the burn was Lean or Rich(oxygen level, too much oxygen is lean, too little rich)
Computer then adds more open time, 11ms, or less open time 9ms and checks O2 sensors again.
This happens very very fast, engine is never running Lean or Rich, the code are to let driver know that the computers calculations(0) are getting too far off so there is a problem.
Usually -15 to +15 won't set a code but outside of that range for any length of time will

LTFT(long term fuel trim) is used so engine and system can age well, lol, a new engine won't have lower compression or fuel pressure, won't have small air leaks or sensor voltage a little off new voltage.
LTFT is used by the computer to offset the 0 calculations so if LTFT is +4, computer will off set it's 0 by +4 so new 0 is actually +4, which is very normal as engines gets older, it has nothing to do with MPG, there is usually more air or less fuel flowing so +4 on old engine would be exactly the same economy as 0 on new engine.


The "popping" noise on letting off on the gas pedal is a misfire on a cylinder and its unburned fuel is being ignited in the exhaust manifold by hot exhaust from a cylinder that did fire.

O2 sensors use a chemical reaction to detect Oxygen in the exhaust, Car batteries also use a chemical reaction to store electrical charge.
And as we all know car batteries wear out, lol, 5 to 7 years
Well O2 sensors also wear out, 100,000 miles is recommended change period, but engine problems can shorten their lives, i.e. rich conditions
O2 sensor tend to fail lean so cause computer to run engine Richer than it needs to be, so MPG starts to go down a bit, not much but it is costing you money.
So if in doubt replacing O2 sensor cost will pay for itself in MPG over the next 100k miles
This is the Upstream O2 sensor, closest to engine
Downstream O2, after Cat converter, will usually last much longer, it "sees" cleaner exhaust so chemicals don't wear out as fast

 

So I was able to get a fuel pressure check done. Key on engine off, the reading are fine... between 35-40 psi. But Engine on, the readings drop to 30 psi. Is that enough of a discrepancy? And if so, since the PSI is right with engine off, does that mean it isn't the fuel pump/filter (both are less than 8 mos old) but instead more likely the fuel regulator?

 

As long as fuel pressure stayed above 25psi off and running then all is well, Fuel Pressure Regulator(FPR) should be fine as well, pressure would drop to 0psi when engine was off if FPR was leaking/bad

 

Well, I am really baffled then. O2 upstream sensor is new, MAF is cleaned and both very silver/shiny, smoke test and removing IAC power suggest no vacuum leak, fuel pressure good, PVC and IAC replaced.

Sigh... so, that leaves clogged cat, leak somewhere between exhaust manifold and 02 sensor, leak in the air filter box itself, and clogged fuel injectors. Am I missing anything?

Thanks again for your time.

 

For an air leak to effect air/fuel mix it must be between MAF and intake valve in head.

An exhaust leak would need to be between exhaust valve in head and Upstream O2 sensor to effect O2 sensors data back to computer.

A $20 vacuum gauge can tell you ALOT about an engine's condition, even tell you if exhaust is partially blocked.
Good read here on testing engines using a vacuum gauge: Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine

A gasoline engine is still just a self power air pump, fuel injection, distributorless, or any other changes they make to the outside doesn't change a thing about "why and how it works".
And how would you check an air pump?
Pressure gauge, in this case a vacuum pressure gauge

 

Thanks RonD for great info:
My 2000 4L OHV with 62k miles (really) was running fine (no codes), but I wanted to do some routine maintenance. So I:

• Replaced plugs
• Replaced plugwires
• Cleaned MAF sensor

Since then it runs well, except that after it's well warmed up I get low idle that becomes rough, almost (but never actually) to the point of a stall.
I'm looking for advice on how to troubleshoot. Here's what I've already done:

• Disconnected the IAC valve while idling - it promptly stalled (as expected)
• Checked for vacuum leaks as best I could - not finding anything. (I was extra careful to examine where I'd been pulling plug wires, but didn't find any leak.)

Can you suggest an approach to diagnosing the problem? I've got a code reader w. phone app, basic tools and skills.

Any help appreciated!
John B.