Leaking IAC and EGR Valves

[betard_foosier]

Hello all, my 98 4.0ohv with 148xxx miles has exhibited symptoms of a vacuum leak since I bought it, LTFT's both at +25 at idle, falling as engine load increases to 0 at WOT. Classic vacuum leak symptoms. I did upper and lower intake gaskets this summer, with injector orings as well. Still have the high fuel trims and slightly shaky idle. Not terrible, but I can feel it miss here and there.

I decided to smoke test the intake yesterday, and saw smoke from 2 areas; the little breather on my IAC valve, which is only a month old, and under the EGR valve, looks the be from the little diaphragm holes. I definitely have a leak, even when I plug the intake and IAC breather, I am able to suck air in through the vacuum like, sounds like its leaking in either through the EGR valve or somewhere near it. My question is, are these 2 valves supposed to leak?

If I cover the IAC breather while truck is idling, the idle doesn't really change, but the IAC starts vibrating and makes a faint hooting sound. The reason I replaced IAC was the old one was hooting very loudly.

 

[RonD]

The IAC valve connects two openings in the throttle body that are on opposite sides of the throttle plate.
The IAC's step motor slides a valve open or closed allowing more or less air to bypass the throttle plate.
It also has a vent(breather) on the IAC's body to equalize pressure, that vent could have some smoke come out in a smoke test, idle wouldn't change if you cover this, "idle air" is coming from the MAF side of the throttle plate not the breather.

EGR valve should be tight, no smoke.
I would suck on the vacuum hose, or use a vacuum pump, to see if it can hold a vacuum.
And more puzzling is that the vacuum line to the EGR valve should be closed with engine off, so no smoke should have got there via the vacuum line, smoke would have to come from the intake opening for the EGR valve.

Vacuum leaks can be tough to track down, I often cap openings/lines one at a time.

 

[betard_foosier]

I think the vacuum leak has to be through the egr outlet back into the intake, because one by one I plugged every other vacuum port on the intake plenum and my +25 LTFT never changed and the idle stayed shaky. No smoke came out through the manifold gaskets, and I had the TB closed off. Where else could vacuum be leaking?

 

[RonD]

Well any "extra air" means it is not passing thru the MAF, so from MAF to intake there could be a possible ported vacuum leak which is often over looked or ignored.
Also MAF should be cleaned, it could be performing poorly at lower flow.

Throttle body gasket leak is often hard to find.

I would be tempted to do a reset and see if any changes occurred.
LTFT = Long Term Fuel Trims for those that didn't know
These numbers are what the computer "thinks" they need to be for best operation, so they are learned from computer monitoring O2 sensors over time.

There was an older trick to find leaks.
Use a shop vac with engine off to create a vacuum in the system, then listen.
Often putting the shop vac elsewhere, so less noise, is best.
I have used a garden hose and duct tape to put the shop vac inside or outside so i could hear better.

 

[betard_foosier]

I cant tell if smoke is getting to egr through intake pipe or vacuum line. Im going to pull the egr off and try to suck air through it to see if the pintle is leaking. Would an egr thats not closing at idle cause a lean condition, even though the air coming from the exhaust is oxygen-poor?

 

[betard_foosier]

I did cap each vacuum outlet from the intake one at a time as the engine was idling, and the idle and fuel map didnt change. That means its not leaking through the vacuum side of the egr, it would be the exhaust inlet to the intake. I could hear a leak in that vicinity as i sucked on a vacuum hose, but couldnt tell for certain whether it was the egr.

 

[cp2295]

Yeah iac has a controlled vacuum leak, so should be leaking. But the egr shouldn't, probably the issue

 

[cp2295]

And it would cause it to be lean because it's getting too much air than what the computer knows the truck should be getting under those conditions.. Even though it's not rich in o2 it still has some and so it can cause it to run lean

 

[cp2295]

There was an older trick to find leaks.
Use a shop vac with engine off to create a vacuum in the system, then listen.
Often putting the shop vac elsewhere, so less noise, is best.
I have used a garden hose and duct tape to put the shop vac inside or outside so i could hear better.

I'm going to have to try that next time I look for vac leaks

 

[betard_foosier]

I haven't had a chance to pull the egr yet, but I don't know what else it would be. Wasn't getting any symptoms when I sprayed carb cleaner or propane around the intake, and I blocked off all vacuum lines leaving intake. Still has shaky idle and high LTFT. Could it be anything else, like a valve or compression issue? One thing I forgot to mention, I get terrible pinging from 2-3500 rpm from about 45 - 65% throttle, in any gear.

 

[adsm08]

Didn't read the whole thing. I just want to point out that small amounts of smoke from the areas you noted are normal. They are the breathers for those spots.

Start by unplugging the IAC with the engine running. If it stalls you are not pulling fresh air from anywhere and might have a computer issue.

 

[betard_foosier]

Will do, I appreciate you taking interest in my case. I'll let you know what I find!

 

[betard_foosier]

Pulled the IAC while idling, the truck stumbled out and died. The strange thing is, sometimes it feels doggy and underpowered, and sometimes it runs like a scalded dog with very little throttle input. Fuel trims are consistent regardless of how truck is driving, always positive at idle.

 

[elmonero]

Did you ever find out what was wrong with your truck?