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IAC leak smoke test


CAW

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I have been having some trouble codes in cold weather p0171 and p0174. I know these codes are indicative of a lean condition. I decided to do some troubleshooting to find any possible vacuum leaks. So I built a smoke leak test machine out of a paint can, a glow plug and an air compressor. It works pretty well. There was smoke from the egr valve and the IAC valve. I got a new IAC valve and replaced the old one. I did another smoke test to see if there were anymore leaks. However, smoke poured out of the iac valves vent/breather/ cap. I did some internet research and found a few forum posts that said smoke from the IAC vent was normal during a pressurized smoke test. There were also several forum posts that said any escaping smoke indicated a vacuum leak and a defective valve. I have no idea what to believe. What do you guys think?

I made sure that I didn't over pressurize the intake components and damage them. the air compressor regulator was set as low as I could get it and still have any flow, only a couple psi. The new iac is a BWD brand. 1997 4.0 OHV engine.
 


tmcalavy

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Don't know the ins and outs of smoke tests, all that really tells you I think that a valve is open...so you see the smoke. The computer tells stuff whether to open or close, based on inputs from sensors, engine temp, etc. If it was setting lean codes, sounds like an O2 sensor problem or a MAF problem. I'd start there and work forward to the intake...IAC do fail but they are a minor culprit...EGR and MAF and O2 sensors have been more likely culprits when I had problems with mine. And a vacuum leak will throw lots of stuff off...check your vacuum lines real careful. Mine had a big hole in the underside of the air intake tube, right back from the intake. Couldn't see it and couldn't feel it until I pulled the entire air intake hose and looked real good.
 

RonD

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IAC valve "vent" is there to equalize pressure allowing the valve to move easier, if you apply smoke to the intake it will come out of this "vent", that is normal.

EGR valve should not have smoke coming out
 

CAW

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Thanks for the replies! Curious why smoke would come out of the IAC vent? Is my understanding of the function of an IAC valve correct? I was under the impression that the function of the IAC was to bypass the throttle plate when it is closed, as opposed to allowing unmetered air in. This way all the air that enters the engine passes through the MAF and is thus measured. I don't see the reason why an IAC should allow outside air into the intake and bypass the MAF?

As for the EGR valve, it has been very problematic to remove. I believe I will need to replace the EGR pipe too since the valve and pipe nut is severely rusted together. I have been spraying penetrating oil on the exhaust side of the EGR pipe retaining nut. Hopefully it will be less stuck than the valve nut.
 

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