A week or so ago I posted about having swapped out my O2 sensors and how my mileage seems to have actually gone down since then. Somebody responded that I could potentially have a vacuum leak even though I am not throwing any codes, causing the engine to dump more fuel.

My idle RPMs are right around 750-800. Is this higher than normal? It seems logical to me that if there is more air being dumped into the engine via a vacuum leak, and more fuel being dumped in to compensate for this, that my idle would be much higher than if everything was as it should be. Is this correct?

 

Not necessarily - the IAC would just close down more.

 

But, if the IAC closed down more, wouldn't that allow LESS air into the motor, and less fuel would be put into the motor as well? It seems to me that it takes a certain amount of air and a certain amount of fuel to get 750 RPMs. Whether the air comes via the airbox, through the IAC, or whether it comes through a vacuum leak, the same amount of air would go into the motor, and thus the same amount of fuel. I guess it just doesn't make sense that there could be more air, and thus more fuel, going into the motor, and still be sitting at 750 RPMs.

I may be totally off here. Logically I guess more air plus more fuel has to equal more RPMs to me. What am I missing?

For those of you that have the famous PCV elbow go bad - did your RPMs sit much higher when it leaked? Did they drop after you replaced the elbow?

 

A vacuum leak will generally correspond with a rougher idle, and then depending on what it is, it can change the idle speed.

 

How tight is your exhuast? Any leaks at the engine to manifold or around the O2 will make it dump fuel.

Quite offten you end up making a leak trying to wrench out the old O2's

44

 

I think I'm the one that posted about the vacuum leaks, I did not have any difference in the idle. What I did notice was the tranny shifting at higher rpms and from take off it felt like I had a 2800 stall converter. I had two leaks in lines and the notorious elbow, plus one splitter jumbled in with the heater hoses that was collapsing under vacuum. I had no related codes or CEL, when Ford ran a driving diag with a computer hooked up the were reading a lean condition on both banks followed by the comp over compensating with additional fuel. The colder it was the worse it got.

 

I've also read here an easy test is to just spray starting fluid near the lines, intake gaskets, elbows etc., if the RPMs go up you have a vacuum leak.

 

That's true ^^^ I had one leak on me w/new sensors. Lucky for me it was a rear 02 Bung. I removed the sensor , cleaned the anti-seize off , applied HH RTV W/Copper to the threads and around the top and it's not leaking so far. I plan on doing the exhaust a different later, so I hope it holds until then.

But , yea , it's possible. It's hard to tell sometimes what's leaking on an exhaust system. The only way I could is with the exhaust cool, engine off -apply 10lbs of air to system through the tailpipe. - That's plenty to give away any leaks you might have.

Good Luck

 

stuff a rag in the tail pipe and feel around the pipe conections hold your hand about 1 inch away if you feel a puff you found your leak. You could also use a smoke generator but there $5

44

 

jbrew - how did you put 10 lbs up the pipe? I assume someone else did as you listened for leaks up by the motor?

 

No - did it myself , not hard to do. I thought for sure I had a picture of that somewhere - can't find it ? I have to many dang pics - can't find anything !

Well, anyway - I use a snorkel adapter with insert I made and plug an adjustable air chuck into it and regulate it to 10lbs and let her fly. The I get under the truck with a squirt bottle that contains air leak detection solution (90% water/10% Dish Soap) and juice up the joints and watch for bubbles.

You don't really need the special fittings . A compressor and adjustable air tool or shut off valve of some sort and just a little pressure. You have to regulate it somehow, shove it in the tail pipe along with a bunch of rags (if that's all you have to work with) to hold it in the pipe while you check for leakage.

I've also heard of people using a shop vac in blower mode to do the same thing ..

But yea, it will show you every little leak in the system. Don't worry if there real small leaks , those usually seal themselves with a little rise in temp.. When you actually do it for yourself , you will definitely know what ones need to be fixed.

 

Thanks jbrew. I'll give it a shot.

Another question - would these types of exhaust leaks be similar to a vaccuum leak? Seems like the intake "sucks" and creates vaccuum leaks, but the exhaust "blows" and wouldn't have any extra air coming in. If anything, it would just have air going out, maybe even before it even got to the O2 sensor. Wouldn't this cause the engine to dump less fuel in?

Maybe I am making this too complicated and need to just start checking for problems. All this fuel/air stuff is interesting to me though.

 

An exhaust leak can act like a syphon and draw air in from the outside.

JMC