I'm starting to think it'd be better if I just lied... Lets try it.


So I JUST finished putting on my headers and, ehh, not much of a difference. I'm going to take them off and leave them in the garage for no good reason

 

A vacuum leak w/ SD is the equivelant of opening the throttle plates. The MAP sensor senses the increase in air entering the manifold/plenum then adjusts the injector pulse width the make up for that air then the O2 sensor fine tunes the air/fuel ratio. This is why vacuum leaks with SD = high idle. With MAF rigs on the other hand, a vacuum leak is UNMETERED air. Any air that enters the motor that does not pass over the mass air flow sensor is air that the computer does not know about and the result is a lean mixture. The O2 can only do so much before it has added as much pulse width as it can...which really in the big picture isnt much...all it is really made for is the fine tuning of an already close to lambda mixture. The exception here is a vacuum leak on the UNDER SIDE of the lower intake manifold in which case the manifold is sucking in crank case vapors as opposed to air. This can cause the motor to stall out, idle roughly etc. depending on the size of the leak be it a SD rig or a MAF rig.

If you have enough of a vacuum leak and the system is a MAF, then all that unmetered air can cause the amount of power generated by each power stroke to drop significantly. This could result in a somewhat low and steady vacuum reading at idle, plus the fact that air is getting sucked in without an increase in rpm.

The MAP will sense the leak instantly then responds by adding fuel. Look at it this way.....at 2000 rpm a healthy engine will be at about 19-21 inches of mercury. Take a speed density motor and start disconnecting stuff at the vacuum tree at the plenum and leave it unhooked....this should put you at about 2000 rpm. You have a big vacuum leak, your idle speed is now 2000 rpm and you have high vacuum. This is because the MAP does not know the difference between opening the throttle and a vacuum leak. If you were to do the same thing on a mass air flow motor the motor would be running very lean and the vacuum reading would not likely be steady. The idle speed would be either the same as it always was if not a little bit lower and a bit rough.

 

The problem with this method is that unless the emissions are too lean before spraying, then the engine speed may stay unchanged meaning that you would need an exhaust gas analyzer to detect the leak OR you have to monitor the O2 sensor to check for the richened mixture caused by spraying. With SD the leak will NEVER result in a lean mixture meaning that this method will NEVER be effective witout monitoring the exhaust content somehow. With mass air though, the mixture will be lean from the vacuum leak and thus the propane / starter fluid / carb cleaner method may be somewhat effective.

 

^ hes one smart clown. i would have to agree with eco on this one.

 

Do you have a STEADY 16 inches (as opposed to fluxuating)?? If so and if I were doing this I would look into the possibility of ratarded valve timing, retarded ignition timing, a leak on the underside of the plenum and low compression.

If you exhaust smells off then thats just a matter of analyzing the exhaust....ideally with an 5 gas analyzer or the poor mans method would be to monitor the O2 sensor and see it reads lean or rich (assuming you are able to reach closed loop) and that would at least point you in the right direction.

 

yeah, I have a pretty steady 16 inches. I have not checked timing, but i can do that. The last time timed my ride I accidnetally grabbed the metal piece that connected the light to the dizzy and the truck bit me pretty good and freaked me out!!

But, if I do have a leak on the underside of my plenum, then I am going to take that as your guess for where my vacuum is leaking!

How do you change valve timing? Isn't that my timing belt?

 

well it would be your timing chain but yes that is how your valves are timed relative to the position of the piston

 

You should have a timing chain as opposed to a belt. If you are going to go there you might as well just replace it. If it is sloppy and wore out then that could explain the retarded valve timing. I have heard of people checking the timing chain by removing the dizzy cap, and turning the crankshaft back and forth and watching the rotor to tell how responsive it is in relation to how much you turn the crank. If the crank turns 1/4 turn (or turn much at all for that matter) before there is movement at the rotor then odds are the timing chain is sloppy.

You might want to check compression as well.

 

Thats what I said in Lehman's terms. For those without exhaust gas analyzers, leaks can be found with carb cleaner.

I've found leaks on lawnmowers, outboard engines, and EFI rigs this way. Cheap, painless and not advertised as fool proof.

Never say never. Found an upper plenum leak in an SD 1989 300I6 this way. Never looked at exhaust. Pulled intake and gasket was compromised right where carb cleaner snuck in and idle wobbled.

No fancy tools needed for this particular fix.

 

yeah, i meant timing chain. I have belts on the brain right now from work.

but, if the chain is off slightly, it's not going to change anytime soon. I won't get to that until next spring. I'll check ignition next week though and see what it says. I don'tknow about compression though. I haven't done that before.