vacum leak = rich?

apple

Would a tiny vacum leak make my car run rich? I just installed a new intercooler and piping and I think there may be a small leak as the car just started driving really crappy and running pretty rich. Help? Ideas?

Well go over your install. Loosen all the aprts up and reseat all the piping.
then check for fittment again.
Then look over all your Vacum lines.
Not knowing what year you have, it could be an EGR issue, but that is a HUGE stab in the dark... like ising a pencil to what a pinyata in a treee, hoping candy will fall out.
You can try spraying card cleaner where you think the leak is, and see if the RMP jump up.

On a 1990 Miata, an air leak between the Air Flow Meter (AFM) and the throttle butterfly will cause a lean air/fuel mixture. Although not strictly speaking a vacuum leak, since anything before the throttle butterfly is not in a vacuum, the un-metered air cannot be compensated by added fuel from the ECU, since the ECU does not quot;knowquot; that air is coming into the engine.

that would be carb cleaner... just so you know... i know it was a typo, but i dont want anyone in autozone looking for card cleaner... or boost fluid. lol.  that being said... it would have to be a verrrry tiny leak for you to run rich and not have the car break up. i always test my setups with a homemade ic tester... just put together some pvc and a hose coupling tapped into it with some teflon around the threads... pressureize to 7-10 psi, and spray your hoses with soapy water.... be sure not to over pressurize your motor!

does the intercooler have beading on the pipes? if not, your really gonna want to weld on some for anything over 8-10 psi...

I am running 8psi on a 1990 AFM still in use. This happened with my previous setup too. My gas mileage suddenly tanked, my idle was all over the place but my turbo spooled way faster too... same issues here. Would a leak in the hose going to the idle control valve have anything to do with this or would it react the same if there was a leak anywhere between the AFM and the throttle body? Thanks for the suggestions though guys. Please continue to ponder my situation.


Would a leak in the hose going to the idle control valve have anything to do with this or would it react the same if there was a leak anywhere between the AFM and the throttle body?

All the air entering the engine needs to come through the AFM, otherwise the engine will run lean.
Why did you think it was running rich? Is there black smoke from the exhaust when the engine is warm? If so, it is running rich, and that cannot be due to a vacuum or air leak. It could be due to a bad Water Thermosensor, i.e. an open circuit either within the WT or a bad connection at its connector or in its wire run.

A vacuum leak can cause a rich condition and raggedy running.  If the leak is medium, the ECU will detect something is wrong and run in one of its mapped safety modes.  Most likely will not show a CEL.  Think the best suggestion is to take the plumbing all apart and reseat everything.  If running a turbo, suspect you also have a turbo/vacuum gauge installed.  What is your idle vacuum?
Since you also run a turbo, suggest running your hand under the intake manifold all the way back to the firewall and should locate an unused vacuum nipple with a rubber cap.  Ensure that nipple is capped.  From testing, that shold show a drop of idle vacuum by about 4 -Hg.
Sounding good in theory, spraying the suspected vacuum leak with carb cleaner or propane I have newer found to work on my Miata.  What seems to work for small suspected leaks is liquid dish soap.  The dish soap is thick enough to form a temporary seal at the leak and should hear the engine RPM change.
Aside from that may be time to check or adjust your AFPR.


Originally Posted by Bill Strohm
All the air entering the engine needs to come through the AFM, otherwise the engine will run lean.

not necessarily true on this point... obviously, the person is running turbo... depending on how he is managing his fuel, he might have a stand alone ecu which would delete the maf or map, depending on the year of his car...
i.e. the op needs to give more information...

No standalone ECU, just a FMU and larger injectors. I have a narrow band that reads rich most of the time, even under vacum, the exhaust is very quot;poppyquot; and there is black smoke when I get on it at all. The thing is that the car was running awesome a few days ago, even the narrow band A/F gauge showed ideal A/F mixtures even running on the lean side and my idle was better than it had ever been. I was almost hit by someone that ran a red light and I had to lock up all four wheels to keep from hitting her then after that my car ran like crap. No fluid leaks and no audible vacum leaks. I had to disconnect my boost gauge b/c the line had a small leak in it and made the gauge read incorrectly. The line was patched and still the car runs poorly; driveable, but poorly.

how old is the Fuel filter?
This is a shot inthe dark, but mabye it is blocked up. after the hard braking.
Something might have been dislogged and not clogging the fuel system.
I know this is a huge shot in the dark...


Originally Posted by TheMcCoy14
No standalone ECU, just a FMU and larger injectors. I have a narrow band that reads rich most of the time, even under vacum, the exhaust is very quot;poppyquot; and there is black smoke when I get on it at all. The thing is that the car was running awesome a few days ago, even the narrow band A/F gauge showed ideal A/F mixtures even running on the lean side and my idle was better than it had ever been. I was almost hit by someone that ran a red light and I had to lock up all four wheels to keep from hitting her then after that my car ran like crap. No fluid leaks and no audible vacum leaks. I had to disconnect my boost gauge b/c the line had a small leak in it and made the gauge read incorrectly. The line was patched and still the car runs poorly; driveable, but poorly.

thats weird, i don't even know where to start on that one... you lock up your brakes and your car doen'st idle any more? i would check all the vacuum fitting on all the hardware... i.e. dissasemble reasemble... maybe something came loose?

Have a look at your Water Thermosensor (post #7) connection, and if you can, probe the blue wire with white stripe (while connector is connected) with a DC voltmeter (positive lead to wire, negative lead to chassis ground). When coolant is cold (68°F) the voltage should be about 2.3 volts. As the engine warms up, the voltage should drop to about 0.4 volts. If the WT is open or its connection to the ECU is bad, your engine will run VERY rich.

Could the insta-rich condition be due to my FMU crapping out and sticking open or closed?

if the inlet pipe work had developed a leak due to the suden stop, i would think it would be lean when not on boost, not rich,
so i would suspect soemthing external, such as the temperature reading, or the FMU,
How does your fmu work? is mechanical or electrical.  can you check what is doing?

it's mechanical, It's a pretty simple device and I don't know how I would go about seeing if it is leaking. The car is driveable and I plan on checking/redoing all connections soon so if there are no leaks I will start to search for a BEGI adjustable FMU instead of my crappy one. But please still throw suggestions at me if you guys have any.

ok  what does the FMU do
increase fuel pressure?

The FMU has a plunger that essentially blocks off the fuel return line when it senses boost and thus increases fuel pressure to the injectors.

can you check the fuel pressure at the injectors?
if the plunger is stuck in the quot;return line blockedquot;  position you will get too much pressuer at the injectors
how does it sense boost,  if that signal is somehow stuck on boost, you will have the same issue

Should have posted your question in the turbo section . BTW , how do you know your vacuum leak is tiny ? I think there is more to this story.......

This went from my asking if a vacuum leak would cause my car to run rich and then I went on a question asking adventure and here we are. I am going over all connections today (hopefully) so we will see if my redoing all intercooler couplers helps at all.