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Topic
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2004 Corolla LE 1.8L
This problem occurred after someone else removed the engine and replaced the clutch.
I’m feeling very stupidly stumped over this issue though…the car lops/surges very bad at idle. I unplugged the MAF and it smoothed out, but fuel trims were still pegged fully positive and the O2 remained lean (I know the O2 isn’t lying because I could drive it rich with propane). I scoped the MAF and the signal appeared normal with no drop outs. And was in the normal 1V ish range at idle.
So I don’t think it’s the MAF…I think it’s a vacuum leak and unplugging the MAF and forcing the car to use a substituted value helped it out. The fuel trims also got more negative as RPM increased which would also indicate a vacuum leak.I could hear a loud hissing under the hood and looked everywhere and tried water, propane you name it and I couldn’t locate any vacuum leak…it was driving me crazy. Then finally I pinched off a hose and the hissing went away and the idle smoothed out, the STFT went fully negative to counter the LTFT and it appears to be fixed…BUT I have no idea what the hose is and WHY it was there in the first place. It basically comes off the air box, bypassing the MAF, and allowing air to go straight into a nipple on the intake just behind the throttle.
I was able to find a picture of it online and I attached it
Why would this hose be allowed to take air from the air box, bypass the MAF and dump all that air just behind the throttle???
What the heck is this hose for??? I don’t see how it has anything to do with PCV or Emissions.
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One thought is that the guy who did the clutch may have hooked it up where it wasn’t supposed to go, but it’s like it’s molded rubber and it fits and routes perfectly onto the nipple on the air box and the nipple behind the throttle.
The car is gone for the day so I can’t look at it right now, but I’m getting it back tomorrow. Found another picture online of an 03 that may indicate that this hose should actually connect not to the airbox but to what looks like the purge solenoid. Tomorrow I may get lucky and find a vacuum hose diagram sticker under the hood (should a looked for it but I guess I was rushing around too much).
Although that still doesn’t explain why there is a nipple on the air box…could it be for some kind of accessory that needs a vacuum port and if the accessory isn’t there maybe the port should have a cap on it?
Mystery solved…there are two molded rubber hoses that fit absolutely perfect in the wrong place. The guy who did the clutch hooked them up wrong and when the owner complained of it running rough…rather than looking into it he claimed he didn’t do it and it was the spark plugs :blink:
Turns out there are two evap solenoids right near the air box. He hooked these solenoids together with a hose. When actually, the lower one gets hooked into the air box and the other one (purge/VSV) gets hooked up to the mystery hose that runs just behind the throttle body and connects into he intake.
It was a really tough vacuum leak to find because I suspected it may be internal but after pinching off the PCV hose and nothing happening I moved away from that direction and started to look for an external leak where I could hear the hissing :dry:
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