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Vacuum leak? Where should I be looking?


spiderw31
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So a whole pile of work later, the paperweight is running and back on the road. I've got two issues with it though. First is the vents; with A/C on, i'm getting air from both the floor and the dash vents. When I'm on the throttle at all, the airflow shifts more towards the floor, and anything like heavy throttle pushes it all the way to the floor. From what I know of the system, that means a vacuum leak, so where should I start looking for it? Second issue is a low idle in drive. According to the tach, i'm right at 500 when idling, and the car has a bit of a vibration along with a voltage drop from the alternator. I'm thinking that the two are connected, though I would have expected a vacuum leak to give a high idle. If it helps, the last repair done was an ECM replacement. Thoughts, advice appreciated!

 

Edit: Cruise control isn't right either. It tries, but can't keep the car up to speed. It's doing something though, because ifI turn it off, the car does suddenly slow down a lot quicker. It's almost like i have some vacuum but not much. Now I'm wondering if maybe I have one of the vacuum lines in the wrong port?

Edited by spiderw31
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Oh, and if anyone happens to have a vacuum diagram it would be helpful. The original owner had a hood lock installed, and they mounted it right over the diagram on my car and ruined it :mad:

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Quick google search yielded this for vacuum diagram. I would have taken a pic of my Lumina, but it's since long gone. I would unplug the line leaving the MAP sensor going to the HVAC and cruise and check it with a vacuum tester. I'm guessing a chunk of the hard line that runs along the firewall or down to the vacuum ball is broken as I had the exact same issue on my '92 Z34 when I first got it.

post-3334-143689185968_thumb.jpg

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the two issues should be unrelated, the ECM should be opening up the IAC if it didn't like the engine speed it is seeing at idle. to me, that says borked IAC/wiring or for whatever reason, the ECM is commanding that idle speed based on the inputs it is seeing and is normal("normal").

 

did you swap your original MEMCAL into the new ECM?

 

the line running to the vacuum reservoir, the reservoir itself and then the lines that split off for cruise and HVAC usage are what I would be looking at for the vacuum leak.

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Thanks guys for the help! That vacuum diagram isn't complete, but at least let me verify which line was headed to HVAC/cruise. I found it was not seated well at the MAP sensor, and that improved the airflow a bit but not fully. I traced the line down from the cruise unit to the vacuum reservoir, and found it indeed had broken. Looks like it was leaky for a whiling when I moved the wiring around disconnecting the battery I managed to break that line. Patched it up, and air is better than it has been for a while! I'll test the cruise tomorrow, but I suspect it should be fine now too.

 

Oh, and you were right Saar, the idle was a separate issue. Or more accurately I think a non-issue. I don't think I was patient enough this morning with the idle relearning process. By the end of the day it was much better and smoother. It may perhaps be a bit lower than before, but not by much. And yes, I swapped the MEMCAL into the new ECM. As a side note, the "new" ECM is actually quite old. Got it from Rock Auto, and it was remanned in 1997!!! They've definitely got some old parts sitting on their shelves!

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the default IAC minimum value in the factory calibrations has always been low in my experience.... normally in the 10 range when the ECM usually ends up "learning" it into the 20 range. the code only allows so many steps of change(usually 2) per usable runcycle(above x temp for y time at idle), so it usually takes between 5 and 10 good, learnable cycles for it to be where it should be. until then, the ECM relies on spark advance changes(both positive and negative) more than it should, causes that awesome surging/unstable idle that tends to happen after an ECM reset. changing the default IAC minimum value is one of the first changes I make to any calibration so that should the ECM lose power, it doesn't make the car drive like a bag of ass until the ECM sorts out a new IAC minimum.

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The part that I don't understand is why the airflow diverts to the FLOOR when you accelerate.

 

I've fought weak vacuum to the HVAC and cruise control; I've had multiple vacuum tube breaks on the vacuum supply to the reservoir "ball" in front of the LF wheel. When that tube breaks, I have weak cruise control--it tends to lose speed. It also causes the air to divert to the DEFROST position, as is the normal operation when there's weak or no vacuum. Air coming out the defroster vents, not the floor, is normal for vacuum problems.

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Well, I cant explain why the floor:shrug:, but the air was redirecting to the defrost as well. I just hadn't checked those vents until much later (and forgot to mention it describing the fix). I'd say it was 50% defrost, 40% floor, and 10% dash. Ultimately though, a splice in the line fixed it, along with the weak cruise. Zero airflow to anything other than the dash vents this morning.

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