1996 3800 series II, elusive Vaccuum leak - Page 2 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 04-06-2011, 10:47 PM   #11
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Originally Posted by BillBoost37 View Post
I've got one more trick... the elbow at the brake booster is a check valve. If it goes bad...you have a vacuum leak.

I'm stretching pretty far here, we gotta get this one soon.

And I wouldn't expect the smoke to get past the pcv checkball either. It'* in there to keep the crankcase from pulling air.
BillyBoosta, you just might be a Genius. I got smoke coming out that valve.

I smoked it again tonight. I checked the PCV valve and everything checks out. The pcv valve does flow in both directions, hence the smoke coming out the oil fille cap when removed. I verified this with anothetr new PCV valve.

I left the booster check valve connected to the hose but out of the grommet in the booster and at first nothing came out. However when I blew into it with my mouth, the smoke started pouring out when I released it from my mouth. The smoke slowly came down to just a trickle on it'* own. I also released the oil filler cap a few times to release the low pressure and each time the booster valve billowed lots of smoke and came down to a slow trickle. This seems to make perfect sense as the car stalls a lot now on decel when braking.

I still don't understand how all this started after the reseal, unless the reseal is allowing the engine to make a lot more vaccuum and now it'* very sensitive to it'* weakest link. So far since the egr tube replacement, I have done 4 short road trips up to temperature and no check engine light. One of the trips revealed a STFT of 0.0% and long term fuel trim a 16.3%. Another trip revealed 20% short term fuel trim with the long term still at 16.3%. The exhaust doesn't smell as bad either. I think I'm winning the battle.

I will pick up a new booster check valve tomorrow and have at it again.

Thanks.
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Old 04-07-2011, 11:30 PM   #12
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I was unable to get a new booster check valve today, had to order it. So I disconnected the valve and plugged the line (no leak) just to see if it would help. It did not. The problem still exists. So I'm back to square one. Man, thought sure we had it nailed.

Also on my ride home today on the highway the check engine light came on. I pulled 3 different codes in one trip. Everthing still points to a vaccuum leak. I think I'm gonna smoke it again, for the third time.
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Old 04-08-2011, 01:25 AM   #13
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PCV valve has the spring and an O-ring in place? EGR valve can cause a vacuum leak as well.
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Old 04-08-2011, 09:40 PM   #14
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PCV valve has the spring and an O-ring in place? EGR valve can cause a vacuum leak as well.
Yes PCV valve has spring in place and both o-rings.

EGR Valve? Leaking? I've never gotten a code for the EGR. What if the EGR pintle was sticking open?

In the past, when it was running fine I did occasionaly get a P0420, but I would always clear it and it wouldn't come back till 3 - 4 tank fill ups.

I just smoked this thing again and cannot find a leak.

The erratic idle does not start until it reaches 138 deg. F. When it stalled I scanned it for the coolant temp. I immedialtly smoked it at that temperature and found nothing. It seems like the IAC cannot respond quick enough to keep it running. It'* a brand new IAC.

I'm wondering if the lower intake gasket might not be sealing properly. What would be the signs of an internal vaccuum leak? The same as what I got?

The challenge continues. Glad this is my back up ride.

Any ideas?
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Old 04-09-2011, 08:21 PM   #15
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Smoked it again and still no leaks.

I took a look at the EGR valve. I found the pintle sticking. Thought sure this had to be it. So I cleaned it up and got it moving freely and smoothly. Put it back on and same problem.
Here are the live data stats on one of my scans when it was idling smoothly in park.

Load Value = 2.3%
Coolant Temp. = 194 deg.
STFT Bank 1 = 20.2%
LTFT Bank 1 = 16.35 %
Manifold Pressure = 10.2 in.hg
Engine RPM = 733
Vehicle Speed = 0 MPH
Ignition advance = --16.5
Intake air temp. = 64 deg.
MAF Rate = 0.5 lb/min.
Throttle position = 0.3 %
Bank 1 o2s 1 output = 0.255v
Bank 1 sensor 1 STFT = 20.2 %
Bank 1 02s 2 output = 0.945v
Bank 1 sensor 2 STFT = 99.0%

Anyone care to comment on the numbers?
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Old 04-09-2011, 11:45 PM   #16
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How old are the plugs and wires? do you have an exhaust leak up front?
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Old 04-10-2011, 09:21 AM   #17
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How old are the plugs and wires? do you have an exhaust leak up front?
Plugs & Wires are 3 months old (1500 miles)

Will double check for an exhaust leak. It'* crazy. Car ran perfect before I messed with it.
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Old 04-10-2011, 08:56 PM   #18
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Found two exhaust leaks. Will look for more, as I ran out of time.
Got one right at the donut for the front pipe to manifold which is about 2 inches downstream of the front 02 sensor. Not a huge leak. The other one is right in front of the rear 02 sensor. It'* actually leaking at one of the welds for the shield that protects the rear o2 sensor. This one is leaking a little more than the one up front. Both look to be an easy fix. Both leaks were found when stone cold by spraying soapy water. Found them both within 2 minutes of engine start up.

Would these really affect the fuel trim numbers this much to create this problem?
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Old 04-13-2011, 09:17 PM   #19
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Do you think an exhaust leak on both sides of the rear o2 sensor and the leak up front at the donut could be my problem? Damn factory welds. I cut off the shield and peeled back the tabs a bit for the O2 sensor in order to gain access. The leak is much worse than what I first saw. Ouch!! See Pics.
The rest of the exhaust is solid. Rear muffler shoud be replaced soon but it is not leaking.
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1996 3800 series II, elusive Vaccuum leak-img_5272.jpg   1996 3800 series II, elusive Vaccuum leak-img_5273.jpg  
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Old 04-24-2011, 09:46 PM   #20
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So the exhaust leaks are fixed and the problem is still there. Bummer. I checked all the wiring to all the sensors and found no issues. I tested the throttle position sensor as that was the only one I hadn't replaced. It was a little iffy on the bench test using the ohm readings so I replaced that as well with a new one. It acts a little differently now but still the same errattic idle and stalling. Short term fuel trims are now reaing 0 % which is probably due to the exhaust leaks being fixed.

So now I went back to smoke test it again and this time I removed the PCV valve and closed off the passage into the upper plenum at the PCV valve. I left the PCV valve and map sensor out. Smoke test revealed smoke still comig out of oil filler cap and PCV Passage from lower intake. ( HMM!) I then removed the upper plenum and smoke tested each injector port and found no leaks. No smoke at oil filler cap or PCV port. Nice, now I don't have to pull the lower intake.

What I found was the passageway from the PCV leads all the way to the thottle body in front of the throttle plate. To me this is a bypass that does not make sense as it is drawing in fresh air all the time ( vacuum leak). Hence the smoke test revealing smoke at the oil filler cap. I'm either missing something or the PCV valve is defective. I verified all seals to the PCV. At a steady 800 RPM Idle I have 20 inches of vacuum. At a steady idle if I just touch the throttle a little bit (a short blip) it starts searching for an idle and stalls. It'* wacky and frustrating.

See PICs. below.
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1996 3800 series II, elusive Vaccuum leak-img_5290.jpg   1996 3800 series II, elusive Vaccuum leak-img_5291.jpg   1996 3800 series II, elusive Vaccuum leak-img_5221.jpg  
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