MAF/MAP and vac leak - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 04-17-2010, 07:47 PM   #1
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Default MAF/MAP and vac leak

%[email protected]#.....

The '98 SSEi has had (what I think is) a phantom vac leak forever. It suddenly appears and then disappears as it pleases.

The way i know it'* present, I will come to a light and stop, but the boost gauge won't go to the bottom (left). It will float above it. Or i will be stopped at a light minding my own business, and suddenly it just moves up. Nothing is noticeable without a scantool. The MAF value does not change, but the MAP reading jumps. Also, when it is present, if the car is in Park, it'* fine. Put it in gear and it jumps up. When it'* not present, put in gear and the MAP doesn't change.

Normal
MAF=.60 MAP=8.3
Abnormal
MAF=.60 MAP = 10.3

I could live with this as a "feature" except for one thing. It destroys the best fuel trims I have had with this car. Recently, I did the LIM gaskets and everything which goes with it was cleaned, new Orings etc. And replaced most of the vacuum lines and connectors. Everything is better, except when this thing appears. (It also did it before the work)

My fuel pressure does run a little low at 43 psi instead of 48 psi (for a '98 ), but I want to finish up on the air side and I missed something.

The 3-connector cap on top of the */C... one line goes to the brake, one line goes to the boost actuator.

Where does that 3rd one go? It disappears into the main loom with the electricals on my car. I don't have my FSM available. Is that headed for the trans?

Any help would be appreciated.. Thanks.
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Old 04-17-2010, 11:55 PM   #2
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What'* the condition of the O-ring for the PVC? Is it possible it got twisted as it was inserted?
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Old 04-18-2010, 01:05 AM   #3
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What'* the condition of the O-ring for the PVC? Is it possible it got twisted as it was inserted?
I have opened that and reset it a couple of times to check the PCV. The setup for the PCV on the L67 is a little different than the L36. It has a a heavy spring load, and I'm fairly confident the valve and Oring are ceded properly. Plus I have gone over every inch of the contacts of the LIM, */C, lines etc with throttle body cleaner.

Before I start pulling that loom apart, I want to know where that one vacuum line goes, and what vacuum service it provides to whatever it connects to. I wouldn't be bothering you guys if i had my FSMs, but they were in the trunk of the '95, which I lent to the Vet hospital.. Thanks for the thought though, and keep 'em coming..
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Old 04-18-2010, 10:32 PM   #4
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I think that might be the vacuum line that goes to the climate control system inside the car. I had a phantom leak also and it turned out to be that hose further inside the loom
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Old 04-19-2010, 07:48 AM   #5
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Charlie...

What'* a happenin buddy? Are you driving around with the defog/defrost or the AC on? Symptoms fit perfectly with the AC cycling.
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Old 04-19-2010, 05:05 PM   #6
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Quote:
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I think that might be the vacuum line that goes to the climate control system inside the car. I had a phantom leak also and it turned out to be that hose further inside the loom
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Charlie...

What'* a happenin buddy? Are you driving around with the defog/defrost or the AC on? Symptoms fit perfectly with the AC cycling.
Bill and Richard..

OMG. How about if I add in the symptom that when the Phantom Air appears, if I turn on the ECC, it bumps the Map sensor even higher. Without even looking yet, I think "We have a winner!".

I may not be the sharpest knife in the drawer, but I'm clever enough to know where to go to ask the question. And lucky enough to have the people around who can answer it.

Bill, you have a PM coming..
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Old 04-20-2010, 02:28 AM   #7
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I opened the loom and checked the plastic line, which was ok. The rubber lines which go from it to the connector on the firewall were foul. Barely touched them and crusty black came off my hands. Replaced them.

It really didn't change anything. #@%&. Is there anything I should check behind the firewall to the ECC?

There is something I haven't learned yet, that should snap this into focus.

When I go from Park to Drive, why would the MAP pressure jump, when the MAF is seeing no additional air?
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Old 04-20-2010, 09:18 AM   #8
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Only thing that comes to mind is you reduce rpm and add load to the motor. A change in map there should be normal.

Charlie, got your pm. I'm thinking you should tell her to make it soon.
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Old 04-21-2010, 08:30 AM   #9
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Only thing that comes to mind is you reduce rpm and add load to the motor. A change in map there should be normal.

Charlie, got your pm. I'm thinking you should tell her to make it soon.
I hear ya. But I need to back up... I put 50 miles on the car today and have a whole different read from the other night.

First, i have the elevated MAP reading at idle virtually all of the time. My -10 on the Boost gauge is equal to 10 or less inHG on my scantool. I'm getting 10.9, so the needle is up. Ratz. Plus my LTFT at idle is at 9-10. Double Ratz.

But here'* the VERY good news... before i replaced those HVAC vac lines, when I went on-throttle towards WOT, the composite of the LTFT and STFT would head screaming towards the P0171 range. Now they do the opposite, they head to (and bounce around) net 0.0. General driving around I'm a little lean in the LTFT 4 range (STFT near or at 0.0). But if the LTFT starts bumping up higher, the STFT immediately goes negative, to pull it back down.

(Note: I had to disconnect the battery while working. I know the computer needs a good 75-100 mi to get its act together. But IMHO, you can get general trends before the computer closes in on its hard numbers.)

Correct me if I'm wrong, but I believe this is good news for the INTENSE mods which have been sitting in a box over in the corner for 2 weeks, while I rewrote the script to this air/fuel soap-opera.

This still leaves me with issues in the low band. The MAP reading is not in line with the air coming through the MAF at idle, which ends up with the trims compensating to out-of-range readings. Idle is a distinct "cell" where the computer "learns", while it appears I'm getting love from of all of the on-throttle "cells" (accel, decel, WOT, etc).

I'm starting to think its time to revisit the lower than normal fuel pressure (low 40s psi vs high 40s+). One of the reasons is that I accidentally noticed something while in death-on-wheels mode. (That'* when I'm driving around scrolling and reading the scantool, while veering all over the road.)...

When I sit at a light and my foot slightly touches the gaspedal and gets the TPS from 0.0 to 0.4..., the boost gauge drops, and the MAP reading drops to 8.0 from 10.9. Where it should be at idle. ??? Weird.

That being said, I don't consider it a big showstopper, and the top-end is safe for mods now..

Any comments would be appreciated. Thanks.

PS. Bill, negotiations for a firm date are on-going. Trying to get those two to make a decision is like trying to herd cats. I just mentioned to them that I'd like to get this settled while I'm still young enough to drive.
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Old 04-21-2010, 09:10 AM   #10
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Charlie.. What'* the boost gauge reading at idle in gear? Gimme one number.

LTFT'* are sounding very good. Better than most actually. At idle things will move around, it'* normal enough. Enough so, that I typically don't look at them at idle. Dan probably has some scans that can be looked at though for comparison. Plus, I'm not sure that my data would be a good compare value.. my trims are probably horrible at idle with the lift I'm pushing.

Suggest May 14th-16th.. it'* only NEBF and a perfect time to meet a ton of people. Tell you you hate her family and never want to visit. That should spark a trip instantly....
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