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Mystery Leak? Lean condition


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#1 wstefan20

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Posted 31 December 2017 - 06:48 PM

Alright, so I'm getting to the point where I might have to break my rule and take my beauty in to the shop.... hopefully someone can help and save me the hassle!To preface, the car runs "ok" and has since I got everything rebuilt. However, I've always noticed that it struggles going up hills and doesn't have much "get up and go" which has bothered me. I finally had enough time to hook up my scan tool and check things out in detail this weekend. There are NO check engine lights, but after bringing everything up to temperature, I get around 15% lean long term fuel trim at idle and when my foot is off the accelerator, which immediately makes me think vacuum leak. The bad is that there is no auditory leak either vacuum or exhaust. The MAP pressure is normal vacuum and responds with throttle accordingly. The fuel pressure is around 55-60 psi and increases to 70 when regulator vacuum is removed or throttle is snapped. MAF sensor is in range and responds as it should (and I just cleaned it). IAT sensor is at correct readings. EGR seems to be functioning correctly. O2 sensors are operating normally. Idle rpm is around 750-800 (I think that's normal).I used the propane test EVERYWHERE and no leaks detected. Unfortunately, I don't have access to a smoke machine.There isn't excessive crankcase pressure, and no detectable blow-by by removing the oil fill cap.Basically, I'm lost... help! lol



#2 Imp558

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Posted 31 December 2017 - 07:09 PM

CTS, IAT,O2? All healthy?

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#3 wstefan20

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Posted 31 December 2017 - 07:14 PM

CTS, IAT,O2? All healthy?

Yup! That's what's killing me. I've never seen something like this before. I know I'm missing something!

#4 Galaxie500XL

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Posted 31 December 2017 - 08:08 PM

A misfire will show lean, because the oxygen didn’t get consumed. Bet a nickel you have an ignition issue, most likely a flaky coil.
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#5 wstefan20

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Posted 31 December 2017 - 08:20 PM

A misfire will show lean, because the oxygen didn’t get consumed. Bet a nickel you have an ignition issue, most likely a flaky coil.

That's what I thought for a moment too. I've been driving it for weeks and had a scanner on for several periods of 10 minutes or more. There is absolutely zero kr and no historical or current misfires on any cylinders. Engine idles smooth as butter too. This issue is killing me!



#6 wstefan20

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Posted 31 December 2017 - 08:39 PM

Here's a small snippet of some fuel trim data while I'm driving around town. Tell me what you think! lol

 

 

https://www.youtube....h?v=nPWwf0EJnzQ



#7 Nas Escobar

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Posted 01 January 2018 - 08:30 PM

A misfire will show lean, because the oxygen didn’t get consumed. Bet a nickel you have an ignition issue, most likely a flaky coil.

I second this. When the coils fail the car still runs but it does weird things.

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#8 Nas Escobar

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Posted 01 January 2018 - 08:32 PM

A misfire will show lean, because the oxygen didn’t get consumed. Bet a nickel you have an ignition issue, most likely a flaky coil.

I second this. When the coils fail the car still runs but it does weird things.

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#9 wstefan20

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Posted 01 January 2018 - 09:44 PM

I second this. When the coils fail the car still runs but it does weird things.

I wish it was that simple, but there's no misfire :(

#10 55trucker

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Posted 01 January 2018 - 10:59 PM

If you're absolutely sure that the issue is not electronic related then start looking for mechanical issues.

 

You mention that you *rebuilt* the engine? Are you positively certain that the cam timing is correct?

 

Judging by your sig I take it that the engine is the 3.8L N/A or is it blown........

 

You didn't mention what engine you're dealing with here....


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#11 wstefan20

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Posted 01 January 2018 - 11:10 PM

If you're absolutely sure that the issue is not electronic related then start looking for mechanical issues.

 

You mention that you *rebuilt* the engine? Are you positively certain that the cam timing is correct?

 

Judging by your sig I take it that the engine is the 3.8L N/A or is it blown........

 

You didn't mention what engine you're dealing with here?

Sorry. Forgot to mention that. Thanks for pointing that out! It's a L36 with L26 upper intake swap (aluminum intake so it's basically a L26).

 

I think I might have just answered my own question here. I did change the vacuum for the pcv valve which is a "metered vacuum leak". So if I increased the amount of vacuum it was able to flow, then that would also increase the leak and boost the airflow and cause it to run lean. However, this would also explain why it changes when I accelerate since it only marginally affects the total airflow at wot.

 

Not sure if it's ok, but I think one way of "tuning" for this would be to put a ball valve in line and therefore I could restrict the amount of vacuum and just adjust this manually until my fuel trims are correct. Anyone see anything wrong with doing this?



#12 Schurkey

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Posted 03 January 2018 - 07:36 PM

 I've always noticed that it struggles going up hills and doesn't have much "get up and go" which has bothered me. I finally had enough time to hook up my scan tool and check things out in detail this weekend. There are NO check engine lights, but after bringing everything up to temperature, I get around 15% lean long term fuel trim at idle and when my foot is off the accelerator, which immediately makes me think vacuum leak.

This doesn't sound like a PCV problem.

 

Define "15% lean".  Is this the ECM commanding the injectors to supply 15% extra fuel, or 15% less fuel?


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#13 Schurkey

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Posted 03 January 2018 - 07:36 PM

It's a L36 with L26 upper intake swap (aluminum intake so it's basically a L26).

 

I did change the vacuum for the pcv valve which is a "metered vacuum leak".

 

I think one way of "tuning" for this would be to put a ball valve in line and therefore I could restrict the amount of vacuum and just adjust this manually until my fuel trims are correct. Anyone see anything wrong with doing this?

1.  WHAT CAR???

2.  What does "change the vacuum for the pcv valve" mean?

3.  Why not just progressively clamp the hose with a pinch-off pliers and see what happens to the fuel trim?

 

 

 

I'm suspicious of the PCV system causing a "15%" fuel trim change.

What is the cranking compression pressure?


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#14 bluegtp91

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Posted 03 January 2018 - 07:58 PM

I'd like to see a picture of the engine bay.

 

Is the L26 manifold vented from the top right or is it capped?

 

Are you still running EGR or no?


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#15 scottydoggs

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Posted 03 January 2018 - 08:15 PM

from gpf post, hes got a catch can hooked up  and what i from what i can tell the evap port is connected to a vac port, instead of being capped off at the tb for one, the rest could all be done wrong too. theres no need for any of it, i suggest tearing it all off and hooking things up the right way. 

 

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#16 Imp558

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Posted 03 January 2018 - 08:34 PM

What's that happening with the MAP sensor? Yeah it may be better to start over and put it closer to factory. Still don't understand the catch can, or the breather.

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#17 wstefan20

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Posted 03 January 2018 - 08:37 PM

This doesn't sound like a PCV problem.

 

Define "15% lean".  Is this the ECM commanding the injectors to supply 15% extra fuel, or 15% less fuel?

+15% ltft is defined as increasing the injector pulsewidth by 15% therefore increasing fuel. Negative numbers decrease the injector pulsewidth thereby decreasing fuel



#18 wstefan20

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Posted 03 January 2018 - 08:45 PM

1.  WHAT CAR???

2.  What does "change the vacuum for the pcv valve" mean?

3.  Why not just progressively clamp the hose with a pinch-off pliers and see what happens to the fuel trim?

 

 

 

I'm suspicious of the PCV system causing a "15%" fuel trim change.

What is the cranking compression pressure?

1. 2002 Pontiac grand Prix GT with L26 upper

2. I re-routed the PCV system to incorporate a catch can. Long story short, I routed the pcv vacuum externally through a catch can. You can check out the process at my thread here: http://www.grandprix...ap-PCM-question

3. I actually did just that and will post the results in a few minutes once all my tests are complete.

 

Cranking compression on all 6 is around 150-160psi (crappy harbor freight gauge so those numbers could be way off)



#19 wstefan20

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Posted 03 January 2018 - 08:50 PM

What's that happening with the MAP sensor? Yeah it may be better to start over and put it closer to factory. Still don't understand the catch can, or the breather.

I actually did reset it to factory, and no change. It looks like the ceiling for stft is 20% and ltft is 15%. The short term fuel trim should oscillate, but it's not, which is strange.

 

The MAP sensor is reading normal vacuum pressure for the pressure here which is around 30kPa (not inHg) and decreases as the throttle is opened (which indicates vacuum is decreasing which is normal)

 

I'm going to try bypassing the catch can to see if that improves the numbers or not. The problem is when I pinched off the hose (actually used a ball valve), I could only go so far before the MAP sensor stopped reading anything correctly (vacuum too weak because valve is basically shut and reading pressure backing up from pcv). If it improves, I know the catch can is leaking.



#20 Schurkey

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Posted 03 January 2018 - 09:42 PM

The "catch can" could decrease vacuum to the PCV valve, but not increase it.  If the vacuum is decreased, it could be because of a leak which could add additional, un-metered air to the intake manifold.  If it's decreasing vacuum because it's plugged, there'd be less air going to the intake manifold.

 

If it's leaking enough to screw-up the fuel trim by 15%, I suspect you'd hear it.  I am not inclined to blame the PCV system for this.

 

I'm used to seeing LT and ST fuel trim numbers, not percents.  (i.e., 128, which would be no fuel added, no fuel subtracted.)

 

The last time I dicked with unreasonable fuel trim numbers with no obvious air leaks or fuel system problems, I had a faulty O2 sensor sending incorrect data at highway speed.  The O2 sensor "tested" good, including reasonable cross-counts at lower speed--but fell on it's ass as speed went up.  OTOH, this reliably set a code, which you aren't getting.

 

Do you have an AIR system?  Any chance it's blowing upstream when it should be downstream?

 

If vacuum hoses are routed incorrectly as others have implied, that's gotta be corrected or you're just chasing your tail.


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