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'79 coupe deville, '04 Deville DTS White Lightning
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Discussion Starter #1
Pretty self explanatory. I need to start working on what the issue could be. Light came on last night and I called onstar immediately. P0171 and P0174. No mods. I tried a search but there are way too many other caddy's that have this issue and the ones I found with the STS-V were all modified. Thanks guys.
 

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2006 STS-V, 2004 Pontiac GTO, 2008 Pontiac G8 GT, 1999 Chevrolet Suburban LT 4x4
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1,417 Posts
State of modification is really irrelevant other than any part you installed (OEM or aftermarket) and missing something to reconnect may be driving it. It is a lean code, it is generated due to something driving your LTFT's (Long Term Fuel Trims), over a pre-determined threshold in the PCM for greater than a pre-determined time frame. This code is standard GM code, and the concept of this system is the same regardless of GM vehicle. The only real difference may be the thresholds.

Step 1: Did you perform a Diagnostic System Check? Yea, yea, yea (this is like asking if you plugged it in)

Step 2: Monitor LTFT's are they greater than the specified value (this is usually somewhere in the 15% - 19% range)? Review any freeze frame data. If it's greater than that 15% - 19% for a long period of time. Move to step 3

Step 3: Monitor the HO2S's. Are they within the specified range (200mV - 800mV) and fluctuating (or referred to as "switching")? Lets assume they are moving/fluctuating, or switching normally, move to Step 4. If not, go to step 5.

Step 4: Insepct vacuum hoses for splits, restrictions, and improper connections. Ensure that there is sufficient fuel in tank. Verify fuel pressure. I've seen this on other N*'s and had it be either a fuel pump or FPR due to low fuel pressure. So from here we go to Step 6. If you did find the problem, go to Step 7.

Step 5: Inspect the HO2S for proper installation. Make sure the harness/wires are not contacting the exhaust. Insepct for a short between the signal circuit and the low reference circuit (I'm sure I can find that if need be). If this fixed it, I think you be done. If not, start looking into the fuel system (we can cover that if need be).

Step 6: Any missing, loose, or leaking exhaust? Any vacuum leaks at the intake manifold and injector O-rings. Check the air induction system and intake ducts for vacuum leaks/restrictions.

Step 7 is post repair and clearing the code, then running the vehicle for the conditions to run this DTC (or your freeze frame data), and see if it re-triggers. If it failed, start over. If it didn't, go to fridge, obtain beer, open beer, enjoy.
 

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'79 coupe deville, '04 Deville DTS White Lightning
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Discussion Starter #3
Thanks for the info. Unfortunately, a lot of what you say assumes I am much more knowledgable than I actually am. I'm thinking it also assumes I have certain tools that I don't.

Diagnostic system check? Is this like plugging a tech 2 into it? If so, I don't have one and I haven't taken the car to the dealer. If not, what are we talking about here? Will this identify where the fault is?

Definitely don't have anything that'll monitor LTFTs and HO2S. And yes I know what that stands for. Thanks google. Haha.

Step 4 seems like something I can do. Can you elaborate on where these hoses are that I need to inspect? How do I verify fuel pressure? If fuel pressure is no good, how do I determine whether it's the fuel pressure regulator or the fuel pump? Can any of these parts be replaced with a better aftermarket one?

Can you elaborate step 5 to someone who isn't as knowledgable as yourself? Pics would be great by the way.

Step 6.. I have been noticing a hissing noise especially around startup when the car is running over on my passenger side towards the back. Is this something? I'll inspect the intake. Maybe one of the intake manifold bolts loosened?

Went to the store yesterday.. Sam Adams octoberfest came and went. They're already on winter lager. Damn.

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By the way reading what you say it's seeming like this is going to end up being a dealer trip. We'll see hopefully it's something simple. My car is getting the d3 springs, new tires, alignment done right now so I'll get it back tonight. Oh and since I was on my last day on the old tires, I figured what the hell.
http://m.youtube.com/watch?v=YQlg7zEXaQg
 

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2006 STS-V, 2004 Pontiac GTO, 2008 Pontiac G8 GT, 1999 Chevrolet Suburban LT 4x4
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1,417 Posts
Diagnostic system check is plugging in any scan tool that has Bi-lateral controls (I.E. Tech 2, Snap-On MODUS or similar, etc)

You'd need said scan tool (or even just a read only scan tool), to monitor LTFT's and the HO2S.

Step 4. The intake on our car only has one hose leading to it from the drivers side of the engine/cam covers. I would check that, but then also check all connections to the plumbing making sure that all tubes are properly seated, clamps tight, etc (some GM vehicles any leak post MAF sensor can trigger this code if it's a sizeable leak). Leaks in plumbing on an PCV system can also cause this. Heck even ensure that your MAF harness is securely booted. Probably doesn't hurt to clean the MAF too.

To monitor fuel pressure, you'd have to hook a gauge up to the fuel rail, turn the ignition on, let the pump pressurize the system and see what reading you get. I know regular N*'s like a higher level of PSI, something in the mid to high 40's if I recall. So I would imagine ours would want more. Then start working your way back through the system. It's always better to work through the cheaper and more common failure parts such as the FPR first, and troubleshoot/replace a fuel pump as a last resort. No idea on aftermarket units. Maybe Walbro makes something. If worse comes to worse, you can always get an inline helper pump. I think for anyone really wanting to modify, one of the popular things to do is modify an 8.1L truck pump and can. But I have no experience with that in this application, so don't ask.

Step 5: In order to do this, you're need to get under the vehicle, inspect where the HO2S's go into their bungs in the exhaust making sure that they're threaded in properly and sealed (as a properly threaded, seated and torqued down and surface rusted HO2S should be). The 4 wire plug off of it, should not be touching it, and the wiring harness should run away from it (the sensor and the hot exhaust), and then be secured within a shield in the driveshaft tunnel (if I recall on our car, as I had to remove one of mine and recall re-routing my wiring a little to ensure no issues), running forward to the engine bay and into the main wiring harness. You may want to see if perhaps, that wire harness where it tucks in behind the shield, may have rubbed against that shield over time/movement from the exhaust and perhaps rubbed insulation off the wire, causing it to short out. Just visual checks here. To check circuits though, you'd need either a DMM (Digital Multi-Meter), or a noid light, and then we'd have to get the circuit layouts. Unfortunately I don't know what pics I could give as I don't have any and my car has been put away for winter.

Step 6: This sounds like the EVAP system running it's purge functioning maintaining vacuum in the fuel system (A whole different topic if you'd like to discuss), but if that were an issue, you'd also have EVAP codes.

I need to run to Binny's and see if they have Leinie's Honey Wheat for Thanksgiving...

Also waiting for the customer service to come in here, apoligize for the problems you're having and then asking you to take it to the dealer and to contact them for any assistance in resolution between you and the dealership.
 

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'79 coupe deville, '04 Deville DTS White Lightning
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Discussion Starter #5
Hahah very nice. Alright then thanks so much for taking the time. I should be getting the car back tonight so I'll look into this stuff tomorrow probably. Thanks again.
 

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'79 coupe deville, '04 Deville DTS White Lightning
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Discussion Starter #8
The one you see is comp driving mode. I went for it a few times after and the tires hooked up pretty good. So I turned off the stability system, same deal. The one you see is the first run. I think I warmed up the tires with that first go and so they were too grippy for any further ones. So yeah the one you see is comp driving mode.

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And I do realize the purpose of the stability system so I don't know why I thought turning that off would allow me to spin the tires more easily.
 

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06 STS-V & 06 STS 4 (sold 2016)
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Cool. Might as well try everything when you get the rare chance to trash your tires.

I've never done that because my tires are still good. I always do a roll out to keep them that way.
 

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I had those codes on my STS4. Took the dealer a few tries to get it right.
They ended up replacing the intake gasket after they over torqued the intake manifold bolts. If I remember correctly that finally solved it.


Sent from AutoGuide.com App
 

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I had those codes on my STS4. Took the dealer a few tries to get it right.
They ended up replacing the intake gasket after they over torqued the intake manifold bolts. If I remember correctly that finally solved it.


Sent from AutoGuide.com App
Which leads to distortion of the gasket, leading to air being sucked in post-MAF. See Reply to Step #4, in Post #4 by me.
 

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'79 coupe deville, '04 Deville DTS White Lightning
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Discussion Starter #13
Well the engine light is back off. I still checked out the intake manifold bolts. Seemed good. We'll wait and see what happens.
 

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I think I've posted this before. After a fault is triggered, it obviously sets the DTC commands the MIL on. That MIL will shut off after three consecutive, successful operating cycles subsequent to the fault. It then moves it to a history file.

But that doesn't mean the problem is fixed. It just means that the last three operating cycles the fault hasn't been present, and/or it has passed an I/M system test. If it failed once before, it'll come back eventually.
 

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Also note that my car was faulting these codes without ever illuminating the CEL/MIL.
Mine were only found while trying to diagnose my still unresolved speed limited to 120mph DIC message...
 

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'79 coupe deville, '04 Deville DTS White Lightning
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Discussion Starter #16
I think I've posted this before. After a fault is triggered, it obviously sets the DTC commands the MIL on. That MIL will shut off after three consecutive, successful operating cycles subsequent to the fault. It then moves it to a history file.

But that doesn't mean the problem is fixed. It just means that the last three operating cycles the fault hasn't been present, and/or it has passed an I/M system test. If it failed once before, it'll come back eventually.
So I take that to mean whatever the problem was that caused the CEL to go on, has gone away consistently enough for the light to go back off. I have a pretty good feeling that it will indeed come back eventually, but I'll cross that bridge when I get there. As of now, if I start trying to mess with stuff to "fix" a problem that may or may not exist any longer, I 1. May cause another problem, or 2. won't really know the problem is gone because the light isn't on now, so it won't shut off when the problem is "solved." Am I flawed in what I'm saying here?

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Also note that my car was faulting these codes without ever illuminating the CEL/MIL.
This would change things. I'll shoot onstar a call tomorrow to see if there is still a code even though the light is no longer on. Thanks.
 

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Yes, In three consecutive, successful operating cycles, either the fault did not repeat and/or the I/M system ran the test and it passed.

From there it turned off the MIL, moved the DTC from current to a history file. After 40 consecutive, successful operating cycles, it'll clear from the history file.
 

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'79 coupe deville, '04 Deville DTS White Lightning
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Discussion Starter #19
Just to follow up I checked for any ill fitting connections throughout the air intake. Looked good. I bought maf cleaner, cleaned the maf. Codes were still there. I had a gut feeling and I figured before I start feeding the dealer my money, I'll replace the maf altogether. Ordered a maf sensor from Chris over at Rippy. Replaced it and the code hasn't returned. Thanks for all the help.
 

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Sorry for digging up an old thread guys.

I'm dealing with a P0174 code now (just one side).

It happened after I replaced a bad coil pack on cylinder 2. That was a hard failure with very rough idle. Replacing it fixed that.

A little while later I got the code. Have cleared it and had it come back intermittently.

I have checked all the air intake clamps that I messed with to lift the CAI out of the way of the coil. Not sure what to make of it.

One other thing is that right before the coil failed (I don't think this is related to that failure) I got gas from a cheap Bradley station that I have never used before. I doubt that would be it, but it is another variable.

I would like to get another tank full back at Shell just to see, but I've been driving it so little right now that I'm only down to 2/3.

I'm thinking I will just wait until I can get a tank full and then see if it comes back (probably will). Anything else simple that I could check that you guys know of?

Or can you think of something I might have messed up when I changed the coil?

BTW, it runs great.

Thanks.
 
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