Trouble with my 1999 C2500 Suburban again...P0174 code

[travis]

I posted here a few weeks ago about the P0340 code on my suburban. After replacing the cam position sensor and the crank position sensor it ran great...I had no issues with it even during a 5 hour each way trip just last week. Last night I went and put gas in it...about halfway home from work this morning it just falls on its face. Massive loss of power, idles and runs rough...feels like it’s only hitting on 4 or 5 cylinders. My first suspicion was water in the fuel, so I stopped and put a couple bottles of iso-heet in it, then limped the last 12 miles home. Broke out the tools...seeing about the same fuel pressure as always, but with the scan tool I got a P0174 (bank 2 lean). The bank 2 O2 sensors are reading quite a bit different than bank 1 (which according to what I am reading online they are reading correctly. Bank 2 however, doesn’t look right. The front sensor reads between 0.220 and 0.440V, the rear bank 2 sensor only varies between 0.880 and 0.890V. So it appears that the rear sensor has failed. Since the rear O2 sensor is reading so rich, could that be why the front sensor isn’t fluctuating like it should be, and could 1 failed sensor absolutely kill power like this?

It is dual pipes and cats fwiw, coming together into a dual in/single out muffler and single tailpipe

[cv67]

Do you have an exhaust or intake manifold leak? Check clamps from the air cleaner to the tb

[Newold1]

I believe on the O2 sensor banks on a GM sub or truck the rear sensors will not read at the same values as the front sensors, they are both measuring different conditions and as such they will and do not read the same. Both sets of sensors should have values that are moving up and down constantly and generally a bad sensor will stay at or very close to one value and not move for example when the throttle is blipped.

Sounds like your engine was going into the limp mode or you have an injector problem that was so far out of normal range that the overfueling or under fueling was causing a cylinder miss and loss of power. Did the cylinder miss codes show on the scan or in history?

Which engine is in your Sub, the 5.7L or the 8.1L?

[rebelrouser]

The O2 sensors in front of the converter should bounce around .5 volts, with 1 volt being rich, 0 volts lean. take some carb cleaner and spray into the intake hose, to drive the sensors rich, if you do not get at least .9 volts they are junk. The 02 sensors after the catalyst should read very little voltage if the catalyst is working as it should. If the front and rear sensors read the same the catalyst is junk. Exhaust leaks really mess with sensor readings because you are introducing oxygen into the exhaust stream that was not part of the combustion process. So look for exhaust leaks. A miss fire will also mess with the O2 sensors as again you are not burning the fuel so extra oxygen is in the exhaust, and will be read by the computer as a lean condition.

https://www.motor.com/magazinepdfs/102000_04.pdf

[GARY C]

Would O2 cause limp mode?

Both my 1997 and 2001 2500HD's had O2's go bad and it gave an O2 sensor code but never effected the way they ran.

[makin chips]

I've never seen a bad O2 cause limp mode in any kind of gm vehicle.

It almost sounds like the egr valve was stuck open if it has one. My '01 Chevy has one. I'd suspect your' 99 does also. That usually doesn't set a p0174, though, AFAIK. Though Ford does specify a stuck open egr can cause it to set P0174 in their ecm's. Gm could possibly do the same.


I'd start there.

[CGT]

Isn't the post cat sensor used soley for catalyst monitoring? I wouldn't think that would cause any driveability issues.

[makin chips]

It won't. I've been missing the cat in my truck for years, light shining brightly with P0420 saved in ecm, and it drives better and makes more power than it ever has. Best mileage I've gotten, also.

But I'm a stickler about maintenance, too. So...

[travis]

I believe on the O2 sensor banks on a GM sub or truck the rear sensors will not read at the same values as the front sensors, they are both measuring different conditions and as such they will and do not read the same. Both sets of sensors should have values that are moving up and down constantly and generally a bad sensor will stay at or very close to one value and not move for example when the throttle is blipped.

Sounds like your engine was going into the limp mode or you have an injector problem that was so far out of normal range that the overfueling or under fueling was causing a cylinder miss and loss of power. Did the cylinder miss codes show on the scan or in history?

Which engine is in your Sub, the 5.7L or the 8.1L?

5.7L, no history as it just started this morning. It was running great before

[travis]

Coolant (*F) 176
Engine Speed (RPM) 678
Fuel Sys 1 Clsd
Fuel Sys 2 Clsd
IAT (*F) 79
IGN Adv (*) 21.5
LT Ftrm1 (%) 8.6
LT Ftrm2 (%) 25.0
MAF (LB/M) 0.95
MAP (“HG) 10.3
O2S11 (V) (cycles between 0.100 and 0.880
O2S12 (V) (cycles...)
O2S21 (V) (cycles between 0.230 and 0.440)
O2S22 (V) (cycles between 0.880 and 0.890)
ST FTRM11 (%) 0 to -0.8
ST FTRM21 (%) 7.8 to 10.9
ST FTRM1 (%) 0 to -1.6
ST FTRM2 (%) 7.0 to 10.9

These readings I took this morning. Now that I’ve got some sleep, it’s time to troubleshoot again.

I’m nowhere close to an expert, but doesn’t that look like it is adding an awful lot of fuel trim on bank 2?

No exhaust leaks that I can tell. I will be checking for leaks on the intake side though

[Dirtybob]

Long time listener, first time caller here.

Could be a bad (front) O2 sensor. Try swapping sides and see if the fuel trims follow.

[Curtis Mc]

Looks like a vacuum leak to me. It may be a single port, but may be a series of very small ones. Since you the G1/2 based SBC with that spider injector intake, it's can be a pain to diagnose. (we used to have a '99 Burban)

Other things I see is that your vacuum is low as is the MAF grams. The STFT is 10% on the one bank and reasonable on the other side. If you have a fast/sophisticated scanner, you can crack the throttle to 1500-2000 rpm to see what your trims say there. If the high one rolls back to 5% or less, that is even more of a indication.

If the data you provided is the freeze frame based on the code setting event, then it's even better evidence that it's a vacuum leak since it's off idle.

Does the truck run better when warm? May not since it appears you may have tipped a limp mode criteria or your spider injector(s) are messed.


Also, as others have noted, your downstream O2's are for 'catalyst efficiency', they should be very stable with normal cruising and trend based on heavy throttle. They should not mirror the upstreams, if they do, and if you have cats (you do) they your cat(s) are not working properly. (different code though)

[travis]

I found a vacuum leak (same one that has bitten me before)...the only rubber hose on this thing runs from a solenoid looking doohickey on the firewall down to a water control valve down by the passenger side exhaust manifold. I replaced the hose thinking this was it...it made absolutely no difference Still throwing the same P0174 code, still misses at idle, and still runs like it is down several cylinders. Scan readings are somewhat better though

I put intake gaskets on this 2-3 years ago, and at the same time replaced the spider injector assembly with the upgraded Delphi unit. I have sprayed carb cleaner all around every gasket surface and have noticed no change at all. Doesn’t mean it isn’t a gasket issue but it certainly isn’t looking like it.

This is what I am getting now...

Coolant (*F) 172
Engine Speed (RPM) 672
Fuel Sys 1 Clsd
Fuel Sys 2 Clsd
IAT (*F) 81
IGN Adv (*) 20.4 to 26.4, much steadier than before but still fluctuating rapidly
LT Ftrm1 (%) 2.3
LT Ftrm2 (%) 25.0
MAF (LB/M) 1.0 to 1.10
MAP (“HG) 10.3
O2S11 (V) (cycles between 0.032 and 0.981, smooth oscillating curves)
O2S12 (V) (cycles between 0.200 and 0.772, smooth oscillating curves)
O2S21 (V) (cycles between 0.246 and 0.832, erratic choppy curve)
O2S22 (V) (cycles between 0.890 and 0.920, rapid fluctuations)
ST FTRM11 (%) 0 to -1.6
ST FTRM21 (%) 1.6 to 7.6
ST FTRM1 (%) -1.7 to 1.0
ST FTRM2 (%) 0.3 to 6.2

[travis]

Ignition advance is *mostly* in the range listed above, but then monitoring it longer shows random dropouts down to like -65* and then up as high as 32* at idle??? Random electrical issue, or is the timing chain about to fall off?

[In-Tech]

The O2 sensors in front of the converter should bounce around .5 volts, with 1 volt being rich, 0 volts lean. take some carb cleaner and spray into the intake hose, to drive the sensors rich, if you do not get at least .9 volts they are junk. The 02 sensors after the catalyst should read very little voltage if the catalyst is working as it should. If the front and rear sensors read the same the catalyst is junk. Exhaust leaks really mess with sensor readings because you are introducing oxygen into the exhaust stream that was not part of the combustion process. So look for exhaust leaks. A miss fire will also mess with the O2 sensors as again you are not burning the fuel so extra oxygen is in the exhaust, and will be read by the computer as a lean condition.

The area in red is incorrect. The post cat o2's should be a fairly constant voltage, usually around 7-800 mv if the cats are good. Not trying to start an argument so all I can say is 90% of what I do every day is tune EFI and mainly GM late model stuff since the early 90's. I haven't followed this entire thread but remember someone mentioning to swap the pre cat o2's and see if it follows. You stated you have a misfire, chase that. Fuel pressure is critical on the early spider type injection systems. If the o2 swap doesn't change anything at least you have narrowed it to a bank.