GM truck running very rich

[phucheneh]

That's a great point ISAslot

And he may be right, but can you disable OBD2 monitors on a stock PCM? I wouldn't think so; at least, not with just a software flash.

And when it runs the catalyst monitor, it should set P0420. Or maybe it has a rear O2 simulator- which is basically just a resistor to mimic the heater and a constant voltage supply for the O2 signal (PCM wants to see switching at front but a more constant voltage at rear). I dunno how well they actually work.

Can you confirm that all of the monitors have run? What do the front and rear O2 signals look like? Is it in closed loop (should be a simple PID for closed loop yes/no)?

Is the ECT reading normally? Around ambient temp after sitting overnight? Then rising steadily towards operating temp (prob 190-200 or so) during a reasonable warm-up period?

IAT reading ambient-ish? It should also rise as the engine warms up, but not a whole lot. Especially if it has a plastic manifold and the intake is ducted to fresh(er) air in the fender.

Assuming there's an IAT...I think there is. I want to say GM just stopped using MAP, but continued with IAT, after modern MAF's were introduced.

Speaking of MAF...don't bother watching that one. A good MAF and a not-quite-right one are pretty damned hard to tell apart. But in the absence of a DTC, 'new MAF' should equal 'good MAF.' A defective new part would likely be pretty FUBAR, rather than just being inaccurate-but-within-spec.

This is a bit of a tough one. In my earlier post, I was assuming everything was stock except for the air intake. Now I'm wondering if it's even POSSIBLE that all the parts are stock. As I think I already stated- if the truck is getting into closed loop, any change to the engine should be corrected for. Inaccurate mass meter or ECT, out of spec fuel pressure, different throttle body or injectors...anything. If O2 feedback indicates it needs to adjust the long term fuel trim away from baseline, it will do so. Right up until it hits the limit for the amount of deviation it will allow. Then it will set a fuel trim code.

edit: P.S. check the fuel trim numbers. There is at least one short term and one long term PID for each bank. They will be in a 'x.xx' number format. Some software may show it as a normal percent ('110%' instead of '1.10'). The max deviation should be 25%...so LTFT should be 0.76-1.24 and roughly the same on both banks.

 

[NutBucket]

Yes you can. I have some disabled on one of our cars and all the monitors are set.

EDIT: But I'm sure the ability to disable things is highly dependent on the ECU in question.

 

[phucheneh]

What make?

If that's the case here, I guess he can just check and see if the monitors are set, then check to see if all the stuff is there and giving proper signals. If the former but not the latter, I would assume it's been flashed with a custom program. And modified hardware is, of course, possible. You can generally tell if a factory ECM has been cracked open. I dunno where it's at on that truck; I see they didn't mount it in two of their common, totally-not-retarded spots: on top of the battery or on top of the air cleaner.

edit: I forgot on top of the coolant tank. GM, you so crazy.

 

[nerp]

This is why people who put CAIs in are usually making a mistake. I bet that 2HP gain was AWESOME!!!! sheesh.

Good luck with your repairs, sir.

 

[NutBucket]

phucheneh,

Subaru. This is a list of the CELs that are disabled:

P0037 - Rear O2 Sensor Heater Control Circuit Low
P0038 - Rear 02 Sensor Heater Control Circuit High
P0137 - Rear O2 Sensor Circuit Low Voltage
P0138 - Rear O2 Sensor Circuit High Voltage
P0139 - Rear O2 Sensor Circuit Slow Response
P0420 - Catalyst System Efficiency Below Threshold
P0545 - Exhaust Gas Temperature Sensor Circuit Low
P0546 - Exhaust Gas Temperature Sensor Circuit High
P1301 - Misfire Detected (High Temperature Exhaust Gas)
P1312 - Exhaust Gas Temperature Sensor Malfunction
P1544 - Exhaust Gas Temperature Too High

 

[NutBucket]

You think GM would ass rape the OP if he wanted a the ECU flashed to a stock tune? Or maybe it would be cheaper to just buy another ECU?

 

[manimal]

Trying to read entire thread on my phone and its not workng very well. Did op Disconnect battery and reset?

If the PO used a flash tune it would reset or if it was a custom tune from guys like westons or black bear or nelson then they can go back to stock for him.


I wonder if the truck had a tune done for specific parts and some of those parts are now off the car?

I picked up a gsxr750W a few weeks ago cheap because it ran like steamed ass. PO had a yosh and no airbox with single filters and put everything back to stock with the bigger jets in. Doh! lol

 

[Sluggo]

Back out in the garage today.

ECT, and IAT seem normal, ambient when starting, then ECT rises and settles in around 200 (F) degrees, the IAT was about 20 (F) degrees above outside temp, but it was idling in the garage with the hood down.

Below are screen shots, after starting it stayed in open loop for maybe 4-5 minutes then went to closed loop. Although it shows separate rear O2 sensors for both banks, it only has one rear O2 sensor.

OPEN LOOP:

REST ARE CLOSED LOOP:

 

[phucheneh]

Best ATG ask-for-help ever. Posting graphs of sensor data?

Shit...normally it's like pulling teeth just to get someone to check battery voltage or something.

That said, though, those graphs are hard to use. No time interval on the x-axis, and the y needs the min/max tweaked and maybe some additional labelling. But I can still give some comments.

The first bit of your post sounds totally normal. As for the graphs-

OPEN LOOP:
IAT never changes, ECT changes an unknown amount, neither have any kind of baseline reading. Not terribly useful. But I'll take your word for it regarding their function. Nothing jumps out at me as 'bad.' STFT seems to be switching a lot, but I wouldn't pay much mind to it, either.


REST ARE CLOSED LOOP:
O2 graph seems normal. The important thing is that they're switching. And hitting a normal voltage.

Second pic is more of the same, but then...LTFT! ...what the fuck is going on there. Totally constant? Any idea of the time period on that graph? Or the actual numbers? Looks like between 1-3%, which is actually a suspiciously low deviation.

If there's only one downstream sensor, why are there two different graphs? The top one does not looks like a rear O2 graph that would result in a passing catalyst monitor.

 

[Sluggo]

Those graphs represent about a 2-3 minute time frame.

I assumed the long term fuel trim didn't move much since it was just idling in the garage, and as stated, it only represents a 2-3 minute time window.

Now I am second guessing myself on the rear O2 sensors, I only saw one, but it may have one kind of hidden around the transfer case, I need to look around a bit more.

Also need to get a fuel pressure tester form Autozone, and see what the pressures are.

 

[Sluggo]

If there's only one downstream sensor, why are there two different graphs? The top one does not looks like a rear O2 graph that would result in a passing catalyst monitor.

My mistake, there are two downstream sensors, I was looking from the wrong side. Which side of the vehicle is considered bank 1 or bank 2?

Guessing I have at least one wonky downstream O2 sensor?

 

[phucheneh]

I doubt it's the O2. What it is, is a catalyst that's not functioning properly.

If your cats are totally gutted, the downstream O2's should more or less look like the upstream ones. Assuming there are also no exhaust leaks. The oxidation/reduction reactions in the cat should cause the graph to be fairly smooth.

Bank 1 on a Chevy V8 is always driver's side, AFAIK.

 

[phucheneh]

http://www.bosch.com.au/content/language1/downloads/Section_A.pdf

meant to post that yesterday.

Look at the part with a standard front (feedback) O2 graph. Skip the wideband one and look at the standard rear (catalyst monitoring) O2 graph.

Your upstream graphs look fairly normal. They should ideally be a bit smoother, but PC graphs usually look pretty irregular, as the update speed for the PID is limited and just won't produce a smooth waveform...the software just draws lines between each data point, so something that may look like a smooth sine wave a scope might be a jagged spike on a PC.

Your rear O2 graphs are weird, though, and as stated, I would be strongly inclined to think that one if not both of them should result in a catalyst efficiency (P0420) code being generated.

 

[Sluggo]

Minor update, the saga continues.

Left the battery cable unhooked overnight Sunday night to let everything reset. Not much changed, the average MPG (just the dash readout, not scientific, just something to set a base observation) went up slightly based on driving today, but probably too short of a sample to base anything off of.

Got some time after work today and checked the pressure at the fuel rail.
Showed 50psi KEY ON - Engine OFF
Showed 60psi ENGINE ON blipped the throttle a few times and the pressure stayed steady.

Book (the GM shop manual for my old 99 Silverado) says 55-62psi KEY ON - ENGINE OFF so it's a tad low, not sure if its low enough to worry about. I am guessing most of the info in that manual are still applicable to the newer models.

The weirder part of today is the downstream O2 sensors. Got to looking at them, then checked my old 04 Silverado which I still have, they looked different. Someone has put the downstream sensors into some kind of extension tube (WTF?) That may explain some of the abnormal readings from above, the sensor isn't even the exhaust flow, just getting a tiny whiff of the exhaust through a small hole?

So I went up to NAPA and got 2 new downstream sensors and put those on. Leaving the battery unhooked again tonight and will reset the AVG MPG thing again to see if anything changes.

 

[phucheneh]

Ah. While I am still surprised that it passes the catalyst monitor...that explains it.

I'm not super familiar with that methodology, but I've definitely heard of it. Basically, I think you're decreasing the sensitivity of the sensor by pulling it out of the general direction of airflow. You're just gonna hold a little pocket near the sensor that does not flow normally; ideally, this will cause the 'switching' to slow down and not throw a code.

I would think the voltage would be off, though. Yours is fairly normal (~500mV is nominal), minus the little spots on the one that drop to 0v.

Unfortunately, new O2's are a waste. All those do is monitor the catalyst...they will not impact mileage. You will, however, start setting P0420, I'd wager.

I'm quite intrigued by this vehicle. Hard to give much useful input without getting hands-on with it.

Remind me again...exactly how bad is the mileage? Are you going ONLY off of the trip computer?

Does it smoke black? Can you still smell fuel? How easily does it fire up in the morning?

Also the battery thing is unnecessary. Disconnect neg terminal, bring over to positive terminal and hole them together for about 30 seconds. Wiggle it a bit to make sure you get good contact.

That will discharge all the voltage in any electronic module. If the terminals don't reach, use a metal tool (wrench or extension works) to bridge them. Obviously, just make sure you don't actually bridge the battery posts. Disconnect both terminals before touching them together if it makes you feel more confident.

 

[NutBucket]

Exactly right phucheneh. Those tubes/spacers are used all the time when folks play with the cats. This keeps looking like a truck that had more mods prior to being bought by the OP.

 

[Sluggo]

Remind me again...exactly how bad is the mileage? Are you going ONLY off of the trip computer?

The computer is just a baseline for me, not the actual mileage, I am actually doing some math.

Haven't had a complete fill-up in a couple of weeks. My mileage keeper iPhone app initially put this truck at about 14.0 MPG

Now my older Silverado with 100k more miles and lower gears 3.73 compared to 3.42, same motor and trans, and an open bed was at 14.9 with the iPhone app. From reading some forums, this Avalanche should be pulling closer to 15-16 or more MPG with my driving loop.

Does it smoke black? Can you still smell fuel? How easily does it fire up in the morning?

Fires right up in the morning, always fires quickly, never an extended or multiple cranking required.

No discernible black smoke, but the smell is still strong as ever. It's hard to describe how bad it really is. Even idling in the garage, me sitting in the truck with open windows my eyes will burn a bit.

If you are old enough to remember carbureted cars, it smells like a fully choked car on an ice cold winter day. You can smell it all around and as far as 25 feet away when it's still outside. So I know something is pushing a lot more gasoline than the truck has any idea what to do with.

I never recall an aftermarket throttle body, or oversized injectors being wildly popular for these trucks. Everything besides the cats and the O2 extension tubes appears to be stock, so I keep falling back on faulty/dripping injectors, or a programmer as suggested above.

 

[NutBucket]

I still think it has no cats and/or a aftermarket tune. Even my properly tuned Subaru running catless wasn't nearly as rich as you describe.

 

[Sluggo]

I still think it has no cats

Oh it has them, pretty sure they are just the hollow shells of their former glory.

Literally.

 

[phucheneh]

I would say it's about 99% certain on the 'no cats' thing. But that should only matter if he needs to pass emissions...and apparently the old O2's and their little spacer tubes had that covered.

I don't think decreasing exhaust backpressure on an NA SBC is going to hurt gas mileage. Certainly not to any measureable amount...

...that said, 14mpg vs 16mpg isn't exactly significant. That kind of decrease can be explained with driving habits.

But the fuel smell is abnormal. Does it do it when it's warmed up? Say you've been driving awhile and stop at a drive-thru? Or maybe just in traffic with the windows down? Any smell?

It sounds to me like it just has a cold-start issue. Leaking injector is possible. Hard to diagnose without pulling fuel rails. What you do, is take out the retaining bolts, and anything else in the way (leave fuel connected), and wiggle the rail/injectors loose. Lay it down with the tips of the injectors pointed down and resting on a piece of white paper. Prime the fuel system up, then let the truck sit overnight. Look for stains on the paper in the morning.

What was the LTFT like, again? Need the actual number. Gotta have some driving on the truck after resetting eeverything, or it will just be 1.00.

 

[phucheneh]

Oh. Also:

A leaking injector, or a bad vacuum regulator (not sure if you have one), will obviously cause pressure loss. So just hooking a fuel pressure gauge up overnight is a good test. It just won't pinpoint an injector.

 

[Sluggo]

But the fuel smell is abnormal. Does it do it when it's warmed up? Say you've been driving awhile and stop at a drive-thru? Or maybe just in traffic with the windows down? Any smell?

Yes, it's there all the time..cold, warm, and hot the smell never goes away.

In traffic with the windows down at a stoplight, it's there, that's what makes me think it's hurting the mileage.

Will try the fuel pressure gauge tomorrow night. From my reading, the pressure regulator on this truck is in the fuel tank, not the fuel rail.

 

[phucheneh]

I figured it was returnless. Which just means one less possibility.

Are you sure 60psi is spec? That's quite a lot for most EFI systems. I can't recall seeing one that had normal pressures that deviated much from the 30-40psi range. Maybe call it 25-45.

But four bar (I'm gonna go all wonkey Euro for no reason)? That's a lot of pressure for a run-of-the-mill port fuel injected engine.

If the gas smell is that strong, you should at least be able to locate it to the engine bay versus the exhaust. It almost sounds like it's NOT the engine running rich, as gas fumes out of the tailpipe are more likely to be smelled by those behind you. With a proper rear-exiting exhaust, you don't really smell your own vehicle at a stop light.

 

[iamwiz82]

Those are spark plug defoulers. Google 'spark plug defouler o2 sensor" and you will see thousands of hits on how to fool the sensor.

A de-catted engine will have a VERY strong odor. You can tell when you are behind one on the freeway at 80mph.

 

[thefordmccord]

I think the only way you are going to fix this is to remove the defoulers and install new cats. Every fuel injected 96 and newer small block Chevy that I have been around (with the cats removed) stunk. Always a strong gas smell.