GM truck running very rich

[Sluggo]

I bought a newer work truck yesterday, an 05 Chevrolet Avalanche; 5.3 motor auto trans, 4WD.

In good condition, but the truck has that smell of a vehicle that runs really rich, like the old carbureted vehicles smell when the choke is on.

Everything looks stock under the hood with the exception of some type of cold air intake that some previous owner has installed. Now that seems to be the first culprit for running rich, but I would think that the computer would automatically compensate the fuel mixture for any variation in air intake.

I haven't looked around it too much, just wondering if there is anything really obvious to check besides the CAI.

 

[phucheneh]

First thing I'd do is check to make sure the CEL is working (key-on self test). If it's working and not illuminated, the car should be getting into closed loop, which means any minor corrections to the fuel trim should be getting made. Closed loop simply meaning the PCM is taking feedback from the O2's and tweaking the mixture accordingly.

It sounds like your truck is running MAJORLY rich, though. Which usually (if no CEL) indicates one of the primary sensors used to calculate base fuel trim is bad; but not bad enough for the PCM to raise a red flag and disregard it. Common culprit is the coolant temp sensor (ECT). That's basically the EFI equivalent of a choke that won't open- an ECT that is reading much lower than it should will cause the PCM to fuel as it would during a cold start.

Could also prevent closed loop without illuminating the MIL (CEL).

If you have access to a scan tool with live data, these things can all be displayed.

The only thing CAI-related would be the MAF; usually they read low and cause lean conditions, though. If you have a K&N-style (oiled) air filter on there, I would replace it ASAP regardless. I'm gonna really go out on a limb here and say that PERHAPS oil and gunk accumulating on the MAF's wire could add mass to it and cause readings to actually be elevated?

...ECT is the likely culprit, though. I doubt a visual check will find anything; you'll likely need to just observe the data.

 

[Sluggo]

CEL functions properly. Lights with key on, then goes out when started.

Fortunately for the CAI, they only took off the stock air cleaner box, so finding one of those to put back on should by quite easy.

 

[phucheneh]

So basically, it's not a CAI. They just installed an 'intake loudener.'

You can buy dry cone filters to replace the oiled K&N, if that's what it has. If you don't mind the noise, there really no other issue besides the oil (and extra dirt due to the low filtering efficiency of those things).

See if you can beg/borrow/steal a scan tool and read the temp sensor data. There should also be a spec for MAF at idle, but those are pretty rough estimates. Will expose a sensor that's grossly out-of-whack, though.

You can also just try unplugging the ECT. If it runs leaner with no input, you know that little guy is reading too cold.

 

[Meghan54]

Been told by my brother and others that a clogged vapor canister and/or bad canister purge valve will produce that symptom. Worse after refueling?

 

[phucheneh]

There's not a tremendous amount of fuel in the vapors running through the evap system. A stuck-open purge valve should actually produce a lean condition, as it's essentially a vacuum leak. Stuck closed (more common, since there's a spring that should make the solenoid fail in that position) will cause no running issues, just a CEL for insufficient flow (since it's a GM, it will probably just throw p0440 for anything evap-related...).

Not sure about a clogged canister. I've seen them full of gas from people 'topping off' after the auto shutoff, but it's always just manifested as an evap problem, not an intermittent rich condition (it would be intermittent because the purge valve isn't always open). Probably because it generally causes evap monitor failure, which will result in the PCM turning on the CEL and inhibiting normal evap system function.

If it's not a sensor related to air/fuel measurement (on the intake side; the O2 'feedback' shouldn't cause an extreme rich condition), it's probably related to the actual fuel injection parts. Either the fuel pressure is too high (bad regulator) or an injector is stuck open. The latter usually also causes starting problems, though.

 

[Sluggo]

Been told by my brother and others that a clogged vapor canister and/or bad canister purge valve will produce that symptom. Worse after refueling?

Haven't put fuel in yet, so I can't speak for that.

 

[SparkyJJO]

Don't go by smell as that can fool you. If it is really running rich the mpg will suck. And don't go by your first tank either, wait until you've run a couple tanks and see what you're getting.

 

[Sluggo]

Don't go by smell as that can fool you. If it is really running rich the mpg will suck. And don't go by your first tank either, wait until you've run a couple tanks and see what you're getting.

The smell is unmistakable, it's not like a faint odor, it's full on '72 Plymouth beater mode running rich smell. Even today as I was shuffling cars around the driveway, the smell of the newer one compared to my '04 Silverado is night and day.

Driving it home I noticed the OBC showed 9.5 average MPG, whereas my current work truck, same motor and 4WD drivetrain shows 15.6 There is definitely an issue with the amount of fuel it uses.

 

[SparkyJJO]

Is it getting fully up to operating temp?

Check the fuel trims. It could be thinking it has a vac leak, or maybe the O2 sensor is out of whack.

Usually this stuff has a fairly simple cause but you don't want to wait too long. excess unburnt fuel, aside from hurting your mpg and wallet, can damage the cat converter in the long run.

Actually, first thing I wonder about is how much did that aftermarket intake change? If it messed with the MAF sensor or its location at all that could easily cause it. Also, if it is a K&N oilable filter, DITCH IT and get a paper one, then clean that MAF sensor with MAF sensor cleaner. Those K&N filters are easily over oiled, which can then coat the sensor in some oil and screw the readings up. Plus, K&N doesn't filter as well, and there really are no HP gains worth speaking of, not at the risk of increased engine wear.

 

[phucheneh]

If you've got a strong fuel smell coming out the tailpipe, you're absolutely wrecking the cat. It's running majorly rich.

Again, if the CEL is functioning and is not on, you should look for something that will hold it in open loop. If it was getting to closed loop, you'd have a fuel trim DTC.

ECT is #1 culprit. MAF, MAP, and IAT may also be possible (you may only have a MAF and no MAP. Some vehicles have both, some [usually older] ones only have MAP).

 

[96Firebird]

I'm going to go ahead and ask - Does the truck have a catalytic converter? Any exhaust leaks?

 

[exdeath]

The intake probably screwed with the MAF or IAT assuming that's all that was done to it.

 

[iamwiz82]

I'm going to go ahead and ask - Does the truck have a catalytic converter? Any exhaust leaks?

I'd look to see if it has a cat for sure.

 

[Sluggo]

FFS, I can't even fathom what I think I am seeing.

Took the truck in for a diagnosis, shop confirmed what was surmised earlier, that the ECT, and MAF need replacement. ECT, was just worn, but the MAF was corroded from moisture getting in through the stupid cone filter, which the PO had pointed down toward the ground.

Replaced those parts last weekend in addition to a stock airbox. Drove it this week expecting good things, I was horribly let down. The computer average MPG dropped from 14.8 to 14.1 fffuuuuuuu.

Drug out my scan software and loaded it onto the garage laptop, and looked to see what i could see. Oddly enough most things looked kind of OK. Drug out my garage stethoscope to see if I could hear an injector stuck open, they all tick along happily.

Came back and read this thread, and kept seeing Cats, surely it has cats, I would have noticed if it didn't...surely. Besides I took it up a few weeks back and had the magnaflow crap cut off and a normal muffler welded on, the muffler guy would have said something...but it sure was loud for a regular muffler...

So tonight I crawled under, and I THINK some idiot has cut open the cats, took out the innards, and welded them back closed.

I hate people.

 

[Slacker]

The joke will be on the thieves who cut your cats off for scrap.

 

[iamwiz82]

Did you try knocking on it? If it sounds hollow you are probably right. Sure looks that way though.

 

[phucheneh]

That doesn't seem like your issue as much as it seems like a RESULT of your issue. As in, the cats melted down and someone gutted them. I would think those giant holes would be big enough to get all the material out...the truck won't run bad because of a lack of functional converters.

Just two cats on the truck? Single exhaust? Do you feel a good amount of pressure at the tailpipe? Fairly constant pressure, at that? A plugged exhaust will sometimes produce an erratic 'PUFF.....PUFF....PUFF PUFF....PUFF'....like the exhaust is coming out in gasps.

I thought you said the CEL wasn't on?

...did you confirm that it worked?

Why did the shop do MAF and ECT? And, more importantly, why did you pay them and/or not return? Their diagnosis was obviously incorrect. If they give you a 'well, it turns out it was the MAF, ECT, and...' kind of thing, tell them to cram it up their asses.

 

[michal1980]

why didn't you have the car inspected when you bought it?

 

[phucheneh]

Oh, and if you're paraphrasing their diag, find a new shop. ECT's don't 'wear,' and MAF's will not get visibly corroded...especially not when an air filter soaked in oil is installed.

Their repair notes should list any DTC's and relevant sensor data. Not 'hur-dur it was worth a shot.'

edit: also post pics of your engine bay.

 

[Sluggo]

That doesn't seem like your issue as much as it seems like a RESULT of your issue. As in, the cats melted down and someone gutted them. I would think those giant holes would be big enough to get all the material out...the truck won't run bad because of a lack of functional converters.

There is the fact they look cut open, and there is an "X" pipe installed and the magnaflow exhaust makes me think a PO thought it would improve performance.

Just two cats on the truck? Single exhaust? Do you feel a good amount of pressure at the tailpipe? Fairly constant pressure, at that? A plugged exhaust will sometimes produce an erratic 'PUFF.....PUFF....PUFF PUFF....PUFF'....like the exhaust is coming out in gasps.

Two cats into a single exhaust run now, exhaust flow seems normal to me. Before I had the new muffler put on, it was 2 cats -> "X" pipe -> 2 pipes -> magnaflow -> 2 pipes. Now it is 2 cats -> "X" pipe -> 2 pipes -> normal muffler -> tailpipe.

I thought you said the CEL wasn't on?

...did you confirm that it worked?

Was not on for my original post, PO may have cleared it. It began to appear intermittently but never for a long period.

Why did the shop do MAF and ECT? And, more importantly, why did you pay them and/or not return? Their diagnosis was obviously incorrect. If they give you a 'well, it turns out it was the MAF, ECT, and...' kind of thing, tell them to cram it up their asses.

They did not do the work, I replaced the parts. I paid them for the diagnosis and was prepared to let them do the work if their diagnosis of the CEL coincided with your thoughts. They wanted to jack up the parts prices too much, so I did the work. Made a thread about parts prices.

Oh, and if you're paraphrasing their diag, find a new shop. ECT's don't 'wear,' and MAF's will not get visibly corroded...especially not when an air filter soaked in oil is installed.

They said ECT was bad, it still read correct on the temp gauge so I said "worn." They said MAF was "rusted", when I took it apart, the screen was gone, and the the wires looked caked with crap, could have been rust, could have been K&N oil, so I just replaced it.

why didn't you have the car inspected when you bought it?

It was a take it or leave it cash deal on a Saturday. It was a good enough deal to take it even if I have to do some work to it. Knowing what I know now, I would do it again.

Giant Pic:

 

[phucheneh]

I apologize if I led you in the wrong direction- I was just going off of your info/descriptions and my own logic/experience.

Unfortunately, it sounds like the shop had the same thoughts as me...but the key part is following through and actually verifying your suspicions, which it seems like they did not. Also, TBQH, I would've told you to avoid the 'diag but don't repair' scenario. If they don't think you're going to do the work, the mechanic is just going to collect his hour of diag and write some stuff down; he doesn't care if he's right, 'cause he's not on the hook for it if you do your own repair.

I wanted the engine pic just to verify that nothing else had been modified on the truck. I kinda suspected something would jump out at me...but that looks stock.

If the ECT was the cause of your rich condition, you would see grossly inaccurate readings while viewing sensor data with a scan tool.

The MAF is usually a lot harder to peg as bad. It's also less likely to cause a noticeable rich condition (usually the complaint is a CEL due to a fuel trim code and maybe a mild decrease in MPG).

Did you say you had a scan tool?

If the cats have been gutted, the CEL should come on with P0420 after the catalyst monitor is run. And it will stay on. But that will not cause a rich condition.

 

[Sluggo]

I apologize if I led you in the wrong direction- I was just going off of your info/descriptions and my own logic/experience.

I don't think that, I asked for opinions, and you offered.

I have been horribly busy at work and home lately, so I did intend to let the shop do the work. I had also just done the water pump and thermostat in my old Silverado, and didn't feel up the the draining the coolant hassle again.

For the stock airbox & filter, MAF, and ECT they quoted $760 parts and labor. I thought that was on the far side of reasonable, so I declined their service.

I have this software which can provide some live data, sensor readings, and show & clear codes.

Currently shows CEL no codes or pending codes.

 

[ISAslot]

Just a thought, not sure of the specifics of this vehicle... since the PO made some 'performance' mods, is it possible they could have added some 'performance' programing? Something that would cause the symptoms and yet not cause the CEL to come on?

 

[NutBucket]

That's a great point ISAslot