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Discussion Starter · #1 ·
Repectfully request advice: Low O2S2 voltage & high LTFT caused by bad Catalytic Con

vehicle: 2011 Cruze LT 1.4L turbo, 67K miles (This car was not properly maintained by the previous owner.)

Long Term fuel trim ~ 12.7
O2S2 (down stream O2 sensor) voltage .67
vacuum at idle after warm up ~ 16"-17" Hg
vacuum at 3000 rpm increases to ~ 20" Hg and snaps to 26" when throttle position goes back to idle. Vacuum then returns to 16"-17".
Vacuum and long term fuel trim go down when AC is on and load percent goes up from 18% to 32%
Long term fuel trim goes down to ~ 3.5 when engine is at 3000 rpm.
This is classic indication of vacuum leak.

After replacing the AC Delco throttle body gasket with the MAHLE G32627 gasket my engine pressure leak down time from 15 psi to 5 psi is 31 seconds. This is an improvement of 14 seconds from the 17 second leak down time with the ACD OE gasket that came with the new intake manifold. (I must order the check valve bypass kit from Extreme Revolution to fix bad PCV design in new intake manifold.)

I cannot find a vacuum leak with propane, spray bubbles, or starting ether. I have yet to fabricate and use a smoke generator.

My question is can a partially malfunctioning low efficiency catalyzing catalytic converter cause a low O2S2 voltage that results in a high long term fuel trim? I think there's too much oxygen in the exhaust because there's unmetered air getting into the intake, but if a partially bad Cat can cause high LTFT, then how can that possibility be checked?

Please forgive my ignorance. I've long thought catalytic converters could not malfunction unless they were clogged or melted or the internal matrix was broken. Mine is not clogged and was not melted or broken or restricted when I had it out of the car and washed it out with soap and water.

Perhaps I need to go to Harbor Freight and buy a laser thermometer to check the input and output temps of my car's first Cat. By visual inspection, I am unable to see oxygen sensors on the down stream Cat.

So off to Home Depot I go to buy and empty paint can from which to fabricate a smoke generator.

Thanks in advance and Best Wishes for Continued Success.
 

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Failures of convertors were usually caused by lean or misfire conditions. Or running leaded gasoline.

Since leaded gas became obsolete. Convertors have been pretty good at lasting practically forever.

Have no idea on the smoke generator. Never heard of it. But if you can't find a leak using propane or ether or carb cleaner. You're probably in good shape. EXCEPT.
The gen 1's seem to be famous for pcv problems. Including whatever name you want to call that cover that sits on the top of the engine. And replacement of the intake manifold.

Do you have any codes. CEL light on????
 

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Discussion Starter · #4 ·
Beyond me, maybe one of these three can help:
@Robby @ChevyGuy @jblackburn

In the interest of helping others later, would you please document your smoke generator and how you use it?

How-To: Write a Tutorial
Hello Good Evening. Thank you for your responding remark. The smoker can I set up was very basic. I bought an empty one gallon paint can at Home Depot. Drilled two .5" holes in the lid and screwed in air coupler fitting and hose barb, and then glued them in place with JB Weld Epoxy. After the JB Weld cured and was hard, I burned three charcoal briquettes in a food can until they were covered with ash. Then I put the charcoal can into the paint can and poured 2 ounces of mineral oil on the charcoal and put on the lid. That made much smoke. I connected an air compressor with a regulator to the air hose coupling and a 1/4" tube to the brake booster vacuum port on the engine's intake manifold. I opened the air tank valve a bit so smoke was forced into the engine. After a few minutes I noticed smoke escaping from the intercooler tube where it connects and fits onto the throttle body. That hose clamp is as tight as it can be without stripping out, so I think the hose has a crack.

The intercooler tube or boost charge tube is part 13345223 which one can see at

https :// www.gmpartsdirect .com/auto-parts/2011/chevrolet/cruze/lt-trim/1-4l-l4-gas-engine/cooling-system-cat/intercooler-scat

To R&R it one probably has to remove the front bumper cover. Perhaps it can be accessed from below after removing the front splash shield.



Thanks and Have a Great Week.
 

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Discussion Starter · #5 ·
Hello and Thanks for your responding remark.

//Failures of converters were usually caused by lean or misfire conditions.// - My Cruze has been running lean for a long time. The driver info display on the instrument cluster shows the vehicles gas mileage MPGs at 24.5 with average speed of 25 MPH.

After R&R of the intercooler tube or boost charge tube-hose
13345223 the vacuum/boost leak I found using the smoker can I set up, the long term fuel trim might go back down to less than 5%. I'll see to that later.

Over on Youtube Jim The Car Guy's presentation of diagnosing bad Cats was informative. Mine exhibits the same sort of behavior where the downstream Oxygen sensor voltage ranges or oscillates up and down instead of showing a steady value. I used the Harbor Freight coupon to buy a cheap laser thermometer. Both the Cats are hotter at their inlets than their outlets by 80-100 degrees, so they're in need of replacement.

Thanks and Have a Great Week


Here's JimTheCarGuy in action.

https:// www.youtube .com/watch?v=wrNtn3MkwG8
 
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Informative video.

Back in the day. Convertors were checked with emissions. And cylinder power balance tests. (Killing a cylinder) A good convertor could lose 1 cylinder and still pass emissions. A bad convertor would have a significant increase in HC's. (hydrocarbons caused by lean or misfire) There was sometimes a rotten egg smell also.

Of course. It was common for people to knock out there fuel neck restrictor and run leaded gasoline as it was cheaper then unleaded. Leaded had bigger fuel hose nozzles.
People also had a tendency to take off the belts that ran the smog pump. Mostly fords. For some reason. They thing they got better mileage without the pump.

If you getting that low of a mileage economy. You might find out why before condeming the convertors. Your temps might come out better. But who knows. Maybe it's the convertors causing the entire problems. Certainly do some investigating before replacement.

Don't forget. Jim knew WHY the convertors went bad and fixed before testing and replacing. (MISFIRE conditions.)
 

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Discussion Starter · #8 ·
When AC is on and engine load increases, LTFT goes up and intake manifold vacuum decreases. But, the AC controls in the HVAC Control Module are electric stepper motors and not vacuum solenoids.

Further questions are raised by the result of my little experiment with a propane torch. I flowed propane fuel gas into the intake manifold brake booster port and observed the Short Term Fuel Trim and Long Term Fuel Trims go negative while the O2 Sensor voltages both rose to more than .9; thus the PCM and O2 sensors are working and wiring is not faulty. So where is the vacuum leak. I've put engine starting ether all over the intake and boost tubes, manifold, throttle body with no increase in RPM, and have flooded the engine with smoke at 3 psi for a 20 minute period with no observable smoke leaking out. This was after I had replaced the throttle body gasket and installed a home made rubber grommet-band around the throttle body inlet tube so the Intercooler-Boost-Charge Tube would seal to the Throttle body. I speculate that their may be a leak sucking in air at the Pre-Cat to Secondary Cat Exhaust Pipe Flange. I had the Pre-Cat out when removing the bad Turbo, and reinstalled with a new GM ACD metal gasket. The new GM ACD metal gasket there may not be keeping air out, so I will modify it by applying layers of Permatex Copper High Temp Silicone Sealant to the gasket and after allowing it to cure overnight then reinstall and retest for vacuum and LTFT. I'll post results on Sunday.
 

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vehicle: 2011 Cruze LT 1.4L turbo, 67K miles (This car was not properly maintained by the previous owner.)

Long Term fuel trim ~ 12.7
O2S2 (down stream O2 sensor) voltage .67
vacuum at idle after warm up ~ 16"-17" Hg
vacuum at 3000 rpm increases to ~ 20" Hg and snaps to 26" when throttle position goes back to idle. Vacuum then returns to 16"-17".
Vacuum and long term fuel trim go down when AC is on and load percent goes up from 18% to 32%
Long term fuel trim goes down to ~ 3.5 when engine is at 3000 rpm.
This is classic indication of vacuum leak.

After replacing the AC Delco throttle body gasket with the MAHLE G32627 gasket my engine pressure leak down time from 15 psi to 5 psi is 31 seconds. This is an improvement of 14 seconds from the 17 second leak down time with the ACD OE gasket that came with the new intake manifold. (I must order the check valve bypass kit from Extreme Revolution to fix bad PCV design in new intake manifold.)

I cannot find a vacuum leak with propane, spray bubbles, or starting ether. I have yet to fabricate and use a smoke generator.

My question is can a partially malfunctioning low efficiency catalyzing catalytic converter cause a low O2S2 voltage that results in a high long term fuel trim? I think there's too much oxygen in the exhaust because there's unmetered air getting into the intake, but if a partially bad Cat can cause high LTFT, then how can that possibility be checked?

Please forgive my ignorance. I've long thought catalytic converters could not malfunction unless they were clogged or melted or the internal matrix was broken. Mine is not clogged and was not melted or broken or restricted when I had it out of the car and washed it out with soap and water.

Perhaps I need to go to Harbor Freight and buy a laser thermometer to check the input and output temps of my car's first Cat. By visual inspection, I am unable to see oxygen sensors on the down stream Cat.

So off to Home Depot I go to buy and empty paint can from which to fabricate a smoke generator.

Thanks in advance and Best Wishes for Continued Success.
Is there a spot around here that has numbers to watch with a scan tool? I get code 0420 with lower fuel economy and would like to try narrowing things down before just spending money on replacement parts. Would like to know o2 numbers to watch to see if a sensor is bad before replacing them and/or the catalytic converter.
 

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If your seconday is bouncing around. The cat is probably going bad.

Do a youtube search for testing cat. Requires a laser temp which can be found cheap at harborfreight.
 

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A code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for (unlikely)
A damaged or failed oxygen / O2 sensor
Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
Failed or under performing catalytic converter (likely)
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings
Leaking fuel injector or high fuel pressure
Cylinder misfire
Oil contamination


Read more at: https://www.obd-codes.com/p0420
 

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Discussion Starter · #14 ·
Add 1/2 gallon HEET in the yellow bottle to a fill up with good fuel, not the cheap gas from Walmart. Drive normally. CEL might clear within 350 miles. If so , good; if not use spark plug non fouler on downstream O2S2 sensor, reset PCM to clear code and do drive cycles to reset I/M monitors.
 

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The 2nd o2 is normally up around high 70's (.72 - .81) roughly... the 1st one bounces around more and hits lower numbers.

Last time i cleared the p0420 or went away for nearly a month and recently came back on. My fuel economy has not improved though...
 
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