05 Tahoe no cats

Dcolli9881 · 2025-02-02T19:03:44-0500

Have a massive drop in mpg
Smoke from exhaust on start up
Goes away after you start to drive
Rough idle at start up stops after so long rough idle starts back when you go to park

Replaced 02 sensors 2/3 times before doing a cat delete cats were bad
Gas problem was before cats got deleted

Replaced fuel injectors
Coil packs
Spark plugs and wires

Long term fuel trim reading 25
Short term fuel trim reading 55

Any insight on what might be the problem

Scottydoggs · 2025-02-02T19:37:24-0500

ltft reading 55 in what? never seen a device that reads over mid 20's. most will show a - aka neg if no - its pos.

when neg its adding fuel, when pos its taking fuel away via the pcm trying to make it right.

normal is -5 to 10 at idle.

Dcolli9881 · 2025-02-02T19:40:39-0500

Scottydoggs said:
ltft reading 55 in what? never seen a device that reads over mid 20's. most will show a - aka neg if no - its pos.

when neg its adding fuel, when pos its taking fuel away via the pcm trying to make it right.

normal is -5 to 10 at idle.

It’s a blue driver plug in that read it being at 55

Scottydoggs · 2025-02-02T19:45:21-0500

will either way, its pig rich. like a injector is stuck open. that could have clogged the cats in the first place.

what injectors? lets hope you didnt get china clones that suck.

Dcolli9881 · 2025-02-02T19:52:02-0500

Scottydoggs said:
will either way, its pig rich. like a injector is stuck open. that could have clogged the cats in the first place.

what injectors? lets hope you didnt get china clones that suck.

Got some from a premium haven’t had a issues with ones I’ve used in previous cars

Scottydoggs · 2025-02-02T20:09:17-0500

you could pull the plugs to see if one comes out wet with gas, or pull the rails off and leave the fuel line on and the injectors plugged in, turn key to run. they should all spray for a couple seconds then turn off. if one keeps spraying its stuck open.

Fless · 2025-02-02T20:42:56-0500

Negative fuel trims means the ECM is taking away fuel -- the O2 sensors are reporting a rich mix in the exhaust and the ECU is trying to compensate.

Positive numbers (like yours, if I read that right) means the exhaust is lean and the ECM is adding fuel.

Does the fuel system go into closed loop? If the trims are extremely positive like this on both banks, suspect a large vacuum leak, or a mis-reporting mass airflow (MAF) sensor, or a leak between the MAF and throttle body, or exhaust leaks around or before the upstream O2 sensors. Other causes may be present, but with trims like that it's something relatively big or a combination of some of the above.

Dcolli9881 · 2025-02-02T20:46:42-0500

Fless said:
Negative fuel trims means the ECM is taking away fuel -- the O2 sensors are reporting a rich mix in the exhaust and the ECU is trying to compensate.

Positive numbers (like yours, if I read that right) means the exhaust is lean and the ECM is adding fuel.

Does the fuel system go into closed loop? If the trims are extremely positive like this on both banks, suspect a large vacuum leak, or a mis-reporting mass airflow (MAF) sensor, or a leak between the MAF and throttle body, or exhaust leaks around or before the upstream O2 sensors. Other causes may be present, but with trims like that it's something relatively big or a combination of some of the above.

Maf goes between .6/.-9

Dcolli9881 · 2025-02-02T20:49:16-0500

Fless said:
Negative fuel trims means the ECM is taking away fuel -- the O2 sensors are reporting a rich mix in the exhaust and the ECU is trying to compensate.

Positive numbers (like yours, if I read that right) means the exhaust is lean and the ECM is adding fuel.

Does the fuel system go into closed loop? If the trims are extremely positive like this on both banks, suspect a large vacuum leak, or a mis-reporting mass airflow (MAF) sensor, or a leak between the MAF and throttle body, or exhaust leaks around or before the upstream O2 sensors. Other causes may be present, but with trims like that it's something relatively big or a combination of some of the above.

This is at idle

Fless · 2025-02-02T20:56:17-0500

All that gas that was being added probably caused the cats to go bad.

What color is your startup smoke? Blue or black? Probably blacker than my java...

If your scan dongle and app can read it, when it's warmed up and at idle speed, what is the MAF reporting in grams/sec? Has the MAF been cleaned recently? Is the intake tube intact from the MAF to the throttle body, with no leaks? All vacuum lines connected and in good condition?

Do a standard fuel pressure test (key on, engine off, turn the key to "on" but don't start the engine). Then what is the fuel pressure at idle speed?

05 Tahoe no cats

Dcolli9881

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Scottydoggs

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Scottydoggs

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Dcolli9881

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