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Ask Chad Farhat Your Own Question
Chad Farhat
Chad Farhat, ASE Certified Technician A6 - Electrical / Electronic systems A7 Heating / Air conditioning systems
Category: Chevy
Satisfied Customers: 3829
Experience:  Owner at Carolina Auto Service & Sales , 20 years Experience in Foreign & Domestic vehicles A6 - Electrical / Electronic systems A7 Heating / Air Conditioning systems
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Bill, 2004 Trailblazer 4.2 FWD, I am a mechanic also but

Customer Question

Hi Bill
JA: What is the model/year of your Chevy?
Customer: 2004 Trailblazer 4.2 FWD
JA: Are you fixing your Trailblazer yourself? What have you tried so far?
Customer: I am a mechanic also but would like some input.
JA: Anything else you want the mechanic to know before I connect you?
Customer: Idles great, put it in drive, turn air on starts to ratlle like pinging.
JA: Where exactly do you think the noise is coming from? And how long has this been going on?
Customer: Like you have bad or cheap fuel from the engine
Submitted: 1 year ago.
Category: Chevy
Customer: replied 1 year ago.
Vacuum drops to 5" because throttle opens, map voltages goes to above 3.0v like you have your foot on the throttle. All the data changes like you have your foot on the gas with the brakes applied. The Rpm is at 1100rpm only in drive with a/c on. Otherwise the rpm is 630 no a/c steady, turn a/c on goes up slighhtly and stabalizes at 630. I lower the rpm with the scan tool and all the data return to normal and rattliong goes away. Get out of idle control mode every goes back to like your foot being on the throttle and brake the same time
Expert:  Chad Farhat replied 1 year ago.

Hi, I'm Chad. Welcome to JustAnswer. I'm reviewing your question and I will be posting your reply ASAP.

Expert:  Chad Farhat replied 1 year ago.

hey man how are you

1-what is your fuel trims long term & short term at idle ?

2-what is the oxygen sensor readings at idle all 4 sensors reading ?

and i take it no fault codes pending ?

have we checked for vacuum leaks ,intake manifold & vacuum lines ?

Please let me know

Thank You

Chad Farhat

ASE Certified Technician

Customer: replied 1 year ago.
118000 miles, Smoked intake no leaks, Fuel trim -+10 pr less, O2 switching excellent and no pending codes either. Simulated codes show up recognised by PCM.
Expert:  Chad Farhat replied 1 year ago.

Is it + 10 Or Minus 10 on fuel trims , is that long term or short term ?

Customer: replied 1 year ago.
If I lower the idle with the scanner to 600 rpm, In drive with foot brake applied, the rattling goes away and idles perfect.
Customer: replied 1 year ago.
ST less than -+10 Usually around 0-5
LT the same roughly.
Expert:  Chad Farhat replied 1 year ago.

you keep Putting + & minus , is it plus or minus it can not be both

Customer: replied 1 year ago.
It switches faster on ST between +5 to -5
Long term is slower and usually close to +1 to -1
Expert:  Chad Farhat replied 1 year ago.

let me ask you this way, i m trying to figure out is the car Running rich or lean or running good on a/f ratio.

if the short term is switching between -5 to + 5 , so we good there .

if short term is stuck more than - 5 or + 5 , like - 10 all the time running Rich , pcm pulling fuel away ,or + 10 adding fuel.

the long term Usually is more of a steady number like 1 , 2 ,5 or 10 or 25 - or plus .

at idle while car running . where these number sits .

if you saying - 5 t 5 on short and fuel trim 1 , we prefect then .is this case

Expert:  Chad Farhat replied 1 year ago.

if you drive down the road , what you have ,lack of power , misfire , back fire ?

fuel pressure checked ? Fuel Pressure (Key ON, Engine OFF) .................... 345-395 kPa (50-57 psi)

with gauge hooked up and running if you rev it up what happens to the fuel pressure

Customer: replied 1 year ago.
We dont have a rich or lean condition at all. The pcm is commanding the throttle to open(1100 rpm)when you put it in drive and A/c is turned on. The vehicle acts like you are pressing the gas and brake at the same time. The engine starts PINGING AND RATTLING. If you command the idle to 600 rpm with the scanner the rattling goes away and all the data return to normal. ONLY IN DRIVE WITH A/C ON.
Expert:  Chad Farhat replied 1 year ago.

that is a tough one , never ran into it , I will open your question up for another expert may be they have

sorry ,

good luck


Customer: replied 1 year ago.
Drives fine FP=50psi with a qt delivery in 30 seconds. It was at Felix chevrolet (LA, CA) for 13 days. They gave up.
Customer: replied 1 year ago.
Throttle body replaced PCM programmed with latest software, New oil, coolant and thermostat. New CAT.