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Engine tries to stall with A/C on at idle, not throttle body caused

Tiny
NREDDING
  • MEMBER
  • 2003 CHEVROLET TRAILBLAZER
  • 4.2L
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 189,500 MILES
Inline cylinder four wheel drive.
no check engine light or codes. The engine is constantly trying to shut off when vehicle is at idle with A/C or defrost on. It is a hundred times worse when going through a car wash!


had the throttle body removed and cleaned by a reputable shop (had the battery disconnected for twenty minutes while they were cleaning the throttle body) no change.

had throttle body replaced with official AC/Delco throttle body, still no change.

Tries to die, you feel the entire truck shudder, see the rpm's go down to 300, then rev up over 1,000. Keeps repeating this behavior constantly when the A/C or defrost is on and you are in a car wash, still does it outside of a car wash, but not as bad.

Please help!
Nathan
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Saturday, December 24th, 2016 AT 6:53 AM

11 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Scan for codes and check fuel pressure with a gauge first. auto parts rent gauge. see link.
https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator
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Saturday, December 24th, 2016 AT 9:00 AM
Tiny
NREDDING
  • MEMBER
  • 5 POSTS
No codes
Fuel pressure test results - test was done at the test port on the inline fuel filter under the drivers door.

When vehicle was shut off with the climate control on foot only, key in the on position, engine off - 52 psi

When the vehicle was shut off with the climate control on defrost, key in the on position, engine off 50 psi - same result 3 times in a row

With engine idling, vehicle in park, 56 psi reguardless of climate control setting, noticed the needle on the gague shaking but staying at 56 psi when the defrost was on.
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Sunday, January 1st, 2017 AT 9:48 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
I think your fuel pressure may be low, spec is 55-62 psi so it may be a clogged fuel filter or weak pump/regulator. So try a fuel filter first then check electrical connections by engine/pcm as they may be not connected properly. Also if this vehicle is driven only short distances where it can't fully warm up like less than 10 miles it may be the evap canister loading up with fuel and is trying to get rid of it causing the problem. You might also put dielectric compound on electrical pcm connections and check belt and tensioner for condition as the belt needs measurement with a tool not by cracks. Recommended replacement is 60,000 miles for both as tensioner loses tension so belt slips when you are needing more power for a/c.
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Monday, January 2nd, 2017 AT 6:21 AM
Tiny
NREDDING
  • MEMBER
  • 5 POSTS
Replacing the fuel filter helped, tachometer is still wavering between 500 and 900 RPM
I will be replacing the fan clutch, and serpentine belt with tensioner and idler pulley this summer.
(fan clutch is making the telltale sounds, and the belt, tensioner, pulley are overdue per the manufacturers specification)
If it is still acting up I will have to look at the fuel pressure regulator, then fuel pump.
Thank You
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Saturday, January 21st, 2017 AT 5:29 PM
Tiny
CJ MEDEVAC
  • MECHANIC
  • 11,004 POSTS
Maybe your compressor is locking up? Hence dragging the belt on the pulley and dropping the RPMs

Compressor does come on in defrost modes too.

All is fine with A/C and defrost off?

Can you see the A/C pulley when when someone else turns the A/C on?

The Medic
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Saturday, January 21st, 2017 AT 7:43 PM
Tiny
NREDDING
  • MEMBER
  • 5 POSTS
Only acts up when A/C or defrost is running, a/c compressor was replaced 1 year ago, I will look at the a/c compressor pulley to see if I notice a chang in the belt / pulley speed and report back
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Saturday, January 21st, 2017 AT 7:57 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 42,895 POSTS
Hey NREDDING,

If I could jump in here it sounds like it could be the mass air flow sensor that needs to be services. Here is a guide that will help you.

https://www.2carpros.com/articles/mass-air-flow-service

Please let us know what happens so it will help others

Best, Ken

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Tuesday, January 24th, 2017 AT 4:42 AM
Tiny
NREDDING
  • MEMBER
  • 5 POSTS
Hi Ken,
Thank you for your advice, I cleaned the mass airflow sensor per the instructions you provided, unfortunately it did not have any effect.
Please don't hesitate if you have any other ideas that I can try myself
Thank You
Nathan
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Tuesday, January 24th, 2017 AT 10:35 AM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Some of these had a throttle body problem that can cause this but not sure in this case. Look for a vacuum leak as that may cause it but it sounds also like high humidity things cause it. Try putting dielectric grease on pcm contacts then reinstalling harness it's on the side of engine also may take the pcm off and look on back for cracks it may be getting pcm wet and causing this as well.
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Tuesday, January 24th, 2017 AT 11:55 AM
Tiny
MRREGULAR79
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I know this is an older post but I was having similar issues with my 2004 Trailblazer ls. Ended up being my up range O2 sensor was going out but wouldn't throw a code. Replaced it and it ran great.
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Monday, June 21st, 2021 AT 2:44 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 42,895 POSTS
Excellent addition to this thread! Please feel free to use 2CarPros anytime!
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Tuesday, June 22nd, 2021 AT 10:51 AM

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