Poor idle, engine shaking

FORDSMB454

MEMBER
2005 CHEVROLET TRAILBLAZER
4.2L
6 CYL
2WD
AUTOMATIC
120,000 MILES

The vehicle listed above is an LS model. So basically, it has poor idle nearly stalls at stop signs or in traffic waiting at red lights. On the freeway though it's fine drives great when A/c is turned on its worse engine shakes at idle. So, things I've done; taken off throttle body cleaned it, checked with ohm meter all good, new MAP sensor and checked readings all good. Taken off intake cleaned, fuel injectors, new filters, and O-rings all good, checked and cleaned rail, all good. I tested the vacuum on throttle body and intake all at 50 PSI vacuum then I did a fuel pressure test I did a on position test only first it spiked to 60 then dropped ASAP all way down to 40 PSI, then I did test with truck on was hover perfect at 61 PSI. I'm thinking maybe the fuel filter or fuel regulator is bad. Or maybe fuel pump, no idea. Any help would be great, thank you.

In addition, I have also changed coil packs, and new sparkplugs and I've ran ohm meter test on each individual coil pack all checked perfect.

1 Reply

KASEKENNY

MECHANIC
18,907 POSTS

When you say you checked the vacuum on the throttle body where did you tap into it? Was the needle steady and then recovered quickly after you spiked the throttle?

The most likely cause of this would be a vacuum leak so I would suggest we run through these guides just to be sure we completely eliminate this.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

https://youtu.be/4ktw1X4W_-k

Next, we need to monitor the o2 sensor data to find out if this is rich or lean when it is going to stall.

Basically, we need to watch the engine vacuum and o2 sensor data when this issue is happening to better understand what may be going on inside the engine.

This is going to be necessary to determine what direction to go.

As for the fuel pressure dropping from 60 to 40 after the engine shuts off, it is not overly concerning. The fact that it happened immediately is a little odd but it was able to hold pressure and supply pressure when it is on so I would not go to a fuel pump replacement yet.

Let's start with this and go from there. Thanks