ey guys,
I have some performance concerns I'm triyng to demystify that may be related to a vaccum leak on my 2002 Blazer.

When coming to a stop applying the brake, I feel a sudden RPM drop and shudder just before the truck comes to complete stop. My research has lead me to believe this is a vaccum leak in the system somewhere.
On a cold engine, this happens almost every time reliably. On a hot engine, it happens rarely.
I noticed that this tends to be worst having removed the air intake on the throttle body and putting it back on. My guess is that the air intake gasket is bad, but I haven't been able to find a replacement.
I took the throttle body off at one point to clean it. I replaced the idle air control valve and the throttle body gasket. I also installed a new MAF sensor. After having replaced these parts I noticed an overall improvement in idle performance. There's still that shudder when braking.

I know for a fact that I have an audible vaccum leak in the AC controls. I already have an idea how to fix that. I just have not bothered to pull apart the front dash yet. Or should I fix that ASAP? Does a vaccum leak in the accessories affect engine performance? I understand that there's a vaccum reservoir that the accessories draw from so the engine isn't affected. But I feel the AC blend doors change air direction when I'm in reverse trying to back into my garage on an inclined driveway. Is that more evidence of a serious vaccum leak?

I have some stats on fuel trim and O2 voltage I wanted to share and get some feedback on. See spreadsheet attachments or charts.
I've done tests at both 1500 RPM and 2500 RPM on a hot engine. O2 sensors look fine. They oscillate between 0.02 and 0.8 voltages. I believe that's normal.
Fuel trim looks off. I see both short term and long term hitting -10% and beyond. Fuel trims seem to be mostly in the negative, so I guess that means the system is running rich.

Fuel trim at 1500 RPM


Fuel trim at 2500 RPM

 

Another thing I wanted to point out. Engine RPM seems to fluctuate weird. I made a chart from one of the csv files I recorded from the scan tool.
The points between 1200 - 1800 is just me adjusting my foot on the pedal and getting comfortable. The sharp dips downward are the weirdness I'm pointing out.
Bad RPM sensor?

 

Using fuel trims to diagnose a vacuum leak is discussed here:

https://blazerforum.com/forum/2nd-ge...-trims-102906/

You can pinch of the vacuum lines until you isolate any leak and retest.


George

 

Good read, George.
Now I'm hoping if someone has time to take a look at my O2 sensor data. I think they're running right. O2 sensors are between 0.02v and 0.8v. I just need confirmation on this.

Based on what I researched, my Blazer has a rich condition. I read that rich conditions are caused by the following:

• High fuel pressure
• Leaking fuek injectors
• Malfunctioning MAF sensors
• Leaks in the exhaust system
• Loss of compression in at least one cylinder
• Occasional misfires
• Defective O2 sensors

I measured fuel pressure to be about 65 psi at it's highest. Not sure if that's too high. I've replaced the fuel pump in this thing several times before.
Not sure about leaking fuel injectors. Still running the factory original injectors. I've seen posts where guys have installed the updated spider-injector design. Not sure if that's something I need to do. I'm actually not sure how to benchtest a spider system.
MAF sensor has been replaced. Noticed an improvement in idle stability when I did.
Exhaust pipies throughout the chasis seem intact, so I don't suspect an exhaust leak.
Not sure about loss of compression. Have never done a compression test on this truck before. Do these 4.3L's tend to get loss of compression in their old age?
I don't get any misfires, but do get a stutter/hesitation while braking at low speed where the engine almost stalls (and has stalled before and started right back up). Symptom of a vacuum leak, I believe.
Not sure if O2 sensors are defective. The data seems to be right. Still running the original factory components.

I'm also not sure if maybe the catalytic converter is clogged. I found a test kit online I can get, but I'm not sure if it's necessary. Do the cats tend to clog in the S10s?

 

Discussion of O2 sensors:

https://blazerforum.com/forum/2nd-ge...82/#post733375

You are running marginally rich. Your fuel trims are -1X. +/-25 sets a code, anything over +/-10 should be looked at. The best fuel tril data is at idle and 2500rpm. Your list are things that can cause a rich condition but there are more such as other sensor problems like the ECT, some discussion there:

https://blazerforum.com/forum/2nd-ge...50/#post731519

Your stumble can be a lot of things including some of the above. I would start with checking those sensor values especially the TPS and then clean the throttle body and plate.

With a vacuum leak your trims are usually lean with a rich command that improves at higher rpm.


George

 

Very good read on O2 senors, George. I'm looking at O2 voltage more carefully.
I recorded my last batch of O2 to a CSV file using my BlueDriver OBDII scanner, plotted the data points to a chart to see the waveform, and I'm not liking what I see.



I plotted this from 22 seconds into the sensor recording to 82 seconds. 1 full minute out of 3 running at 2500 RPM.
I'm not seeing a clean up-and-down waveform like in your O2 sensor post. Does this mean my O2 sensors are failing?

I did the same for the down stream and it doesn't look good either.


So failing sensor, or bad cat?

 

The O2 graphs don’t always look like well behaved sine or square waves. The key is switching between around 0.15v and 0.85v at a minimum of once per second with nice crisp rise and fall times. Yours are probably close enough to call good. The last test of those up stream sensors is to force them lean and rich and make sure that they respond properly, but you would need live graphing. If I can find a video I’ll post it. You force lean with a vacuum leak and you force rich with propane or a snap throttle.

The cat has to be at operating temp to learn anything from the O2 graph. The voltage is trending down, was this cat really warmed up? After it’s up to temp the voltage should be hovering around 0.45v.


George

 

Here is one of my upstream sensors at around 175,000 miles. Truck ran pretty good with a little bit of a rough idle. Original sensor I believe:





George

 

Oh yeah. It was warmed up. The Blazer had just came back from a drive when I benched these numbers in the garage before shutting it down.

The 2500 RPM graph does trend down but plateaus around 0.6v during that full 3 minute duration.
The 1500 RPM graph tells a far more odd story during those 3 minutes.

 

I'm going around asking questions and trying to dig up answers for how to test the fuel injectors. Someone told me that the injectors are likely stuck open resulting in the low fuel trim.

Based on the downstream O2 sensor graph I posted, I'm thing about just replacing the cat and add in new O2 sensors from RockAuto and the spider-injector-thingy. Would those actions improve these numbers here and restore performance, or would it remain more or less the same?