4.2 Sluggish Acceleration and Odd Cold Startup

FlyFish3rman

Newbie
Original poster
Aug 4, 2022
8
USA
Hi All,

New to the forum. I have a 2007 Trailblazer LT SWB 4WD 4.2 I6 with 180k on the odometer. I've had it for 1-1/2 years and it has been awesome. I've looked around for my specific issue(s), and it has been somewhat addressed in other posts, but I've not seen my two issues together specifically, nor an explicit resolution. Hence my novel below.

For the past couple months I've had two issues:

1. On cold startup the exhaust makes a gurgling/sputtering noise. The RPMs do not jump around or fluctuate when this happens. You can hear the SAIS pump working during this time. Once the SAIS pump shuts off all the gurgling/sputtering stops and the vehicle idles smooth. The person I bought the car from had replaced the exhaust from after the CAT to the tip, and did a poor job. The flex pipe was just shoved in the muffler. I had my shop weld the pipe to the muffler, and I thought this would fix this issue but it has not.

2. When the engine is hot or cold it is slow to accelerate in 1st gear. I'm not an aggressive driver, but the few times I've needed to punch it (e.g. onramps, merging) it will not take off. The RPMs slowly rise and the car feels bogged down until about 4500k RPM, then it takes off. Almost feels like turbo lag.

I have a decent OBDII reader, and have looked at live data. I have no codes showing at all. My STFT and LTFT never leaves +- 5%. My upstream and downstream 02 are in the acceptable range of 0.1 - 0.9. This is at both idle and load. My STFT B1S2 is reading 99.2% which tells me my cat may (?) be working fine. My warm coolant temp hovers around 190 per my reader.

I'm wondering if issue 1 and 2 are related.

Within the last 20k miles I've replaced the following:
Thermostat, alternator, power steering pump/lines, water pump, idler pully, serpentine belt, trans cooler lines and new trans filter, rear diff serviced, new front wheel hubs, and new air filter.

I've yet to test vacuum and exhaust back pressure, as I have to rent those tools. I haven't changed plugs or coil packs. I haven't changed 02 sensors. I just don't know where to start since my live data is not, to my limited understanding pointing me anywhere specific. I'm thinking I have an issue with SAIS valve, a partially failing CAT, or both.

Looking for some advice for those more familiar with this platform.

Thank you!
 

christo829

Platinum Donor
Dec 7, 2011
436
Fairfax, Virginia
Welcome to the Nation!

You might want to start by disconnecting the SAIS hoses at the pump and seeing if they're full of water (just don't be right underneath the hoses when you disconnect them!). When my valve failed, it ended up allowing so much moisture to accumulate at the pump that it eventually killed it. That valve not sealing can cause other issues in the exhaust stream, so you might want to test that and get it sealed up first if it is leaking.

You can also do what a number of us here have done, removing the entire SAIS assembly, block off the port on the engine, and get the SAIS related codes removed from the ECU.

Cheers-

Chris
 

mrrsm

Lifetime VIP Donor
Oct 22, 2015
6,153
Tampa Bay Area
Welcome to GMT Nation...

Give this "Once Over The World" View of a "Sluggish on Acceleration" Information and Video Primer for the Tralblazer 4.2L Engine a "Look-See". And in addition to what Chris mentioned above... Investigate an Inoperative or STUCK SAIS Solenoid located on the Center Passenger Side of the Aluminum Engine Head... especially if a P0171 Code were to appear in Stored Codes.


If replacing the SAIS Solenoid becomes necessary... Fortunately...it is one of the more "EZ-2-DO" R&Rs for this LL8 Engine.
 

Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,842
Ottawa, ON
Checking for a clogged cat wold be my first place to look for a slow acceleration and low power. Would be worthwhile to replace the plugs with REAL ACDelco 41-103 (there are fakes floating around). Coils I wouldn't bother with unless they are giving you trouble like misfires.

For the SAIS, I would just delete it and have the codes tuned out. It's a useless and trouble prone system.
 

FlyFish3rman

Newbie
Original poster
Aug 4, 2022
8
USA
I'm going to inspect the SAIS and do a backpressure test on my exhaust in the next day or two. I'll also just replace the plugs with Delco's, since I'm not sure when that was done last. The sluggishness is getting worse and still no codes/bad live info readings, so hopefully I figure this out soon.
 

FlyFish3rman

Newbie
Original poster
Aug 4, 2022
8
USA
Also...

Will I experience any performance issues if I block the SAIS port with the GM cover and not immediately tune the PCM, or would I just have an annoying CEL? I'm not sure what the computer would do in that scenario.

I ask because I have an upcoming trip and probably wont have time to send it out, unless I could find a local to me tuner.
 

mrrsm

Lifetime VIP Donor
Oct 22, 2015
6,153
Tampa Bay Area
Before you perform the task of examining any Back Pressure issues... and because the Prior Owner had previously 'manhandled' the CAT-Back Hardware so badly, after the Engine and Drive Train have Cooled Down, take a moment to apply a "Dead Blow Hammer" to the center underside of the Stainless Steel CAT-Can with just a few Gentle Taps.

Listen for any 'Re-Bound Sounds' similar to that of 'Dropping Coins into a Ceramic Bowl'. If the Rare-Earth Impregnated Ceramic Honey-combs have Fractured and fallen down all akimbo into the bottom of the CAT Can... their Broken Ceramic Matrices will "Ring Loudly Like a Bell" if they've been damaged.

Such a resonating, ringing noise will confirm them as being a primary factor in having High Resistance Exhaust Back Pressure. Remember... The key thing here is to apply VERY LIGHT TAPS... or you may end up doing more harm than good with this Diagnostic Tactic.
 
Last edited:

FlyFish3rman

Newbie
Original poster
Aug 4, 2022
8
USA
Update: Gently tapped the exterior of the CAT with a dead blow hammer and there is no sounds of anything broke or loose in the cannister. I'm going to rent an exhaust backpressure gauge today on my lunch break to identify or eliminate the CAT from my investigation.

Would a stuck open SAIS valve cause the type of sluggish acceleration that I've described? I'm eliminating it either way, but I'm curious.

If the above checks out clean, then I'll change spark plugs and check the fuel filter/fuel pressure. Let me know if you all would do anything different.
 

Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,842
Ottawa, ON
It could if it's introducing air in the exhaust stream and screwing with the O2 sensor. If it thinks it's lean, it will inject more fuel to compensate which can also kill your cat and MPG.
 

FlyFish3rman

Newbie
Original poster
Aug 4, 2022
8
USA
Thank you, Mooseman. I'm going to do one more test on the 02 sensor at WOT and see if I'm rich or lean. I can't find anyone who rents a back pressure tester in my area, so I may have to take it to a local exhaust shop to test.
 

FlyFish3rman

Newbie
Original poster
Aug 4, 2022
8
USA
Took it for a test drive after lightly tapping the CAT and NO acceleration issues. I had my code reader on live data to check my upstream o2 (trying to parse between an exhaust or fuel issue). I did 10 runs in first at WOT after the car warmed up on an old country road. No hesitation or sluggishness at all. My upstream o2 hovered at .912v at WOT, then dropped down to the standard alternating voltages when the RPM dropped back to cruising speed (1500rpm), which tells me the engine was getting the necessary fuel at WOT.

I'm wondering if the few gentle taps I did on the CAT loosened up a blockage. It sound far fetched to me but who knows. I'll test over the next few days to see if things change. Otherwise, I think the CAT is the culprit.
 
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mrrsm

Lifetime VIP Donor
Oct 22, 2015
6,153
Tampa Bay Area
Good News!

If the CAT-Back Piping that the Prior Owner had so forcefully inserted was canted slightly and impinging upon any of those Ovate, Ceramic "Briquettes" nestled up inside of that CAT-Can... they may have been TILTED just enough to Misguide the Exhaust Stream and back it up into the corner of the CAT-Can ...instead of allowing it to Head Straight Out through the Exhaust Outlet. Let's Hope it Holds. :>)
 
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Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,842
Ottawa, ON
At WOT, the O2 sensor does not play into the equation. That's why you got the readings you did. What would have been more important to see is the fuel trims. Positive trims indicate it's adding fuel (running lean) and negative it's removing fuel (running rich). At least the sensor seems to be working correctly.

So now you're saying it's not sluggish anymore?
 
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Mike534x

Hobbyist
Apr 9, 2012
754
My guess is a partially clogged cat, thats really the first thing that comes to my mind. I guess the easiest way to tell, is to remove the upstream O2 sensor and see if that helps eliminate the problem entirely.

Not to derail the thread, but what was the actual purpose of the SAIS?
 
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FlyFish3rman

Newbie
Original poster
Aug 4, 2022
8
USA
At WOT, the O2 sensor does not play into the equation. That's why you got the readings you did. What would have been more important to see is the fuel trims. Positive trims indicate it's adding fuel (running lean) and negative it's removing fuel (running rich). At least the sensor seems to be working correctly.

So now you're saying it's not sluggish anymore?
A high o2 voltage (.9v) would indicate a low oxygen condition or (rich with fuel). If my upstream o2 is reading high during WOT then I can be somewhat confident that the engine is getting fuel for the requested load. I was trying to use this metric to rule out a lack of fuel issue (pump/filter/injector) in the sluggish acceleration problem I'm experiencing.

Went out again tonight to test. The car does not feel sluggish anymore, or at least it is a lot better than before. Again, I've done nothing but the previous "tap" of the CAT to check for broken internals.

My fuel trims are all normal. STFT never leaves +-5% and my LTFT does not leave +- 2%, and my upstream o2 is properly oscillating between .1-.9v.

However, my downstream o2 is consistently reading .8v. My previous vehicles were supposed to be around .45v. What is the downstream o2 on a 4.2 supposed to read at idle and under load?

.8v to me means low oxygen (rich condition) coming out of the CAT, which could either mean a improperly function o2 sensor, or a CAT that is no longer working properly.
 

mrrsm

Lifetime VIP Donor
Oct 22, 2015
6,153
Tampa Bay Area
@Mike534x ... FWIW... The Purpose of the SAIS (Secondary Air Injection System) was made as an addition to the Original 2002 LL8 Engine (originally fitted with a Block-Off Plate and a Tiny Gasket under Two Studs & Nuts) in order to allow the Engine to "Gulp" in additional Air (O2) and then spew it into the Exhaust Stream ASAP right after Start Up. The idea here was to get the Catalytic Converter running up to Higher Operating Temperatures as quickly as possible after each Start Up and improve its efficiency.

The EPA and the California Clean Air Standards were pushing to have the LEAST amount of unconverted Fuel escaping as unprocessed by not having an inefficient CAT making nothing other than CO2 and Water Vapor as the only By-Products moving through the Tail Pipe. So in typical GM Engineering Style, this SAIS Solution seemed like a good idea for them to achieve these goals.

However, the placement of the SAIS Air Pump so low on the Driver's Side of the Chassis - Frame though invited other problems for Boat Owners (and the occasional Weekend U-Boat Commanders) who backed down Boat Ramps and were dousing, dunking and essentially drowning many of those SAIS Pumps in Local Lakes and in Salty Seaways, often doing the system more harm than good.

The other problems of the P0171 Code and its frequent companion the P0300 Code occurred whenever the SAIS Solenoid located on the Passenger Side of the Engine Head would stick wide open.

After becoming non-responsive, that 'open valve' allows enough Unregulated Air to enter the exhaust system and LEAN out A/F Mixture enough to confuse the Upstream O2 Sensor and this can cause the PCM to ADD IN More Fuel under those conditions to get better Stoichiometric Parity. Having Broken Exhaust Manifold Bolts also contributes to this problem as well.
 

Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,842
Ottawa, ON
The downstream sensor measures for cat efficiency. I don't think it looks to lean or rich but only if it fluctuates the same as the upstream sensor. It it fluctuates, that means the cat isn't doing it's job. Since yours is steady, that says the cat is doing its job. However, this does not tell you if it's clogged as the exhaust that is passing through can still be cleaned so the readings are steady. A quick way to tell if it's clogged is to remove the upstream O2 sensor and drive it. If power and responsiveness improves, it's a sure sign it's clogged.
 

budwich

Guru
Jun 16, 2013
1,749
kanata
not sure of the PIDs "integrity" for timing, but you might want to monitor that for a while and see if there are any correlations in and around that area.
 
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FlyFish3rman

Newbie
Original poster
Aug 4, 2022
8
USA
Thank you, everyone. I haven't had issues with acceleration since "tapping" on the CAT can.

I'll do the upstream o2 removal test when I get a chance to buy the o2 slotted socket tool and see if that makes any more of a difference.

Also, I plan on removing the SAIS next week. Is this the block off plate for the SAIS check valve (found on GM parts direct SAIS Cover Plate)? I could not find it on the lime-swap website as mentioned in other threads. Otherwise, I'll try an track down an 02 TB in a junk yard and pull it myself.
 
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Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,842
Ottawa, ON
Yes, that is the correct cover plate. Hopefully it's still available since it was used only on 02-03 trucks. A yard is also a good option.
 

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