I6 performance issue

[Black08TB]

Hello guys, been some time since I logged onto GMTNation. My 08 TB has been running just fine and has logged 155K as of last week, but has started to hesitate at highway speed. It won't hold 65 MPH on cruise control or just manually modulating the throttle. It works through it, after hitting 70 mph the cruise control works fine and doesn't downshift at all. Regular city driving is fine, no noticeable driveability problem. Whatever the issue is it hasn't lit up the check engine light. I was thinking it might be a misfire in a cylinder or two as the coils are original to the truck, as are most everything else. It's been a stone cold reliable vehicle for me. Any suggestions or experiences, and possibly a cure? As an added bonus the exhaust manifold cracked yet again and needs to be replaced. This will be the fourth one since I've owned it. Any help is appreciated, thanks!

Mike

 

[Redbeard]

How long ago since the last time the throttle body was cleaned? Cause if it's dirty it won't throw a code either. Just remember to disconnect the battery for about 30 minutes during the cleaning of the throttle body to reset the computer.

 

[Black08TB]

That's a good idea, I can't recall the last time I cleaned up the intake and throttle body. Will have to get that handled asap. Thanks!

 

[Mike534x]

When were the plugs last done on it? Usually its best to put new coils in when doing the plugs, to avoid any unforeseen issues down the line

My memory is bit a blurry on this one, but I think you're suppose to let the truck idle on its own for 10-15 minutes after re-connecting the battery so the PCM can re-learn the new throttle perimeters. Its been awhile since I worked on an I6, but I recall doing something similar after doing my first throttle body cleaning. It also probably wouldn't hurt to replace the CPA/Cam Actuator Solenoid, and Cam Position Sensor. When the cruise control stopped working on my 02 Envoy way back when, it never threw a CEL for either of these sensors, but after swapping them out everything was working as it should.

 

[Sparky]

I never changed my coil packs. Factory originals with 215k.

The two biggest things that I noticed when mine got down on power a bit was plugs (they say good gor 100k but after about 80k is when I began to notice it) and also I had a sticky caliper that was causing just enough drag to make it feel off on the highway and have to downshift when it didn't used to. So check those things just in case.

 

[Chickenhawk]

Where is your coolant temp gauge pointing when its warmed up? Dead center, slightly left of center, or slightly right of center?

 

[mrrsm]

If you've ticked things off all of the items mentioned on this list ...and the SUV STILL hesitates at or above 65-70 MPH.... But NOT during Local Around Town Driving...

(1) Clean the Throttle Body (Pull the #10 & #28 Fuses for 15-30 Minutes to re-set & re-learn).
(2) Tested the COPs (Coil On Plugs) and then swap whichever P30X with a NON-Matched Fire COP.
(3) Replace all the Plugs with GM OEM Only Flavor... if you are so inclined ...at 150, 000 Miles.
(4) If you got a P0014 Code and have R&Rd the CPAS Solenoid and performed a "6 Quart Cheap Oil + 1 Quart Motor Oil Flush" and then performed a second 7 Quart Mobil1 5W30 & Mobil1 Oil Filter Change.

...then this problem might be getting caused by a Failing Torque Convert Clutch Solenoid (and/or TCC Valve)...of the TC Clutch itself is slipping when the transmission gears are feeding through during 4th Gear Over-Drive stress...making the Clutch slip. When it gets REAL Bad... the 4L60E will slip out of 3rd t 4th and refuse to 'lock up'.

 

[limequat]

How do we know the cracked exhaust manifold is not causing the issue?

 

[mrrsm]

Hmmm... Now THAT is a very interesting observation... Because if the Crack is high up on the EM and close enough to the Engine Head Exhaust Flange during the slight Valve Overlap... it COULD suck in enough unmetered, Ambient Air. It follows that instead of the Piston Vacuum pulling in Metered Air strictly through the Intake Port ...that 'Extra O2' might throw things off with the A/F ratio. And if this ever happens in VERY Cold Weather... the Exhaust Valves can occasionallly get Warped and not seat properly from the sudden shock of Temperature Change as well.

 

[Mooseman]

If it was a misfire, it would throw off a code so I doubt it's that.

All the above suggestions are good and I would add check the cat for a possible restriction. A backpressure test would be good here. The manifold should be replaced first as that could be masking a problem with backpressure not showing as much a problem at lower temps but as it gets hot, like at highway speeds, and expands to close the gap, the backpressure manifests itself more.

 

[Black08TB]

Thanks for all the suggestions guys, I'm going to get the exhaust manifold changed next week.

The TB ran awful this morning, wouldn't hold highway speed wherever I tried to keep it, but darned if it didn't run fine this evening on the ride home. I stopped at the local parts house when I got into town and had the computer checked for trouble codes and it's empty, no misfires or other issues there. Was kind of hoping for a bad coil and be done with it. After that I was thinking it may be a fuel issue and bought a couple bottles of Sea Foam and double dosed the fresh tank of Shell 89 octane gas. Replaced a filthy air cleaner and hosed down the throttle body with CRC cleaner for a few minutes. When it started she belched smoke badly out the tail pipe but cleared up just fine.

Clean the Throttle Body (Pull the #10 & #28 Fuses for 15-30 Minutes to re-set & re-learn).
Is the truck shut off and dormant when I do this, or is it running, or key on, engine not running? Please advise.

At 100K I replaced the 6 plugs with OEM stock from the dealer so I'm pretty confident that they are ok. Will be commuting as usual for the rest of the week and hope to see some improvement and maybe find I'm on the right track with the fuel. If it improves some, I'll probably have the fuel system cleaned while it's in for the exhaust manifold.

Thanks again for the help so far guys, it's appreciated.

 

[Mooseman]

Clean the Throttle Body (Pull the #10 & #28 Fuses for 15-30 Minutes to re-set & re-learn).
Is the truck shut off and dormant when I do this, or is it running, or key on, engine not running? Please advise.

To do it properly, you have to follow these directions. It.must be removed.
http://gmtnation.com/forums/threads/how-to-clean-your-throttle-body.10607/

 

[Black08TB]

Thanks for the link, will get this done asap.

 

[Chickenhawk]

What was the make and stock number of the plugs you used? No such thing as OEM plugs. They are the ones they put in at the factory. Not likely they will put them in at the dealer. AC Delco 41-103 are the ONLY ones that should be in the I6.

Also, did you see my above question on your temp gauge?

Work through problems logically, starting with the most common and the most inexpensive solutions first before you start throwing parts at it. BY FAR, the most common reason for loss of power at highway speeds, while climbing hills or accelerating on the highway, is a plugged cat.

 

[Black08TB]

Where is your coolant temp gauge pointing when its warmed up? Dead center, slightly left of center, or slightly right of center?

Left of center as it always is.

What was the make and stock number of the plugs you used? No such thing as OEM plugs. They are the ones they put in at the factory. Not likely they will put them in at the dealer. AC Delco 41-103 are the ONLY ones that should be in the I6.

Also, did you see my above question on your temp gauge?

Work through problems logically, starting with the most common and the most inexpensive solutions first before you start throwing parts at it. BY FAR, the most common reason for loss of power at highway speeds, while climbing hills or accelerating on the highway, is a plugged cat.

They are AC 41-103 from the dealer parts dept, they have 50K on them.

I've no plans to throw parts at it, but with the advice provided about the exhaust manifold possibly being the culprit, and needing to be replaced anyhow, that is the first thing to get done. I'll mention to the shop owner to look at the CAT while it's there as I have no idea how to test a cat.

The air filter was filthy so that needed to be changed and I don't remember the last time the fuel system was serviced. So I'm tackling the fuel and exhaust manifold at this time and if nothing improves onward and upward. I need two more years out of my TB until I'm ready to trade up. Will report back as things progress, thanks again for the help.

 

[Sparky]

How far left of center on the temp guage? Ideally, straight up at 210 is where you want it, although my old 02 always sat almost 1 tick left and was perfectly fine and normal. That was the exception though. Anything lower than 1 tick left and I'd be replacing the stat/sensor.

 

[Mooseman]

You would need to get actual temp readings as the gauge is a "guessitgauge". A bluetooth OBD adapter and the Torque app on Android works very well and cheap.

 

[Blckshdw]

Left of center as it always is.

my old 02 always sat almost 1 tick left and was perfectly fine and normal.

A bluetooth OBD adapter and the Torque app on Android works very well and cheap

Mine has always sat slightly to the left, call it half a tick, even after my t-stat replacement a few weeks ago. Temps stay between 195-200 after a short time driving.
Every now and again, I consider yanking my cluster, to pull and adjust the needle

 

[Chickenhawk]

While the coolant temp gauge is not linear and is not always 100% accurate, especially if the stepper motors have ever been replaced, I have found over the years that the gauge reading for most people is a fairly accurate indicator as to the health of the thermostat, and hence the cat. This is why that was my first question when I read your symptoms.

With some exceptions, I would still tend to say that a temp gauge left of center means the thermostat is likely bad. If it has been like that for a while, then the cat is probably plugged. Cats run on a very narrow range of mixtures, and as these platforms age, we are going to see more and more plugged cats. If the driver has also had a few misfires in the past or if the vehicle has ever been driven even just a few miles with a check engine light flashing, the cat is almost certainly bad.

A plugged cat will not throw a trouble code.

Testing will confirm this. Coolant temperature on a scan tool needs to be 205 to 210 when warm. If it is anything below 190, then your sensor is bad or the thermostat is bad. If it has been like this for a long time, the cat is almost certainly plugged.

The cat can be tested with an exhaust backpressure test. They pull out the front oxygen sensor and replace it with an exhaust backpressure gauge. Backpressure at idle should be 0 PSI and no more than 1 to 1.5 PSI at 2500 RPM.

This is also a good time to replace the front oxygen sensor. We recommend a new sensor with every plug change, if for no other reason than to prevent them seizing in the manifold after many years.

Don't let the shop test the cat by just feeling it or reading the temperature. A backpressure test is definitive. (I own my own exhaust backpressure gauge and an entire set of oxygen sensor sockets because we have two Trailblazers in our family and that's how sure I am that your cat is partially plugged.)

 

[Black08TB]

Got the truck back from the shop yesterday afternoon, it took a few days to replace the exhaust manifold. He broke a couple bolts and took some extra time to get them out. Anyhow, he drove it down the highway a couple exits and it ran fine. I picked it up and took it down the highway with the cruise control set at 70 and was passing traffic up a slight grade with a big cross wind. I drove it this afternoon for 30 miles on the highway and it held speed just fine. The manifold gasket was leaking at cylinder #3, and the manifold itself was broken completely just above the bung for the O2 sensor. So maybe it was sucking air and baffling the system. I don't know, but it's running good again and I'm happy.

 

[Mounce]

That's interesting. Thanks for the follow up.

 

[Black08TB]

Have put 700 miles on it since the repair. Took two round trips to Rutland, VT. with my family and a load of luggage, gear and the truck ran flawless. Had plenty of power and held speed on the grades without issue. The old TB is running as well as it's ever run and I'm real happy. Did have one problem. After taking the wheels off to rotate the tires, grease the U-Joints and front end, I found the cause of the little clunk that would happen when backing up. The left rear, top locator bar for the rear end was broken in half. I took it off and welded it back together with some 1" x 1/8" flat stock. I patched it on both sides and one across the top, bent it over both sides and welded it solid. Will probably hit the salvage yard for a used one (or buy a new one) come spring time since it was pretty corroded. But for now it's solid and I'm not worried about it. I checked the passenger side while I was at it and it's fine.

Passed through Ludlow, VT and saw two TB's for sale at Benson Chevy. If my TB was on it's last leg I'd have driven this one home. The frame had surface rust, no doubt from the VT winters, but it's an 04' with 69K asking price $5995.

https://www.bensonschevy.com/Vehicl...t-TrailBlazer-4dr_4WD_LS-Ludlow-VT/3324249603

they had one other TB, an 07' with 126K miles for $6995.

https://www.bensonschevy.com/Vehicl...t-TrailBlazer-4WD_4dr_LS-Ludlow-VT/3350272833

 

[Sparky]

I wouldn't buy a salvage yard one with how relarively cheap they are brand new. I had to replace both of mine a year or two apart from each other when I had my TB.

 

[Black08TB]

I wouldn't buy a salvage yard one with how relarively cheap they are brand new. I had to replace both of mine a year or two apart from each other when I had my TB.

Good point, will check the dealer first.

 

[Sparky]

You can get a Moog or Mevotech for around $50 or ACDelco for $80 ish from Rock Auto or Amazon. I would suspect the dealer would want north of $100 given historic pricing structures at those places.

 

[Black08TB]

Just checked, $55 for AC Delco at Rock Auto. $125 form GM Parts Direct on the internet. Rock Auto it is.