SOLVED! Low Power idle to 2500RPM

#1
05 4.2 with 220k

First noticed the issue when it wouldn't pull jets skis up a steep hill. It doesn't miss, stall or hesitate. No check engine light or codes are stored except for ABS Module. Once it reaches 2500 RPM power increases and by 3000 RPM it starts pulling all the way to a WOT shift in every gear. I first suspected it was not shifting down into first but I pulled down a steep section of my yard and tried reverse with the same result. Virtually no power off idle.
Ideas suggestions?
 

littleblazer

Gold Supporter
#2
Make sure the TC isn't slipping and do an exhaust back pressure test. A plugged car would exhibit low power symptoms.
 
OP
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#3
Definitely not the cat. I spilled it’s guts over 100k ago.
Not sure what you mean by the TC slipping. The lack of torque is only at low rpm. It very similiar to starting in second gear.

I suspected the vvt as a retarded cam could affect low rpm power, however it starts well, mileage is normal and no codes stored.
Usually performance issues manifest themselves at full throttle and high rpm
This one doesn’t. It runs good at high rpm 🤔
 

littleblazer

Gold Supporter
#4
If you had a vvt issue it would idle rough and sound bubbly. Like you said.

Have you tried manually dropping it to first and seeing if it seems better? To me it sounds like some component in the trans is slipping but that again would only be worse with more rpms...
 
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#5
I agree I used first and reverse rpms do not flare
Normally it would spin in the yard until traction control engaged
I just verified full throttle by watching the throttle blade while my wife floored it.

Compression test coming up
 
OP
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#8
Pulled the CPAS and screens were pretty clean. It was replaced year or so ago.

I’m really not sure what direction to go.
 
#10
There would be cam/crank correlation codes if the chain jumped or stretched or VVT acting up.

When was the maintenance done? Plugs? ACDelco 41-103?

Check the fuel trims. Also check the intake manifold bolts.
 
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#11
New acdelco at around 20k ago. Plugs looked great yesterday when I pulled them for the compression test. Eyeball says slightly lean by color
Oreilly’s isn’t keeping the Bosch scan tool on loan a tool anymore so I had to go to autozone and theirs is worthless except for a code reader.
What’s the best deal going for pcm monitoring these days?
Will spend some time today on intake manifold leaks.
 
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#15
Used propane and starter fluid all around intake manifold without any apparent change in rpm

It rained today and stabilized at 3500 RPM in first gear I opened the throttle and it spun the tires until 6000 RPM... traction control off of course. Flooring it from idle it just doesn't have enough torque to do anything until the higher revs.
 

littleblazer

Gold Supporter
#16
That idle vacuum seems low.
 

MRRSM

Lifetime VIP Supporter
#19
In addition to @Mooseman 's suggestions about performing Fuel Trim Checks at Mid Range and Higher RPM... Three other things might be added into consideration:

(1) Examine the Small Vacuum Line Hose on the Fuel Rail Fuel Pressure Regulator for being loose.
(2) Perform a Dynamic Fuel Pressure test via an Honest Analog Fuel Pressure Gauge.
(3) Replace the Fuel Filter.
 
#21
What's the maintenance of the transmission been like? Fluid level and condition?

Even though you gutted the cat, it is possible that you may have backpressure from the muffler or resonator packing letting go. Try pulling the downstream O2 sensor and running it like that. If it improves, it's usually from backpressure. You could also do a backpressure test if you have access to a tester.
(I normally recommend pulling the upstream O2 sensor but since in this case, the cat is gutted, this would affect fuel trims the least)

At WOT, the O2 sensor and fuel trims do not come into play as the system just goes into full power mode. That's why the FT's are at 0.0.
 
#22
What's the maintenance of the transmission been like? Fluid level and condition?

Even though you gutted the cat, it is possible that you may have backpressure from the muffler or resonator packing letting go. Try pulling the downstream O2 sensor and running it like that. If it improves, it's usually from backpressure. You could also do a backpressure test if you have access to a tester.
(I normally recommend pulling the upstream O2 sensor but since in this case, the cat is gutted, this would affect fuel trims the least)

At WOT, the O2 sensor and fuel trims do not come into play as the system just goes into full power mode. That's why the FT's are at 0.0.
If you pull the o2 when cold you could probably get away with it in open loop and be okay...
 
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#23
Pulled the .O2 sensor while it was cold in open loop and no change
The pics of the scanner earlier in the thread show wot against a curb. Around 875 is all it will do.

Transmission is due for fluid change as it’s been around 100k.

I’ll check some of the other items when I get back home later this week
Thanks again
 

MRRSM

Lifetime VIP Supporter
#24
One other thought comes to mind...

Back in Post #16... you mentioned trying Propane and Starter Fluid to no effect...and that makes me wonder if your MAF and IAT either need a good cleaning or possible replacement. Even though your only 300 FASL ... a sketchy MAF ..which actually alerts the PCM as to how much air is passing through it ...BY WEIGHT...and a Dirty IAT would not be giving the right responses if clogged with atmospheric dirt.

So if their responses are sluggish... spraying more Fuel through the TB would not help if the PCM does not know how to achieve a Stoichiometric balance by correctly sensing the AIr passing into the system. It cannot hurt at this point to replace your Air Filter and even clean the Throttle Body, too while you are it if this all seems like a worthwhile avenue of investigation. If you try this on... CRC MAF Spray is the only acceptable cleaning product for those sensors. Fuel... AIR... and Spark. In the end... it never gets any more complicated than that.
 
#25
EnTow, this is a very interesting thread. Your symptoms of no power in reverse on an incline match a problem my daughter's little truck is experiencing. We have a steep drive way and I often go up in reverse to park the truck in a particular way. So in reverse from standing still the truck is often gutless and struggles to go up the driveway. I started a thread on it. https://gmtnation.com/forums/threads/2004-tb-surges-at-idle.18875/ I will be following your thread for developments. Good luck!
 

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