99 malibu with erratic poor acceleration...can't figure this out

Hi everyone. Knowledgeable at home mechanic here. Have a 99 malibu with the 3.1L engine with 165k miles roughly.

Has a very odd acceleration problem that only happens when warm or at temperature. There are Never any codes nor pending codes. Not seeing anything odd with short and long term fuel trim at idle. MAF has been swapped with a known good one. Car has new plugs and wires. We also redid the top end: new head gaskets and lower intake.
There are no vacuum leaks as far as i know. Transmission shifts great, no issues there.

Ill try to describe this as best as possible. Its so hard to accurately describe the issue because of how odd it is. There are times when it just wont accelerate. From a stop i could floor it and it just kinda sputters...slowly accelerating while jerking the car gently and then....out of no where....itll snap out of it and accelerate decently.
It does this in city traffic but also at interstate speed. Ill approach a hill, begin to accelerate, TCC will unlock, still not moving faster, downshifts to 3rd...STILL not really moving...then like i said all of the sudden it kinda snaps out of it and begins to move.
I was told it may be a transmission issue but i highly doubt that. It shifts great and never slips.
I think at this point i need a graphing scope and/or scan tool to see whats causing this.

Any help is very much appreciated. Thanks guys.

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Im kinda pissed with the shop i took it to.

They basically test drove it, didnt hook a scan tool up to it, brought it back and said, "you have a transmission problem"
I asked how? They said, "your torque converter is staying locked up when it shouldnt be"

1: i call serious bull crap to that. It locks and unlocks perfectly on the highway. I watch it lock and unlock a hundred and more times.
2: most torque converter issues are a bad converter clutch or similar.

3... the problem used to be pretty erratic but lately...its been happening at every acceleration

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Yeah, I also disagree with a transmission issue. Doesn't match the symptoms you're describing. A TCC sticking applied would cause the engine to stall when coming to a stop, not sputter on acceleration. :silly:

What kind of scanner do you have right now? I ask because graphing would be nice, but not necessarily required. If you're looking for suggestions, BlueDriver is an excellent option. I also use an ELM247 adapter with OBD Auto Doctor regularly.

After hearing your symptoms, my mind immediately thinks about a weak ignition coil. It's a Type 2 system, and these coils fail all the time. :lol: I see you did plugs and wires, but you can still have a weak coil that can't deliver under load. IMO, the symptoms match because initial acceleration is when the ignition system sees the most load. The throttle just opened, the mixture in the cylinder is (briefly) lean, and the ignition timing gets dialed up. Suddenly, a coil that works well enough for idling and cruising can't cut it any more.

I'd recommend doing a power balance test on all six coil towers with an incandescent test light. Even if the engine isn't misfiring at idle, you'll still be able to pick out a weak tower or coil. Here's a video of Paul demonstrating the idea:



Different coil design, but the concept is exactly the same.

Thanks tyler.

I am using a very cheap actron scan tool...nothing fancy by any means but i can at least view live data.
I actually did swap the entire coil pack and ICM assembly. I know this doesnt necessarily verify anything but it came from an impala with the 3.4L (same platform pretty much) the impala has no issues whatsoever...

Problem stayed.

When the car is cold it drives great, it is very very peppy..
As soon as it warms up it has the problem.
It also doesnt even matter if i floor it which forces open loop mode, it still isnt correct until it "snaps" out of it

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do u have A tach,, if it is a transmission problem, the tach will tell you, if its revving and not moving, its a transmission problem. Ive heard a failing tps sensor can also mimic transmission issues and not throw a code. also dont forget abt the 02 sensors., closed loop.

Thanks. O2 sensor is confirmed good.
Tps on the actual scan data is good but the actual electrical pattern i have not checked yet.



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This issue has me so frustrated and stopped in my tracks...and its a silly malibu LOL not a very complicated car but yet here we are.

I will check the TPS for sure. Any other ideas?

Still not convinced its a transmission issue... or even a torque comverter issue. If the stator was bad then it would cause issues all the time, not erratic or intermittently right?

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they can check out good electrically, and still be bad and not throw a code. in dealing with a similar issue right now on a truck. they are very precise com ponents

also , dont know if that engine has a camshaft or crankshaft reference sensor sometimes both, but they can cause sqiurrely problems as well. usually throw a code though,. bad converters are usually noisy.. the whine loudly and make the vehicle strain to get moving. I think AAMCO will scan your transmission for a minimal fee.

does your scanner detect transmission codes. there are TCC circuits in valve bodies, that are hydraulic, and some are electrical. if you are having a pressure rise malfunction it could be a transmission issue. some solenoids in transmissions are electronically controlled.
Does the car have a tach?

have you checked for a failing converter. sounds like a clogged converter to me. there good cold then warmer they get the worse they accelerate.

Thanks...i have not checked exhaust backpressure yet. Thats one thing i did not do as i dont have a gauge. I will definitely have to check.

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Wacoeagle...
Cam and crank sensors are good and check out ok.

Im still not onboard with a transmission issue..
I do realize TCC is a pulse width solenoid and hydraulic valves.
On this vehicle when its up to temp, TCC is applied in 3rd gear and 4th gear when above 40mph.

This issue is happening at a stop at 0mph. If my TCC clutch was the issue then my engine would be stalling. It does not stall and never has.
Plus, while driving on the highway i can visually watch the tach as the TCC locks or unlocks. Its the same way everytime.
There are no DTCs for the engine or transmission.

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HI
By the sounds of what you are sating, you most likely have a fuel problem, I would test the psi and amount of flow. If you have a scope test the pump.

I would also check all the powers and grounds to the pump.

This is a good starting point , oh if you have a filter change it before you test then re test.

Good luck

Doc