Low power at low rpm's

[cchase]

This is a strange one, and I'm not even sure I understand the symptoms entirely.

Our 2002 Malibu has felt down on power for a few months now. I've been trying to figure out what's going on by at least isolating "what" it's doing specifically, but even that has not been easy.

I came to the conclusion today that I think it's only down on power from about idle to ~3000 rpm. Above that, it begins pulling with normal gusto (or at least noticeably more). It drives like a vehicle with a lot of turbo lag, to me.

Additionally, the power delivery in that range under normal acceleration is almost "lumpy". It's almost a surge. Not to a degree that I think a passenger would question it, but it's noticeable.

I am not getting any codes. Fuel trims are no different than ever, about +2.34% to +3.12% long term on the highway at ~2000 rpm. Throttle position sensor readings are responding correctly. MAF values look realistic to me, up to about 150 g/s at WOT and near shift point. The timing seems to respond correctly.

Originally I thought it seemed just, down on power, but after flooring it a few times today I noticed that at almost all at once, nearly at 3000 rpm, it really starts pulling hard (okay, as hard as 170 hp will pull).

Any ideas? Things to check?

 

[NHGUY]

Vacuum readings?

 

[cchase]

I hooked a vacuum gauge up and it's sitting steady at about 19" at idle. Looking at sensor data again, everything looks good. Drops to near 0 at WOT.

 

[901Memphis]

Plugged cat?

 

[The_Eric]

Engine?

TPS can give similar symptoms and wont always set a code.

 

[Mr Nice]

Lazy 02 sensor without triggering an OBD code ?

 

[Kuato]

Old Chryslers used to act like that when the timing chain stretched. Timing belt been changed?

 

[cchase]

Originally Posted By: The_Eric
Engine?

TPS can give similar symptoms and wont always set a code.


3.1 V6. I monitored TPS signal through OBD and confirmed that it was behaving properly, so I had ruled that out. Any reason to think twice?

Originally Posted By: LT4 Vette
Lazy 02 sensor without triggering an OBD code ?


I looked at the O2 sensor data but have not plotted any with the oscilloscope yet. But the data did not lead me to believe the O2 sensor was behaving strangely. I realize seeing the numbers alone do not give the whole picture, but it seems odd that it would result in the symptoms I described.

Originally Posted By: Kuato
Old Chryslers used to act like that when the
timing chain stretched. Timing belt been changed?


Pushrod, timing chain.

 

[cchase]

Originally Posted By: 901Memphis
Plugged cat?


I considered this but again, only seems to be at lower power. Also, the vacuum gauge seemed like a good place to start for catalytic converter issues and it responded normally for changing engine speeds. Anything else I should look at?

 

[spackard]

Intake manifold vacuum leak.

 

[The_Eric]

Originally Posted By: cchase
Originally Posted By: The_Eric
Engine?

TPS can give similar symptoms and wont always set a code.


3.1 V6. I monitored TPS signal through OBD and confirmed that it was behaving properly, so I had ruled that out. Any reason to think twice?



Yes. I battled my wife's Grand Am for some time because some driveability issues- similar yours, but not entirely the same.

Long story short- I had checked the TPS several times with a scan tool (appeared normal). I even went so far as to graph it's output with the scanner and still nothing. Also, there were no definitive codes (PCM had some issues too). Initially I was positive it was a PCM issue, so I ignored the TPS code (there was a TPS high and TPS low code simultaneously and scads of other codes too) Finally the problem got worse, so I decided to put a TPS on. Driveability problems solved!

Our Hyundai also was acting a little extra pooky- a quick look with the scan tool found it to be out of whack, but it never set a code. A new TPS brought it back to the way it should be.

 

[19jacobob93]

I had what sounds like the exact same thing on my car only at about 2,000rpm under load, and it also cleared to full power at about 2,500-3,000rpm.
I got a lot of people telling me it was either the coil pack, leads or spark plugs, and since they're all pretty cheap, and the coil and leads were 14 years old I replaced the lot and the problem has gone completely

 

[Nick1994]

Low power at low RPMs? My car has low power at all RPMs! Lol

 

[cchase]

Originally Posted By: 19jacobob93
I had what sounds like the exact same thing on my car only at about 2,000rpm under load, and it also cleared to full power at about 2,500-3,000rpm.
I got a lot of people telling me it was either the coil pack, leads or spark plugs, and since they're all pretty cheap, and the coil and leads were 14 years old I replaced the lot and the problem has gone completely


The spark plugs and wires are fairly new (~30k miles ago). The coil packs are ~$70 each (3 of them) so I'd rather be sure before just shooting in the dark to replace those.

 

[cchase]

Tonight, did the following:

- cleaned the IAC valve
- cleaned the EGR valve
- cleaned the MAF
- wiped down the inside of the throttle body and TB valve
- replaced the TPS

Everything was fairly clean, I think I remember cleaning the throttle body about 3 years ago or so.

I didn't notice a substantial improvement when I was done, although I didn't have a chance to go on the highway.

 

[artificialist]

What are your compression test numbers? Sometimes a weak engine fails a compression test.

You don't need an expensive tester if you DIY. A generic one from an auto parts store will last long enough for a DIY-er.

 

[cchase]

Originally Posted By: artificialist
What are your compression test numbers? Sometimes a weak engine fails a compression test.

You don't need an expensive tester if you DIY. A generic one from an auto parts store will last long enough for a DIY-er.


I have not compression tested the engine or pulled the spark plugs. But I will do that this weekend if I have the time.

I somehow doubt the compression is low because it pulls quite well as soon as it magically crosses the rpm threshold. It really is quite "sudden" in the power change.

 

[Eddie]

Sounds like the timing is being pulled back for some reason. Ed

 

[cchase]

Pulled a few spark plugs this past weekend and all looked decent.

Tested fuel pressure, book says 52-59 psi key on, engine off, I measured 56 psi. Book says it should drop 3-10 psi at idle, mine dropped 6-8 psi.

Not sure where to look next.

 

[cchase]

Someone out there must have some fresh suggestions for me...

Tonight I did a compression test on 2 random cylinders. I realize it's not the most comprehensive method, but I really don't think the engine is "worn out".

My results for both cylinders tested were 190 psi. Spec only calls out a minimum of 100 psi with no more than 30% deviation between cylinders. One of the plugs I pulled enabled me to look at the top of the piston which was pretty darn clean. Some very light carbon but still clean enough to see the machining marks on the top of the piston.

I put a vacuum gauge on it and it sat at about 18" and varied by about +/- 0.5" at idle. Twisted the throttle briefly, vacuum went to 0, snapped the throttle shut, vacuum went to ~24" then settled back at 18" - although, and hopefully someone will make something of this - the engine did "sputter" ever so slightly right when it hit back to 18" vacuum before recovering. I don't think if you were in the car you would have noticed it, very slight.

This afternoon I unplugged the MAF sensor to see if things "changed", but the symptoms were unchanged.

To summarize the symptoms:

- car has a "lumpy" acceleration at low RPM's and heavy throttle but when it hits a magic RPM (and I'm not usually looking since I'm accelerating but I think it is somewhere between 2500 and 3000 rpm) it takes off normally. Very on/off in its nature. One second there's nothing, then suddenly it takes off - it's noticeable enough that you can really hear/feel the engine start pulling.

- the car seems to be generally low on power, but it's not noticeable around town. I have noticed that it is downshifting more than it should going up long hills on the highway, and I even had one instance where I thought I should speed up and realized (with cruise control on) that the pedal was all but on the floor already...

- at steady state throttle going up long inclines on the highway at about 3000 rpm (perhaps the same as when the "switch" happens under acceleration), the engine almost feels like it's surging - or as if the transmission was slipping ever so slightly and then catching repeatedly. I do not think it is a transmission issue however, I think it's tied to the same thing that's causing the other problems.