Low power at low rpm's

[The_Eric]

I went back and re-read your post about the vacuum gauge readings. That to me isn't entirely an accurate test.

I like to plumb a pressure gauge in the O2 sensor port and measure that way. Diagnostic methods vary, but the way that has served me well so far is to install the gauge and start the engine. Give the throttle a couple of soft whacks. If you see 4 or 5 psi you've likely found your problem. If that's inconclusive, I'll rev it hard two or three times back to back. Bad cats will give 6+ psi back pressure. We had one in the old shop I worked at that was so bad it pegged a 15lb gauge and stuck it there! That's why I start off easy now. That car struggled to achieve 40mph.

In fact, we've got an 06 Kia Spectra at the shop now, awaiting authorization to repair. It came in with lackluster acceleration and power. I suspected a bad cat and investigated. First clues were visual. Melted tape on a wire harness in front of the pre-cat. Next was the intermediate pipe/flex pipe that went back to the secondary cat- it was drooping down. It got so hot, it drooped! Pulled the upstream O2 and was able to see the catalyst in the pre cat seemed fine. Back pressure test at the down stream O2 showed 0-1 psi at idle, 2-3 at around 2500 rpm and 12psi when revving hard. Yup, we've got a problem. Called for permission to tear the pipe off so I can verify it's the cat and not the muffler and found the media was broken and blocking the converter.

 

[Clevy]

My mustang showed very similar symptoms with a car that was shot. Once I removed them the car ran perfectly.
Now I'm not suggesting cutting off your cats,however if it's possible to check for an exhaust restriction I would look at that if possible.

 

[OVERKILL]

For those saying the catalyst, this flies in the face of my experience with that issue as per the following line from the OP:

Quote:
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.


When I was dealing with a plugged cat, it was down on power everywhere. And trying to accelerate, when you brought the rev's up, it was like running into a wall. Basically the polar opposite of what the OP is experiencing, which is a loss of power when flow through the exhaust is low, and then the power comes back at a point where the flow would be increasing (3k+).

 

[Cujet]

Clogged cat would be my other guess

 

[cchase]

I appreciate all the feedback I'm getting on this.

My plan, for now, is to try to disconnect the exhaust before the catalytic converter and see how it feels.

Although I do like the idea of a pressure gauge, I don't really have access to that kind of equipment easily.

 

[cchase]

I was able to "test" and check a few things today.

I was able to cobble together a pressure gauge using an old 0-10 psi fuel pressure gauge and connect it to the upstream O2 sensor hole.

At idle, it wasn't even registering 1 psi of pressure, and at 2000 rpm it was the same. If I popped the throttle WOT it went to about 3 psi for 0.5 seconds and then immediately went back to basically 0. I did verify that the gauge operates correctly the way I had the system cobbled together.

I pulled ALL the spark plugs, and found that 1, 5, and 6 were severely worn - as in the little electrode tip was almost completely eroded away. The other 3 looked normal, or at least normal for their 40,000 miles in service.

I replaced all 6 plugs and did observe that it is "better" than it had been recently, but probably still more in line with the way it felt when I originally posted this thread.

 

[Scott_Tucker]

I think you have a single cylinder misfire.

 

[cchase]

Originally Posted By: Scott_Tucker
I think you have a single cylinder misfire.


Is there a way for me to more readily narrow down if this is a possibility?

I do have to say though, I hooked an oscilloscope up to the 24X crankshaft position sensor and the squarewave looked nice and even to me, which seems like it would rule out a misfire.

 

[Scott_Tucker]

You need to do a cylinder balance test by cancelling out 1 cylinder at a time and see if there is a difference in the way the engine runs. A good firing cylinder will cause the rpm to drop when it is cancelled. If there is no change in rpm, then you know that cylinder is not contributing power.

You can do this on your engine by disconnecting the coil pack or injector for each cylinder.

You need to figure out the basics before you begin using tools like an oscilloscope. Even though the pattern may look visually OK to you, doesn't mean it isn't misfiring.

The cylinder balance test used to be the first thing anyone would do when diagnosing engine performance problems. It was easy with the old Sun scopes.

 

[Scott_Tucker]

Actually, does it idle 'lumpy' as you keep saying? You seem to be describing the rough running of a misfiring of an engine. It's possible that spark is leaking through a coil pack boot and arcing to the spark plug tube.

The spark will take the path of least resistance. As a professional, I have seen many times where I have arcing in one cylinder at low rpm that decides to fire through the spark plug half way through the rpm band. This would correlate with what you are saying about it lacking power in the lower rpm but picking up in the higher rpm.

 

[OVERKILL]

Originally Posted By: Scott_Tucker
You need to do a cylinder balance test by cancelling out 1 cylinder at a time and see if there is a difference in the way the engine runs. A good firing cylinder will cause the rpm to drop when it is cancelled. If there is no change in rpm, then you know that cylinder is not contributing power.

You can do this on your engine by disconnecting the coil pack or injector for each cylinder.

You need to figure out the basics before you begin using tools like an oscilloscope. Even though the pattern may look visually OK to you, doesn't mean it isn't misfiring.

The cylinder balance test used to be the first thing anyone would do when diagnosing engine performance problems. It was easy with the old Sun scopes.


It was easy even with some of the old ECM's. You could trigger a cylinder balance test on EEC-IV quite easily and the computer would let you know which cylinder was the culprit.

 

[cchase]

Originally Posted By: Scott_Tucker

You need to figure out the basics before you begin using tools like an oscilloscope. Even though the pattern may look visually OK to you, doesn't mean it isn't misfiring.


Is it safe to be disconnecting spark plug wires with a running engine? Something I remember hearing told me that it was bad for the ignition coils, and I certainly don't want to shock myself either.

For what it's worth, I hooked the oscilloscope up to the crankshaft position sensor to make sure that part was working properly, not to diagnose a misfire potential.

Wouldn't a misfire code eventually be set if a cylinder was not behaving correctly? I understand it has to misfire a certain number of times in a period of time, but still, this has been an ongoing issue for quite a while so I would think that if a cylinder was mis-behaving, eventually it would trip a code.

I will try the cylinder balance test you've described when I get a chance, in either case.

 

[Scott_Tucker]

It's safe as long as you don't do it too long. Does this engine have coil on plug? Unfortunately, misfire counter effectiveness varies by manufacturer and model. Some manufacturers allow you to kill cylinders with a scanner. Do you have access to misfire data?

 

[asand1]

EGR sticking open?

 

[cchase]

Originally Posted By: Scott_Tucker
It's safe as long as you don't do it too long. Does this engine have coil on plug? Unfortunately, misfire counter effectiveness varies by manufacturer and model. Some manufacturers allow you to kill cylinders with a scanner. Do you have access to misfire data?


It has shared coils, 2 cylinders per coil.

I don't have access to misfire data, but given a good recommendation, would probably go out and buy a scanner that had that ability (for other reasons as well - not just for this particular job).

Originally Posted By: asand1
EGR sticking open?


A good idea, and I went through that thought process too, but I tested the EGR and it is operating correctly.

 

[Scott_Tucker]

So you have a waste spark system. It has spark plug wires that lead from the coils to the spark plugs?

 

[cchase]

Originally Posted By: Scott_Tucker
So you have a waste spark system. It has spark plug wires that lead from the coils to the spark plugs?


Yes, that's correct (plug wires). Not sure about whether it is a waste spark system or not.

 

[circuitsmith]

If it's two plugs per coil then it's a waste spark system.

 

[Scott_Tucker]

Then you can just pull each individual spark plug wire and ground it to see if a cylinder isn't firing.

 

[cchase]

I will try to do that this weekend, I am also planning on manually operating the MAP sensor with a vacuum pump and voltmeter to confirm once and for all that it is operating correctly.

In the meantime, here are 2 photos that show representative plugs (one is from cylinder 1, 5, or 6 and the other is from 2, 3, or 4).