Hi all, I had a 2005 AT, on 2nd engine.

about two years ago, I suffered from both low MPG(13-20) and p0300 code, so I replaced all the spark plugs, wire set, and ignition coils. after I replaced the plugs, the code was gone but the MPG is still bad, and after I replaced the coils, everything was good until now. (NGK plugs and BWD coils)

I recently found the car cannot hit the redline, and rpm is always below 7k (just sometimes not always), and when if fails, the CEL light pops, and I receive p0300 code. starting the car is also a little "rusty" I guess. However the MPG is quite good, (16.5-24). It's quite annoying if you cannot redline the car. Right now both the plugs and coils have 2 years, 9k miles on them (havent driven a lot, but pretty hard driving..). should I look at the plugs or should I look at the coils? it's really frustrating if I have to replace both, since there isnt even 10k miles on them?

Thanks for your feedback!

 

additional question, if I replace plugs or coils, instead of replacing all, can i just replace some false ones?

 

It's unlikely to be ignition parts. You might have a clogging cat, which will smoothly put a cap on how much power you can produce. If you can rev in neutral to redline, but can't when it's in gear, then it's likely that your cat is starting to clog on you. Other possibilities would include a failing front O2 sensor, a filthy air filter, and failing intake valving.

You can test the coils and plugs to make sure they are working right though, to confirm whether or not you have had a parts failure.

 

What is the redline on the AT's?

 

7.5k

 

Thanks for the input. The reason I suspect ignition more is that the misfire happened only sometimes, and sometimes, it redlines just fine. A clogged cat shouldnt do that, right?

Back to two years ago I also had issue with not being able to redline, but replacing the ignition seems to solve it very well. It's just so bad if I wll have to do it again after 8k miles

 

Intermittant problems would point more toward intake valving or O2 sensor than cat.

You aren't very clearly describing your symptom though. There are LOTS of different ways and feelings to "unable to reach redline". Could be smooth and just never gets there, could be jerky and unstable, could be hitting an RPM point easily but then getting blocked from going further, could be getting choked with fuel till it's sputtering, etc...

If you can describe the sensations and reactions more clearly, it would help with the diagnosis.

 

What about fuel pump? Do fuel pumps exhibit intermittent performance deficiencies, or do they just quit when they reach a threshold?

 

A fuel pump failure itself just quits when it overheats. In theory a leaking fuel line could produce similar problems, but it would also cause a heavy fuel smell with any fuel system pressure and limited car movement. If the owner isn't commenting on that, then I leave it off the diagnosis list.

 

RIWWP, I beg to differ.

When I replaced mine, at the freaking track, with a rock and a stick for tools, it solved the left turn issue as expected, but MPG went up a bit too and fuel trims quit wandering around... My guess is that a worn pump is not providing a constant pressure and that will really **** of the ECU.

Just a thought..

 

Hmm. Interesting. A certain amount of logical sense to it. I'll have to keep that in mind.

 

Notice the 2 years ago part... Was the last time you did the tune up two years ago? How many miles since then? And lastly did you clean off your MAF and ESS sensors as well as reset the computer by 20 brake pedal stop method or disconnecting the battery for a few minutes? You may very well be due for new plugs and coils already if its been more than "x" number of miles.

 

alright folks! thanks for all the input. I've keeping a close eye to the car, but since my last post (Nov), it's been good. I redlined almost every time but never had any trouble reaching it, and no CEL. until recently I disconnected battery and clean MAF, then did some redlines, some are just fine, but some have clear hesitation after 6k rpm. In the later situation, the CEL will flash but will clear pretty quick. I didn't check the code, but should be the same one.

Some other information:
1\ when I have trouble reaching redline on 1st gear (as CEL pops out), there is no trouble to reach redline in neutral.
2\ I checked the CAT. I redline the car multiple times in a run, then check underneath the car in night. didnt see anything glowing. so the CAT is still good?
3\ The plugs, wires, and coils have 2 years plus few month on it. but there is only 8k-9k miles on them. I didnt drive the car a lot. do you think 8k-9k miles sounds right about wasting these ignition parts?

 

1. Again, intake valving or cat. (I'd also suspect a weak fuel pump as well).

2. Diagnosing a cat by running the car hard isn't really a definitive diagnosis. The only 100% sure means is by physically removing the cat and inspecting the honeycomb structure. This is especially true in an AT's case, since you have 2krpm less headroom vs a MT to heat the cat.

3. It's not out of the question that a failed cat killed your first ignition set and now this second one. Verify cat health, then get the ignition system tested. Use a timing light or spark tester to determine the health of the coils. Pull the plugs and make sure the ends look brown-ish (assuming you don't premix) and the tips aren't broken off or missing.

First, start with pulling the cat, that's the simplest one to diagnose with just hand tools. Examine both ends of the pipe. I say this because you are still experiencing symptoms consistent with cat failure. Second, test each intake valve for smooth operation. They should open and close easily with no hitching or sticking. Third, test the ignition system. Fourth, either pressure test the fuel system or replace the pump entirely if it's the original. (OEM one's just plain suck and it's practically a regular maintenance item on the car.) Fifth, get an OBD-2 reader that can read AFR and examine the reading under load and cruse. If the reading "bounces" erratically, replace your front O2 sensor. Get a helper with this if the one you use lacks a logging feature (eyes on the road, please).