02 hesitation under acceleration

jeffsmls · Jan 10, 2011

02 3.8 hesitates when throttle is past one third to WOT. Runs smooth with light throttle up to about one third open. No codes. Car has 109,000 miles.

I have recently replaced: plugs, wires, fuel filter, DPFE, EGR solenoid, right after-cat O2 sensor, cleaned MAF & air filter and replaced EGR valve about one year ago. I have recently tested with another EGR that I know is good with no improvement. O2 sensor was a guess, so don't want to guess anymore.

Engine idles fine, will smoothly acclerate with light throttle. Will miss in any gear and neutral, greater than about one third throttle.

I have looked at some of previous posts but did not see same problem, some similar but not same.

I would appreciate any help or direction to previous applicable posts.

Thanks in advance.

wmburns · Jan 10, 2011

jeffsmls said:
I have recently replaced: right after-cat O2 sensor.

The rear O2 sensors only job is to monitor the cats. It does not affect driveiblity. Your $$ is better spent replacing the FRONT O2 sensors.

Tell us about any mods. Underdrive pulleys?

How about the routing of the spark plug wires? Routed using factory looms and standoffs? Touching any metal parts?

Try this old racer's trick. Run the motor in a TOTALLY dark location. Look for the blue hue of escaping spark. Inspect the plug wires for white spots which is evidence of arcing.

Consider testing the base engine's health. Perform a compression test.

Consider using a fuel injector cleaning service such as injectorrx.com. Once cleaned and flow tested, the injectors will be as good as new.

Check for an exhaust restriction. Hit the cats with a rubber mallet. Listen for anything moving inside.

Do you have access to an ODB2 scanner capable of monitoring operational data? Would be a great help to know the fuel pressure, long term fuel trim, and air flow.

jeffsmls · Jan 10, 2011

wmburns,

Thank you for the O2 sensor info.

Two mods to the engine consisting of a BBK CAI and an SLR Motorsports Performance Chip.

The SLR chip was wired into the two outer wires of the MAF plug wires by using blade-type connectors that pierce the plastic to establish contact with the wires. The car ran with this mod for some time before this problem developed. I removed this chip after the problem developed with no luck. If these two outer wires leading to the MAF were damaged by this installation, could this cause the problem?

Plug wires were replaced into original factory looms and standoffs. Not touching metal parts. Will try racers trick you suggest as well as checking cats. Will also look into injector cleaning.

I only have a basic scanner that retrieves codes. Does not monitor operational parameters.

Thanks for advice.

wmburns · Jan 11, 2011

jeffsmls said:
If these two outer wires leading to the MAF were damaged by this installation, could this cause the problem?

Yes. Without knowing the wire colors I can make and educated guess. On the MAF connector, Pin #5 is "MAF sensor input". Pin #6 is the IAT. I suspect the chip works by fooling the computer to think that the air temperature is lower than it actually is. I suspect the problem is that the PCM is not receiving any IAT input. In such cases, it assumes a very high value. This is limiting the timing advance and thus your symptom. Fix the wire damage and go from there.

This is why it's important to list all mods.

jeffsmls said:
I only have a basic scanner that retrieves codes. Does not monitor operational parameters.

No. A basic code reader can will only read DTC codes.

jeffsmls · Feb 27, 2011

Update

OK, I removed the injectors & had them serviced. I also repaired the wires that were damaged leading to the MAF. I did the old racers trick which revealed a bad plug (which I replaced). Running better but still will hesitate & backfire thru intake when you floorboard the gas pedal putting engine under load (like 5th gear at 30 mph).

I appreciate the help so far. Would welcom any further thoughts.

Thanks.

splatteredguts · Mar 10, 2011

Check the coil. I had the exact same problem. Turns our my coil was bad and causing a misfire. Replaced it and now the motor runs smooth under load.

Hope that helps!

02 hesitation under acceleration

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