Use the correct O2 sensor

[ES_97Sport]

So, four years of rebuilding ... I finally got the exhaust replaced (OEM down pipes, CAT and elbow before the muffler and 2.25" w/Flowmaster after) and picked up (but didn't install) a pair of new OEM O2 sensors to replace the ones in there now. Everything in the vehicle that's related to engine management has been replaced with new OEM in the last year.

Running like a top, except ....

I had a weird, intermittent 'delay' when pressing on the gas from a stop or letting off the gas. I've seen this before with improperly adjusted - or plain worn out - TPS. 'Cept, I just replaced it and had it adjusted twice by two different people - that wasn't it. So, what do you look at when EVERYTHING is NEW??? In addition to this, it seemed to idle a little rough sometimes. OH, and the exhaust was kinda smelly intermittently - which I figured would go away after the new CAT was install, but didn't. Everything was very intermittent.

Over the years the CAT has been replaced several times as have the O2 sensors. When we replaced the CAT the last time, it was with a different CAT that relocated the bung for the rear O2 sensor. The OEM O2 sensor wiring wasn't long enough to reach and the rear sensor was dead. I didn't want to put in another aftermarket one because those weren't working for me.

I had plans to replace the entire exhaust but that time was a ways in the future. So, I figured no problem, I'll extend the harness. That's easier said than done. The wire is stainless - not copper - so its very difficult to solder. That explains why I've always read and been told this is a stupid idea. While difficult, its not impossible. I extended the harness eventually and put everything together. Been driving on that for a while now.

Yesterday, during a brief reprieve from the crappy rainy weather, I crawled under the big Sport and replaced the rear O2 sensor with a new OEM - unhacked - sensor. Second to last thing I need to do before I'm officially done with 4 years of reconditioning. (didnt' have time to replace the front O2 sensor)

Anyway, started it up, let it idle for 10 minutes and then took off to work. Didn't pay much attention. Parked at work for the day, took off last evening to drive home. Hmmmmm. Running a little 'funny'. Not bad exactly, just a little different. Still didn't pay a lot of attention. Got up this morning for work, started it up and sat for 5 minutes while searching for an address and then took off. Ah, yes. By the time I got 30 miles to the dealership it was definitely running 'different'.

No smell. No hesitation. No rough idle. Absolutely nothing. Runs EXACTLY like my '03 3.5L. Glass smooth and dead quiet. Took it from 60 to 85 degrees, highway and town - same behavior. Perfect.

Which all makes sense and is why I wouldn't have ever done this if I had a choice and absolutely never would have done this if I needed a permanent solution. O2 sensors are ultra sensitive pieces of equipment and changes in resistance in the harness is going to totally skew the reading from the sensor itself. So, at least the behavior is explainable.

This, yet again, IMHO, clearly demonstrates that the rear O2 sensor IS used in fuel trim calculations, too. Contrary to what all the Mitsu techs claim.

Something I thought I'd pass on before the weekend. Ok short term solution if you're desperate, but that's all. I'm pretty sure it wouldn't have passed emissions even though it never did trigger CEL or pop a code.

Edward

[redraif]

Good to know Edward. Thanks for that! Too bad I didn't know about this when we replaced them all with aftermarket years ago. Add it to the list of things to do to the sport... lol!

Please let us know if it running good also improves gas mileage. With these big tires I would love to tweak everything else to maximize the power and gas mileage.

[ES_97Sport]

Jul 30, 2015 12:05:05 GMT -5 redraif said:

Good to know Edward. Thanks for that! Too bad I didn't know about this when we replaced them all with aftermarket years ago. Add it to the list of things to do to the sport... lol!

Please let us know if it running good also improves gas mileage. With these big tires I would love to tweak everything else to maximize the power and gas mileage.

Not sure what's happening with performance and mileage yet. I just figured out that just a few days after I had the exhaust completely redone, I managed to crack the passenger side manifold almost completely in half so I've had an intermittent exhaust leak almost ever since. We had some pretty serious flooding at the office (1' of water on the roads) and I soaked the entire engine and engine bay with ice cold water for probably 1-2 miles. I'm surprised I didn't crack the other side, too.

I don't expect any huge change, though. Although leaky and spliced in so many places I couldn't count them all, I already had a Flowmaster with 2.25" exhaust. My mileage and performance HAVE gone up, but I'm positive that's got more to do with getting rid of all the leaks, hack job joints, the chopped up down pipes, and putting the factory CAT back in.

(See Note at bottom of this post) If you haven't already done the 2.25", I REALLY recommend it. I've been driving my '03, my new '99 and my friends '98 the last few days while the big Sport is in getting a couple things done before we leave for Moab. Its amazing that I can drive my big Sport up hills in 4th that the others have to downshift into 3rd to go up. And the others are unloaded - my big Sport almost weights almost 1500lbs more than any of the others. Its been a lot of years since I've driven my big Sport with everything freshened up to new and this is the first time I've driven it when EVERYTHING is in place all at the same time on a basically new vehicle. Its kinda scary how much of a difference the FIPK intake and the exhaust make over the stock setup.

The sensor I hacked actually was an OEM sensor. Using aftermarket sensors is what actually created the problem.

Over the years I'd switched to Nippon and Bosch sensors from OEM and in the same time frame we'd replaced the CAT 3-4 times, fixed leaks, replaced the muffler, flex joint twice, installed the 3.5L which resulted in the down pipes being chopped to shreds and welded back together. Inadvertently, 'stuff' moved around and changed which resulted in the rear O2 bung being in a different location.

All the aftermarket O2 sensors I've found or used have a harness about twice as long as the factory harness. If a shorter CAT is installed or a CAT with the O2 bung in the rear of the CAT, an aftermarket O2 sensor will still reach, but an OEM sensor will not.

So, my issue wasn't with the sensor itself, it was with the harness. What I did was take an old aftermarket O2 sensor, but the harness off a few inches from the sensor, cut the plug off the new OEM sensor and solder the aftermarket harness to the OEM sensor.

And, yes, it was extrodinarly difficult to chop the crud out of a $170 brand new OEM sensor, knowing I'd have to replace it as soon as I installed the new exhuast.

Note: The rear factory O2 sensor goes in the elbow that crosses above one of the cross members to the rear behind the CAT. At some point we'd removed the elbow and ran directly from the CAT to the muffler, and the replacement CAT had the O2 sensor in the rear of the CAT. Something like 10-12" further forward. The OEM sensor harness was something like 6-8" too short.

After going through all this and taking a good hard look at the factory exhaust vs my custom exhaust and pipe diameter and such, I'm changing my exhaust modification recommendations.

Rather than replace everything from the CAT to the rear with 2.25", I would strongly recommend replacing everything from the end of the ELBOW - leaving the elbow with the O2 bung in place. This will allow installation of either OEM or aftermarket O2 sensors. It will also be easier to replace the muffler when needed because there's a standard joint on the end of the elbow. Instead of having to cut both ends of the pipe to get the muffler out, the inlet can be unbolted. If a similar joint is installed on the outlet, then the entire thing becomes a matter of removing four bolts - as it is with the stock exhaust. The entire rest of the exhaust will last (usually much) longer than the muffler so it makes sense to make it easy to remove and replace just the muffler.

One reason I'm changing my recommendation is that the elbow is mandrel bent. Very, VERY, few exhaust shops mandrel bend pipe and if they do you pay a huge premium usually. Even though the OEM elbow is 2", it will still flow better than a 2.25" replacement bent on a standard tubing bender. Originally, I had a REALLY good exhaust guy who could design exhausts and do bends on a stander tube bender that produced dang near as good of flow as a mandrel bent system. AFAIKT, bending exhausts is becoming another of the lost arts because once he disappeared, I haven't found anyone even close to that good since. So, in the case of the elbow, you'd be better off leaving that piece in place. It won't detract from performance and it'd make at least your life easier.

Edward