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Discussion Starter · #1 ·
My E got 25 mpg in town and 28-30 highway...before a scumbag stole my catalytic converter (to be referred to as CAT from now on). Now I'm getting 16 in town.

I have my CAT theft ordeal detailed in other threads on this forum but I put in a CARB rated Walker CAT my own self using clamps, with no welding. I'm sure all connections aren't as leak-proof as all welded would be but my CEL doesn't come on, it starts as before. It has the original upstream sensor but a new Denso downstream sensor (maybe they aren't playing nice with each other?) Runs a bit rough but doesn't miss, hard to describe what the low rpm acceleration sounds like but I don't think I'm imagining that. I drove a Toyota Camry while I was repairing my E that was super quiet and smooth so maybe I've just forgotten how noisy my E was before the theft.

One thing for sure is my in town mpg has plummeted. Is that now an EMU issue, like it isn't used to the "new normal" DIY CAT replacement and hasn't reset itself to allow the engine to now run so dam rich?

Dealing with the ECU is not something I've ever done. Is there some sort of driving ritual that would get it to relearn the proper engine settings? If that's what it is, of course.
 

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I would hope that the Walker CAT does not have enough extra restriction to affect mpg that drastically. Air (and O2) should not be leaking into the exhaust system since it is at higher pressure than ambient. This would leave the O2 sensor as the primary suspect for me. Honda sensors cost more, but this is one case where you get what you pay for.
 

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Does not seem like the downstream O2 sensor should affect this though, from what I read all it does is monitor the cat efficiency and cause the CEL to light up if the cat is not doing its job. Plus, albeit this is a sample of one, I'm running a Denso downstream one with the original upstream on my '03, with no issues. Any chance the upstream one was damaged in some way when the cat was sawed off?
 

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Have you ever had your valves adjusted? I just don't think a cat would affect your MPG that much or lead you to run rich. I guess you could kill battery and try to reset the idle/ ECU the way Honda recommends, can't hurt.
 

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Discussion Starter · #5 ·
Does not seem like the downstream O2 sensor should affect this though, from what I read all it does is monitor the cat efficiency and cause the CEL to light up if the cat is not doing its job. Plus, albeit this is a sample of one, I'm running a Denso downstream one with the original upstream on my '03, with no issues. Any chance the upstream one was damaged in some way when the cat was sawed off?
The upstream sensor was about 1 1/2" above the cut. I didn't actually inspect it for damage. The cable from it to the connector above was still plugged in. In my endless research I remember reading the two sensors should be replaced in pairs (probably to sell us two instead of just the one we really need!). The fact the new one is a Denso is really a good thing, that's de facto OEM, right?
 

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Discussion Starter · #6 ·
Have you ever had your valves adjusted? I just don't think a cat would affect your MPG that much or lead you to run rich. I guess you could kill battery and try to reset the idle/ ECU the way Honda recommends, can't hurt.
I'm comparing the mpg I was getting with the valves as they were with the valves still adjusted as they were, only after my CAT debacle
 

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Does not seem like the downstream O2 sensor should affect this though, from what I read all it does is monitor the cat efficiency and cause the CEL to light up if the cat is not doing its job. Plus, albeit this is a sample of one, I'm running a Denso downstream one with the original upstream on my '03, with no issues. Any chance the upstream one was damaged in some way when the cat was sawed off?
That would be my guess. Maybe just not quite bad enough to set the light. Walker should be less resrtictive if anything
 

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Well for what it's worth I traded my restrictive cat for custom header, downpipe and magnaflow cat/ injen catback exhaust. I didn't notice a drop in MPG. If anything it was improved.

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Discussion Starter · #10 ·
To be honest, if you got 25 in town your better than Ive ever got in town by a long shot
I've gotten minimum 25 in town since it was new in 2004 and 28-30 highway as well. Heck, I've gotten 35 mph highway at times. I keep reading the mpg woes of other owners on these forums but never had a reason to complain...until now

I put in new spark plugs earlier this year. Could I have fouled them with a bigtime rich mixture while driving my E without my CAT (back from the scene of the crime and around my driveway) and they're still fouled? I'm grasping at straws here as you can tell
 

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Is your car an automatic? I suppose if the car was driven for awhile with not cat and with no cat there would be less torque and one would have to use more throttle to accelerate, then the computer would be re-learning your new driving style with more throttle and adjust the mapping to give more fuel and possibly hold lower gears longer before upshifting and thus increase fuel usage.

I remember my mechanic telling me I could disconnect the negative battery terminal and let it sit for awhile and reconnect it and the computer will re-learn my next new driving style and adapt to it. I know whenever I drive one of my automatic cars be it the Element or my previous Acura with a heavy foot, it will adjust and will accelerate better and downshift sooner and hold lower gears longer but when driven gently for fuel economy it learns that style and is very slow to downshift, I really have to mash the throttle but it gets much better fuel economy but is slow to accelerate, it almost feels like there is something wrong with the car but it seems to be in a fuel efficient mapping
 

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Discussion Starter · #13 ·
It's a 5-speed. I'm surprised no one has addressed my original question of "should I try resetting the ECU?" My plan would be to get under the hood (after the engine has warmed up) pull the ECU fuse, wait ten minutes and put it back in. At least that's what some Y'Tub videos I've seen say to do. Of course, I've never done this. Is there any downside to trying this just to see if it helps?
 

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My understanding is that disconnecting the negative battery terminal will disconnect the ECU and allow it to reset and relearn, no harm in trying that
 

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It's a 5-speed. I'm surprised no one has addressed my original question of "should I try resetting the ECU?" My plan would be to get under the hood (after the engine has warmed up) pull the ECU fuse, wait ten minutes and put it back in. At least that's what some Y'Tub videos I've seen say to do. Of course, I've never done this. Is there any downside to trying this just to see if it helps?
I did suggest this in post #4. Just disconnect battery and do idle re-learn procedure. Also don't believe in coincidences if you have had poor gas mileage since changing spark plugs, then maybe prior plugs were bad and now you are running on all cylinders, 16mpg was normal for me when I lived in stop start city traffic.
 

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Discussion Starter · #16 ·
I did suggest this in post #4. Just disconnect battery and do idle re-learn procedure. Also don't believe in coincidences if you have had poor gas mileage since changing spark plugs, then maybe prior plugs were bad and now you are running on all cylinders, 16mpg was normal for me when I lived in stop start city traffic.
I changed the plugs late last year and my mpg didn't change at that time. Only after my CAT was stolen

So I just copied down the OBDII Drive Cycle Pattern and will give that a shot soon (after pulling the ECU fuse)
 

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It's a 5-speed. I'm surprised no one has addressed my original question of "should I try resetting the ECU?" My plan would be to get under the hood (after the engine has warmed up) pull the ECU fuse, wait ten minutes and put it back in. At least that's what some Y'Tub videos I've seen say to do. Of course, I've never done this. Is there any downside to trying this just to see if it helps?
I would think disconnecting the Battery, pulling the ECU fuse, or if you have a scan tool that has “ECU reset” Would all do the same thing. Only downside I my amateur brain can think of is that you would lose the fuel trim data currently there that might be of help For your troubleshooting? Also, all your obd2 readiness criteria would get reset.

I lost track of your cat replacement thread. Did you ever post any pictures after you finished? I’m curious how it all turned out.
 

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Discussion Starter · #18 ·
I would think disconnecting the Battery, pulling the ECU fuse, or if you have a scan tool that has “ECU reset” Would all do the same thing. Only downside I my amateur brain can think of is that you would lose the fuel trim data currently there that might be of help For your troubleshooting? Also, all your obd2 readiness criteria would get reset.

I lost track of your cat replacement thread. Did you ever post any pictures after you finished? I’m curious how it all turned out.
I was thinking getting rid of any fuel trim data that was set during the time there was no CAT installed would be better gone, so I can start from the beginning with the Walker CAT. The fact that the CEL isn't coming on is one thing but I'm sure there's still a ton of bogus stuff going on anyway

Fun Fact: when you look at online diagrams of the fuse box in the engine compartment the F1 ECU fuse is labeled "IGP". It's labeled F1 ECU on the fuse box lid, thankfully

So I figured you all had lost interest in my CAT ordeal (which went on and on, me learning as I went). My goal was to not pay anyone for their labor and to buy the necessary parts for me to get a CAT installed, knowing that even it could get stolen the next day. I knew the final DIY weld-less install would be ugly and not permanent but at the very least I can find a welder to actually weld this properly down the road

The plumbers straps are not a theft deterrent, they're there to assure that the whole assembly won't bounce/pull off the two ends, which is not much of a possibility but what the heck. I painted it bright red a few days after this pic

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Discussion Starter · #20 ·
ajchien's comment about perhaps leaving the current fuel trim data would be the way to go. IOW, not resetting the ECU as the CEL is not on which I take to be a good sign but what do I know?

So, after reading his comment I did the usual internet search to find other opinions of if I should indeed just leave the ECU as it is and trust the onboard computer to successfully reset itself if I did a complete OBDII driving procedure...and never found anything definitive

So, any opinions about me resetting my E's ECU (by pulling the fuse which I've located...I did read that the removing the negative off the battery to reset the ECU was the preferred method back in the day but doesn't work so well with modern cars) before I do the full driving procedure? You know, just starting with a cleared out engine computeer RAM and letting the ECU work with the new normal that is my repaired CAT?
 
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