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Discussion Starter #1
All started last night, went to slow for the turn onto the highway and I didnt get any regen. This baffled me but I couldn't figure out why that was, at least not while driving.

Continued on my way home and was amazed at how the car was running. Felt so peppy and just rolled along, I thought it was from the tire pressure I raised a day ago. A also noticed the MPG homed around 75. You know how 50MPG tends to be "home", that is its pretty much the lowest it will go without really pushing it, now it was sitting around 75. Often found it running around 80 MPG and even drove at 100 + for a bit. This was all around 70 MPH at that!

MPG I would normally struggle to get drafting was easy running out on my own. All in all the car was just floating along the road, none of the normal struggling with the gas pedal to keep it at speed.

Got to the offramp and setup for my long coast charge, and again got nothing for regen, not even a bar. Pulled to a stop and noticed the engine was idling at 3000 RPM. DAMN!

Figured it was a vacuum leak, praying I didn't hurt it running it so lean on the 50 mile trip. As soon as I pulled in the drive, I hoped out to check the exhaust manifold, half expecting the entire exhaust to be glowing, I was amazed that I couldn't see the least bit of red on the manifold.

I spent an hour tearing it down, checking, and spraying everything with starting fluid. Couldn't find a leak anywhere, I could even stall the engine with my hand and it would hold a suction for several seconds before the PCV valve leaked down. Everything I saw told me there was no vac leak, but nothing else explained the high idle. Tried resetting the ECM, looked for the IAC valve, pinched off vac hoses one at a time, nothing.

I finally pulled off the Autovox cruise control that never worked in the first place, I was going to take it apart and see what was wrong. All I did was unhook the throttle cable for it and the vac line and plug it. I started the engine just for giggles and everything was fine.

I dont think the throttle cable was stuck, I had looked at it prior, and everything was in place. I swear I had also pinched the vacuum line, in fact that was one of the first things I checked, so I dont know what exactly it was, but im guessing it was leaking vacuum inside the unit.

I took the cruise control apart, everything appeared to be fine, maybe it was venting vacuum through the solenoids, I dont know, all I know is the car was fine on the way to work today, although with significantly worse mileage, fighting a headwind, I ended up with 55MPG.



This makes me wonder though, what about a forced lean burn? The cars lean burn is weak and very finicky, but what I experienced last night was great, I didn't even know there was anything wrong the whole way home. If I had a wide band O2 setup, I would recreate the leak and drive it to see exactly what ratio it was running at. A long term setup would require a EGT (exhaust gas temperature) gauge to ensure you didn't melt anything, and you would have to be careful not to run things too hot for too long, but its certainly a very very tempting idea, what I experienced last night was amazing (does that sound dirty to anyone else? lol)

Anyone interested in experimenting, the engine would hold a 1800-2000 RPM idle when stopped, and about 3k on a rolling idle. Anyone interested in loaning me a WBO2 or EGT setup, please let me know.
 

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Interesting. I'm pretty sure the Insight, like most modern Honda's, uses speed density to determine fuel injected from the factory tables - ie, the fuel is a function of engine speed vs intake manifold absolute pressure. A vacuum leak would serve to increase the manifold pressure, and thus the fuel used. A vehicle with a MAF sensor would run leaner with a vacuum leak, but not the Insight.

Another possiblility is that throttle position plays a heavy role in determining the fuel used, and that the lower throttle position that you could drive with, caused by the vacuum leak, allowed it to run leaner.

However, I'm pretty sure the Insight has a factory wideband O2, like the Civic VX/HX of old. When in "lean-burn" mode, it functions in closed loop to hit a target lambda value, preprogrammed into the ECU. If this is indeed the case, I can't figure out why a vacuum leak would allow the engine to run any leaner.

Edit: A quick check from the parts pages reveals the Insight does have a factory wideband in the primary O2 position.

But you are right, a long term evaluation would require an EGT sensor. You can probably use a ScanGuage2 to view the real-time A/F ratio as read by the ECU, since this data is usually available on the OBD2 diag port.
 

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Why would a vacuum leak cause such a high idle speed unless the ECM was making up for it by supplying more fuel to the engine via the injectors. Perhaps the MPG gauge shows false readings when this happens as well. For that much change in MPG, you should be able to calculate real MPG by the fuel tank amount vs the miles driven to see if your findings are true.

JoeCVT - Just your average CVT owner
 

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joecvt said:
Why would a vacuum leak cause such a high idle speed unless the ECM was making up for it by supplying more fuel to the engine via the injectors. Perhaps the MPG gauge shows false readings when this happens as well. For that much change in MPG, you should be able to calculate real MPG by the fuel tank amount vs the miles driven to see if your findings are true.

JoeCVT - Just your average CVT owner
A vacuum leak is essentially the same as a partially open throttle. This will raise the manifold pressure, and the ECM will compensate. That's how speed / density works.
 

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Discussion Starter #5
Speed density engines will still run lean with a vacuum leak, trust me.

And DAMN! $180 for a map sensor? If mine ever goes out, I'm rigging in a $30 GM sensor, I bet they run on the same signaling range.

I do find it odd that in going for a super efficient engine, Honda went with a speed density setup, kinda makes you wonder if they figured out something no one else did...
 
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