Unburned Fuel

[rh77]

So, I have an exhaust leak, which I believe is between the header and cat, so I can smell unburned fuel pretty strongly when stopped. So I get to thinking, unburned fuel is fuel wasted, decreasing economy, right? I know, the CO is probably not the best for my health, but what about economy? Is it normal for there to be an unburned gas smell with an exhaust leak? The plugs and wires are fairly new. What could be the problem?

RH77

[diamondlarry]

That sounds fairly normal. The ECU dumps extra fuel in to cool the engine and insure that there is enough fuel for the cat to work properly. That's why most gas saving devices won't work unless you intercept the O2 sensor signal and fool the computer into thinking that everything is still normal. This would probably confirm that the leak is before the cat.

[SVOboy]

Yeah, more fuel to cool the combustion charge is correct. If you hook up water injection you can substitute water/methanol for the extra gas to cool the charge, therefore leaner/less waste.

[rh77]

So at idle, the air coming into the engine needs to be cool? That would explain why the fumes have been more pronounced lately -- with the warm-air intake. The ECU is probably dumping more fuel at idle. Bummer. So is it agreed that cold air needs to enter the combustion chamber at this point? Secondly, if I rig-up something so draw-in cold air at idle, this will perhaps decrease the fuel used at idle? Finally, is all of this worth a respectable gain in economy, or more trouble than it's worth? I'm thinking of a manually-controlled, vaccum-driven intake air diverter. I'm leary of "fooling" the ECU with electronics, since I want the engine to last.

RH77

[DaX]

Hmm, good point. I was thinking exactly what you were - some kind of dashpot that controlls a flapper to direct intake air. I know that some company made an exhaust cutout that worked on vaccum...don't know how it works though.

[SVOboy]

So at idle, the air coming into the engine needs to be cool? That would explain why the fumes have been more pronounced lately -- with the warm-air intake. The ECU is probably dumping more fuel at idle. Bummer. So is it agreed that cold air needs to enter the combustion chamber at this point? Secondly, if I rig-up something so draw-in cold air at idle, this will perhaps decrease the fuel used at idle? Finally, is all of this worth a respectable gain in economy, or more trouble than it's worth? I'm thinking of a manually-controlled, vaccum-driven intake air diverter. I'm leary of "fooling" the ECU with electronics, since I want the engine to last.

RH77

It's not that the air coming in needs to be cool, but the fuel does. If there is less fuel being combusted is causes the engine to heat up, which is why leaner engines run hotter. Engines therefore inject more than they need so that the explosion is not as hot, it's somewhat of a dampening affect. With turbocharge engines this is more pronounced because a lean engine will blow up, yet make more power. On of the ways the FI people try to push the limits of going lean safely if by using water injection to compensate for some of that extra fuel.

[Matt Timion]

So at idle, the air coming into the engine needs to be cool? That would explain why the fumes have been more pronounced lately -- with the warm-air intake. The ECU is probably dumping more fuel at idle. Bummer. So is it agreed that cold air needs to enter the combustion chamber at this point? Secondly, if I rig-up something so draw-in cold air at idle, this will perhaps decrease the fuel used at idle? Finally, is all of this worth a respectable gain in economy, or more trouble than it's worth? I'm thinking of a manually-controlled, vaccum-driven intake air diverter. I'm leary of "fooling" the ECU with electronics, since I want the engine to last.

RH77

There was a post on an insight forum a while ago where a guy made an automatic warm air intake. It would switch to WAI when the outside temp was below 65 degrees.

http://www.insightcentral.net/forum/viewtopic.php?t=1769

Isn't it also possible to do the same thing but make it switch when the car is idling? Perhaps use a vacuum switch?

[SVOboy]

I still don't understand what air has to do with any of it. The extra fuel that is dumped in to cool the charge doesn't burn up well and gets tossed out and burnt in the cat, but since the leak is before the cat you can smell it. It's normal from what I know.

[rh77]

So it seems like the ECU is trying to keep the combustion area cooler, even if the coolant temp is well below the threshold for the fans to kick-on (which I believe is 205F). The thermostat is currently 180 (192 is on it's way). The fans haven't come on since August (aside from the A/C-Defroster) so, am I to assume that it's preventing detonation? Is the intake air temperature sensor perhaps the source, or does it just dump fuel regardless at a certain RPM with no throttle input?

I read the article above about the Insight owner that setup a dual-stage intake and found it helpful (but the setup is based on temperature and not speed). Since the outside temp is cold (and will remain so for a while), I could rig a cold air adapter and perhaps a "Y" where cold air comes from the outside and meets the warm air, using a manual vaccum switch hooked to a butterfly valve (like a throttle body) to cool the incoming air. Would a fuel map offer an answer?

I still haven't determined how much fuel can be saved, and if the effort is worth it.

RH77

[SVOboy]

am I to assume that it's preventing detonation? Is the intake air temperature sensor perhaps the source, or does it just dump fuel regardless at a certain RPM with no throttle input?

Yes, the extra fuel is to prevent detonation. Just dumps fuel regardless, the IAT is only a correction factor that affects the fuel uniformly based on temperature. Different fuel maps are definately and option, that combined with ignition maps and water injection could net some power and economy. Thinking of going obd1?

[CruiseControl]

My guess is that the reason the cylinder temperatures are lowered during idle is that there is less coolant(oil and water) moving around the engine at low RPMs to disipate the heat. If the cylinder temps stay high at idle, the oil probably burns, and the engine could need to turn the fans on, which in very cold weather, with slow coolant velocities could freeze the radiator. Checked my 94 Honda DX intake hose, and it has a plugged intake, that looks to be, in about the same place as my 74 Ford's was. The Ford had a bimetal valve that controled a flap to allow warm air from the exhaust manifold to enter the intake plenum. The Hondas and other 4 cylinder vehicles used the same type of system, so checking a wreck from the 70s or 80s should turn up some choice parts. While this may improve your MPG, don't ask me what it will do the engine at idle. Come to think of it, I believe that the valve was, blown shut, by the higher air velocities above idle, and was designed to improve idle performance in cold weather. Back in the daze of Sunoco 260 octane, they used to put a spark plug at the end of the exhaust pipes and detonate waste gas as a show.

[SVOboy]

The Honda CRX HF and most newer hondas (and other cars) have Exhaust Gas Recirculation which reburns the exhaust **** to the extra gas combusts again instead of dying in the cat. This equals more MPG for Honda, but less for some of the ****ty US manufacturers, go figure.