UltraGauge and EOC / engine kill with ECU on

[Floordford]

This is sort of a two part question.
Why with the engine off, ECU still on and im moving, does the UltraGauge not calculate in MPG?

And

Is there a way to stop the engine but leave the ECU on without using the ignition switch? Like many, my car wont chart mileage with the ECU off. But I don't want to prematurely wear out my ignition switch.

[JethroBodine]

Don't know about the Ultraguage, but shutting off the engine without the key switch can be done with a switch inline with the power or ground wire to the injectors, making your own DFCO(Deceleration Fuel Cut Off). I did read somewhere on here that it might be a good idea to switch off the fuel pump as well( and that does happen when the keyswitch is used), but I don't remember where I read it. I'm thinking that is the direction I'm going to take.

[SentraSE-R]

The UG does still calculate average and trip mpgs with the engine off and ignition and ECU on. The only display that doesn't work then is the instantaneous mpg.

As JethroBodine says, you can rig up a kill switch to the fuel injectors, which leaves the ECU on and the displays should continue working. Ignition swiches are cheap. I've saved well over $1800 in fuel costs by hypermiling - enough to buy all the clutches, starter motors, and ignition switches you all worry about replacing. And over 42,000 hypermiling miles, I've never replaced a one of them.

[Floordford]

Well my thought goes to the fact that car parts dont heal themselves. They all wear out. And if you use a part 20 times a day as opposed to only 2, then the life of that part will take a hit and need to be replaced much sooner. I do prefer clutch starting from a roll over using my starter. My starter is a days journey to get to as I know other tightly made car are too. So I dread the thought of replacing it.

Ill have to try and hunt down my injector ground. I wonder if the coil ground would work just the same. Or even the crankshaft position sensor. My Mustang has 8 coils over each spark plug and disrupting the crank signal might work. The crank sensor only has two wires going to it.

[JethroBodine]

You might not want to shut down the engine with turning off the spark as the injectors may still pump fuel until the ECU decides there is a problem, which it may or may not, depending how smart your ECU is. Unburned fuel in the exhaust system can have explosive side effects.

[Floordford]

Seems like a catch 22. Cut the fuel pump and the injectors will cycle dry until the engine stops. Also the spark plugs would still fire as the engine turns. Unless they all cut out at 300rpm. And on the other hand cutting spark will momentarily keep the pump at pressure and un-needed fuel will enter the cylinder.

[JRMichler]

Do not kill the ignition. If the injectors keep working, you will pump a flammable fuel air mixture out the exhaust while the engine spins down. BOOM!

I found a fuse in my truck labelled "fuel injector". When I pulled that fuse, the engine quit without setting a CEL. So I wired a kill switch into that circuit. It's been working well for 2.5 years now.

[Floordford]

I was thinking keeping fuel pressure up for when I start it again might be a good idea. Because this switch will be mostly for when I coast. And I don't have catalytic converters to worry about reigniting unburned fuel. Its long tube headers to 2.5" full exhaust. So there is plenty of breathing room even if it backfires. And I suppose I could set it up one way and if it doesn't work well then I could set it up the other way. I just didn't know how others had their cars set up.

[JRMichler]

On my truck, like most if not all vehicles, the fuel pump is on a separate circuit from the fuel injectors. Cutting the injectors leaves the fuel pump running. The engine fires instantly on a bump start.

Cutting ignition squirts raw fuel into the engine while the engine is spinning down. This does bad things to the oil film on the cylinder walls. My engine takes a full two seconds to spin down from 2000 RPM. That's about 33 revolutions during which fuel is being injected but not burned.

If you cut the fuel pump, you can expect some poor running while the fuel pressure drops.

Somebody told be a story years ago about tow starting a 1960's Cadillac. The Cadillac driver neglected to turn the ignition at first. When he did turn it on, the fuel in the exhaust system went BOOM and blew open three of the four mufflers.

[JethroBodine]

Totally agree with JRMichler. I had to put my cut off switch inline with the main injector ground and found a fuse for the fuel pump. Firing two relays from one switch, I cut both signals at once. Fires up instantly on bump or key start.

Back in the early 80s, my cousin's favorite game was taking his Chevy Impala up to 75mph, turning off the ignition in gear, putting his foot on the floor to flood the exhaust with unburned fuel as he coasted down to about 50, and turn on the ignition making a nice, loud explosion out his tail pipe. No catalytic converter needed. And yes, it did eventually blow the exhaust off the car. No injuries. Your mileage may vary.