salad

So I've been watching way too many Youtube videos about big trucks and listening to my friend brag about how his parents' Corolla saves gas and it got me thinking. From what I've read, the 4.0L ECU doesn't completely turn off the fuel injectors when engine braking... just the low air flow with the shut throttle plate triggers the MAP to reduce it to idle levels.

Tonight while tooling around town I found a lonely stretch of road and killed the engine while in gear: Lo and behold it slowed down faster then if I just let my foot off the pedal. Turned the key back to run and kept going. Lots of fun lol

My big idea is to wire up some normally-closed relays and install toggle switches in the dash somewhere that will break the circuit to the fuel injectors. For enhanced engine braking, just flip the switch and enjoy the fuel and brake savings lol. I think this would have even better value at higher RPM. Anybody else ever done this before? Any input?

PNWjeeper

Sounds like a good idea IN THEROY but the real test will be in the practice. Out of curiosity though when would you use this switch? Seems like any time you would use it, would be when the engine is already doing it.

salad

Thanks!

On a diesel with jake brakes, the fuel flow is shut off to the injectors, and at (or near) the top of the power stroke the exhaust valve is opened up early to release the air pressure. This is where the noise comes from. Otherwise the air that's been compressed (quite a lot if it's turbocharged) will push the piston back down on what would normally be the power stroke. This way the engine works as a giant vehicle-driven air compressor.

So, the idea would be to use it when engine braking. Normally when you release the throttle, the MAP tunes everything back to idle. With so little fuel and air going in, the engine is struggling and it forces the vehicle to slow. Even at a normal idle, the 4.0 has enough power to climb a slight incline at a few hundred RPM without stalling out lol. If you kill the fuel entirely then you should get even more of a slowing effect.

A couple thoughts have already occurred to me... partially disabling the injectors (like 2 or 4 at a time) would probably cause the ECU to dump more fuel into whatever's still working since the O2 sensor is reading a lean condition. Might not work that well unless all 6 are turned off at a time. Either way it's gonna cause some CELs lol

salad

Anyone else?

XJwonders

Don't know what's gonna happen to the fuel pump when it's pressurizing the lines without something letting go. Just a guess.

salad

You troll... lol

I've learned more about how the fuel injection system works since. The injectors are automatically shut by the PCM during "deceleration". Pressure is relieved internally in the pump.

XJwonders

ah u could have updated this!!' You selfish Canuck!

BNJeepsta

Honestly, I think you're going to end up causing yourself more of a headache than anything. You would have to kill fuel delivery to all of the injectors at the same time for it to work and if you don't get the proper AFR back quick enough, it's going to stall. If you had a manual trans, it may be somewhat beneficial but, with the auto, there is enough slip that it probably won't make much of a difference.

Larry's XJ

I tought the pump just recirculated back to tank? If you aren't "using" fuel it would just go back.

I've used engine braking on plenty of vehicles by downshifting trans, so if zero fuel should work even better.
Momentary switch on sterring wheel should work.
Can see use on both street and trail, a 4l air compressor should have lots of drag.

XJwonders

97+ cherokees have return less fuel system

Larry's XJ

Learned something else. I thought all FI had return lines. So you would have to cut power to pump and injectors?

salad

I guess that doesn't read as clear as I thought it did. XJs already work like that from the factory, even back to Renix!

When the PCM or ECU enters "deceleration" mode - zero throttle input and declining vehicle & engine speed - it stops operating the fuel injectors and ignores O2 sensor input. This continues until you do "something" or it reaches idle RPM again.

So all of our XJs have had this feature since 1987.

The fuel pump has an internal pressure bypass to keep it from blowing up. All the PCM does is stop powering the injectors.

jedijeb

Not sure how much slippage is in the newer AX autos but in my 85 the A904 has quite a bit of engine braking capability even with the 2.5l.

s14unimog

ahh, you're thinking of this for an auto-magic; eh... I was going to say, when I shut mine off at speed, there is no difference, but I have a manual.

I say you're on to something, for you AW4 guys. I wonder if the trans would have any issue with taking a higher decel load. I can't imagine interrupting the injector 12V source would be an issue. Try it!

salad

IIRC unless you hit the brake pedal the torque converter stays locked. PCM does the same decel shutoff of the injectors