LT1 Fuel Cut one decel?
Thread Starter
Team Owner
Joined: Aug 2004
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From: Park City Utah
LT1 Fuel Cut one decel?
I know EFI cars have programing to cut fuel during decel. In every EFI car I've owned, the fuel cut, and fuel "on" during decel down to about 1000 RPM has been noticable -more in some cars, than others. But even in my '05 CTS-V -a pretty modern and refined car compared to any C4, it is a noticable event.
In my '92 LT1, I can't feel it happening, ever. Furthermore, (and this may be a completely bogus indicator) the instant mpg display shows worse gas mileage when coasting down a hill in a lower gear than a higher gear...which makes zero sense. The car should show whatever mpg as I lift, then regardless of gear selection, w/in a few second it should cut fuel and show "99mpg". But that doens't happen. On a given hill, it may show "99" in 6th gear, "97" in 5th, "85" in 4th and so on. If the fuel is cut, shouldn't matter what gear I'm in...I should be getting infinite mpg, basically.
Any thoughts on why I can't feel fuel cut and why my instant shows what it does?
-Tom
In my '92 LT1, I can't feel it happening, ever. Furthermore, (and this may be a completely bogus indicator) the instant mpg display shows worse gas mileage when coasting down a hill in a lower gear than a higher gear...which makes zero sense. The car should show whatever mpg as I lift, then regardless of gear selection, w/in a few second it should cut fuel and show "99mpg". But that doens't happen. On a given hill, it may show "99" in 6th gear, "97" in 5th, "85" in 4th and so on. If the fuel is cut, shouldn't matter what gear I'm in...I should be getting infinite mpg, basically.
Any thoughts on why I can't feel fuel cut and why my instant shows what it does?
-Tom
Last edited by Tom400CFI; Jul 14, 2011 at 10:27 PM.
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In my '92 LT1, I can't feel it happening, ever. Furthermore, (and this may be a completely bogus indicator) the instant mpg display shows worse gas mileage when costing down a hill in a lower gear than a higher gear...which makes zero sense. The car should show whatever mpg as I lift, then regardless of gear selection, w/in a few second it should cut fuel and show "99mpg". But that doens't happen. On a given hill, it may show "99" in 6th gear, "97" in 5th, "85" in 4th and so on. If the fuel is cut, shouldn't matter what gear I'm in...I should be getting infinite mpg, basically.
Any thoughts on why I can't feel fuel cut and why my instant shows what it does?
-Tom
Any thoughts on why I can't feel fuel cut and why my instant shows what it does?
-Tom
Take my 6-speed for example.
Your saying as I let off the accelerator going down a slight grade, the fuel should "cut off".
If it did that, and you pushed in the clutch the engine would die.
It does not, it stays at idle. Couldn't do that without fuel.
And it makes perfect since to be getting better "instant" mpg in a higher gear going down a hill.
Higher gear = lower rpm = higher mpg
Drop to a lower gear, and watch the rpm go up and instant mpg go down.
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From: Indiana PA
Tom, it sounds like it is working ok, my 93 does the same thing.
Rolla-Mo LT4 you are dead wrong, I have yet to come across a EFI system that does not have fuel cut off. If RPM's get too low, like you said,push in the clutch it fires the injectors and keeps running just fine. Deceleration with more than 1K or so RPM and you will get fuel cut off everytime with throttle closed unless something is wrong. Keep in mind this fuel cutoff maybe a short time or a longtime, all depends on programming and the inputs of various sensors.
Tom, the only way for sure to see if it is cutting off is to put a scan tool on it and observe the injector pulse width and the 02's.
Some cars also have Fuel Enleanment on Deceleration. It all depends on what was programmed and built into the car. Consult your closest Field Service Manual.
Rolla-Mo LT4 you are dead wrong, I have yet to come across a EFI system that does not have fuel cut off. If RPM's get too low, like you said,push in the clutch it fires the injectors and keeps running just fine. Deceleration with more than 1K or so RPM and you will get fuel cut off everytime with throttle closed unless something is wrong. Keep in mind this fuel cutoff maybe a short time or a longtime, all depends on programming and the inputs of various sensors.
Tom, the only way for sure to see if it is cutting off is to put a scan tool on it and observe the injector pulse width and the 02's.
Some cars also have Fuel Enleanment on Deceleration. It all depends on what was programmed and built into the car. Consult your closest Field Service Manual.
Thread Starter
Team Owner
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From: Park City Utah
I'm not making this stuff up; GM cars (and most other brands) ARE designed to CUT FUEL during decel to save gas,reduce emissions and eliminate backfires through the exhaust. That part is fact and not what I'm asking about. http://www.diycardoctor.com/deceleration_enleanment.htm
What I'm asking it is; in all other cars I've owned/driven, the fuel cut/engage is perceptible to the astute driver. In this car, it's not perceptible, and the instant mpg indicator shows evidence that I'm truly not getting fuel cut. And I was wondering if any other LT1 owners had noticed this characteristic?
Last edited by Tom400CFI; Jul 14, 2011 at 10:31 PM.
Thread Starter
Team Owner
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From: Park City Utah
And you're right; the scan tool will tell me all. I'll have to try it and see. Thanks for inputting your experience.
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Rolla-Mo LT4 you are dead wrong, I have yet to come across a EFI system that does not have fuel cut off. If RPM's get too low, like you said,push in the clutch it fires the injectors and keeps running just fine. Deceleration with more than 1K or so RPM and you will get fuel cut off everytime with throttle closed unless something is wrong. Keep in mind this fuel cutoff maybe a short time or a longtime, all depends on programming and the inputs of various sensors.
It certainly isn't perceptible in any of the vehicles I own.
Have owned plenty of carbureted cars too.
Thread Starter
Team Owner
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From: Park City Utah
Last edited by Tom400CFI; Jul 14, 2011 at 11:39 PM.
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Race Director
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From: Texas
Same here. In any of the tuning software you can mess with this. I've always left it alone, no reason to change it.
Drifting
Joined: May 2006
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From: Chewelah Wa
I recently changed to full siamese runners and went from 19# injectors to 24's on my mildly modded 85 L98. I now notice that the engine "dies" during decel, very noticeable in both sound and feel. Quite irritating actually. I did notice it somewhat before the changes....now it is bad. I have been studying up on DFCO, and I see no parameters to play with in tunerpro....I have raised the idle to no effect. I have the 870 ecm with $1F. I am going to play with IAC settings (reset) and see if this will help. Any suggestions?
Drifting
Joined: May 2006
Posts: 1,428
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From: Chewelah Wa
I reset the IAC, still does the same thing. I am thinking this is not DFCO, except for the fact that it relights itself when the clutch is pushed in (or the motor would stay dead). It will almost come to a complete stop before it refires. My understanding is that DFCO will not enable below a certain RPM or speed. Wish I could see the tables for this on my chip. Bueller?
