Overspeed Engine?

[Giachino]

Guys, probably me being parinoid, but please let me know your thoughts.



Driving from LA to SF 2 nights ago..... was managing 20mph (overhead)... for the first part of the trip. Going over the grapevine (at 90mph)... was putting a big dent in the fuel economy! ... . so on the downside of the grapevine, but the truck in neutral and coasted... . still maintaind 70-80mph..... finally when the truck slowed to 70... . I got anxious and put her back in drive..... she revved to about 2600 (that I saw)..... and it didnt sound too hot (engine make a diff sound upon engine braking?)... . and then I got a funny smell... . (not sure if this was from a big rid riding the brakes next to me or not???)... .



To the point: Will the 48RE allow you to overspead the engine or hurt either the engine of transmission by going from N to D while going 70mph?



Also, what is up with the tow haul mode on these things? Mine absolutely has a mind of it's own... . when I want it to provide engine braking, it doesnt... . and on this trip... . later on... . I was doing 70, put on tow/haul... . and the thing instanly goes into 3rd gear... . revs up to 3k... . stays on for 5 seconds and this shoots into OD... . WTF logic do they use in the programming?

 

[Bekim]

Sounds like they mad it idiot proof. 70 is a pretty high rate of speed to drop an auto into gear!
From an idle to an instant 3000 rpm!! WOW
Mike
###

 

[Prairie Dog]

If you over speed the engine it will set a code - if you dont see the MEL on then you should be ok. Now as for the transmission, there's no telling...



Good luck,

Scotty

 

[BErmoyan]

I dunno, it might have gone from neutral right into OD causing the rpm's to jump all the way up. I hope all is well though.

 

[jwilliams3]

Its fine I wouldnt worry.

 

[johncameron]

When I got my truck in 2000, I downshifted from 6th to 3RD at 70MPH and all I remember is hearing the rear tires squeel and the engine make noised I'd never heard before... ... So with that being said, your engine is FINE.

 

[klenger]

When I got my truck in 2000, I downshifted from 6th to 3RD at 70MPH and all I remember is hearing the rear tires squeel and the engine make noised I'd never heard before... ... So with that being said, your engine is FINE.

Did you have to clean the seats? :-laf

 

[johncameron]

No, it didn't make it through my drawers

 

[cojhl2]

I dont think its a good idea to go down a hill in neutral anytime anyway for sure. In fact I think it may even be agaisnt the law. . I know in a big truck we would never do that!!

 

[PVasilauskis]

If you were trying to save fuel by coasting downhill in neutral ya didn't. You would have saved more with your foot off the accelertor pedal coasting in gear. Someone correct me if I'm wrong (I'm sure they will) but if the RPMs are above idle speed and the accelerator pedal is in the idle position (your foot is off it) the ECM turns off the injectors i. e. no fuel flows. But if you were idling the engine on the way downhill in neutral, you would be burning what ever it takes to idle that whole time. Now there may be some threshold as the RPMs need to be X far above idle before the fuel gets cut off but I think that's how it works.

 

[Bob4x4]

If you were trying to save fuel by coasting downhill in neutral ya didn't. You would have saved more with your foot off the accelertor pedal coasting in gear. Someone correct me if I'm wrong (I'm sure they will) but if the RPMs are above idle speed and the accelerator pedal is in the idle position (your foot is off it) the ECM turns off the injectors i. e. no fuel flows. But if you were idling the engine on the way downhill in neutral, you would be burning what ever it takes to idle that whole time. Now there may be some threshold as the RPMs need to be X far above idle before the fuel gets cut off but I think that's how it works.

:-laf if fuel doesnt flow the engine shuts off



Bob

 

[betterthanstock]

If you were trying to save fuel by coasting downhill in neutral ya didn't. You would have saved more with your foot off the accelertor pedal coasting in gear. Someone correct me if I'm wrong (I'm sure they will) but if the RPMs are above idle speed and the accelerator pedal is in the idle position (your foot is off it) the ECM turns off the injectors i. e. no fuel flows. But if you were idling the engine on the way downhill in neutral, you would be burning what ever it takes to idle that whole time. Now there may be some threshold as the RPMs need to be X far above idle before the fuel gets cut off but I think that's how it works.

In our engines, no fuel flow above 950 rpm when coasting. (with stickshift you can feel the fuel come on at 950 as you coast to a stop in gear) This is true of all computer controlled engines (gassers too).



No damage to engine. High idle is 3500, and no damage below 4000.

 

[klenger]

From a fuel economy point of view, I think putting it in neutral is a trade off either way. If you coast in gear, the truck slows down faster becasue of the engine braking and you have to add power sooner to keep going. With the trans in neutral, you will coast further, but the engine uses some fuel to idle. I coast in neutral fairly often and it seems to really help the milage, but would take a scientific test to see which is more efficient. Any time you are coasting, whether in gear or not, you are saving fuel.

 

[AMink]

In fact I think it may even be agaisnt the law

I was looking through Nevada state code and sure 'nuff, there's a law against coasting in neutral! Now I know!

 

[PVasilauskis]

:-laf if fuel doesnt flow the engine shuts off



Bob

If the glove doesn't fit you must... . sorry. :-laf



Unless of course you're in gear being pulled downhill by gravity, in which case when the ECM cuts off the fuel (because the RPMs are greater than 950 [thanks betterthanstock] and the accelerator pedal is in the idle position) the drivetrain keeps the engine turning and it simply becomes a positive displacement air pump. Until of course RPMs drop to 950, then the ECM will begin feeding the beast again in order to maintain the engine idling as requested by the accelerator pedal. Now we can get interesting and throw in an exhaust brake into this 5. 9L air pump but that's a whole other thread.

 

[Mhannink]

well i know that some of the truckers used to go into portegue overdrive (neutral) to save fuel and they could actually go faster down the hill since the rpm wouldn't limit the engine and they would have enough speed at the bottom to climb most of the next one w/ little effort. but of course that was 30 40 years ago when they laws wheren't near as strict