So I put the Coupe in diagnostic mode today while driving around, watched the tach (because I thought it was idling high at a stop in drive) <--- it was normal.

So I would be dirivng, say 50mph and take my foot off the gas, RPM goes down to 1,050RPM and stays there... normal.

But, when I bump it into neutral, the RPM goes *up* to 1,500RPM, and if I hit the brakes to come to a stop, it will s-l-o-w-l-y go down, like at 30mph it would be 1,400RPM... until about 4 or 5mph when it would go from 1,300RPM to the 600RPM idle speed....

I did this many times, and all with the same thing...

Is it supposed to? Only thing I could think of is... the computer knows the car is moving by the speed sensor, and when in nuetral, it keeps the RPM up to keep the accessories spinning faster... yes? Thats the only logical thing I could come up with other then... something is wrong.

I know its a totally different generation here, but when I did the same thing with my '79, no matter what speed, the RPM would drop to idle (I had a tach in it) which makes me believe it may be a computer/electronic thing then mechanical, if in fact nothing is wrong.

Yes, mine does exactly the same. I got fed up with this, disabled the ISC motor and set the idle to 500 rpm. I do have to hold the throttle down a little when starting the engine but I also gained about 2 mpg city driving. I also notice that there is much more engine compression braking now which will translate into less brake wear.

One possible explanation might be to reduce pollution by always ensuring that the CAT is being supplied enough fuel to burn off exhaust.

It is the throttle follower, part of DFCO, Deceleration Fuel Cut off. A pain and doesn't work for the good of fuel economy or a good coast down.

If you watch your fuel injector PW, watch it as you decel and count the seconds it takes it from cruise or acceleration to actually bring the injectors to a 0 PW. It doesn't do it much, if ever. I am working on my LT1 to get more agressive DFCO, but it is still a pain to get working right. GM likes this 2.5 second delay before DFCO even starts. So look for that, you are likely fighting the DFCO delay.

My '94 Deville does the same thing. I just don't coast in neutral! :)

If I ever get a hold of the stuff to break the code on it, and the time, that is a key item to "fix".

My '94 Deville does the same thing. I just don't coast in neutral! :)
I usually have to switch to D3 or D2 while coasting down a hill.

A GM engineer tells me that Cadillac programmed their ISC step down to do just as you've described. Engine brake deceleration, especially with front drive cars, was considered too harsh and uncomfortable.

My '87 practically runs away from me going down hills.

Some people have reported 2-3 mpg better (it would have to be lots of city driving) with agressive DFCO. I can't stand the coast on problems and GM is one of the worst for it.

On a 99 Tahoe we drove at work I would litterly get the engine warmed up and then disconnect the ISC motor. It drove great then. You might be able to do that on the Cad 4.x V8's, IF the ISC stays put when you disconnect it.

So its part of the engine braking system?

The Coupe will coast forever when in drive...

the only way to use the engine as a brake at all is to downshift... 3rd only works if I am going 60+ on a big hill, even then 2nd is where it'll do alot of the braking.

so why again does the RPM go up when in neutral? the engine is getting more throttle when the car is moving, then when its stopped?

So the ISC motor will open the throttle blades more all the time while moving?

To switch to D3 smoothly I usually slow down to 40 MPH, or downshift manually while pressing gas pedal. It's the only way to coast down a hill without overstressing the brakes.

its an automatic, no need to hit the gas or anything, just pop it into 3rd while going down hill.

I used to do that... but here in FL, our elevation change is about 15 feet, and thats only when going over the bridge...

its an automatic, no need to hit the gas or anything, just pop it into 3rd while going down hill.

I used to do that... but here in FL, our elevation change is about 15 feet, and thats only when going over the bridge...
I meant downshifting smoothly, without a harsh thump...

yeah, I know what you ment... I was just saying... it will cause no harm, and personally, it dosn't bother me at all.

i remeber when this slowly bringing the throttle plates closed thing was more to do with ensureing the engine doesnt die if the plates snap shut ....Chrysler and a few other makers had issues where after running ont he interstate , and pulling off on an exit that when the driver did the sudden lift the engine would sometimes die ....esp if the driver was to suddenly nailthe throttle after the snap shut .....GM if you ask me is just makeing sure the engine is ready for anything ....

i like the fact the damn thing will coast forever ....

only thing i really want to change with my ECU is the cooling fan turn on temp and a way to swtich between a 87 octaine timing curve and a 93 timing curve .....oh and to get rid fo the 115 speed limiter

The DFCO has a slew of settings to make it function well.

DFCO Delay
DFCO Enable RPM
DFCO Disable RPM
DFCO Enable MAP
DFCO Disable MAP
DFCO RPM Drop Rate (if the engine drops rpm too fast it will add fuel back)
DFCO Ignition Timing

So with all of this you can do a pretty good job at cutting off the fuel on decel. Unfortunately, GM has it setup so it RARELY ever activates too. So that is pretty bad in my book....

i remeber when this slowly bringing the throttle plates closed thing was more to do with ensureing the engine doesnt die if the plates snap shut ....Chrysler and a few other makers had issues where after running ont he interstate , and pulling off on an exit that when the driver did the sudden lift the engine would sometimes die ....esp if the driver was to suddenly nailthe throttle after the snap shut .....GM if you ask me is just makeing sure the engine is ready for anything ....

i like the fact the damn thing will coast forever ....

only thing i really want to change with my ECU is the cooling fan turn on temp and a way to swtich between a 87 octaine timing curve and a 93 timing curve .....oh and to get rid fo the 115 speed limiter

Yeah, I like how it'll coast on and on too... it allows me to do a WOT run to 60, then let it coast about half a mile and still maintain sufficent speed to make the U-turn etc...

cooling fans, the manual bypass but still maintaining automatic control would be cool.

I personally have no desire to run 87 octane in the car...

Fast Chips will remove the speed limiter and they claim +20hp and +30ft-lbs torque... for $160... if I decide to keep driving the Coupe, I will get one...

My car is quick until 100, after 100 she slows down alot, by the time it gets to the 112mph limiter, I don't know how much is left... it *may* get to 120, but thats pushing it.

Yeah, I like how it'll coast on and on too... it allows me to do a WOT run to 60, then let it coast about half a mile and still maintain sufficent speed to make the U-turn etc...

cooling fans, the manual bypass but still maintaining automatic control would be cool.

I personally have no desire to run 87 octane in the car...

Fast Chips will remove the speed limiter and they claim +20hp and +30ft-lbs torque... for $160... if I decide to keep driving the Coupe, I will get one...

My car is quick until 100, after 100 she slows down alot, by the time it gets to the 112mph limiter, I don't know how much is left... it *may* get to 120, but thats pushing it.

A lot of that is cam my friend.
The cam for the 4.X L engine series was designed for low end performance and economy.
Then came the NorthStar. A completely different (yet better) design.
The only way to get higher performance from the 4.X L engines is to regrind the cam and use the Allante intake and heads. Even then, you will not be able to justify the gain from the conversion expense.

I really have no reason to boost the power of the 4.9 other then to make it faster. IE. It is more then powerful enough for a daily driver.

i have an 88 coupe deville with the digital speedometer, there is a way you can look at the rpms on it? how do you do that?

don't even need the digital speedo, it uses the FDC and ECC.

Just gotta go into diagnostic mode and then select tach..... do some searches for diagnostic mode, plenty of info on it.

On my LT1, idle is typically 600 rpm or so (varies with a lot of conditions) in gear, and 775-800 in Park/Neutral. If the PRND switch is messed up it will set idle wrong too. My 91 would occaisionally come up with a fault for PRND switch, and the logic of it when I diagnosed it was off on one gear, but honestly, I think the FSM was wrong on it, as it always was that, even when "good".

On my LT1, idle is typically 600 rpm or so (varies with a lot of conditions) in gear, and 775-800 in Park/Neutral. If the PRND switch is messed up it will set idle wrong too. My 91 would occaisionally come up with a fault for PRND switch, and the logic of it when I diagnosed it was off on one gear, but honestly, I think the FSM was wrong on it, as it always was that, even when "good".
NODIH,

I occasionally switch to N while coasting down a hill and sometimes get E91 (PRNDL) code. Nothing to worry about. You could really have a PRNDL problem, but it could be set for the reason I mentioned as well.

My idle is at 575-625 RPM in gear and 600-650 RPM in neutral when engine is at 180F plus.

don't even need the digital speedo, it uses the FDC and ECC.

Just gotta go into diagnostic mode and then select tach..... do some searches for diagnostic mode, plenty of info on it.
WHAT!?!? Digital RPM in a DeVille?? Tell me how you do it Rick!!! I have a tachometer anyway, but i gotta know!!

its easy... just enter diagnostics mode, not sure on yours, it dosn't have as many features as the '93. ut once in I push LO, them HI intuil I get to P.0.8 and that is the digital tach. Its only 3 digits, so it idles at 65, which would be 650RPM, under WOT shift it may go to 510, 5,100RPM.... usually not much past 4,900RPM tho. Its not all that fast to respond, but still very effective for getting RPM at certain speeds and shift points etc...

Don't know why at 75 there is a 2-3mph deviation between the actual digital dash speedo, and the digital speedo in diagnostic mode tho...