When you pronounce breaking and braking, it's about the same, isn't it? I can really see that even you understood originally what I meant. Then when I just said for later reply (Rob's 73) that his reply had some real knowledge, you took offence and started barking me because of my (hopefully) understandable mistake with this one little word albeit there was remarkable difference in meaning I really understand what MPG is and how I can calculate MPG or litres/100km myself... I'm not that idiot

And still, my question is (if we can finally go back to topic) what could be wrong if it doesn't shut down the fuel injection during engine braking? If I drive let's say 40 MPH with 6th gear and release gas pedal (or how on earth it needs be to said in English ) the DIC shows e.g. 60 MPG.

 

Oh yeah, now I can see. I talked first about MPG, then about liters/100 km and then again MPG and then I said it should be always 0. But I meant 0 liters/100 km (because with MPG you cannot really see "zero" consumption because it doesn't show you infinite) But it doesn't make difference as you still thought kilometers per liter Idiot :P

 

It's the same reason when you release the gas pedal going downhill it reads 99.0 MPG. The DIC is making an estimate on current fuel usage based on the speed you are currently traveling at that moment in time. The DIC is not accurate.

If the engine is running, it is using fuel (albiet a small amount)... even while engine braking. There is no problem

 

veppe, After reading all the posts I believe what you are asking about is a function of the DFCO Deceleration Fuel Cut-Off also referenced by Rob's 73 and mseven. It's quite involved in how it's achieved. There are several tables, functions, and enablers that contribute to it. The reason it's done this way is to make it adaptable to the conditions the car is experiencing at any given time, making it's function variable. These variables can be altered by reprogramming the pcm. I am no expert on DFCO. I just turn it off when tuning my car and back on when finished so it doesn't skew my AFR readings. If you'd like to learn more about it I would suggest posting in the Scan and Tune section, Google-ing 'C5 DFCO', or joining EFILive and/or HPTuners forums and discussing it there too. C5 Tech isn't as PCM-centric as it could be, I think it's more oriented towards the mechanical end of the C5.

The PCM makes a calculation of the MPG or L/kmh by using the injector flowrate table, the speed of the car, the VE table, and the DFCO.
Also, Welcome to the forum.

 

Thanks for the posting! This basically gives good information, so as also earlier said, some tune up might be reason for that. I don't know the exact history of this car so I cannot say if someone has done something or not On the other hand, because fuel injection should be totally cut off during engine braking fuel consumption should be anyway shown as zero during engine braking (when reading litres/100km of course) because injector flowrate should be zero as well (if I understood correctly what this injector flowrate means ) Unless someone has left DFCO off after tune up (intentionally or didn't remember to turn it back on). So actually my question is, how can I check this, and how can I turn it back on if it's off? Do I need some analyzer/programmer (or whatever they are called) practically meaning to get my car to some tune up mechanics or dealership service...



For people who still insist engine needs always some fuel to run even in engine braking: First use some common sense (if you have any) to realize why it doesn't need fuel to "run" in engine braking. If you couldn't realize, then please study a little bit more the modern engine technology and how engines with fuel injection are working. I think they have had fuel cut off in engine braking already since early 80's or whenever fuel injections came And I don't really believe that GM hasn't included it as well because they have done also some definitely more expensive things to get fuel consumption lower, like 1to4 shift feature...

Actually when I tried to google some good web pages to give you more information about this, I found one reason not to use engine braking (just search 'engine braking' in that FAQ). But I think you have more benefits of engine braking over this issue
http://www.nesteoil.com/default.asp?...,547,5597,5760

Some basic information of engine braking can be actually found from this stupid "tip" to avoid engine braking to save fuel. You just need to read a little bit farther in the topic to find correct answers
http://www.dailyfueleconomytip.com/d...ngine-braking/

Actually I realized now what are the reasons people e.g. in this forum (and in that stupid "Fuel Economy Tip - Avoid Engine Braking" which definitely seems to be American) don't know how engine braking is working or why it's a good way to save some fuel (I don't mean all the people in this forum don't know but at least majority of replies to this topic):
1) People in US always use MPG (of course because metrics would be little bit harder to use ) and DICs (always?) limit it to show 99 MPG even if actual fuel consumption is zero (engine braking). So you don't really "see" the zero consumption in any occasions!
2) In US you don't need to have any driving lessons to get your driver's license. At least here in CA where I just went to DMV, filled the needed forms, did the written test using my common sense (I just did couple of pre-tests in DMV web site, not reading the actual flier), and passed the test (-6 points so just enough to pass the test ) Of course I had about 10 years driving experience in Finland, and there are not so many differences in laws and rules..
E.g. in Finland you need to take lessons (both class and driving lessons) in order to apply for driver's license. One part of that is fuel economy lessons where basic engine technology is clarified basically for everybody (yeah, some blonds don't maybe understand it ) to clarify benefit of engine braking with modern cars (all cars nowadays have some kind of fuel injection) are explained how to save fuel.
3) Gasoline has been (not anymore!) so cheap in US that it hasn't made any sense to learn how to save some small amount of gasoline. E.g. in Finland gasoline costs right now about 1.4 euros/liter, meaning about 5.5 euros/liter which is about $9/gallon I think it's about the same price everywhere in Europe so it really matters then if you can save something I think with nowadays prices here in US people are also thinking about ways to save some fuel.

So this was long posting but I hope this was useful for readers to clarify engine braking and ways to save some fuel

Still waiting for some more hints how to fix my problem Maybe it's around that DFCO feature, if it's turned off in my Vette or if there is some real problem somewhere Just need to know how to fix it then For me this actually matters because I'm driving to work through the mountains. Nowadays I'm driving downhills with neutral or just clutch engaged because in engine braking DIC shows bigger instant MPG.

 

Are you really sure? Try doing engine braking and change to metrics to see instant l/100km... In all of my previous cars having DIC (not all of my cars have had it) it has shown 0 l/100 km during engine braking. Of course in the beginning (some 1-2 seconds) when you start engine braking it shows some consumption because it calculates instant MPG from some driven distance (like 0.1 miles or something).

So as I already said, I don't really believe GM hasn't used this "feature" to cut off the fuel injection which is used by any other car manufacturers And like already said by ArKay99 and Rob's 73 there is this DFCO feature in the Vette's fuel injection control too

 

I think it depends on the rpms, speed and rate of deceleration. If you are at 6000 rpms at 75mph in second gear and let off, then you will see 0 fuel. There may also be built in protection of the cats (so they dont go lean for too long and overheat them) when the rate of decel is slow......like going down a long hill in 3-6th dropping only 100 rpms every 5 seconds.

 

I think it should always cut off fuel when you are above some specified limit, like Rob said somewhere 1500 RPM. Cats shouldn't heat during engine braking because engine is not producing any heat. Actually it should just cool it because it's just taking fresh air which is passing to exhaust. Of course pressuring the air in the cylinder will heat it but especially in gasoline engines shouldn't heat so much that could cause any problems to cats. Ok, I'm don't know so much about cats and how they work but if engine braking works with any other cars, why not in Corvette

Actually I'm wondering nobody has just gone out driving his/her Vette, check l/100km in engine braking and get back to this topic with some comparison results Then we wouldn't need to speculate how Vettes are really doing engine braking (I still rely on Rob's and ArKay's knowledge about DFCO)

 

When I get home this summer I'll try downshifting on a hill to see if the DFCO is actually kicking in. The thing is, I never find myself in a situation where DFCO will actually matter. The car is practically idling when I'm slowing down. I would see a far better decrease in fuel consumption if I left stoplights a little slower. Again, I've never relied on the DIC to give me 100% factual data because it's already proven to be somewhat inaccurate, especially when it comes to mpg or l/100km calculation.

A better way to see if the DFCO is actually working is to drive with a scantool and monitor the injector pulsewidth, rather than the DIC. Searching around on the topic I read that DFCO is dependant on coolent temp, Baro, TPS, and speed. If it is cool out, it will not kick in. Perhaps Arkay99 can verify....

It still sounds like splitting hairs to me when squeezing every possible mpg out of a Corvette. The C5 has great MPG to performance ratio; but if gas economy was that important to me I would buy a smaller engine, economy car. Still, I hope you find what you are looking for

 

The metric measurement of fuel consumption is litres used per 100km. If you're going to be a condescending Mr Know It All on the forum then you'd better be right. If not, then you only prove you're a complete idiot too.

Peter

 

To address your question Veppe, even though I think others have.

It appears to me the decel fuel cut-off is less aggressive on these cars compared to some others. So, it really doesn't cut the fuel as much or as often as you might see in another car.

Something not addressed here is you mention 6th gear which is saying you have a standard transmission. I haven't seen it addressed if DFCO is enabled in the stick cars. If it was, the engine could stall if you clutched in while the fuel was shut-off.

Peter

 

Yes, I think also that it may not be so aggressive on these cars, maybe just to ensure good accelerating if I decide to kick down. But it should really show even something.

About engine stalling. Shouldn't be a problem. If you think it more, it cannot stall. Because like Rob & co said, it will start injection again below some 1500 RPM. So practically it cannot stall because if you clutch above that, it still have enough inertia to "start" engine again And DFCO is used also in stick cars. All of the cars that I have driven and seen DFCO working, have been stick cars, (ok my dad's Buick was automatic, but it had also DFCO) always have had DFCO. So shouldn't be caused even by that.

Basically I'm almost always accelerating very quickly e.g. from stop lights, so yeah I haven't bought this car to save fuel So good fuel economy is not my only concern with this. If something doesn't work like it should, it means that there is problems somewhere (some sensor or whatever) which can then cause problems to some other things as well. Actually I realized just today really high fuel consumption in the freeway which I haven't seen before. With 6th gear, driving flat road some 75 MPH, MPG was 20-22 Normally it has been even over 30. Maybe this is related to same problem...

And yeah, DefenderC5, DIC is not so accurate to tell the truth, but some good estimate it should provide. (for "some" reason DICs are always optimistic )Of course one possibility is that DIC has just problems and engine is actually working fine. I have all the receipts from refuellings from last December till today, so I should actually just calculate also the average. It would maybe tell if DIC is working badly wrong

Maybe I should also check the codes, if it could help So, I have following codes right now:
PCM P1571
TCS C1277
BCM B2592

Hmm.. Both PCM and TCS codes are for traction control and BCM code related just to column lock... No help from, this... BTW, my car is not putting new codes so much! I think I reset the codes some one month ago. Most often I'm getting TCS failures if I have passenger with me. Problem with passenger door wirings..

Investigation continues...

 

First are the codes current "C" or history "H"?

P1571 ASR Desired Torque
C1277 Requested Torque Signal CKT Malfunction

This can be caused by corroded grounds. Do a search for Bill Curlee's thread on cleaning the electrical grounds. Also, couldn't hurt to check the battery. P1571 has a TSB I believe and is a problem with the 97 PCM itself, I know you have a 98 but perhaps it was an early year 98? Bill Dearborn is a good contact for this stuff... http://forums.corvetteforum.com/show...6&postcount=11

B2592 Column Lock/Unlock Drive (B) - Have you done the column lock bypass and/or recall? If so, I would just keep clearing this one.

If they aren't affecting operating I would just clear them and drive.

 

They were just history codes. Had to be I think because I didn't start the engine when checking the codes (or is it giving sometimes current codes also when just having ignition switched on?)

Recall has been done, I checked last time in dealership from their service log. Seems that also differential has been changed in 16k miles.

BTW, how did you know this is 98? It's Indy PC, I'm not sure if it was manufactured already in 97, but I think mine is from May 98.

Actually battery was almost empty when I was 5 days in Dallas and I forget radar detector on. It had almost eaten the battery.. E.g. it cleared something from DIC, like average MPG, although it wasn't totally empty (I could still start the engine).

So far it has been enough to stop and start the engine for those TCS problems

 

I tried to see if I could get the DFCO to kick in. No such luck. I tried coasting down from > 70mph with rpm's above 3000rpm and the display never dropped below about 2.5l/100km. I also never felt any change in the deceleration passing through the 2000rpm range. My car is a 98 6-speed.

With my truck, I can feel it around 60mph. there is a very noticeable chaange in the deceleration but the rpm's don't change so it's not changing gears or unlocking the converter so it's most likely the DFCO. It's a S-10 with an auto.

Peter

 

I was also first wondering if it's some feature in Corvette to not cut off the fuel (just to give better acceleration). With me it's also exactly this around 2.5 l/100 km (sometimes 2.4 or even 2.3 but not below) But I have also asked this same question in another Corvette forum (Finnish) and there people said they can see 0 l/100 km during engine braking. Maybe GM has just done different configurations for US and European models Of course it would be nice if we get more "test results" from other Vette owners just to compare