This will be a lengthy post, not interested in responses from anyone unwilling to read thoroughly and contribute intelligently.

My interest in the question posed in the subject is twofold.

Initially it was because I religiously track actual fuel consumption and log estimated consumption as reported by the DIC, and have noticed the DIC tends to report somewhat optimistic numbers when comparing AVG to actual miles divided by gallons. While I do not expect the DIC data to be absolutely accurate, I've logged over 12k miles over 49 fill-ups, and consistently find the DIC AVG to me roughly 1 mpg better than actual.

https://www.fuelly.com/car/chevrolet...autsch/1122602

This is absolutely the least pressing part of my curiosity, but unlike my C4 I find no constant in the PCM calibration that will allow me to skew the data used to calculate this telemetry. I'd love to make it more accurate, but here's my more pressing question.

At the end of the day today I got in the car to go home. Started and took off without issue. About a minute into the drive I see just about every light on the cluster illuminate. The speedo, tach and voltmeter are the only ones reading accurately, while the fuel and oil gauges are flatlined and the temperature gauge is pegged. The DIC is displaying every warning imaginable including reduced engine power, low tire pressure, service traction control, service srs, service vehicle, etc. I cannot control the door locks or window motors from either control pad.

I know these are a bunch of false warnings likely caused by a lack of communication on the ALDL, so I pull over and shut it down. Upon restart everything appears normal so I take off and start driving. Still no control of the windows or locks. After about another minute the fireworks resume. I pull over once more and repeat the same except I remove the key and let the seat retract to egress position. I put the key back in and press #1 memory and the seat goes to my normal position. I repeat this a couple times with no issues.

I start it back up and get on the interstate. Since I know I'm chasing ghosts I just keep driving normally. After between 5-10 minutes all of a sudden everything starts working again except a few modules. Only the security and traction control lights are illuminated, and the DIC is alternating 5 messages:

Service vehicle soon
Service active handling
Service traction control
Svc tire press monitor sys
Low Tire Pressure

Window and door lock controls work as normal again.

This condition persists for the remainder of my 62 mile drive home. Other than all the warning messages flashing, nothing at all seems wrong. I am even able to use the cruise control.

So my question about where the fuel consumption is calculated is because one thing I noted was that each time I shut the car off and restarted, the AVG fuel economy estimate was grossly lower than when I last parked it. It was reading 24.9 mpg when I got back to work after lunch break, and when I re-started it the first two times it was showing around 12 mpg. My thought is that whatever module this average is stored in is possibly the culprit.

My assumption is this is the IPC, though I had previously assumed it was kept in the BCM since that's where it resides in my C4 (called the CCM, but same thing).

I've read tons of info on stuff like this, and have checked grounds and the connectors inside the door accordions long ago. Ruled out the speaker ground shield (sarcasm). I have Bill Curlee's "My C5 dash has gone nuts" post bookmarked and have read it beginning to end at least twice today alone. I did for a brief time have a (visually confirmed) bad ground on the negative battery terminal, but have since resolved that. This is the first time I've seen the fuel consumption data reset since fixing that over four months ago.

Other than a handful of times when the door control modules seem to take a "vacation" for no apparent reason, and one spurious code from the EBTCM on a day with 102F ambient temps, there have been no other "gremlin" like problems.

But this one has me baffled. Looking for intelligent discussion / ideas.

 

pull the DTC codes first.

 

As far as your more “pressing” question looks like you have an intermittent serial data bus issue…my recommendation to you is when everything goes haywire remove the bus bar off both splice packs near the BCM and with your DVOM with key ON see if you see 12 volts or a ground on each of the terminals…12 volts or ground on the serial data line (the purple wire) will cause this…one of the modules power or ground is being lost and that 12 volts or ground is back feeding onto that data line…the DDM is not the only module that can cause this.


/f_NEEdgZSlE?feature=share

 

I re-read the Bill Curlee post I referenced earlier and noted the location of the ALDL star busses in the footwell, so yes that's a good suggestion, thanks. That'll be fun to do on the side of the road. :-O

I just went for a short drive in it and no issues, so this is definitely something intermittent. I'm still highly interested in what module the fuel consumption is calculated in, because it was clearly reset to zero at the onset of this "fit".

I guess I'll throw a spare meter in the glove box and will post back if I learn anything.

The next thing already on my todo list was pulling both door control modules and reflowing solder joints as they've been intermittent in very cold temperatures, so I'll probably tackle that this week and re-inspect the power connections in the pillar box. It was warm yesterday - in the low 60s. In fact, the only thing different about yesterday is that I left the windows open a bit and when I closed the doors they slammed a bit harder than usual. Hmm.

I guess if nobody knows the answer to my question the next thing on my todo list will be snooping the VPW bus and trying to dissect the network traffic.

 

If either of the door modules are causing this when you’re driving if by removing the Star Connector bus bar with the 4 wires corrects your issue it is on that half of the serial bus…Driver Door Module, Passenger Door Module or the Seat Control module…if you remove the other bus bar off the other splice pack the car will not run because the BCM and PCM are on that side must communicate with each other…if you were to jump those 2 terminals together the car will at least start...what’s your question ??…the fuel consumption ??…that is the least of your worries right now…I wouldn’t trust the number anyway…if it reads a few MPG off just add or subtract that amount…the PCM knows how far you traveled and how many gallons you have left so there is your number.

 

Thanks, I hadn't retained that from reading.

Sorry, I didn't watch the video b/c when someone replies to a question with a youtube video I will universally ignore it. Stupid thrives on YT. Science thrives in the printed word. I realize there's a convergence coming into play, but 90% of the time the YT video isn't as worthy of my attention as something I pick up after my dog.

Though I'd love to correct it, the approximation error is not what I'm bugging out about. It's that I normally keep the DIC set to display average fuel consumption, and when I stopped the car and re-started when this "fit" manifested itself, the average was clearly reset to zero or some other value that wasn't in line with the normal 25mpg that would have been in agreement with the previous 220+ miles logged on this refill.

I appreciate that you've responded to my post, because I've read hundreds of posts you've submitted and I recognize you know what you're talking about. But I'm not your typical C5 (consumer) owner. I've pictures of my 1995 C4 DIC showing an odometer reading of zero. Care to test that - give me some arbitrary number and I'll be more than happy to oblige.

 

Far as I can tell it's the PCM that calculates fuel consumption/economy. I have a DIC in my truck and with stock PCM it worked fine, but when I changed to the P01 as found in 01-03 Vettes, my instantaneous economy was whacked but average worked. It was off by 1mpg as well until I cleaned up the MAF calibration, but I do have a fuel economy scaler available to me in EFILive which I assume would skew the mpg calculations.

 

Just wanted to post back with an update. Nothing productive has been found on the IPC "wigging out" and showing no comms. I suspect I might not see that again for years.

But... I did spend some time this weekend going through the door control modules. Some suspect solder joints were found under the conformal coating, which makes me wonder if anyone who's fixed these by replacing relays first tried re-flowing these solder joints. Also pulled the driver's side-view mirror which I thought I'd broken by popping one of the pivot ***** out of it's socket. Turns out the pivot ball end has a flute that keys the adjuster screw to the socket, and when not aligned one cannot simply pop the socket back on the ball. Could've figured that out with some careful laparoscopy without removing the mirror, but oh well.

I also cleaned up all the "mystery" wires in the BCM cavity. Seems someone had a head unit or standalone backup camera in it before the current setup as the previous install tied the camera trigger into the backup light control on the BCM. Also found and jettisoned a lo-jack or some other type of location tracking module underneath the BCM who's power wires had been cut and left hanging loose.

Anyway, after all the time spent fixing the silly stuff a thought came to mind. Recently local law enforcement must have received some grant money they couldn't figure out where to spend, and decided to squander it on a mobile "your speed is" annoyance they've staked down at the spot where the speed limit coming into town drops from 55 to 45. This made me notice there seems to be a bit of speedo error. So today I did some logging of some known mileage routes and reported vs. GPS speed and find there's a solid 2.0% error. Doesn't sound like much, but when I applied it to the distance traveled on older fill-ups it looks like the calculated versus estimated error is reduced to a much more reasonable number. I believe my Conti Pro Contact 295/35 18 rears are quite a bit shorter than what the car was delivered with. Edit: I'll be sure to report back after I correct the VSS scalar in the PCM and run a few tanks through it.

I haven't spent any time yet snooping VPW comms to see if I can map any of that out, but Supercharged may be correct that some major component (or all components) of this calculation / storage take place within the PCM, because it has access to everything needed (injector flow constant, MAF / engine load, distance traveled). My gut tells me the cumulative average might be tracked / stored in the BCM or IPC because it can be reset from the DIC keypad, which is how the same functionality worked in the C4.

Just sharing what I hope is possibly a small nugget of useful information.