ScanGauge going crazy on finacee's Riviera

[wmjinman]

When my Jimmy got messed up, I moved the ScanGauge out of it to my fiancee's '97 Riviera. Changed the engine size (4.3 L to 3.8 L) while running, and then did the "calibration" routine at the first 2 fill-ups.

By dividing miles travelled by fuel added to fill it, it's been getting in the 24-25 MPG range in mixed driving. But the ScanGauge will go from readings that "look reasonable" - in that range, accounting for instantaneous conditions, to "totally crazy". By that, I mean, it'll show 240 to 303 MPG "current", and as I drive, the "tank" reading will keep climbing like it's trying to catch up to the new readings. Then I'll start it the next day and it'll look like it's giving me "correct" readings again - until - the crazy high readings start again. Any ideas what could cause this, or anyone else have similar problems? The ScanGuage was bought in late October/early November, as I recall.

I don't know if it matters, but the "check engine light" is on, and the code is P0102, which appears to be "MAF". I tried to clean the MAF sensor, but the light soon came back on after "clearing the code" with the ScanGauge. Would this have anything to do with it? I wouldn't think so, but????

Sorry if there's already a thread on this - I only looked at the first 3 or 4 pages of this catagory. Thanx...

[user removed]

Would the super high readings occur during periods when you are coasting? If so then it might be an indication of DFCO in operation. If not then it could just be that your engine is just using very little fuel to keep rotating while the vehicle demands are non existant, like glidng downhill with your foot off the gas.


regards
Mech

[MTXA]

MAF is one of the key inputs the ECM uses in fuel & ignition strategies and should be repaired ASAP, though it would seem highly doubtful that is what is causing the problem with the Scangauge. When I encounter a vehicle with odd electronic issues, the first place I go in diagnostics is to ensure that there is no AC voltage bleed from the alternator. AC voltage makes DC electronics go nutty. Connect a DVOM set on the AC scale to the battery and check the voltage with the engine running. Anything over .02 volts AC is suspect.

[JRMichler]

I think the Scangauge calculates fuel used from the MAF signal. If the MAF is failing, the engine will go into some sort of open loop mode while the Scangauge is working from a garbage MAF signal. That would explain both the trouble code and the goofy Scangauge reading.

You can expect the measured tank mileage to decrease if the engine is in open loop mode.

[wmjinman]

Old Mechanic,
Nope, it's not just when I'm coasting. When it's doing it, it'll even do it going UP the same hill I'd expect over 100 MPG going down. And it seems not to care about throttle input, but only speed. That is, at 70 MPH, it'll show 303 MPG going up or down hill. At 50 MPH, it'll drop to something like 242 MPG. At 25 on a city street, it might be 150 MPG. But it's intermittant - sort of. It will show "realistic" readings (like 17.2 while slowly accelerating at 30, or 36 while easing up approaching an off-ramp), but then the next time I look, it'll be showing 242 or something.

MTXA,
Thanks for that. On the Riviera forum, it sounds like those MAF sensors are like $200. CR@P! After the Suzuki Swift (White Gnat) repair, I'm worse than broke!! But there has been a gas smell in the car lately too, so maybe that's part of the open loop, or running rich. I'm not enough of an auto diagonstician (???) to do the test you're talking about, but I'll surely pass on the suggestion to my mechanic. Thanks again.

JRMichler,
Thanks. Sounds like the MAF situation needs to be taken care of, regardless, so that'll be the next step with this car. Once that's done, we'll see how things are.

Thanks, all. Bad news isn't fun, but knowing the truth is always a good thing - I guess...

[user removed]

I agree with the others about the MAF sensor being the source of the erratci readings. They generally (MAFs) gradually get a buildup of corrosion and particluate matter that insulate the sensor from the air flow and the ECU goes into a limp down mode where it uses other parameters to regulate fuel flow. Get the MAF situation corrected and you will probably see the readings corrected.

regards
Mech

[wmjinman]

Well, here's another potential clue; After reading in the Riviera forum that it could be dirty, as you mentioned, I followed their advice & went and bought a can of CRC "electronics components cleaner" and sprayed the heck out of the thing several times. I sprayed the contacts at both ends of the connection, too. When I put it back together and cleared the code with the ScanGauge, it stayed off for awhile - like at least a half hour of driving while doing errands. But then it came back on and when I checked, it was the same code (P0102), and would come back on almost immediately when cleared again.

When I was working on it, I disconnected the flexible (rubber?) air intake hose, and saw a screen of some sort at the opening of the throttle body. This screen appeared to have a dent, and maybe even a little tear in it right in front of the MAF position (at the top). It also looked discolored - kinda "brownish", almost like it had been burnt. So I douched the heck out of it with the CRC cleaner too, but it didn't really get rid of the color, so I'm guessing it wasn't really "dirt". But I now remember while messing with it, wondering if that was somehow causing trouble, too. ??? What's the purpose of that screen? Just to keep larger particles out, or something else? If some sort of "charge" (electrostatic, maybe?) is supposed to go across it, could a break in it mess anything up?