Incorrect SM and PL data

gunslinger

Member
Joined
Nov 21, 2005
I've been trying to chase down a lean condition, so today I was checking ecm inputs powers and grounds, and I noticed that O2 and battery voltage data don't match the actual inputs.
Actual battery input voltage is at least 1 volt higher than the data stream says it is, and O2 input is 150-200mV higher than scanmaster shows.
I plugged in the MODIS, and it agrees (pretty much) with what scanmaster is reading. I'll check PL tomorrow, but I think it will agree with the other two.

Any ideas??
 
the scanmaster and PL only display what the ECM thinks is happening, they don't actually measure anything (except the analog inputs for the PL)

I think you realize this, just clarifying for those playing at home......

The ECM does not measure battery voltage, it measures at the ignition switch input to the ECM, there are lots of places in the stock harness for losses and voltage drops. Some difference is somewhat normal, and it varies with load and other things. You might get out the schematics and chase it with a voltmeter and see where the losses are.

Post what you find, I'm real curious.


The O2 input is another thing.

Lets back up to about 1990. The "in" tool for reading the datastream was the OTC2000, everyone started using it. It displayed the O2 voltage INCORRECTLY, but became the de-facto standard. Since then, all tools made for the Buick community have mimiced that incorrect scaling. PL and Scanmaster included.

But, the ECM measures the voltage at the O2 input on the ECM, and has its own reference ground wire that goes to the engine. So, make sure the ground is good, but your data is probably fine.

When you say "lean" what data are you using? What conditions? With widebands being so cheap nowadays, there is no excuse to not have one.

Bob
(hows that for info overload...)
 
Thanks Bob. That clears up a couple things.

Do you know what pin the ECM is using to monitor battery voltage?
As far as I cant tell, the ECM only has 4 grounds. I checked them and they all had a 2.4mV drop, so it looks like all the grounds are good.

The lean condition I've been chasing started out with a failing fuel pump. Fuel pressure and O2's were dropping off at WOT. I've got that fixed, but O2s are still in the mid 500s to low 600s. It's lean enough to cause a misfire under load. I've been using scanmaster and powerlogger data to diagnose it to this point.

I realized yesterday that the O2 is cooling off at highway speeds and going into open loop, so I'm installing a heated one today. We'll see what difference that makes.

It's lean only at WOT. MAF is at 255, TPS is at WOT voltage, and O2s are too low. PL files from a couple years ago show O2s on the low 800s, and nothing has changed in my setup.

Thanks for the help.
 
The O2 shouldn't "cool off" at highway speed, so it may just be bad.

Misfires at WOT will make the O2's look lean. You may just have a bad coilpack, wire, or sparkplug.

Post a PL file if you like.

Bob
 
True Bob, while cruising on the highway, my EGT's stay pretty steady at 1,000-1,100 degrees.
 
Do you have a WB? My WB and the stock O2 readings are quite different sometimes. Sounds like the O2 is getting hinky.
 
The O2 shouldn't "cool off" at highway speed, so it may just be bad.

Misfires at WOT will make the O2's look lean. You may just have a bad coilpack, wire, or sparkplug.

Post a PL file if you like.

Bob

I'll post a couple PL files when I get home.

I've already replaced the O2 sensor because it was flat lining at highway speeds. while I was testing yesterday, it took forever to get into closed loop. Checking exhaust temps with a IR thermometer, it was showing header temps in the 350-360 degree range, and the downpipe was around 175. The O2 sensor needs to maintain 400 degrees to work. I'm blaming this on my Poston headers.

I'm thinking that this problem is something I did when I put the engine back in last year. I didn't make any changes to the combo, just re-gasketed it, new timing chain etc. If the cam sensor was out 180 degrees, do you think it would cause something like this? The car runs pretty well, hot idle is a little rough, but that's it. I've already replaced the plugs, the wires are fairly new, and I tried a new coil, but it idles the same.
 
New heated O2, goes into closed loop faster, but still reading lean at WOT. O2s in the 550s. The only thing keeping the head gaskets in one piece is the alky. It's not misfiring, but it's on the edge.

I double checked the cam sensor, and it is installed correctly.

I'm starting to go in circles.

Going to hook up the laptop this weekend and log some data.
 
Maybe you should toggle through the setting in the chip to see what they are set at. Did you try adding some fuel with the chip to see what happens?
 
Maybe you should toggle through the setting in the chip to see what they are set at. Did you try adding some fuel with the chip to see what happens?

I've reset the chip back to default settings, and have added 20% WOT fuel, and it's made very little difference.
I'm going to figure this thing out today.
 
Got another chip to try? I don't have a 42lb one.
 
I just got a reply from Eric on his board. I'm going to check a couple things toady. He thinks there may be a problem with the O2 ground. I'm going to double check that, and go log some data.
 
Looks like it's fixed. It was a problem with the grounds.
All the grounds had a 2.4mV drop with the key off, and ecm unplugged, so I assumed that they were good.
I plugged everything back in and started the car, and the voltage drop increased to 145mV.
I pulled all the grounds off the engine, and tested them to the ecm, and they all had a 0.0mV drop, and engine to B- was 0.8mV.
Put it all back together, and now engine running I get a 3.1mV drop on all the grounds, and everything is reading right again.

Now I just have to find out why the it's breaking up a little under boost. I'm thinking it's a coil.

Thanks everyone for the help.
 
If the coil is OK try closing up the sparkplug gap a little. Glad you found that issue. Grounds on these car seem to the route of all evil.
 
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