Sensor-y overload

Explain to me like I'm 5.

Mass air flow sensors, O2 sensors, throttle position sensors, manifold pressure sensors all seem to me to send similar-ish signals to help the ECU refine how much fuel the injectors send to the cylinders.

MAF detects how much air is going to the throttle body, throttle position sensor detects how much the throttle is open, MAP detects manifold vacuum (which relates to load and throttle position somehow?), O2 detects how much oxygen is in the exhaust (how rich or lean the engine is running).

Is that accurate more or less?

How does the failure or degradation of one or more of the sensors affect the ECUs ability to make accurate and timely adjustments? 

How would one go about testing for bad sensors?

In reply to JohnInKansas :

VR6 in the Jetta? I had a TPS fail in my Corrado. I don't remember the exact symptoms other than it running like crap, but testing it is easy if you have a multimeter: 

1. Remove it from the car
2. Set your multimeter to ohms
3. The sensor has 3 pins - touching the leads to 2 of the pins(outside ones IIRC) measures the total resistance of the sensor, and won't change when you sweep it back & forth. Touch the 3rd lead instead of one of the outer leads & you should see the meter reading change as you sweep it. 
    
4. If the sensor is bad, the reading will jump around drastically as you sweep the sensor. Meaning there's likely a dead spot when the ohms go to infinity, probably either just off idle, or in a position relating to whatever speed the car had spent most it's life it.  

In reply to JohnInKansas :

Following!

Accurate enough.

The whole idea is to make an estimate of the air (MAF and/or MAP sensors), estimate the fuel (fuel injector model, fuel pressure, time), and estimate the spark (MAF/MAP and engine speed).  The O2 sensors in the exhaust provide feedback to the air/fuel models to keep them close.  And on more modern engines, the knock sensor provides some feedback to the spark estimate.

TP is there historically more for the transmissions (as, for some reason, it JUST used that for way longer than it should have).  But knowing a good idea of BP and engine speed, you can use the throttle position as a back up to the MAF/MAP calculation.

But there are a LOT of other sensors- like fuel tank pressure, EGR flow, cam location (which is also used for the air estimate), turbo pressure, exhaust pressure....  There are a lot.

OEM's generally use the same rough method for adjustments, but the specifics is that we all do it differently.

That all being said, thanks to OBDII, if a sensor is bad, the system should detect it, and throw a code.  Not all of them will light the MIL light, so you may just want to check codes to see if there a sensor failure.  ALL sensors have basic circuit decisions on them, and most of them have other more intrusive tests to make sure they are working.  The more modern the vehicle, the better the diagnosis is.

Pete: yep, the VR6 in the GTi. Has developed an intermittent miss at idle as it gets up to temperature that eventually makes it stumble and die if left to its own devices. Cleaned the MAF this afternoon and reinstalled. Ran it a couple miles around the neighborhood and was really running good for the first couple of miles, pulled nicely all the way through the rev range in all gears, but got steadily more hesitant until by around mile 6 it wanted to stall at anything except just off idle. If I put my foot in it, it would either stall out completely or (eventually) fight through it and run up through the RPMs (sometimes pretty jerkily).

I have a wideband with gauge, I may put another bung in the exhaust and see how wonky the ratio gets under different conditions.

alfadriver: Good run down on the way the sensors work with each other, thanks. Unfortunately, I think the interior harness is just prior to OBD2, so I may have to pursue other avenues of diagnosis. I would be interested in pulling the codes, though, will have to see about getting that done. 

In reply to JohnInKansas :

So pre OBDII is a little tougher to check the sensors.  Like Pete said, you need to track the voltages in a way that the computer can still see it.

Adding the WB should tell you a lot, but you are not complaining of it smelling really rich- so it's more as if it's leaning more and more lean.  Which is kind of an odd failure mode- a leak will just be there, if there's an O2 drift, you can just unplug it- but back then, they didn't have that much range of authority to mess it up that bad.  Some think that changing the engine temp sensor can do that, but it's really not that huge of adjustment anyway.

And the fuel pressure can be dropping as it warms up....

The WB will suggest what direction it's going to cause the miss.  BTW, many WB sensors have a simulated NB output- so you can put the sensor in, and feed the signal back into the original input.

That almost sounds more like a fuel delivery problem, like a failing pump, or debris in the filter or plugging the sock in the tank.

Or, even more likely, a weak spark issue, like a coil or power output stage overheating.  The higher the load, the stronger the spark needs to be, so as it gets hot it gets worse.

One problem needs a fuel pressure gauge taped to the windshield, the other can be tested/eliminated with an adjustable gap spark tester.  I say adjustable, because the concern is not yes/no but how strong.

This is what I have, it was like $15 at Advance.  You turn the knob to open or close the test gap.  The numbers roughly correlate to how large of a plug gap the spark has to jump in a combustion chamber, but rule of thumb is you should get big fat sparks with the gap around 15mm or so.  If you have to close it down to 5-10mm to get a thready thin little spark to cross the gap, you have weak spark.

Plot thickens: doesn't want to get stumbly at idle if the MAF is unplugged. Didn't do a road test with the MAF unplugged, but I'm going to see if I can bench test it.

I may very well have other issues too (fuel pressure, spark, grounds).

If it doesn't die without the MAF, look for an air/vacuum leak after the MAF.

What year VR6? The likely suspects are a little different between a 92 Corrado and an 02 GLI. This is one of those cases where specific troubleshooting vs generic will help you more.

Honsch: that is entirely possible, probable even. Will see if I can track down a leak.

Paul: I'm not sure on the year, but it's a AAA engine, so one of the earlier 12v's.

Checked the pinout with a multimeter last night, supply voltage and internal grounds are good, but the signal voltage was pretty low. Like 1/3 what it should have been all the way through the rev range. Suppose that makes sense with "runs okay unplugged" and the potential for a downstream leak. 

I presume, based on your description, that it's either getting lean, or rich, causing it to run badly.  Rich can generally be smelled.  

A maf can drive mixtures lean or rich, and so can an oxygen sensor.  If you unplug one of the two, and it then runs better, it's likely the source.  

I've probably seen more o2 sensors drive the mixture lean over time.  A maf will often be erratic, or so far out of range that the ecu ignores it and drives off the base map and o2.

The possibility of fuel delivery problems is real, too.

JohnInKansas said:

Paul: I'm not sure on the year, but it's a AAA engine, so one of the earlier 12v's.

AAA spans 92-99 and there are four distinctive different engine management versions for that span, all slightly different. 

That being said likely culprits include (in my personal order I would troubleshoot) coilpack, wires, plugs, fuel pressure/delivery, engine sensors (MAF/O2), actual mechanical issue like low compression, etc. 

A lot can be diagnosed with a scan with VCDS for codes and looking at the measuring blocks for the temperature sensors, throttle position, etc.

Okay, some further thoughts. 

I know the exhaust isn't airtight. It's close, but not rock solid. That certainly is influencing O2 readings to some degree. Parts on order, will have time to get them installed next week.

Spark strength tester is on order to make sure the ignition is strong. Will give me a good excuse to check the plugs for indications of mixture and give them a good cleaning.

Definitely runs better at hot idle with the MAF disconnected. 

I'll see about finding someone with a VCDS. Not sure I'm compatible; engine and engine harness are unknown origin AAA VR6, ECU is '94 Passat, cabin harness is '91 GTi (OBD1).

Good thoughts from everybody, thanks. 

The exhaust leaks pre-O2 will mess up fuel trims on OBD1 quite a bit as the adjustment window is quite huge before it throws a code. 

I would spray test the coilpack, they are all suspect.

The MAF a 4pin or 6pin on this one?

Is there any of the emissions stuff missing (secondary air injection, egr, etc)?

Also this a fresh build or a problem w a car that was working fine otherwise?

JohnInKansas said:

Plot thickens: doesn't want to get stumbly at idle if the MAF is unplugged. Didn't do a road test with the MAF unplugged, but I'm going to see if I can bench test it.

I may very well have other issues too (fuel pressure, spark, grounds).

I have no idea if this is true of VWs or not but on a lot of cars that test indicates a bad MAF. 

Paul: Fixing the exhaust as soon as I get the gaskets. When you say "spray test ", what do you mean by that? MAF is a 4 pin. As far as I can tell, ALL of the emissions stuff is missing or at least uninstalled. Car came to me as a third or fourth-hand project via AAZCD, he ran out of interest/patience before he got it running well, unsure if it was ever running as well as it is now. 

APE: that was what I gathered. We'll see. Further troubleshooting and fixing known issues won't hurt, anyway.