1996 850 NA Chug Chug Rough Running

[PeteB]

Bought the the $30 OBDLink SX, it comes with a key for their basic software and
it all went perfectly. Installed the software plugged it in, worked on the first
try. The only code was P0102 MAF Circuit Low Input, which is to be expected since
I disconnected the MAF as a test and drove it for half an hour or so. Cleared the
codes, drove it, and it still chugs, no new codes so far. How long does it take to
report new codes, does it have to see the error a few times.
Not sure where to go from here, still chugs, and it drives exactly the same way
MAF connected or disconnected.

[abscate]

A code usually needs two consecutive drive cycles to set.

Keep AC off
Keep speed under 60 mph
No large throttle applications
Fuel tank between 1/4 and 3/4 full

to complete drive cycles

[PeteB]

Thanks, will do.
Forgot to mention that the software reported a smooth rise to 181 degrees.

[PeteB]

Is there any sensor that I can check in the scanner to help determine if
there is a vac leak?
Anything to help figure out the high idle?
Does a hot temp of 181 suggest an ECT issue, I thought modern engines
ran hotter?

[abscate]

Pete

My 1999 with an 87C thermostat cycles between 105C at idle, and 85 at cruise per the measurements attached here

viewtopic.php?f=1&t=79319&p=427243&hili ... re#p427243

[E Showell]

My '98 V 70 runs in the 180+ F range when driving after warm up.

[abscate]

Is there any sensor that I can check in the scanner to help determine if
there is a vac leak?
Anything to help figure out the high idle?
Does a hot temp of 181 suggest an ECT issue, I thought modern engines
ran hotter?

If your scanner supports LIVE DATA you are in luck

The SHORT TERM FUEL TRIM (STFT( is the parameter that tells you how much fuel the ECU is adding to the mix to make the correct mixture.

The ECU in theory only needs the MAF, since that measures the air directly, ergo the ECU knows how much fuel to send to the injectors (more correctly, how long to open the injectors)

But, the upstream OXS sends a signal back to the ECU telling it how much or little oxygen remains in the exhaust - ergo how close the ECU is to the right ratio.

The STFT is the amount extra/less the ECU adds/subtracts to satisfy the OXS measurement. When everything is right it will float around +-10, no more or less and be within a few percent of 0 at steady idle or steady throttle.

Now we get a vacuum leak. Too much air means the ECU has to dump in fuel. Your car runs well, because the mix is still correct!! BUT...STFT runs up to say, 20% Most cars, at 20% or more, then trip a P0171/4 lean code. (This is isn't exactly correct, but you get the idea)

Heres the trick. At idle, a small vacuum leak is a large amount of extra air relative to the closed throttle, so STFT runs up to 20%

Now we open the throttle plate to 2500 rpm. The vacuum leak is now small compared to the air going through the engine, so the air fuel balance is closer and needs less correction, so STFT drops down below 10%

So, monitor STFT at idle. If it sits near 0%, probably no vacuum leak. If its 10-20%, carry on...

Bring revs up to 2500 rpm, if the STFT falls, you confirm a leak

[PeteB]

Makes perfect sense, thanks! It supports live data and I think trim so I will check.

My son used it for the evening last night, no codes this morning.

Any tips for checking the O2 sensor based on live data?

I'm surprised that the ECU knows that it is idling at 1020 RPM and does not throw a code....?

[PeteB]

Drove it, warmed it up, and getting at idle -1 to +4 % fuel trim.

Shut it off and started again right away then it was -2 to +3 % no idea why it changed.

It varied a lot driving but usually not more than 5%, occasional blip to 10%.

[PeteB]

Going to try cleaning the MAF.

This video gives some hints on diagnosis with a scanner: