1996 850 NA Chug Chug Rough Running

PeteB · Post by **PeteB** » 27 Dec 2016, 20:47

Just finished the PCV repair on our 1996 NA 850 and had hoped that fixing a
vac leak and the PCV would correct the rough running but it did not. I've done
the timing belt, plugs, air filter, cold air flap mod, thermostat, and it does not
report any codes. Compression is good enough in all cylinders. It does not
smoke at all, and the valves looked very clean when I had the intake manifold off.
The PCV was not badly plugged.

I've read:
Disconnect the MAF to rule it out - good idea?

The ECT sensor often goes bad - should I just change it or would we get a code for it?

The O2 sensor often goes bad, it was replaced at 77K now we are at 177K miles.

Any suggestions on how to debug this without throwing parts at it?
I'd like to have an OBDII tester since I work on our other cars also so why not, what
is a good one that will give me the most data and work with other cars? Will it give
me a quick idea of what is wrong?

Post by **erikv11** » 27 Dec 2016, 23:45

MAF: good idea. Car off, unplug it, turn car on, give it a minute to stabilize.

ECT: Pull it out and check it with an ohmmeter at different temps. You can get a chart from the forums (probably in the VRD) with the expected values, it has been posted many times.

If the car has the CEL on, the code reader will help tremendously. Note that your MAF test above will trip a MAF code CEL.

Post by **abscate** » 28 Dec 2016, 04:04

ECT usually does not give codes unless there is a wiring fault, so measure the resistance and compare to the chart.

PeteB · Post by **PeteB** » 28 Dec 2016, 12:33

Thanks everyone, I have an ohm meter and will do.

Forgot to mention that it idles hot very (100%) reliably at 1000 RPM, higher when cold,
never lower for the last year or so no matter the outside temp. Just to be clear cold
startup it is higher as would be expected.

Will a code reader that also reports temp, fuel trims, etc. give a much better idea of
the problem?

Post by **erikv11** » 28 Dec 2016, 16:52

It may, yes. It is great to have one that gives at least fuel trims, mine was stolen about a year ago and I really miss it. You might want to start a separate thread about code reader recommendations, or just hit the search button it gets asked frequently.

PeteB · Post by **PeteB** » 28 Dec 2016, 21:05

Thanks for all the info, everyone, Erik, which one did you have and would
you go with the same one again?

Post by **erikv11** » 29 Dec 2016, 00:43

I had the Autel EU702. Really nice unit, also does SRS codes. You can still get it but I think it is a bit outdated - you can get more capable units now for about the same price. So while if somebody returned my old one to me I wouldn't bother to go shopping for another, I would do more research before buying right now.

PeteB · Post by **PeteB** » 01 Jan 2017, 10:12

The IAC looks fine, kind of a sprung door in there, completely free to move.
Sprayed some carb cleaner in made little difference and it runs the same.

Removed the MAF connection, runs exactly the same and no CEL .... ?
Turned the ignition on for lamp test and it seems to be out!
So I need to read codes, probably a bad MAF, I'll search on what to do,
any specific suggestions for cleaning or whatever?
This thread seems to be good, 2nd post on - I will clean the connector:
viewtopic.php?f=9&t=53998

Seems I have to pull the cluster, again, and see what is up with the bulb.

PeteB · Post by **PeteB** » 02 Jan 2017, 10:06

Cleaned the connector with DeOxit and no difference.
Is there an ohm test for the MAF itself?

Post by **erikv11** » 02 Jan 2017, 10:41

You can test the MAF under operation using back probes at the connector, I have never tried it. Search this forum or the general internet for procedures. Doesn't sound like a MAF problem though, since it is the same unplugged. You did let it run for a few minutes unplugged? The ECU needs to go into default air:fuel mode (which is on the rich side), that can take a few minutes.

Since the bulb is out, it may have codes that can help greatly for diagnosis. I would reset the codes, run it, and then see what codes come back associated with the rough running.

other thoughts:
- Still a good idea to test the ECT when you can get to it.
- FYI cleaning the MAF connector sometimes helps, cleaning the MAF itself almost never does anything unless the sensor surface is visibly oily or dirty.
- What brand is the MAF in there, for the record?
- Does it chug at all engine speeds? Or at just a certain throttle position (wondering about the TPS)?

It absolutely sucks to throw parts at troubleshooting a problem like this, at the same time I always have a couple proven, spare ECT, MAF, CPS, TPS, crank, etc. sensors in a box in the garage. And fuel pump relays. It's so easy to make a couple swaps and then be certain about those parts of the system.