1996 850 NA Chug Chug Rough Running

[PeteB]

Wouldn't no change MAF connected or disconnected suggest that it is not getting
a signal to the ECU - even when connected? I drove it until it warmed up, and
started it a few times, no difference at all.
Yes, I need to get a scanner and see what codes we have, asap.

It chugs the most at idle, smooths out by 2000 RPM.

[erikv11]

OK sounds like you tested the MAF sufficiently, yep.

You've considered this before I know, but chugs-at-idle points most often to a vacuum leak. I know you have looked in detail at vac lines but if it were my car I would keep an open mind to that possibility.

With the MAF disconnected, so when there is no MAF signal at all, and everything else as it should be, the engine will run on a default air:fuel which makes it run smoothly but a little rich. Basically the car runs just fine with the disconnect. A faulty MAF gives bad signal, not no signal. That's why unplugging it "fixes" the rough running, by changing what the ECU hears to "no signal." So with your car, something outside of the MAF seems to be the problem, screwing things up no matter what kind of signal comes from the MAF.

Do you know if your 1996 has m4.3 or m4.4? I am just curious. http://new.volvocars.com/ownersdocs/199 ... n1996.html

[PeteB]

Vin is W5544T
So engine is a 55 with if I am reading it right:
4 = w/o EGR w/air pump, w/OBDII, w/elec. control evap,
Motronic 4.4 w/Rulo., TLEV (55)

I'm open to looking for a vac leak, not sure where to look.
Does a vac gauge help at all?
I don't remember finding a hose going to the firewall for the cruise control,
is that normal?

[erikv11]

Cruise has its own vacuum pump, mounted to the bottom of the battery tray, so it is a separate vacuum.

MVS users have documented that some P80 cars run poorly with the MAF disconnected, I haven't figured out which or why (or if there is a which or why to it), but I think the exceptions may be all 4.3 or earlier. That's why I asked about that.

Vac leak detection can be tricky if it is not an obvious leak at the intake manifold or something, you can smoke it or use the starter-fluid method. Both seem pretty effective, smoke is probably more sensitive but starter fluid easier.

[E Showell]

With it idling hot, the first thing I'd be looking at is a combination of thermostat and ECT.

[PeteB]

Thanks, the temp was low and would cool down more on the highway,
so I changed the thermostat within the last year. Warms up fast and stays
rock solid in the center now.
Isn't the ECT the sensor for the temp gauge and I've seen people report
that the guage is erratic when the ECT is bad. Is an erratic gauge an
indication of a bad ECT?

[E Showell]

Correct -- ECT = engine coolant temperature. That's the temp gauge on the dash. Consistent 3 o'clock is where you want it. If it is jumping around after the engine has warmed to operating temperature, you probably have a bad sensor or thermostat. I always change both together.

[erikv11]

With it idling hot, ...

What do you mean by this part of the comment? I'm not getting it

@PeteB: go back to the very first reply: check the resistance of the ECT at different temps, you'll be glad you did it.

[E Showell]

Erik. Just repeating O.P.'s words.

[PeteB]

Not sure if I'm following you, just to be clear the temp gauge is solid
no jumping around or anything strange at all.