We recently bought a '97 850 with 97000 miles. It is in very good shape and we like it very much. However, since we bought it, it has had numerous and continuous related problems. On the couple occasions that the check engine light lights, the error codes have only shown that the engine miss-fires on all 5 cylinders. Sometimes the engine runs perfectly, but around half the time it "stumbles" which feels like it could be miss-firing or starts randomly upshifting and downshifting to varying degrees. It might be that the stumbling is actually a mild version of the shifting problem. At its worst, when the car shifts, it will stay in that gear for 5 to 10 seconds. The car's speed and rev's changes appropriately due to the change in gear, but the car still is unmistakably being driven down the road at all times.

The car was put on a computer while it was doing the shifting thing, and there were no codes given. But when the throttle sensor was specifically monitored, it would instantly change from 11% to 83% corresponding in time to the shifting with no change in gas pedal. Even while idling, it would similarly change and seemed to be accompanied by a slight change in engine speed.

We’ve replaced spark plugs, leads, distributor cap, and rotor. The electrical connections on the throttle sensor were found to be corroded so "the end of it was replaced" (whatever that means). None of this made any noticeable difference.

On a possibly related note, it was discovered that a vacuum line was found dangling. A possible connection point was found at the PCV valve, but the line needed to be extended to reach it, so we’re not sure that it is the correct spot. It didn’t change the car’s symptoms at all. The hose definitely has vacuum. The pictures show that the hose comes from around the corner of the intake manifold someplace. Note that it is a stiff plastic tube with a large soft rubber anti-chaff cover over it where it goes around the corner of the intake manifold. A short portion of the black plastic tube can be seen by the "6" in the "26" part of the photo's date stamp. Anyone know where the other end goes?

The car is still in warranty and we want to get this stuff figured out soon so that we can drive it enough to feel reasonably sure that all the bugs are worked out. The guy fixing the car doesn't want to replace the position sensor unless he is sure that it is the cause. Can anyone offer any help please?

By the way, I've also got a 740 wagon with 300000+ miles and still going strong.

Finding the bad vacuum line is the solution to all of your problems! Replace it with a new one and make sure it is plugged into the oil separator where it belongs. You vacuum pressure, or lack of it, will bring out all of the little gremlins in the engine management computer. People have reported O2 errors as well as AMM and you name it. Drivability issues will get suddenly better when that hose is plugged back in.

Klaus
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(V♂LV♂s 1995 854T, 1998 V70R)

Klaus-
Thanks for the help, but we had already routed the line to the PCV port and it made no difference. I still think that if the throttle sensor is known to be sending erratic signals, it needs to be replaced. The dealer has just about conceded that he'll have to get one.
Pegs850

I can't believe he replaced part of the position sensor and not the rest.

"The car was put on a computer while it was doing the shifting thing, and there were no codes given. But when the throttle sensor was specifically monitored, it would instantly change from 11% to 83% corresponding in time to the shifting with no change in gas pedal. Even while idling, it would similarly change and seemed to be accompanied by a slight change in engine speed.

We’ve replaced spark plugs, leads, distributor cap, and rotor. The electrical connections on the throttle sensor were found to be corroded so "the end of it was replaced" (whatever that means). None of this made any noticeable difference."

This should be a dead give-away that the entire sensor needs replacing IMHO.
In regards to your vacuum line, that's the pcv (positive crankcase ventilation) hose that runs from the bottom of the intake-to-turbo hose to the oil trap under the intake manifold. You should have it serviced, and possibly replaced. If under warranty, who's warranty? Is it the dealer? If so, is he doing the diagnosis and repair? If so, take it to another dealer.
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1998 S70 T5 Emarald Green Metallic (125K), 2004 V70 2.5T Ruby Red (45K), Previous Owner of Black '94 850 Turbo Wagon (Sold at 140K and miss her). My cars have been running so well lately they've got me worried.

I happened to come across this thread and realized that I never posted the final fix. I replaced the throttle position sensor and it has been shifting flawlessly since- about 100k miles.
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'97 188K+miles 850 grey wagon, '91 340K+ dark grey 740 wagon