For a while i've been noticing that above 50mph, when trying to coast, I get an odd and annoying problem. When I take my foot off the accelerator, the car jerks me forward, and when I press back down, it jerks me back - and i'm not talking about an abrupt and significant change in throttle, i lightly press the pedal and the car jerks & i'm pressed back into the seat, and when I remove my foot lightly the car jerks and i'm pressed forwards. It's frustrating because when i'm trying to coast at 50, i either have to put my foot on the gas and go faster than 50, or remove my foot and go slower due to engine braking, or deal with the annoying lurching back and forth to stay where I want to be.

I thought that the throttle position sensor may have the "closed" position set too far from the actual closed throttle position, so that by the time the TPS gives the signal to the injectors to begin firing again, the throttle is already a bit open so that I get an abrupt jolt from the greater air mass giving more power. Though i'm not too sure how that would explain the inverse happening when I lift off the pedal.

I noticed something yesterday which made me doubt it's entirely throttle related - the issue only occurs at/above EXACTLY 50mph, and doesn't occur at all below that - not at 49mph or lower. So that kind of throws my theories out the window.

Any ideas?

Hav you been doing the partial tranny fluid change ( every 20 or 30 K miles, I forget which) spelled out in the 700 maintenance manual?

Your mention of this happening going above and below 50mph almost certainly suggests it’s related to the lockup torque converter in the trans, which kicks in and out almost exactly at that road speed. It should be an AW71L in your ‘91. The lockup function is hydraulic and completely local inside the trans. There should be a noticeable change in revs, dropping down to a fairly fixed rpm of about 2100 rpm at that road speed, akin to the feel of kicking in/out of overdrive, but not the kind of lurching you describe. A) Check your trans fluid level -there was a recent post here on doing that, you need the engine running and preferably warm. B) Give it a thorough drain and flush -again, a recent post on that here, likely in the same thread, see also the FAQ here. C) If after a few good trips things aren’t dramatically improved then you'll probably want to think about a blockage or, less likely, something broken like a spring in the valve body, perhaps wanting to visit a good local trans shop for further advice if you're lucky enough to still have such a specialty shop in your area, and failing that your favourite, trustworthy Volvo mechanic with experience in servicing high mileage RWD Volvos. As I recall, lockup torque first appeared in 1989 with the AW72L in the 740GLE 16-valve and AW70L in the 740T, which transitioned to the AW71L being used in the later 740s and 940s (turbo only I believe until 1994 when it became standard in the NA as well).I don’t believe it appeared in any of the 240s, but I may stand to be corrected on that.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

What does the tach do at the moments of forward lurch and aft lurch?

Does it act any different if the OD is locked out?

Also, isn’t 50 mph about where a lock-up torque converted locks in? Could it be struggling with that??? (Not sure if your trans even has a LUTQ.)

The more knowledgeable ones on this forum will probably want to know what engine/trans combination you have.