New problem - bucking and stalling at odd times 96 850 Turbo

[Dave in Maine]

My beloved 96 850 Turbo sedan has picked up a new, disturbing trait. On very damp days, after running for a few minutes it will start bucking and stalling. When at idle, it just starts slowing down, then catches itself. When driving it feels like somewhere between fuel starvation and the way my first car (a 61 Olds F85 3-speed standard tranny) would buck before I got my feet and right hand coordinated and smoothed out my clutch, shift and gas technique. The current problem seems to occur more at lower RPM and idle. Adding brakes or sometimes turning the wheel will make the buck and stall more pronounced. Stepping on the brake - say to turn into a parking space - will make it worse and sometimes stall the car. If/when it stalls, put it in neutral or park and it starts right up again. When it happens while driving at a low RPM (1000-1500) a very gentle, consistent touch on the gas gets me through it. This problem will last a few minutes and then go away. Check engine light is on the vast majority of the time but will sometimes go out.

In the last year I've put a new A/C core into it, water pump, timing belt, rollers/pulleys, fuel pump relay, rear brakes, ABS module, exhaust joint(s), thermostat (after the heater didn't work at all in -20F weather - thanks to this site for confirming my suspicions there) and got the odometer running again. Best guesstimate is I'm at about 200k. I don't have a code reader (Would appreciate tips on which to buy - less expensive/Harbor Freight models preferred) so no codes to share.

Ideas?

Thanks in advance.

[sventastic]

Are there any unusual noises emanating from the engine bay?

[Dave in Maine]

Are there any unusual noises emanating from the engine bay?

Nothing out of the ordinary. It's just like it's starving for fuel and/or bucking. But no funny noises.

[WhatAmIDoing]

When I had a similar problem, new cap and rotor solved my issues. May also want to inspect wires and plugs. What fuel pump relay did you buy, Volvo or aftermarket?

[erikv11]

Vacuum leak is my first guess. Maybe the brake booster vacuum hose but check all of them for 20 year old, crumbling and/or cracked rubber. Including that PCV-related nipple on the passenger side of the intake manifold.

[Dave in Maine]

I think it was an aftermarket fuel pump relay, but I'd have to look to be sure. Only have about 20k on it, max. When the old one was going bad, the engine just cut out - no halfway bucking or anything.
My mechanic has checked the plugs and wires and assured me they were ok.
Would a vacuum leak be intermittent like this, and only in very damp weather?

[wizechatmgr]

Spray it all down with a Windex bottle full of only water in the dark... If all goes well you'll get some sparking from where she's jumping.

*DON'T PLACE YOUR HANDS ON THE CAR WHILE DOING THIS - YOU DON'T WANT TO BE THE EASIEST PATH OF RETURN*

[WhatAmIDoing]

I think it was an aftermarket fuel pump relay, but I'd have to look to be sure. Only have about 20k on it, max. When the old one was going bad, the engine just cut out - no halfway bucking or anything.
My mechanic has checked the plugs and wires and assured me they were ok.
Would a vacuum leak be intermittent like this, and only in very damp weather?

If it only happens in damp weather, it is very likely an electrical issue. When my aftermarket fuel relay went bad, it would start starving the engine for fuel and cause misfires when the relay got hot. You can try jumping the pins for the relay when it happens again, but I think the issue is likely in the ignition system.

[erikv11]

I agree about the electrical being more likely ... good thinking