1997 850 Stumped- Bad sensor? Engine cuts out

[Redbox]

Well, this problem has been haunting me for ages. 97 850 base wagon, unknown mileage. The car runs fine, starts fine, idles fine. Every so often the engine will start to shudder, with very little throttle response, then stall. Shut the car off and restart and all is fine. It has done it just idling in the driveway and usually on the road. Doesn't seem effected by weather.

I replaced the throttle position sensor and no more problem, then a few weeks later, it started again. Cleaned every connection I could find. At one point I had the hood up when it started doing this, and I touched the plug for the temperature sensor and the rpm's came up to normal and it smoothed out. Redid the connection and all was well for a while, then it started again.

I recently did the PCV rebuild, oil pan seals, cleaned throttle body, all connections cleaned, MAF cleaned, new fuel pump & filter, front struts, etc. The car has been running better than ever, then today it just died in the middle of traffic. I didn't notice the usual violent shudder this time. Had to crank it half a dozen times, then it started and ran fine the rest of the trip.

I have a CEL, but it shows my usual code for the air pump. I don't know what to try next. Crank sensor? cam sensor? It acts like the timing gets thrown off, or the mixture goes crazy. While it was running today, I unplugged the MAF sensor and not much of anything happened. When I plugged it back in, the engine quit. Does that tell me anything?

At this point I don't trust the car. Luckily so far whatever the problem is, it gets reset when you restart, but I have to find the problem.

Any suggestions would really be appreciated.

[jreed]

Interesting and frustrating symptoms!
You didn't mention replacing or bypassing (temporarily) the fuel pump relay... this has caused stalling and no-starts for many others and if you haven't inspected your relay for cracked solder joints, it doesn't take long and would be a good idea.
A high resistance connection at or inside the engine coolant temperature sensor could make the engine computer think that the engine is cooler than it really is, causing the computer to inject more fuel than needed which could cause it to run rich and rough.

[Redbox]

Good thinking on that, but I did fix the bad solder joints in the fuel pump relay some time back (maybe I should check it again). The coolant temp sensor thought must be on the right track, as some sensor must be sending bad info for some reason. I even took the ignition switch apart and cleaned all the contacts the other day. I'm assuming that any short in the wiring to any of these sensors could cause problems like this?

[Redbox]

Well, I ordered a new coolant temp sensor just to throw parts at it. Cleaned the contacts for the crank and cam sensors, plus the TPS. Tested the crank sensor and the readings were fine. I saw someone on some board say that he had replaced the TPS many times. I can't imagine that. Anyone have experience with them going bad over and over? I did replace the original with a Bosch part.
I don't understand how a car can shudder and die, then be hard to start, but not leave any codes on the scanner.

[erikv11]

TPS failure is uncommon, I have had only one go bad on me. Replaced with Bosch, no problems after that.

Did you check all of the grounds?

Seems to be going around lately, here is another one https://www.matthewsvolvosite.com/forums ... =1&t=69758

For the MAF test, you don't really have a report yet. You want to unplug the MAF with the car off, then drive it for a while. If the problem never comes back, then the MAF is bad. Best to not plug/unplug the MAF while the car is running, that will confuse the ECU.

[Redbox]

Thanks for the feedback
One of the first things I did when I got the car was go through every ground I could find, but it has been a couple years, so maybe I'll give them another check while I'm waiting for parts.