1992 740 W/Regina intermittent power loss

Hi all, I'll try to make this as short as possible. Thanks in advance for any and all suggestions.

Problem description:
-Car starts and runs great when cold
-Runs fine at temp, until it doesn't.
-Car will stop producing power, but not stall. If on highway, it feels like you took your foot off the gas
-Tach still operates normally
-Car will idle a little rough at ~500 rpm, stepping and holding the gas pedal will not produce anything above 1k rpm in neutral
-Issue has happened at highway speed and leaving a stop light.
-Shutting off car and restarting it will cure the issues for ~10-20 seconds. Am able to limp off the highway shutting it off and restarting, using the normal power it produces in between.

The first time it happened it threw two codes, 1-4-3, knock sensor wire or signal faulty and 2-2-1, oxygen sensor not operating. Knock sensor was replaced but the issue remains.

When the issue was taking place, pinching off the vacuum tube to the MAP sensor would restore the idle temporarily.

Current theories:
-Knock sensor retards ignition fully
-MAP sensor intermittent
-O2 sensor intermittent
-Fuel pump intermittent (fuel tank is 3/4 full, filter replaced last year)

As these things go, now that the car is home I can't reproduce the problem. Any experience or testing/troubleshooting advice would be appreciated. If someone has suggestions for trying to recreate the issue I'd love to try. Since it runs great until it doesn't, I don't want to replace everything only to be left stranded when the issue pops up again.

Thanks all

Sounds identical to the symptoms I experienced with a 92 940 Rex/Regina in which the fuel pump brush shunt broke, forcing the pump's current to flow through and overheat the brush spring. The fuel pressure would vary according to the spring making contact with the brush.



http://cleanflametrap.com/ReginaFuelPump/

Loss of power? You need a turbo.

Problem solved.

If you have the shrader valve on the fuel rail. Which a Regina car should have. Connect a fuel pressure gauge and drive around the block or locally until you get the problem. If the fuel pressure is bad it's the problem. If it's good then move on to the other possible issues.

BTW You can also check for codes with that system. I think brickboard will have the procedure or the UK volvo club.

Ok, good news. I was driving around the neighborhood and was able to reproduce the problem. I believe its the oxygen sensor.
Per brickboard faq:
Black (connected w/engine running) should cycle rapidly between 0.1V and 1.2V
Black (disconnected w/engine running) should after a while rise to 1.2V

Disconnected it sat at 1.5. Connected it would sit at 1.5 and slowly drop to .8 and stay there. Most telling is that when it was acting up/unable to rev, disconnecting the black signal wire from the oxygen sensor restored normal operation. Final verification will be tomorrow when I think it will be running correctly and verifying the proper voltages.

Is there any chance I'm missing something here by blaming the oxygen sensor? Its original as far as I know, with 263K miles.

No, an o2 sensor that old should have been replaced about 100k miles ago. Even if it isn't the main problem. Your sensor isn't at optimal service life.
BTW I would make sure you get the correct sensor for Regina. It isn't the same as one used for a Bosch system.

dl242gt said:

No, an o2 sensor that old should have been replaced about 100k miles ago. Even if it isn't the main problem. Your sensor isn't at optimal service life.
BTW I would make sure you get the correct sensor for Regina. It isn't the same as one used for a Bosch system.

Click to expand...

I agree. Though I'm too lazy to read the "FAQs" being referenced, I understand the Regina's sensor to be resistive, so some suggestion to look at its voltage open circuit (disconnected) is going to be misleading.

Also, I don't believe the symptoms deserve to be attributed to an oxygen sensor failure.

Was there ever any resolution on this I'm having a similar issue?

I replaced the o2 sensor and haven't had the issue since, probably 3k miles including a 400 mile road trip. I ordered a replacement fuel pump and still have it sitting in the box for if/when it fails. I will still occasionally get a fault code for the knock sensor wiring so its possible that was a culprit as well but after the o2 replacement I no longer worry about a roadside breakdown.

Sorry for the lack of an update, if you get your issue fixed post the solution in here as well. Maybe this thread will save the next guy with this problem.

Alex