Rotten fuel economy on my 96' 850 Topic is solved

sreynolds · Post by **sreynolds** » 05 Mar 2005, 14:41

Well ... the tires were not changed during the drift in fuel economy, so that's probably not the problem. The Volvo shop here said that they performed a check on the O2 sensors while the car was running, but I really am skeptical that they were looking at the sensors themselves (probably just checked the computer while the engine was running). I live in an area where the mechanics aren't really equiped to hunt down problems like this. For instance, I would like to check the back pressure on the exhaust system to see if there is excessive pressure or not ... I don't know how to do that myself, but none of the mechanics arournd here know either. Nobody can tell me if the EGR is working correctly, just that it doesn't return any codes. It seems like you could somehow take a voltage reading on the wires from the O2 sensors to tell if they're okay, but people look at me cross-eyed when I suggest it. And then there's the possible issue that the computer could be bad, but is acting like it's okay. I suppose if I had an infinite amount of money, I could find the problem by trial and error ... just replace everything until the problem goes away.

Post by **matthew1** » 05 Mar 2005, 14:56

http://www.shotimes.com/SHO3badO2sensor.html

Guest · Post by **Guest** » 06 Mar 2005, 15:54

I had the same problem of poor fuel economy but I had had a code for the front sensor. Changed but MPG dropped and NO code. Solved by replacing the MAP sensor $170 The O2 sensors were able to clean up the exhaust and didn't trigger a code .

Guest · Post by **Guest** » 06 Mar 2005, 20:32

What is the MAP sensor? Is that really the Air Mass Meter (AMM) located on the snorkel between the air filter and the throttle body? Last year, the AMM went bad during a trip and I thought the car was going to blow up. Bought a new one and it ran great again.

Tonight, I measured the resistance on the temperature sensor with the engine warm. It read 400 ohms. I disconnected the two connector in-line socket whose wires led to what looked like a sensor in the thermostat housing. Sounds like this resistance may be low???

sreynolds · Post by **sreynolds** » 07 Mar 2005, 06:46

This morning I measured the resistance on the temperature sensor. At an outside temperature and block temperature of 40F, the resistance was 5500 ohms (engine cold overnight). It measured 400 ohms when the engine was hot. Are these values good ... my temp guage seems to work ok.

sreynolds · Post by **sreynolds** » 07 Mar 2005, 22:14

This is in response to Matthew1 who posted the procedure to test an O2 sensor. Thank you for the detailed procedure! The following are the results from the test:
1) When heating the upstream sensor with a blow torch, the voltage passes 0.6 volts in less than 20 seconds (in fact less than 10 seconds) - Passes test #1.
2) The voltage drops almost instantly to less than 0.1 volts when the flame is removed - Passes test #2.
3) On the two minute heat test, the voltage peaks at 0.95 volts briefly (10-15 seconds) then slowly decreases to about 0.82 volts - The third test says that the voltage should not change, but it does - Is this a fail?????

Further, the voltage will never go back up to the 0.95 volts unless cooled for about 15 to 20 minutes. Over the two minutes, the voltage may flicker up to 0.86 volts MAX, but rarely.

Can anyone speculate if I have a good or bad O2 sensor? Do I also need to test the downstream sensor?

The heater wires showed a 3 ohm resistance which is good.

Post by **matthew1** » 07 Mar 2005, 22:49

I can't help with the electrical readings, but I can tell you my sensor was the rear sensor, and I had poor economy until I changed it.

sreynolds · Post by **sreynolds** » 13 Mar 2005, 21:46

For those of you following the fuel economy problems that I have had, the saga ended last week. Took the car into Volvo again, and they said that their scanner didn't show anything that would cut my mileage by more than a third. But, they did say that my air pump was not working and would require repair before the next inspection. They indicated that this shouldn't affect my mileage at all, and I think that I agree. I pulled the pump to find it partly filled with water (frozen). So, I melted the water, emptied it out, did some small repairs on the wires that were damaged by mechanics, and reinstalled. It is now functioning fine. I don't think that this was the cause of the mileage problem, but it saved me more than $300. I am only posting this portion in case others run into failed air pump problems that could be easily repaired. I think the water got into the pump because the car sat for a long duration while being repaired.

Mileage problem solution:

I bought a can of electrical contact cleaner from our local electronics parts store. This cleaner is made exclusively for electrical switches and other types of electrical contacts. If anyone else out there buys the same stuff, make sure it DOES NOT HAVE LUBRICANT in it. I carefully sprayed gobbs of the stuff on my mass air flow (MAF) sensor, even though it was not returning a code and it looked fairly clean. There were a few spots of a dried "mud" looking substance on the channel around the sensor, but I couldn't see any on the sensor itself. When the cleaning was complete, the car ran like it was new and the mileage (with my lead-foot wife driving) went up to 25.1 MPG on the hiway. Moral of the story: the MAF can have profiound effects on performance and mileage. One can of contact spray can save the day.

Guest · Post by **Guest** » 14 Mar 2005, 12:11

I just posted a question on brickboard.com.....I asked exactly what you discussed in this thread.

I am going to give the MAF a look today. I hope that's all it is.

Glad you figured it out.

Rotten fuel economy on my 96' 850 Topic is solved

Tried everything