98 S70 T5 Intermittent Hesitation / Rich condition

Scholz · Post by **Scholz** » 24 Jul 2015, 20:58

Hi there, my beautiful 98 s70 t5 has this nagging and annoying intermittent issue where all of a sudden the car gets very rich, the ecu dumps fuel off the map to compensate and i end up with a large negative LTFT (-15-20%). The car will have a slightly noticeable rough idle and will surge with no power upon acceleration. Once it gets going however it will run fine and the ecu will slowly correct the LTFT while the STFT is very high. The problem seems to get worse at idle and will sometimes briefly shutter for the most minute of moments like a single misfire. I don't suspect vacuum leaks, (unless the hose that travels from the front of the car to the Cabin air filter maybe? on the passenger side of engine is metered air, there's a small crack in that hose.) I did the PCV service myself 5 months ago and replaced a few worn vacuum lines myself in the process, cleaned the IACV/Throttle Body and have fresh OEM plugs/wires/air filter installed. Pulled the plugs last night and there's no evidence of fouling, nor are the plugs wet, or anything out of the ordinary to lead me to believe of head gasket issues etc. Car pulls great and has amazing power once it gets going.

Fuel pump died a month ago, so that has been replaced along with the filter and the pressure looks good at the rail.

Using the torque app on my phone i have been watching the MAF values at idle along with both o2 sensor voltages. MAF values seem to fluctuate (very slowly, like over a period of 10-20 seconds) from 3.8-4.6 grams/sec. The front o2 sensor seems to oscillate as it should but it does have periods of hanging on a particular voltage.

I cleaned the MAF to no discernible effect. Unplugging the MAF initially does nothing to the idle but after 5-10 seconds the car starts running even richer and begins roughen out a bit.

The only code I've had since this problem began surfacing two months ago was last weekend I sprung a code for the Manifold Absolute pressure sensor, which as i understand it is actually the barometric sensor near the passenger headlight. I cleaned it with MAF cleaner, cleared the code and it has not returned. I live in Portland, Oregon near sea level so as I understand it, this sensor has no bearing on my fuel map.

I apologize for the long windedness of the post, attempting to narrow down a possible list of culprits before i go throwing my limited funds at it. Thanks again!

Post by **abscate** » 25 Jul 2015, 03:24

I would definitely fix that vacuum leak. Isn't that the Fuel pressure regulator vacuum hose?

Post by **erikv11** » 25 Jul 2015, 08:21

+1, first step is vacuum leaks, you are on that case which is great. They can be elusive, I would keep checking. There is no hose to the cabin filter, not sure what you are talking about, sounds like perhaps you mean the hose to the ECU box? If it is a fat plastic hose then no it is not metered, you could post a pic to be clear.

The baro sensor readings are used by the ECU to compensate the air:fuel mixture at high altitude. I don;t know much more about it than that, but I have indeed read of them going flaky and causing rich/lean issues. Since they rarely go bad, and since the car gave a baro code, I would get a replacement from a junkyard and swap it in there. New they are super expensive, but from the yard it should be pretty cheap.

Scholz · Post by **Scholz** » 25 Jul 2015, 09:22

Thank you for your replies, after more investigation Erikv11 you are correct this is the big plastic hose that feeds into the ECU, which I mistakenly thought to be the cabin air filter. Is the barometric sensor the same on all p80's? Or do I need to look for pre 99's?

Scholz · Post by **Scholz** » 25 Jul 2015, 12:45

Had a look through the local junkyard and was unable to locate a barometric sensor. The few S/V70's they had were missing that component and the 850's had the connector but it plugged into a dummy socket. I may end up ordering new it seems. I did however see a 1998 V70 T-5 with a manual that looked interesting.

Post by **abscate** » 25 Jul 2015, 12:49

1999 turbo MAP sensor is certainly different. That actually senses the induction tube pressure post turbo

Scholz · Post by **Scholz** » 25 Jul 2015, 23:41

So I sat in the car for a good 10 minutes or so at idle this evening. After she went into closed loop the oxygen sensors behaved as I would expect them to. Upstream was oscillating nicely between .3 and .7v or so and the downstream was switching at a far slower rate hovering around .4 to .5v. Long term fuel trim was around -4% and the short term looked good too. However, once I started driving around town the upstream seemed to switch slower and the downstream started jumping around the map.

When I returned home and parked her, both oxygen sensors continued to act strangely. Upstream was switching slower, but worse yet possibly the downstream was oscillating around .8v.

I should note I did not notice the intermittent issues nor did the fuel trims change at all. Does this indicate perhaps sluggish oxygen sensors and/or worse yet a catalytic converter issue as well?

Scholz · Post by **Scholz** » 31 Jul 2015, 18:45

Replaced the MAF two days ago with a new Bosch unit. Seems to have solved the problem almost entirely. The readings on the new unit seem to be a tad lower than the old unit in terms of grams per second but it was tough to diagnose I guess because the old unit wasn't sending erratic information, just slightly incorrect.

Thanks for your help, time to continue on the path to stage zero. Not much left to do on that for me aside from Oxygen sensors. Maybe the upgraded Ignition coil and TCV from IPD perhaps?