1998 V70 GLT Automatic with low RPM hesitation, P0171 code, LTFT stuck at -25

NoisyBard · Post by **NoisyBard** » 12 Jul 2016, 12:21

Greetings,

As the title states I have a wagon with some performance problems. Car has 168k miles (~270k km) with a mix of highway and city driving. I have limited experience with troubleshooting fuel systems (much better at electrical), and I wasn't able to find existing threads that matched the symptoms and sequence of events I see.

A little bit of history:

Plugs, wires, and distributor all changed in last 1000 miles after being long overdue. Old plugs were worn out but looked normal with little variation between them. Recently had a reputable shop replace the oil pan o-rings due to visible bubbles in the oil and a persistent lifter tick. They replaced the PCV system and cleaned the throttle body while they were at it since all the extra vapors coming through the oil had fouled things up. Engine ran like new after they were done with no more bubbles or noise.

The car threw a code P0108 (MAP/Barometric Pressure Circuit High). It had done so once before a year ago, but the code hadn't returned since I cleared it until now. I back probed the old sensor and found good ground and 5 volt power but no output on the signal wire. There were no performance problems and the car drove normally for a couple weeks until the replacement sensor arrived. No black smoke and normal fuel economy. On the last trip before I replaced the sensor, the car fired up as always by the fourth compression (ruh-ruh-ruh-vroom!). I swapped in the new sensor, cleared the code, and the real problems began.

The car immediately became very difficult to start. The first attempt would result in cranking for 10+ seconds with no sign of ignition. There was no raw fuel smell from the exhaust. A second attempt at cranking would last longer than normal with six or seven compression strokes before it caught. Idle was unstable, threatening to stall but always catching itself. After ten to thirty seconds the idle would stabilize and everything seemed fine. The engine behaved the same whether hot or cold. When applying accelerator at low RPM the car would now surge and stumble a couple times before making power. This was most noticeable from a stop but also happened when coming off coast. Holding the accelerator and brake with RPM at 1300-1500 for a couple seconds resulted in much easier takeoffs. Once above 1500 to 2000 RPM the car seemed to run normally and has plenty of power.

Finally threw a code P0107 (MAP/Baro Circuit LOW). Back probing the new sensor I get good power and ground and 1.2 volts on the signal wire. I live very close to sea level, and I can't find any info on what a "normal" output would be. I disconnected the battery to clear the ECU but this had no effect on the symptoms and the P0107 came back quickly. I unhooked the sensor out of curiosity and the starting problem went away. Threw the P0108 code instantly, but at least I wasn't killing the starter any more.

However, the hesitation and hunting idle after startup remained. Threw a code P0171 (Fuel System Bank 1 Too Lean). My scan tool reports LTFT pegged at -25, and it remains pegged there up through 3000 RPM (I have not revved higher yet). STFT stays at +/- 1. Front O2 sensor shows the normal rich-lean variation in closed loop, and the rear O2 sensor shows a much smaller variation around 450 mV. Engine idles smoothly once the hunting idle settles down. The exhaust smells a little wet/rich, but there is no visible smoke on acceleration.

Snapping the throttle in neutral results in the engine coughing for a couple seconds before revving up. There is no visible smoke when this occurs. MAF values and RPM seem to be in sync when this occurs, showing a jittery line just above idle values followed by a rapid climb when the engine figures out I actually want it to rev.

I'm sorry for such a long post. At this point I'm getting quite frustrated since it all seemed to come out of nowhere when I installed the new sensor. Nothing else was touched under the hood between the end of the last drive with the old sensor and when I installed the new sensor and noticed the problems.

Any advice on where I should begin? I don't want to be throwing parts at it, and the whole series of events seems like gremlins to me.

Thanks,
Bard

Post by **abscate** » 12 Jul 2016, 13:38

Welcome Bard - great name in literary circles..

That sounds like a classic major induction leak that is screwing up your engine management by letting in unmetered air.

Try (engine off, key out) unplugging the MAF and then starting up and seeing how it runs, report back.

NoisyBard · Post by **NoisyBard** » 12 Jul 2016, 13:56

Thank you for the compliment and your quick reply.

Unplugged MAF and engine seems to have its pep back when in neutral. Plugged in barometer and starting problem gone as well.

Now if that points to unmetered air, my question is why did it wait until I replaced the sensor the first time and then show up? I wasn't anywhere near the air system and everything ran perfectly until I swapped the sensor in.

EDIT: Wouldn't negative fuel trim mean the ECU is trying to lean out a mixture it sees as really rich? I'd think an air leak would result in positive trim as the ECU adds fuel to account for air that slipped in behind the MAF sensor.

As RPMs increase, wouldn't the crazy fuel trim improve as a leak would make a smaller percentage of total air intake compared with idle?

ferocity02 · Post by **ferocity02** » 12 Jul 2016, 22:25

Negative LTFT means it thinks it too rich. I wonder why it's throwing a code for too lean?

NoisyBard · Post by **NoisyBard** » 12 Jul 2016, 22:41

Exactly what I'm worried about. There's no obvious miss, but there's the slight wet/rich smell in the exhaust. Maybe just coincidence that an injector started failing around the time I replaced the barometric sensor? But I'd think a leaky injector would leak down the fuel rail and make for hard starts after sitting a while. That hasn't happened and the crank/no start problem is somehow solved with the MAF unplugged.

All really strange and I may need to take it to a shop. I don't have the tools to be messing with the fuel system.

Post by **abscate** » 13 Jul 2016, 02:30

Spend some time following the air from the air cleaner to the throttle body before taking it to the shop. Look for broken clamps, soft hoses, splits, holes.

You could put your old MAP sensor back in but I'll eat my hat if that solves it. Take it as a fresh problem and forget the history while troubleshooting.

It would have been easy for the shop to have loosened induction stuff during the earlier work that popped on you now, for example

Post by **E Showell** » 17 Jul 2016, 05:29

What's status of engine coolant temperature (ECT) sensor and thermostat? Bad ECT can cause hard starting and over rich mixture.

zmaller · Post by **zmaller** » 30 Oct 2019, 23:55

Did this end up getting solved? I’m having a very similar issue with no boost leaks, negative fuel trim and a lean code.
Thanks