P0171, Getting Closer to the Problem

whirligig · Post by **whirligig** » 11 Mar 2016, 20:43

This P0171 problem on Moriticia, 98 V70, has been plaguing me for months. Earlier this week, I replaced the PCV system and now no smoke is puffing out of the dip stick hole but the P0171 problem is still there. Replaced the O2 sensors yesterday and it's still no cigars. Finally I decided to monitor and log the fuel trim using a bluetooth OBD II adapter and a tablet equipped with a free version of Torque. Yep, at idle the short and long term fuel trims are in the negative meaning too much air in the exhaust and all the ECM will do is dump more gas at the problem. I even dumped the logs into Excel and charted it. The fuel trims were mostly in the negative. But, at slow cruise at 1500 rpm, the fuel trims were normal in fact beautiful, +/- a couple of percent. So, there's a vacuum leak someplace but where can it be? Out with the carb cleaner and popping off the fuel injector cover, I sprayed around the injectors, then the intake flange, and all the vacuum hoses around the manifold but the rpm stayed steady around 840 to 870. No leaks here. Out of frustration, I decided pop off a vacuum hose and create a real leak and spray this here cleaner into it to see if this old mechanics trick really work. It did, the motor stumbled and sped up.

This afternoon, I had a rare but bright idea to plug up the vacuum hose to the evap system and see if my fuel trim improve. If that lead to nothing, then the hose to the EGR. Sure enough, after a few miles of driving, the fuel trims at idle became less negative and started bouncing around +/- 2 or 3% as it should be. This looks promising. I checked this new purge valve that's been in the car since summer to see if it holds vacuum using my trusty MityVac. It held with no problem. I hooked up the valve to a variable power supply and turned up the power until the valve broke suction. That took 8.2 volts drawing 0.25 amps. I took my old purge valve and ran the same tests and it held suction just as well but broke suction at 7 volts at 0.35 amps. I then measure what was coming out of the harness with the car idling and the voltage was bouncing around between the low 6's to 6.9 volts. Bumping the throttle didn't change anything. With voltages so close to the purge valve's, I don't think the valve will fully close leaving me with a constant vacuum leak. Here are my questions:

1. What's the spec voltage for opening the solenoid on a Pierpont purge control valve?
2. What's the typical ECM output to the purge control valve?

With all this said and done, the car has been running just fine. Gas mileage was never very good except on a rare long trip. And, I passed smog just fine recently. Time to look at the wiring diagram.... Any tips, info, or ideas are welcomed.

Post by **chrafael** » 12 Mar 2016, 07:23

The information I have says battery voltage at terminal #A39(+) and A42(signal ground-) with the key on. Duty cycle is 0-33%. You may need a meter that reads duty cycle/frequency. When the water recedes around here I'll go and check mine and let you know what it reads.

Post by **chrafael** » 12 Mar 2016, 11:37

Here goes. Key on engine cold negligible voltage. Engine running, cold 6.5 give or take. Engine running full temp 7.5 to 8.5 volts about 28 cycles per second. What seemed more important was that it was pulsing on and off at about once a second. I could put my finger on it and feel it working, not always on or off, but as demanded by the ECM. Hope this helps.

whirligig · Post by **whirligig** » 12 Mar 2016, 23:49

: Scope probe on ECM's A39(+) and A42(- or signal ground). Looks like duty cycle is about 27% at -12 V. The period for each pulse was 130 millisec and the pulse is 35 millisec. Scope is at 5v/division and time is at 50 millsec/division.

: Scope probes were on ECM's A38(+) and fusebox 11B/5 (-). Duty cycle about 10% and voltage is about -12v. Scope is at 10v/division and time is at 50 millisec/division.

All tests were with the motor idling. What I didn't do was record the quiescent voltage to see where it's at when the signal is not pulsing. I have more logs to review from yesterday and today. Today I took a 30 mile trip and had the vacuum, short (STFT) and long term fuel trims (LTFT) recorded. At idle, the vacuum is between 18 and 19 inches of Hg with deceleration peaking at 25 inches. That doesn't seem bad at all but I need to check this against a real mechanical vacuum gauge. What's scary to me while driving on the freeway is watching the STFT wildly dance around while the vacuum is holding steady. Could this be dirty fuel injectors? Yeah, the check engine came on today. Didn't even bother to check it as I cleared it.

One thing I did forget to do is vacuum check all vacuum valves e.g. the one in the airbox and the EGR. Something to do for tomorrow....

Post by **abscate** » 13 Mar 2016, 06:36

Is the purge valve supposed to be variable duty valve? Unlike the turbo boost control valve, it's function is really just open and shut. I don't see an engineering reason to control it even at low frequency except for the test cycle, maybe.

That scope cal is traceable to NIST, right?? We don't tolerate uncalibrated tools here on MVS

Post by **chrafael** » 13 Mar 2016, 07:34

My highly calibrated finger said CLICK,CLICK, CLICK at idle and when I revved the engine it went to a faster, finer buzz. Felt variable to me.

Post by **abscate** » 13 Mar 2016, 12:17

Interesting. My Scanner reports purge valve as either open or shut, and I think VIDA does the same. Ive always noticed it shuts at idle, which makes sense, as you don't want excess unmetered air into the manifold at idle. Once the engine revs, it is less important.

Why it would open at low frequency rather than DC is beyond me.

whirligig · Post by **whirligig** » 13 Mar 2016, 15:25

abscate wrote:Interesting. My Scanner reports purge valve as either open or shut, and I think VIDA does the same. Ive always noticed it shuts at idle, which makes sense, as you don't want excess unmetered air into the manifold at idle. Once the engine revs, it is less important.

Why it would open at low frequency rather than DC is beyond me.

What kind of scanner are you using? That would be handy to see when the valve should open and close. It would give me a clue if it is sticking.

Post by **chrafael** » 13 Mar 2016, 17:48

Go into VIDA to product specification/design and function/section 28 ignition and control systems/evaporative emission (EVAP) system control/Control, which is the last topic on the page. Bottom line for people without VIDA is yes, it does vary according to engine demand.

whirligig · Post by **whirligig** » 14 Mar 2016, 21:45

I was at the VIDA site this morning. It looks familiar. I think I've downloaded some free wiring manuals from there long ago. Is registration required now to get any kind of info?

I hooked up the purge valve for now. I tested the other vacuum devices, EGR, air box's hot air door and its check valve. The EGR held vacuum just fine. The check valve did not. It was gummed up and so took I it apart and cleaned it. Got it working intermittently but I hated this whole hot air system. It's overly complicated and I don't believe it works. The bimetal (I think) spring overides the actuator that it keeps the flap open to hot exhaust air all the time I think. I decided to plug the white vacuum line at the check valve and propped the flap open to air from the snorkel. Vacuum at idle seemed to improve but I encountered P0171 twice today usually in about 10 miles of driving in both case

By the way, I forgot to mention I did check the spark plugs last week and they look just fine. A slight gray to light tan on all of them. Would that rule out fuel injectors? If so, this would leave the EVAP system which I have had problems before i.e. P0455 with the J-hoses. The charcoal canister and solenoid valve are original. I tested the valve over the summer and works fine however I never checked whether there's a signal on the wires.

I may just have to break down and register and pay for some info on VIDA and get the computer interface or cry uncle and take to my mechanic.