Long and technical story in case anyone has insight here. Have been trying to finally lay to rest lean codes since I purchased the car about a year ago. Since it isn't driven as much as the other two, this has been a 'leisurely process'. Unfortunately, my last emissions tests are now due this month, and I have a conundrum.

Had small leaks many places. Thus far, smoke leaks have been fixed by
1. replacing the corrugated air pipe and gasket
2. replacing the full load breather hose,
3. replacing the part load breather hose (and gratuitous cam cover since I broke the original investigating the breather hose),
4. replacing the throttle air hose.

I also replaced the MAF sensor because, why not?

Seemed like things were going well with fewer leaks, except, the car recently started reporting 'Restricted Performance' before I started the engine. This gave me the idea that the ECM hadn't had time to even ponder air/fuel flow problems and was similar to what happened when I disconnected the MAF sensor and turned ignition on.

So, before I smoked it again, I tried to run through the pinpoint tests for P0171 and P0174 to see if there was a wiring problem (starting p. 1475 in the Second Edition manual). Successfully went through A1-A5. On A6 (short to ground from the power line from the connector (EM20-10) in the engine management fuse box), the resistance was much smaller than required (about 1 kOhm when it should be over 10 kOhm) suggesting a short to ground.

Okay, that sucked, but I wandered the wire back to the fuse box and found nothing, but I did get crispy wiring cover all over the garage. The wiring of that line from the MAF to the fusebox also powers the air assist close valve (AACV) in the throttle body, the canister close valve (CCV), and the cooling fan relay (see below).

Oddly enough, eventually i found if I pulled the connector to the AACV, the resistance increased to about 2k (still not okay), and if I pulled the connector to the fan relay too, it increased to 39k (okay). I replaced the fan relay with one I had around and nothing changed. I did not replace the AACV since I didn't have a throttle body hanging around.

So, the 'short to ground' is in both the AACV and the fan relay? That seems unlikely to me. There's nothing in the workshop manual that suggests disconnecting the other arms of that wiring tree, so, on the other hand, I'm not clear why these 'shorts' are not issues for the pinpoint test. On the other other hand, what are the odds that there are two 'shorts' that appeared to just show up while I was repairing leaks?

Anyone have any insight here?


Portion of the wiring diagram discussed.

 

Have you replaced the two VVT solenoid O-ring seals? If not, I would do that as a next step....

 

Yes, left this one out. Gratuitous, since it did not appear to be leaking there, but since both connectors needed to be replaced...

 

What was the resistance to ground after you replaced the fan relay but with AACV still disconnected? Something closer to the expected value?

I doubt that 1kohm vs 10kohm would really make much difference, especially with actuators and relays. There amount of current lost is not going to be large enough to cause any of the devices in that tree to see a lower than expected input voltage.

An issue with the AACV does seem like a potential cause for the lean condition though. Have you noticed a difference in the fuel trims when cold vs full warm? My 4.2 car doesn't have the AACV but as I understand it's job is to give some extra air in the cold condition and if it were stuck open say, then you might get lean conditions when fully warmed up.

Does the AACV have the same tendency to get crudded up with EGR junk like the rest of the throttle body does?

 

Twas the same as before I replaced the fan relay, about 2K. The replacement had no effect. I was concerned that something was wrong with the AACV, but when the fan relay alone also caused the 'short', that suggested to me that maybe i didn't understand quite what was going on.

I don't know, but makes sense. i've tried looking into the passagways, but not particularly successfully, but nothing looks gunky. I didn't want to take it off, necessarily, since there is likely a gasket which is not available, and it doesn't appear to be leaking from the throttle body...

 

Okay, I 'bit the bullet' and took apart one of the other cars and measured the resistance from MAF power line at the fusebox to ground (Okay, I admit I didn't think of this until this evening, even with another similar car in my way every time I went back into the house). The result...

6.2 kOhm

So, still low compared with required, and the other car fuel trims are around zero under the appropriate conditions. Bottom line, I'm going back to smoking for leaks and ignoring this for now.

Unclear what the lesson I should learn here, but there surely must be one...

 

I find a smoke test is very revealing. O-rings often leak as well as the ones on the oil filler tube.
If nothing shows smoke leaking then I would close off the vacuum line to the fuel tank also as a test. Need to drive it to see if code comes back.
The MAF often times is the culprit, sometimes it is worth a shot to clean it with MAF cleaner. Then smoke test

 

Yes, they get quite leaky after 20 years. The car I'm working on probably sat in the sun for some time as well. For example, there were smoke leaks in the air pipe, junction to the throttle body, part load breather, throttle air hose, cam cover (perhaps induced by me playing around with and subsequently breaking the connection). Now there's one around the dipstick tube (almost certainly induced by me replacing the cam cover and disturbing the delicate balance between the dipstick tube gasket and its surroundings that was maintaining the vacuum for the last decade or so).

MAF here is new and has been electrically and road tested.

It will be intriguing/irritating to see what else will leak when the dipstick tube hole is properly plugged. Probably should just wrap the entire engine and vacuum system in three layers of silicone tape at this point...

 

I concur with wrapping the entire engine. So many potential air leak points on these cars given their senior-citizen ages now....

One great improvement is the Mina Gallery aluminum air duct tube. Far superior to the OEM crappy plastic air duct tube and well worth the cost (I paid $175 with free shipping in April 2019)....

 

Not that this has anything to do with the thread (sorry), but I'm always suspicious of "Performance" intake pipes. Isn't our OEM pipe "Tuned"? I thought the tuning allowed for some supercharging effect by managing the intake pulses on a non-SC engine. Or is the tuning just for sound reduction? Not sure either way, but I've never gone the "After Market Route" here.

 

I was not looking for performance improvement. I wanted build-quality improvement, and the aluminum air duct certainly delivered in that category....