Thank You Z31 Performance

This month marks one year since becoming a member of the Z31 family when my 16 year old son brought home his first car, a 1987 NA/AT base model with relatively low mileage relative to its age (113K) for which he paid the whopping sum of $1,000 of his own hard-earned money. I have been a gearhead/DIY/shadetree mechanic since I was a teenager starting with dirtbikes and progressing to a number of different vehicles of all varieties. I consider myself fairly mechanically inclined and have tackled numerous projects over the years however I was completely unfamiliar with this car and all of its subtle nuances. When my son bought it a year ago, the car started, ran and moved under its own power - just barely. After performing the usual maintenance items one would expect on a 30 year old $1,000 car (oil change, brakes, plugs, distributor rotor/cap/wires tires, decarb throttle body, ignition timing etc.) we embarked on what neither of us could have ever imagined would be a year long quest for basic driveability which has only very recently been achieved. Though at times infuriating, I have grown to like, even love this quirky little car. Without the help of this online forum (by far the best of several out there devoted to the Z series) it would have been an even longer and more arduous undertaking to come to some understanding of the byzantine array of sensors and actuators that control idle mixture and other engine control functions. By way of "giving back" I would like to share some of our experience in hopes that it may help others.

Symptoms included: Hard starting, erratic idle, running rich, intermittent but regular misfire, power loss. Judging by the numerous posts, these maladies seem to be quite common with this vehicle and very difficult to nail down due to overlap amongst the various sensors and components. Much very insightful and valuable information has already been written here on these little doodads, my intent is to add what I have learned and wish I would have known before purchasing and installing the following parts:

Cylinder Head Temperature Sensor (CHTS): Prime suspect for rich running/rough idle woes but check its resistance against FSM specs with an ohmmeter BEFORE attempting to remove and replace what may very well be a functioning sensor. What should have been a relatively simple and straightforward job turned into a days-long ordeal when the old sensor SNAPPED OFF FLUSH WITH THE HEAD upon attempting to remove it with a socket wrench. I suspect this sensor was functioning quite well and the problem was entirely within its wiring harness which was chafed to bare conductor in several places and sported brittle connectors which contacts were corroded to a Christmas tree shade of green.

Throttle Position Sensor (TPS): This is a simple on-off switch, it is not a potentiometer as in most other vehicles very little can go wrong with it. Test for continuity per FSM it turns on at idle and off immediately upon throttle opening. That's it. Save yourself $70.

Airflow Meter (AFM): Often blamed for many of these symptoms but is not implicated in a rough idle condition. The Prior owner installed a reman unit immediately prior to pawning the car off to some sucker er, putting it up for sale, obviously under the belief the AFM was causing the rough idle and other problems. It wasn't. This is a very expensive part, even as a reman item.

Fuel Pressure Regulator (FPR): This was the prime cause of the rich running condition. The rubber diaphragm had disintegrated from being bathed in gasoline and underhood heat for 30 years and allowed the engine to draw untold quantities of un-metered fuel through the FPR vacuum actuation line. With the car running, detach the vacuum hose from the port at the top of the FPR, you should not smell, much less see gasoline, much much less unleash a geyser.

Air Regulator: This device simply admits extra air for a high idle on cold start. It is an expensive (over $100) part. Test it by putting in the freezer for a couple hours, at ambient temps the shutter valve should be completely closed or nearly so and fully open when cold.

Suspected Vacuum Leaks: PCV is cheap and easy enough to replace, EGR operation can be checked by actuating the diaphragm by hand. Though the car gave symptoms of vacuum leaks (rough idle, stumble/rough transition from idle to open throttle) it would idle reasonably, sometimes better, in gear and under load from stalling the torque converter than not.

ECM diagnostics: Definitely do this first, however the last item we replaced which turned out to be the missing piece of the puzzle that brought it to 100% and restored "like new" performance DID NOT generate a code hence it was among the last items considered which brings us to

Crank Angle Sensor (CAS): Though replacing the above parts definitely made incremental improvements (especially the leaking FPR) performance nevertheless remained sub-optimal. Replacing the CAS completely resolved the intermittent erratic idle/hesitation/stumble/misfire/higher RPM cutout issues but, as stated previously, did not throw a code. I wish I would have replaced this one at the outset.

Thank you Z31 Performance community, hopefully this is as helpful to someone out there as your posts were to us.