Wednesday, May 06, 2009

P2187, A Miata Success Story

Recently, Mia's check-engine light has been coming on intermittently. Thanks to my ScanGauge II, I've been able to read the code, P2187, and clear it to see under what circumstances it recurs. P2187, as the internet will tell you, is the OBD-II, code for "too lean at idle," or something to that effect. I poked around on* and** and found that this code could result from a number of sources, including a bad manifold-air-pressure (MAP) sensor, a leak in the intake track, a bad oxygen sensor, and so on. We are still clearing out the garage in the Grondulbarn, so, as of this morning, I hadn't taken a close look in the engine bay to further diagnose the problem. In the last two days, I've also noticed, thanks to my boost/vacuum gauge,*** that the vacuum level at idle was about 10 inches of mercury, higher than the usual 20-ish. Finally, today, I observed that the car's maximum boost seemed to be around 6 pounds per square inch, instead of the normal 8.5. It was then, on the road, that I realized that there must be a leak in the intake, somewhere in or near the manifold. That would explain all three symptoms: Air leaking in at low load, when the gauge pressure was negative, would yield a lean condition and a too-low (in terms of absolute value) vacuum, while air leaking out at high load, when the turbo was spun up, would give too little net boost.

Unfortunately, I didn't---and still don't---know much about the vacuum systems of the car. At my first opportunity, I put the hood up and went right to the only vacuum line I knew of, the small-diameter line leading off the back of the intake manifold and going to the...something, where I had T'ed off a line to the sensor for my boost gauge. As luck would have it, the 3-inch line between the manifold and the T had several cracks! So, I drove to the nearest auto-parts store, bought a few feet of the right-sized vacuum line for a few bucks, and cut and installed the line using the leatherman that lives in my trunk. Et voila, everything returned to normal! I'm Brian with the overhead projector about my parking-lot repair, as I'm sure you can tell.

Anybody need some small-diameter vacuum line? I have plenty to spare.

* The largest online community dedicated to the Miata, which has great resources in the Garage section and also has one of the best-run automotive fora I've visited.
** A forum dedicated to the 5428 Mazdaspeed Miatas produced.
*** This gauge measures the pressure in the intake manifold, so it performs, I think, essentially the same function as the built-in MAP sensor, but instead of being wired to the car's computer, it has an analog display for the driver. Interestingly, the gauge's negative range (called vacuum) is marked mm Hg, and positive range (labeled boost) is marked in PSI. I guess those are the conventional units for these two measurements---at least in the hegemony of the Imperial System---but I think it's strange to use two different units to measure opposite signs of the same physical parameter.


  1. Randall8:51 PM

    Thank you!

    To make a long story (somewhat) short, my CEL recently turned on, but hours later turned itself off. I went and read the code anyway and it came back as P2187.

    It turned on and off a few more times over the following days, only to finally turn itself on and stay on for the past few days.

    Searching around online gave me some vague ideas as to the cause of the CEL, but I was still having trouble tracking down the ultimate source. Then I found your post, located the hose that you said was giving you trouble, and sure enough, mine had a decently sized crack in it.

    I'll be replacing it tomorrow and that will hopefully cure my P2187 problems. Thanks!

  2. Randall,

    I'm glad I could help. Let me know if the repair works out.

  3. Randall7:05 PM

    Once I found the correct sized replacement vacuum line, everything else was quick and simple. I'm happy to report that my CEL turned off today after driving the car normally for a couple of days.

    Thanks again!