What the heck is engine code 16?

Ever since the day I brought the 1994 Miata home there had been nag lights on the dash.  One was infamous in the Miata community, the AIR BAG flashing light.  The usual cause for this was a depleted battery within the computer (ECU or PCM depending on who you ask), it’s da brainz yo.   Standard procedure is to just leave that light flashing, it is official… trust me.  It comes on after startup engine diagnostics and just flashes happily.  So be it, flash on… flash on!

After completing the brake job and clearing all the rubbish out of the intake tube we took the test drive.  It was running and it now could stop well too.  About a quarter way through the test drive a new engine code appeared. Checking the code it was an engine code 16.  Way more about this evil code over here if you just have to know:

https://forum.miata.net/vb/showthread.php?t=460735

These older Miatas were built before OBDII diagnostics, which appeared around the 1996 model year,  so I had to count flashing CEL light blinks.  One flash, then a long pause, then six more flashes.  Indeed, engine code was 16.  Verified this by shorting the Ground and 10 pin on the diagnostic plug under the hood to force diagnostics to flash any stored fault codes. Yep, code 16, confirmed.

Engine code 16 pointed to EGR valve issue. It was winter now so I decided to “chill… heh… ya…” until spring.  In the spring PK and I revisited this engine code. We removed the old EGR valve. The diaphragm inside appeared to move o.k. with only a very slight stick once in awhile. We cleaned it up and put it back on the car and did a test drive.

The EGR valve sits at the back of the intake. You can barely see it in the image over there Circled in RED. >

Again during the test drive the code 16 was there when decelerating and would disappear when accelerating.  It was decided to get a new EGR valve to put on the car.  A couple weeks later I had the New EGR and gakset ($120) , installed it and test drove it.  The CEL light was still present but intermittent now.  Further study is needed.

We proceeded to other issues in the mean time. The next bit would be replacing the valve cover gasket which leaked near the back on every used Miata I examined during the time I had been looking for this one.   Since the plugs and wires had to come off they are scheduled for replacement during the valve cover work.  The valve cover was,  cleaned,  new gasket ($19) ,  Plugs ($15.00),  Wires ($40).  Our parts place was out of NGK plugs or wires so we got the regular stuff. boo  🙁

Once complete I decided it was time to drive it home and continue the engine code 16 troubleshooting there. Home is about an hour away. During the research time I continued to drive the car once every couple of weeks but did not accomplish much rest of the year.  The next year I got back on this project.  I removed the new EGR valve,  cleaned the passages in the valve and put it back on, reset the codes and proceeded to drive the car.  It was 2 weeks before the code 16 showed up again.  GRRRR!

There are some references on Miata.net that the intake manifold has a small passage near where the throttle body connects that feeds some gases to the EGR. This passage can accumulate a bit of carbon in the opening restricting the flow of the gases and cause he code 16.  Not knowing if this had been cleaned before me or not I figured it couldn’t hurt to do it.

The easiest way was to Seafoam it via the brake booster vacuum line.  Essentially this means unhooking the vacuum line from the brake booster while the car is running and letting the vacuum pull some Seafoam into the intake. I couldn’t keep the car running when I unhooked the vacuum line from he brake booster so I enlisted some help to press the accelerator a little to keep the idle up so I could unhook the line without the car choking out. (Thank ya Chev for the assist).   It only takes about half a cup of Seafoam to do this.  The vacuum from the booster sucked up the fluid with little trouble.  Just had to place the line within an inch of the fluid for it to suck it in a little at a time so the engine didn’t quit.  There was a little smoke starting to come from the exhaust as deposits burned off some. Not a lot of smoke, I had read that some people had massive billowing smoke so I was expecting a show! Hmmm, perhaps the intake isn’t too dirty after all.

Once all the fluid was pulled into the intake I let the engine choke out and stop. Directions say wait 15 minutes and go for a drive. Fifteen minutes later I went for a drive. I had some smoke initially, very light, not much but the car ran like a new machine. It had pep, It had gusto, and it wanted to go. So go we did! I hit the interstate to let the Seafoam burn off. No engine code 16 either. But a day later it was back, the dreaded code 16.  GRRRRR!  Again!

 A week later I decided to do a deeper Seafoam clean again from a different vacuum line. The brake booster line connects to the rear of the upper intake manifold.  In my mind I was doubting it cleaned the front part where that tiny EGR hole is.  This time I induced the Seafoam via the extra vacuum hole near the top front of the intake manifold. It had a rubber plug over it I removed and attached a vacuum line and fed the Seafoam in.

<Circled in red again.

This produced some more light smoke. I used the correct amount this time and let it sit 15 minutes again. I again took it on the drive and again it ran great! This time it lasted for a week before the dreaded code 16 came back.

I have noticed a correlation that it happens more when it’s warm outside. In the cold weather I really have to work the car to get the CEL code 16.

I had to do more research and it would be a couple weeks before I could work on this again.

Several weeks later I took the EGR valve off again and removed the throttle body to really dig in.

…Check out Part Deux for the continuing Saga….