September 28, 2023

Code 16 part Deux!

Continuing … Part 2 – Code 16

The next thing I did a week or so later was to again remove the EGR valve and clean the openings. I also removed the throttle body so I could clean the upper intake manifold manually ensuring I cleaned the little EGR hole.

While I had the throttle body off I noticed some brown muddy crud in the cooling lines of the IAC valve on the bottom of the throttle body. I tried to clean that off and cleaned the air tunnel of the throttle body before reassembly.

I used some pipe cleaners (now called fuzzy sticks at some stores) twisted together to make them thicker to clean the openings at the back of the exhaust manifold where the EGR attaches. There are two small holes, upper and lower.

The fuzzy sticks aren’t long enough to go all the way through to the other end (there is a bend in the pipe). I also cleaned the small opening inside the front part of the intake manifold with the fuzzy sticks now that I had the throttle body off. To make sure the passage was clear all the way I got some soft copper wire that I was able to pass the full length of the EGR tube.

When I reassembled I noticed I now had a rough idle and bouncing tach needle. I would idle at 1200 to 1500 RPM on start up and then settle to 1100 RPM. Clearly something bad had happened. I was able to do a couple of test drives ok and still had the dread code 16 too. Great, now I had two issues instead of one and the second was caused by me.

Studying led me to the IAC valve.  The IAC is attached to the bottom of the throttle body and it is the part that had the brown muddy crap I attempted to clean off.  The brown muddy crap was on the two coolant lines.  Essentially, the coolant lines are there to run antifreeze through the valve to warm the air flowing through the valve sufficiently in really cold weather. When the valve cannot read the air temp you get what I got… rough idle and variable idle which will eventually get worse.

Perhaps I had jammed some of the crud back up inside it?  I took the throttle body back off … again … the next weekend and checked it out. It all looked o.k. but clearly it was not.  There are some screws on the IAC that can separate it from the throttle body but it seems to be the consensus that it is a bad idea to try this as the screws are super soft and would likely to be destroyed in the process.

I decided I need professional help (I heard that before…).  I took it to the Mazda dealer for a diag and a clue.  I needed to know what the heck was causing engine code 16.  I already knew what was causing the rough idle. To recap,  I had replaced the EGR and cleaned the intake a few times.  I had also replaced all the vacuum lines and still the EGR code 16 was intermittent.

The shop determined my EGR solenoids were not opening as they should.  They  didn’t charge me for the diagnosis.  In return I ordered the solenoids from them for $170 with tax (ewww).  Parts arrived a few days later and I installed them right away.

Old Funky Solenoid


New Solenoid install – New vacuum line!

Connected all the lines back, now let’s get the idle issue done.


A new IAC is $300-$400-ish and since it is a considered a bad idea to take one off the throttle body it is recommended to buy them already together. The main technician at the Mazda dealer said the same thing.  That is bigger money that I want to spend on new parts until I clear this CEL code 16. I hunted for a known good used one. I found one almost immediately at The Parts Group Website (no longer in business).  Tom had both pieces together from a known good engine for $80. I would need to reuse my Throttle Position Sensor from my old unit. No problem!  It sounded straight forward enough.

I did the change-a-roo and put it all back together.

The old throttle body was cleaned really well when I had it off previously, look at how clean that neck looks inside (…as a whistle!)

I needed to take it off again and remove the TPS (throttle position sensor).


Below are both units.  The top unit is the one I purchased used from The Parts Group and mine is on the one on paper towel catching the antifreeze coming out of the IAC.  On the lower unit the black piece on the left side in the middle is the TPS that I need to move to the top one.


The trick before moving the TPS is to mark where the screws are on the plastic screw hole slots.  This is so you can put it back on and do very little adjusting. Adjusting is done by rotating the sensor left or right ever so slightly and tightening the screws and testing, possibly repeating.  That is why there are slotted screw holes on the sensor.  As luck would have it my markings were good and I was able to attach the TPS and do one little adjustment!!! HOORAY!!!

Back together again!! I did have to rob the throttle stop screw from my old unit, the purchased part didn’t have it for some reason.  Also, the butterfly was adjusted all wrong and wasn’t closing all the way on the throttle body.  A few minutes adjusting the stop screw cleared that up.


Interesting Note:  The screw had a little dab of paint on the threads. The reason is that once the screw is set where you want it engine vibrations can cause the screw to move. Putting a dab of paint where it meets the hole will dry and act as a kind of glue with just enough stick to keep the screw from unintentionally moving yet still easily removable.

I started the engine and no more rough idle. One little tweak on the TPS position and the idle was a sweet 850-900 rpm!

So what about the code 16?   It came again back days later.   :/

…Next… My car has lost its friggin’ mind!!!


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