CarmonColvin

Last weekend I finally got my rebuilt engine running in my '91. At first I fought a terrible idle problem that caused by the midlde intake manifold not seating properly against the lower intake manifold because of the use of a different year model rear side housing. The rear housing I used in the rebuild was different in the area around the top clutch inspection plate. A few minutes with a die-grinder had it clearanced enough to allow the middle intake to settle down properly.



Last night I set the TPS per the factory service manual. I then tried to set the idle down from my 1500 rpm. Everything I tried just increased the idle. I could not make it go down. Mysteriously after giving up on the idle while I was digging for my timing light the idle settled down to around 1100 rpm. The engine was completely warm before I started so that could not of been the cause of the change.



I then set the timing per the FSM. At first I could not get it adjusted to where I could even see the timing marks. After 4 tries of indexing the CAS (the same every time) I finally got to where I could set the timing very close to perfect.



A quick trip to a fast food jonit proved that I had made a significant improvement. The throttle was more responsive and it did not miss as much around idle (still at 1100 rpm).



When I got home and pulled in the garage.. Just before I shut the car off it settled down to a perfect 900 rpm idle. The best I have ever heard the car.



The big problem started this morning. It was just a few degrees cooler than last night. It started with about 30 seconds at 3000 rpm then it settled to a 1500 rpm idle. Half way to work I hit stop and go traffic. The idle began to surge between 1200 rpm and 1500 rpm. It did this in a very quick and rythmic pattern. I tried tapping the throttle, re-starting the car and neither broke the surging pattern at idle. I did discover that if I used the clutch to drag the engine speed down from idle it would sometimes maintain a solid 900 idle. But then the next time I had to pull up in traffic and needed to rev the engine it would return to the 1200-1500 idle surge.



What should I be looking for to fix this?

j9fd3s

make sure the thermowax is working right, its the highest point in the cooling system so it gets air bubbles easy



mike

CarmonColvin

I lived with a surging idle for a few weeks and finally decided to do something about it. I now have 99% of my idle problem fixed.



I removed the throttle body and cleaned it out really well. I even took 600 grit sandpaper to knock off some rough varnish buildup from the main air ways.



I used the factory service manual and adjusted all of the flappers back to factory specs.



I then found that the thermowax (offically called Water Thermovalve by Mazda) inlet was clogged in the throttle body so I was never getting coolant flowing through it. (first major probem). I cleaned that out and then removed the thermowax and tested it with a thermometer and a pot of water in my kitchen. It failed (seond major problem). It never open vented at the prescribed temperature. I tested one from the parts car and it worked perfectly.



I then checked the Double Throttle Diaphram. I had to go buy a hand vaccum pump with a gauge to test it with. It is supposed to be fully closed at 200mmHg. I found it would never stay closed. I could not pump fast enough to ever get it fully closed. The diapharm had a small leak in it !! BING.. MY VACCUM LEAK!. I tested the three spare ones I had in my garage. All of them leaked. I just capped off both ends of the line going to it and re-assembeled everything.



Started right up to a solid 1100 rpm idle. With some adjustment I had it down to around 800. No more surging or eratic idle.

Now that a week has passed I have found that it will surge a bit as it did before on cold mornings (below 50 deg F.) but goes away within a few minutes of driving.



I suppose the following was happening. At idle the double throttle diaphram was leaking enough air to cause the idle surge, this was impossible to find using the carb cleaner metod of vaccum leak detection because I never thought to spray it into the back of the diaphram.

In a good functioning system the double throttle diaphram is taken off of the engine vaccum and just open air vented when the Thermovalve reaches 171 degF(thus opening the secondary throttle plates). Mine was never doing this because of the lack of coolant flow and bad Thermovalve.



I am going to try and get a new double throttle diaphram and hopefully that will take care of the idle surge I get when the motor is cold.

j9fd3s

yah that leak is internal, so you wont find it by spraying the engine with carb spray.



ive never seen a bad double throttle diaphragm, interesting, i usually disable that so its open all the time. the gsl-se's water thermo swtich/valve fails and the double throttle stays closed, and the valve it like $65



mike

vosko

n/a's are fun to fix idle problems. i just let judge ito do it... lol