Fluctuating Low Idle/Vacuum when warm and stopped. Input?
 

Background info... the car is a 94 Rx-7 with 58k miles. Modifications include downpipe, midpipe, greddy smic, egay intakes, greddy catback. Supposedly tuned stock ECU by previous owner. Now here's the issue.

I check my autometer gauge during startup, and it get's a vacuum of 16" from my autometer boost gauge during cold idle. I let it warm up for a few minutes and vacuum stays steady at 15".

After a short drive around the block, I stop the car, and the idle begins to fluctuate between 400-600 rpm for about 10 seconds. Vacuum then drops to 12-13".

It's hard to take off from a stop during that point, and it usually ends up in me dying out. All this happens about 50% of the time I come to a stop. When it doesn't happen my vacuum usually stays at 15".

What could be happening? Vacuum leak somewhere? Bad apex seal(hope not)?

any ideas guys?

edit - also forgot to mention, the idle goes down 100 rpm or so when I press on the brakes as well. Is that normal?

I'd find out exactly what ECU you have and get a compression check to rule that out.

Will do that once I get a chance. You recommend bringing it to a Mazda dealership for a compression test or just do it myself with a normal compression tester?

Also I checked the spark plugs, and here are the pictures. There's also some brown gunk in the oil filler neck. Ugh.

http://imageshack.us/a/img543/2656/img04331.jpg
http://imageshack.us/a/img84/488/img04451a.jpg

A normal compression tester should give you enough info. You mainly want to see 3 even bounces. Those plugs look pretty worn, hopefully you're swapping them out with fresh ones.


Is a good idle when the car is warm your only issue, does the car drive ok when warm otherwise?

Make sure all the mechanics of the idle control are in good shape. Check the throttle body adjustment screws, the idle air bleed screw, map sensor is connected, etc. Also check if your TPS is calibrated properly (search for TPS in third gen section).

Also, the stock ECU can't be tuned. There are aftermarket chipped ECUs like Pettit, Knightsports, M2, etc., and fully programmable ECUs like the Apexi Power FC. If you have a stock ECU and stock boost control and those mods (mainly the mid pipe), you're likely going to hit well over 10psi. You don't want to do that on the stock ECU... I'd recomend getting a PFC (best) or a Pettit ECU (works well), or some good boost control if you want to run that exhaust set-up.

Yes the bad idle when the car is warm is the only issue I have so far. Will check what you've noted, thanks for your input.