Just bought a non-turbo 88 RX7. Got a great deal at an auction on it for 900 bucks. Anyway, I have this problem with it where when I shift out of gear the rpms will drop below idle then come back up. On hotter days the rpms will drop to 1k-2k then come back up to regular idle.

I am thinking this is a vacuum problem cause by the secondary air injection hose not being hooked up to anything and kind of just "hanging out" under the hood. It seems the previous owner installed a cold air intake and maybe just didn't use the right hose/pipe. That's just my guess because well, this is my first RX7.

I was having trouble finding pics online of RX7 engines with cold air intakes and the secondary air injection hose hooked up from an angle that I could see where it was hooked up to.

Any input would be welcomed. And if anyone knows where I could get the proper hose online (if that is my problem) that would be greatly appreciated.

 

What's the car normally idling at 2k+ RPM? When warm, it should idle at 7-800 RPM. When cold, it should idle at ~1500 RPM. A sticking throttle cable or misadjusted throttle stop can make it idle really high.

The secondary air injection system is just for emissions, and that hose you're seeing could stand to have a filter. A small crankcase breather filter works. Alternatively, you'd actually benefit from getting rid of that ebay cone filter, and replacing it with the stock air box. That black hose normally connects to the air box, and it would also prevent your AFM from bouncing around.

 

Off the top of my head a quick decrease in RPMs can be caused by having am extremely light flywheel. Not sure if that is the problem you are trying to describe or not.

 

I guess I should have proofread what I wrote a few more times before posting. I meant, 100-200 RPM not 1000-2000. Apologies. The car seems to idle fine when sitting in the drive way, etc.

My problem comes when I shift from gear into neutral. The rpms will dip below idle then come back up. On hotter days it's much worse than colder days. I am guessing its because of better compression with cooler air but that's just a guess.

On hotter days this has become a more serious problem because the car will be on the verge of stalling out.

Where would be the best place to start looking for a factory airbox?

 

Ah, ok. Vacuum/air leaks can definitely affect idle quality, and if the ECU is constantly having to make corrections with the BAC (idle air control), you could see some fluctuation. My first guess would be the intake elbow before the TB. That area was never very tight on my S4 NA engine, and if your AFM is not secured, it will move around.

When you have 10 posts, you can use the classifieds section to look for an airbox.

 

Thanks for the tips! I have a friend who is a mechanic. He just informed me that he would be able to order the part for me. I'll check out the intake elbow, see if I can adjust it and go from there.

 

I'd still check the classifieds first. New Mazda parts can sometimes be excessively expensive, and there should be plenty used ones sitting in garages/sheds/etc... Lots of people go the cone filter route, but on your typical NA engine, it's not going to have any real benefit (or actually losses if it's pulling hot engine bay air).

 

Thanks again for the great advice. I will try to find a constructive way to get to ten. Haha.

 

Just for clarification, would an airbox work from any model 86-92 or would I need to find one from an 88?

 

I shift out of gear the rpms will drop below idle then come back up


That could be something as simple as the throttle damper not being adjusted right.

Page 4A-51 of the online series four non turbo FSM.

 

im having the same problem except when i shift out of gear and go to stop the rpms drop low then dies

 

You need one from an S4 ('86-'88). The S5 AFM is round, so the mounting is different. You're also going to need the rubber piece that connects to the plastic air duct.

Also, I was going to add that if you're talking about idle speed dipping 1-200 RPM before stabilizing, that's something I see on my own car. But if you're talking about idle speed dropping to only 1-200 RPM (about to stall), that's not normal.

 

Just found a S4 airbox through the classifieds on here for 10 bucks. Not a bad deal at all.

When you say "rubber piece that connects to the plastic air duct" what exactly are you referring to? Something that would connect to the TB? Or the plastic that's above the radiator?

 

pic#2 ..the oval pipe on top of the rad has a rubber coupling that goes to the Stock air box,you need that to hook the box to the "snorkel"(the pipe over the rad).

 

put a breather filter on the air pump. dont want that sucking anything in. replace vacuum lines. my idle will drop like that as well but that as well, but i'll look into it eventually.

 

See pic below. The piece connecting the box to the duct over the radiator.

 

Got it, just what I thought. Thanks again.