Miata idle dip

My 97 Miata has started to experience a very low idle when I let off the gas. It will almost stall and idles at or just below 400RPM and will very slowly climb back up to a normal idle. If I am coasting to a stop it idles slowly enough that I can feel the power steering going away.

Any ideas on the cause?

I know that this has been discussed at great length on other forums, but I can only find discussions about 1.6 cars or 99 and up. I am looking for info on the first gen 1.8 cars.

Thanks,

Sounds like your Throttle position sensor may be bad. I'm not sure how to check those but I know this switch let's the car cpu know when you let off the throttle.

First check the dashpot thingy that slows down the throttle plate. It is little plunger thing that the throttle cable wheel closes onto. You can pull the rubber boot off and work some oil into it if it is seized up.

Other than that, clean out the ISC.

Seems to be a common problem. My '90 does it, and I've yet to figure it out. The ghetto way to do it is to set the idle to ~800 rpm by tightening the throttle cable adjustment. But that is ghetto.

Before anything just clean the throttle plate make sure your air filter is clean and then reset the comp and drive it. Fixed mine like that. Might not be your solution but its free/cheaper then sensors and all that so i would definately do that first.

Then if that doesnt fix it start looking into Tps or whatnot and moneyspending and such.

The tips for the 1.6 cars apply to the 1.8 as well - except for the one with the dashpot, that's only found on the 1.6.

Try giving the idle bypass screw a quarter turn CCW, and go a bit further if that helps. That's the quick and easy and usually effective way. The non-ghetto way to set your idle speed is to use a paperclip to jump GND to TEN then adjust said idle bypass screw. Correct idle speed is 850, by the way.

Don't clean the throttle plate. There's a black lining on there that helps the butterfly seal. Take it out, and a low idle will definitely not be a problem :)

w/e didnt mess mine up and it idles like a champ at 230,000 miles WOOT

i mean i wouldnt scrub it with a brush but sometimes it helps to clean the throttle passage. Str8 up carbon does still get in there like any other car.

Thanks. I'll give the adjustments a try. I had a similar problem on my '90, but it was several years ago and I am fuzzy on what fixed it, but I thought that it had something to do with a temp sensor on the back of the engine.

curtis wrote: w/e didnt mess mine up and it idles like a champ at 230,000 miles WOOT

Then I will refer all the "I just cleaned my throttle body and now my car idles really high" phone calls to you Seriously, you got lucky.

The Idle Speed Control valve under the throttle body can get gummed up, cleaning that sometimes can help solve problems. But if this car was in my driveway, I'd give that idle speed valve a quick twist.

let me just add that ive cleaned more then one miata butterfly in my day. Passages need to be cleaned every once in a while. Turning a screw isnt a fix to the problem. it just opens the flap more which may work because as the passage gets gummed up at the plate it breathes less and turning the screw helps it breath more. Im not saying you are wrong, I know you know what up. But i know that ive done it and it helped me several times but maybe i got lucky several times.

It doesn't open the flap, it allows more air to flow through the bypass passage. It's the factory method of adjusting idle speed - that's why the bypass screw is there. I'm not recommending messing with the throttle stop.

Keith wrote: The non-ghetto way to set your idle speed is to use a paperclip to jump GND to TEN then adjust said idle bypass screw.

Make sure you get GND and TEN - I had a nice paperclip wire sized scar across my pointer finger and thumb after I put the paperclip into the wrong holes and heated it up cherry red in a couple seconds.

Could be worse, I've seen a car burn from that! There's a B+ right beside TEN. Not too bright really.

Keith wrote: Could be worse, I've seen a car burn from that! There's a B+ right beside TEN. Not too bright really.

That makes me feel MUCH better but I suppose I deserve that.

pigeon wrote:

Keith wrote: The non-ghetto way to set your idle speed is to use a paperclip to jump GND to TEN then adjust said idle bypass screw.

Make sure you get GND and TEN - I had a nice paperclip wire sized scar across my pointer finger and thumb after I put the paperclip into the wrong holes and heated it up cherry red instantly.

Fixed that for you. I still have those scars myself. Don't learn the hard way like i did people!

I had a bad idle when I let of the gas too. Found then that my valve cover gasket was leaking into the spark plug well completely saturating everything with oil. Changed plugs and wires and gasket. Worked like a charm.

Just so anyone took it as a comment on them - it's Mazda that wasn't too bright putting that (unfused?) B+ right beside TEN.

Did you know you can also use the diagnostic connector to honk the horn? Seriously.

Keith wrote: Just so anyone took it as a comment on them - it's Mazda that wasn't too bright putting that (unfused?) B+ right beside TEN. Did you know you can also use the diagnostic connector to honk the horn? Seriously.

My car's PO used the diagnostic wiring harness to wire up his fog lights. He did it and covered his tracks nicely. My clue came when I found a nice, charred, 25 amp fuse inside a melted fuse holder last month. Then the fun began of tracing significant portions of the car's wiring to try to fix the car's electrical gremlins.

Keith wrote: Did you know you can also use the diagnostic connector to honk the horn? Seriously.

1. Wire horn to brake lamp relay on buddy's Miata.

2. Laugh from afar.

Tyler H wrote:

Keith wrote: Did you know you can also use the diagnostic connector to honk the horn? Seriously.

1. Wire horn to brake lamp relay on buddy's Miata. 2. Laugh from afar.

Turn signal relay is a better choice. That way he doesn't discover it until he's away from the garage.

I love Miatas!!!

This one seems to have fixed itself! I was driving it up to my shop to try to make the adjustment and I decided to try it with the AC on. It made it idle properly. When I got too cold I turned off the AC and low and behold it didn't dip when I let off the gas!? I have driven it several times since and it is still running like a champ. The only thing I can guess is that this car has sat for several months while I have been addressing some cosmetic issues and it must have gummed up a little. I guess that I just finally ran it enough to free it up.