View Full Version : Rpm bounce

07-03-2014, 09:35 PM
Alright guys,

I went and bought my first Miata yesterday. I test drove that thing into the ground and it ran perfectly fine. I drove it back to my town 100 miles on the interstate...ran like a dream. By the end of my first day with the car, the RPMs are jumping and the car is revving up while idling. This only happens after the car has warmed up and been driven. It will idle fine in my driveway, but drive it 1000 feet and this will start. Once you put the car in neutral, start slowing down, or come to a complete stop, the engine will calm down a bit, but then rev up to 1000 RPM and start bouncing up to 2000 and back down to 1000. This is a steady 2 second per bounce. If you turn on the AC, it will bounce back and forth continuously with no stopping.

This is a 1994 stick shift. I did some research, filled the tank with premium and poured in some seafoam as well. Then i changed out the spark plugs. Then I took off the idle control valve and cleaned out all of that area really well... this made the car seem to idle a bit smoother, but did not fix my RPM jumping problem as soon as I took the car out of the driveway. Also, after replacing the idle control valve, my air bag light is flashing. This is three flashes, then a break, followed by two flashes. And it does this continuously.

I ran the car over to a mechanic in my town that drives a miata...he told me that he thought it was a timing belt issue. That sounds like total crap to me, but what do I know?

I will post a video tomorrow. Thanks for all of the help. I hope I didn't just invest in a world of problems.

07-03-2014, 09:52 PM
I had a similar issue. My car idled at 800. At slight throttle (to make it to to 1,200 or so rpm it would climb until 2000 and then bounce back down. My issue ended up being the idle air control valve.

I have a 1.6 92. While not exactly your problem, I would be it is very similar.

Try setting the idle to 800 and see if It still happens. Then see if it happens at partial throttle.

07-04-2014, 09:59 AM
Alright. Did some research and noticed a hissing sound coming from the engine. Sprayed it with some cleaner and sure enough the engine starts revving up. There is obviously a leak. I don't know if this will fix my bouncing issue but I'm going to replace the hoses and see how it goes.

07-04-2014, 07:50 PM
Turns out the hiss was coming from one of my fuel injectors. Switched it to the first slot and sure enough, the hiss followed. The pintle cap was totally broken and cracked, I'm not sure if that has anything to do with the hiss. We replaced the O Ring and the car started idling MUCH better. No revving, but it is idling around 1500 RPM. Took it driving around to get it hot and that fuel injector started spewing gas. Took it off and replaced the O rings again...now it's idling around 1700, but no gas leak.

I'm going to replace the injector, but there is not a store in a 50 mile radius that has one in stock. It should be in on Tuesday. I'll update after I have replaced the fuel injector.

Ideas... Anyone?

07-04-2014, 08:20 PM
I've pinched those o-rings on a number of occasions. I'm not sure why you are replacing the injector. Do you think it is clogged?

For the bouncing tach, check the ground point just to the right of the throttle body. Clean the leads and this may help the bouncing tach - I had the same issue.

I also had the 32 code for the airbag. More than likely someone took the steering wheel off and damaged the clockspring. 32 means a short on the drivers airbag circuit. Here is a link to the SRS manual for the 94. http://www.mellens.net/mazda/Mazda-Miata-1994/airbags.pdf

07-04-2014, 08:33 PM
I honestly don't know if the hissing injector is to blame for the engine revving/RPM bouncing or not, but it did start idling a hell of a lot smoother once we moved around the injectors. I just can't imagine what else would be causing the issue. And if the thing is hissing, shouldn't it be replaced anyway? We replaced the o rings and all of that and it is still hissing.

07-04-2014, 08:45 PM
Also, I just read this


and that first symptom sure sounds like what is going on with my vehicle. I have never worked on cars before....this is exciting!!! Frustrating and expensive...but exciting!

07-05-2014, 05:39 AM
Ahhh, I didn't realize it was still hissing. I suppose it could be the injector. Do you have somewhere local that can inspect and clean your injectors? It might be better as you'd have 4 clean injectors and they may find them all in working condition.

07-07-2014, 10:51 AM
After moving the hissing injector to a different slot, it seems like the revving/rpm bouncing has subsided. I adjusted the idle speed to bring it down and the car is running much smoother, but still idles a bit high.

I'll be replacing the fuel injector in the next couple of days and see how that works out. If that doesn't solve my problem, I think I'll be replacing the throttle body/idle speed control valve.

07-16-2014, 08:23 PM
Checking in to let everyone know that this problem has been SOLVED!! It was not the fuel injector itself that was the problem, but rather the injector seals. I bought $4 repair kit from Injector Rehab. Replaced the pintle caps, O rings, and seals...car is running like a dream now.

This is my greatest mechanical achievement. Youtube, these boards, and the internet in general is amazing. It's incredible that two guys with no mechanical experience outside of changing oil and brakes were able to research, diagnose, and repair this problem.

I hope that this thread helps others in the future.


kung fu jesus
07-17-2014, 10:28 AM