I have a 1989 RX7 (non-turbo) that runs beautifully -- but... From 3600 RPMs, it begins to misfire, randomly. And then, at about 4100 RPMs it runs smoothly again. Does anyone have a theory?

 

Sounds like the infamous "3800 rpm hesitation". Happens usually due to poor grounds. At 3800rpm, the secondary injectors kick in. At the same time, the ECU halves primary injector duty. Poor grounds can cause a hiccup here where the primary injectors go to 50% duty, but the secondaries don't come online until a second later. This causes what you're talking about.

I recommend you follow Aaron's write-up on re-grounding. I can link it for you shortly. It also includes an optional section at the end about adding a ground that may be helpful.

Also, the ECU grounds out through the harness at the top of the engine. I added some wire and ring terminals to the four ECU grounds and grounded them near the ECU, for insurance. I would try the above solutions first. Only if they don't work would I try regrounding the ECU, since the original grounds should be adequate.

EDIT: Here's the link:

https://aaroncake.net/RX-7/grounding.htm

 

Agreed with wondrous bread. 3800 RPM issue. Usually is an ECU ground. What some people have done also, is "adding" an extra grounding point to the atmospheric pressure sensor on the passenger strut tower. They'll often splice the original harness. Maybe something to look into as well
If you're going to clean the ECU ground, mentally prepare to take off the upper intake manifold. You can get to it with the manifold on, but it's a big PITA

 

Good point, I hadn't thought of that.

Without the vacuum spider it's possible to reach the ground with the intake on, barely.

With the vacuum spider, it would be incredibly difficult.

 

Thank you everyone for the advice.
In the picture below, am I looking at the ECU ground?

 

That does look like the ECU ground wire. Normally though, it's grounded to the top of the engine. They often move around since people service the engine and don't always put them back.

The exact location isn't critical though. I'd clean that up well and see if it improves your symptoms. Even if it isn't the ECU ground (though it does look like it), it can't hurt.

 

Agreed, that does seem to be the ECU ground. Try hitting it with a wire brush and see if it helps your issue. The normal location for that ground is under the UIM, but like Wondrous bread had mentioned people move them around. I believe Mazda themselves actually moved the ECU ground too, depending on the year of your car
If cleaning that doesn't help, I'd point towards the atmospheric pressure sensor (or boost sensor as some call it). Test it per the FSM and make sure it's got a good ground

 

Thank you, Cardinell, I will let you know what happens.

JWP

 

Well, it appears the issue was caused by a bad ground. Very smooth through the entire acceleration range, now.

Thank you all, JWP

 

What a wholesome thread. Love that this forum is still active and helpful and even better that OP got his issue fixed.

 

Its reasons like this thread that have kept me coming here every day for the last 19 years. Still an incredible forum after all these years!