Pretty sure my boost sensor is toast on my s8 rx7. Took the car out for a cruise recently and it started misfiring at idle and struggling to idle, but ran fine at cruising RPMs. When I got home, I noticed my oil injection system was dumping 2-stroke oil into the intake (I was also premixing to avoid running dry), and I had accidentally left the boost sensor sitting loose on top of the intake—not bolted in.

It ran okay for a bit, but once it warmed up, the same issue came back: rough idle, misfires at low RPM, but smooth at higher RPMs. Limped it home, and now it barely idles—same behavior I saw previously when I forgot to plug in the boost sensor.

Car’s parked and not drivable at the moment, so I’m trying to troubleshoot without making things worse. Is there a reliable way to test the boost sensor before I drop $500 on a replacement? I’m fairly confident it’s the culprit, but I’d like to confirm before spending that kind of money.

 

compression is good on both rotors

 

You can likely pick up a used MAP sensor on Buyee for $100. I may have a spare in any event.

Did you, or anyone, touch the spark plug leads or coil harness at all?

Make sure those are hooked up correctly. The USDM FSM is not a guide for s8.

Is the filter on your MAP sensor facing the right direction? Is it even there?

 

Boost sensor for S8 is a newer version than on S6.

 

There is a test procedure in the shop manual. Really the stock MAP sensor is VERY hardy, I don't know if I've ever seen one go out.

Dale

 

Yeah, I was looking at new ones. The S8 one (N3F1-18-211) superseded the S6 (N3A1-18-211) according to Atkins, and they have the S8 one in stock. They're pretty close to me, too. I saw some used ones, but none of them were cheap enough or guaranteed to work. I’d be willing to throw on a used one if it came off a running car.

The prices I was looking at were:

• $300 from Japan, brand new, but with a 4+ week shipping time
  
• $650 from the local dealer with a 2-week shipping time
  
• $475 from Atkins, and I could pick it up same day
  

No one’s messed with the wires since my last post—where my wires hit the harmonic balancer—but it’s really deteriorated since I drove in that cruise with it sitting on my intake. I figured it didn’t like the heat or vibrations.

I also pulled off the filter and tested it without one, but that didn’t change anything. While I’ve got it up and out, I’m gonna go ahead and burn off the carbon from my plugs for now, just to make sure it’s not due to fouled plugs. I’ll also check my wiring fix while I’m in there.

 

I'm with you on the guess on fouled plugs. Running at low load which should be high vacuum w/o the MAP sensor in place will make it run VERY rich.

 

im just not completely sure what else could be causing this because it acts like an unplugged map sensor but it could be a short in the wiring going to my coils, it was like intermittent then it completely failed.

 

I have N3F1-18-211 sensors and all my parts may be returned if not satisfied.
$125 shipped

 

i might be interested i want to do a bit more diag before i purchase anything

 

my secondaries didnt look bad but when i pulled my primaries they were black and wet lol

 

Check your TPS is not wonky,.

What ECU are you running?

If you still have the stock ecu, consider running codes before going much further.

If you suspect a short in the coil harness, you may also have shorted out a coil.

Check the ground in that circuit is well fastened.

 

i got a power fc and im still doing some diag i though that wiring that broke a while ago was going to the coil packs but it looks liuke it was going to my omp and a connector next to the alternator so i gotto check into that more and everything

 

well i charged the battery went over my connections and cleaned my plugs and now its idling. i feel so dumb lol but my guess is that the plug wires weren't completely snapped in and that cause the plugs to get build up on them and it just spiraled until it was barely running. i really appreciate everyone's help and advice.

 

So it wigged out on me again today. I drove it about 15 miles, mostly highway. I stopped, turned it off for a minute, then drove about a mile through city streets with no problem. I turned it off again, but when I started it back up, my PFC read 95°C, so it wasn’t overheating. Still, it didn’t want to drive. It popped and banged, then randomly started running perfectly fine when I hit 3rd or 4th gear around 3k ish RPM.

I think it’s a combination of fouled and cold plugs. The tuner put in 9s all around, so I figured I’d stick with those. It idles in the mid 11s to mid 12s AFR. Last time this happened, a plug wire had popped off and was just sitting on the plug. My trailing plugs were black and oily. I'm assuming that’s from over premixing while I’m still figuring out my oil injection system.

I’ve got new plugs on the way, and I’m looking at ordering an HKS Twin Power to help with the richness. I’m hoping it’s not the TPS because that would suck. But it didn’t do this before I started premixing this much, so I don’t think that’s the issue. Once I get my oiling system dialed in, I’ll cut back on the premix.

I don’t know. I’m just kind of word vomiting everything here because I don’t have any friends who know rotaries well, so I’ve got no one to talk to about this.

 

This is how your coil harness should be hooked up. Please check that and report.

 

Here is a replacement coil harness. Mazda recommends you replace every five years. I replaced mine and it helped get rid of the backfiring I was getting on decel. with the motor hot.

https://buyee.jp/item/jdirectitems/a...n_DirectSearch

 

i think mines hooked up correctly i know the white plug is plugged into the right connector and i double checked it when i had the coils out las, but every 5 years? thiis is probably still the factory one XD

 

i ordered a replacement it's make sense that something is up with the coil wiring. hopefully this fixes my idle cause 12:1 idle sucks

 

The more I think about it, this has happened three times. The engine freaked out with popping and banging—twice while heat soaked, but once even after it had cooled enough to open the cooling system. In both heat-soaked cases, it ran poorly until reaching a certain RPM, then suddenly smoothed out. The third time, after cruising for a few minutes, I slowed down and the issue returned. I drove a mile or two, and it happened again. When I checked the plugs, the trailing ones were pretty fouled. Between these incidents, I took the car to Cars and Coffee, drove about five miles each way, stayed for a couple hours, and had no issues aside from a rich idle.

 

The white plug goes into the T1 coil, which is on the left of the three coils in the picture.

 

You ordered the new version of the coil harness, right?

TPS may still be wonky. Power FC will compensate for not getting a favourable reading and that may be why the engine seems to sort itself out.

Further, some people have experienced that their TPS will test correclty, but then go out of phase when the engine gets very hot.

@Tom Smith I think Tom Smith ha similar issues to you and, in the end, it was the TPS.

 

There are a number of very experienced FD RX7 people in the seattle area and on here. There are a few South of Seattle. You may want to try hook up with them.

 

i meant right like correct as its mounted on the car the white plug is connected to t1

 

hmm ok ill look into it i ordered the right one ii think i ordered it from nengun same part number oe mazda but i might cancel my order cause the estimated ship date is over 3 weeks out and whatever they use for shipping estimates it weird i had that wiring harness in my cart with a pfc mount and shipping was $55 usd but when i added a sticker shipping jumped up to $76