Rx8 won't start without jump, once started no throttle response
 

My bad RIWWP, I've searched the forum high and low and just wanted to take this straight to series 1 troubleshooting but I guess I'll have to wait like everyone else....

I just got my rx8 this week. It's an 04' MT with 80,000 miles. Yesterday I was driving along and all the electronics cut off, including powersteering. The engine was still on and I smelled something burning. I pulled over and the car had a rough idle for moment before it cut off. I opened the hood to see the ground on the alternator was melted. I replaced the terminal, wire, and post and after I got a jump the car ran fine till it happened again today. Only difference is when I changed the terminal and wire today the car started but there is no throttle response. It starts and idles fine for a minute or so, If I press the throttle, even all the way, it jumps from 1krpm to 2krpm but never higher. After a little while the lights start to flicker and the idle gets rough and ultimately the car cuts off I checked all the grounds I could think of and nothing. All I have left to check is the battery terminals but I don't understand how that could cause the throttle issue?

Also I did add a 12v power wire for a sound system to the battery the day before the alternator ground issue occurred the first time. This is my one and only car (I've had my fair share of rx7s and miatas) and I need to get it running before work tomorrow. Any leads, ideas, questions would help a bunch?


tl;dr: Alternator ground melted, replaced, happened again but this time the car starts, but I get no throttle response. Then it dies again and needs a jump to start.

See? Now I can move it to Troubleshooting for you :)


Hmm, something is damaged, and it's likely something electrical. Go through and check all fuses and relays to be sure they are fine. Keep in mind that fuse checking should be swapping each out for a known good one, one at a time since some fuses look fine when they are busted in an unexpected place.

Needing a jump to start the car, I'd first check the battery terminals, and by "check" I mean disconnect them, completely clean them, and reconnect. Often the corrosion that is crippling the battery's ability to send current is between the clamp and the post, and not visible till you remove the clamps. If you can start fine with a jump, this suggests you don't have a solid battery connection, as the jumper cables are ignoring that connection.

As far as the throttle responding to pedal input, the next step I'd honestly take is to get an OBD2 data reader, one that can read live data. A bluetooth one with a smartphone or tablet and an OBD2 app is generally ideal and pretty cheap. You will want to take a look at what the various throttle position sensor outputs are showing as you move the pedal. It should help indicate if one of those is the problem vs an ECU problem. It's possible that if you had an overvoltage spike, you damaged something. It's entirely possible to fry the ECU, although I'd expect that it wouldn't be running if you had. Maybe a resistor for one of the throttle pedal position sensors got damaged?

Thoughts...

First, disconnect the stereo wire you added.

Second, clean connectors, see if that worked...

I'm surprised this wasn't the first thing the OP tried, the causation seems pretty clear.

I definitely gave that a shot, I went ahead and got a new battery with some new terminals and the car starts fine. It still won't rev past 2000 and it's kicking out a CEL, I checked and its p0123 and p0223. Throttle body curcuit a & b high voltage. I was cleaning the throttle body and I noticed that the harness that led to it had melted (right next to the alternator ground) and exposed two of the wires. I'm sure this has something to do with it but would it be at the PCM/ECU side or should I just tape the wires up and try it out? I don't want to make things worse by trying out something obviously broken..

Yep, that could do it. Inspect as much of the harness as possible and see if there are any unbroken wires. It MIGHT just be shorting (the high voltage suggests that is accurate, but that could be a fried resistor in the ECU as well), but you need to validate the integrity of the wires. If they appear to be intact, then you will need to completely shield each wire individually. Then give it a shot.

If they are broken, excessively crimped, the cores themselves melted, etc... then your only option is to replace that section of the harness. Technically the entire engine bay harness is one piece, one part number, but you may be able to get the section you need from a wrecked 8. OR, if you have any ability with electrical work, you may be able to splice in replacements cleanly.

The harness is fine except for a section right before the plug itself, I saw exposed wire on two of the wires and spliced in two new wires. I'm a technician at a stereo shop so splicing wires is no new thing.. I have yet to put everything back together but I realized the throttle body doesn't open when the gas pedal is depressed. Is that normal?

If the engine is off but the key is in the on position, the throttle plate should respond to the gas pedal. You have an issue somewhere. Can you get the actual voltage that the pedal sensors are seeing? I think there are 3 different sensors in the pedal as redundancy, and if sees even 1 off I think it disables the pedal completely.

Keep in mind the problem could be in the harness between the ECU and the pedal, or the ECU and the throttle plate. Getting the state of all position sensors is going to be important for diagnosis, and in live data if you can (it's all reported by OBD2). That way you can see what is responding and what isn't.

Well I brought home my obd2 scanner, it has readouts for ACC pedal D & E, Rel tps, TPS B, etc, etc. They show up as percentages rather than voltages though. Or do you mean reading the sensors with a voltmeter?

the actual voltage will be needed eventually most likely, but those percentages are fine.

Watch them live as you move the pedal, and as you move the throttle plate. One or more are probably not responding.

I'll just start from the top of the list:

Command TAC, responds to pedal from 0 to 100
ACC Pedal E(%), responds to pedal from 20.0 to 67.9
ACC Pedal D(%), responds to pedal from 30.5 to 78.5
TPS B, doesnt respond to pedal or plate. Stays at 100
Rel TPS, doesnt respomf to pedal or plate. Stays at 16.1

Sounds like your pedal and sensors and wiring from there to the ECU is fine.

The problem is going to be at the throttle body, or the wiring to it. Since the throttle plate itself isn't moving on command, it could be a fried solenoid. Swapping in a different throttle body is probably the shortest/easiest way to confirm the problem.

There are a couple different factory inspection/testing articles that could help you though: M A Z D A

Engine -> Intake Air System -> Throttle Body Inspection
Engine -> Control System -> THROTTLE POSITION (TP) SENSOR INSPECTION

I just replaced the TB and now the car starts and idles fine up to 3000 rpm.. I found a post from a while back that you posted that kinda details what I'm going through.. My only CEL's are P0123 and P0223, which translate to Throttle/ Pedal Sensor High Voltage Circuit A, and the other code is for Circuit B.. What do you mean by this being it's own kind of limp mode?

Hmm. Yeah, the ECU defaults to a steady 3,000rpm if it can't figure out what position your pedal is in. It's rare, but it can happen. It's one of the "limp modes" that the ECU can go into.

However, you previous had readings from all APP sensors? Have any of them stopped working?

This isn't a steady 3, it jumps from 1 to 2 and when I press the pedal it stops at 3. It never made it past 2 till I changed out the throttle body though. And I still get the same readings on the reader.. Aside from the one on the pedal itself, what other sensors are there?

Ok, a bit different than what I ran into when my pedal apparently froze, i started the car and it immediately jumped to 3,000rpm and nothing I did made a difference to the RPM. I pumped heat into the footwell as the car warmed up and I restarted it a bit later and I had normal control back

I don't know about that specific result, but it really seems like it's picking up the movement, but isn't sure so it's defaulting to specific RPMs. Best guess is to go bakc to the factory manual and start going through the pedal position sensor test/inspection procedures.