Rough Idle - Stalling after long drive
 

First of all, I'm no expert. Most of what I know I've learned from searching these forums the last month trying to solve my problem.

I have a 2006 RX-8 manual transmission with just over 90k miles, no CEL or codes, oil changed last week and MAF sensor and throttle body have been cleaned. I changed the coils and spark plugs a couple weeks ago as the first attempt to fix the problem. It had been about 30k miles so they definitely needed to be changed, and the car did drive much better, but I'm still having issues.

I am typically seeing the issues after driving the car for about a half hour. My drive to work takes about 40 minutes, it is 38 miles and 36 of that is interstate. The car drives great getting on the interstate and while on the interstate, but as soon as I exit and slow down/idle the car runs rough. The idle is inconsistent and I completely lose power at low RPM's. At its worst the car will actually die at idle. The problem is worst when accessories are on, especially a/c. I have also noticed some flickering of the clock display and dimming of the headlights during the RPM fluctuation at idle. The fans also seem to turn on in sync with the idle revs.

I got on OBD-II scanner and all I've really noticed as being potentially strange so far is the battery voltage. The voltage is consistent around 13.5 during interstate driving but fluctuates from 11 to 14 at idle (see images). I replaced the battery early this year but broke one of the connectors in the process, I replaced both connectors but it's possible that in the process of tightening them I loosened another connection somewhere. I also wonder if the alternator might be going bad and not able to charge the battery enough during idle after a long drive.

I had the car to the dealership last week but they couldn't recreate the issue and said it "wasn't too bad". They said they could do a compression test as the next step, but obviously that is expensive and I don't even know if that will identify the problem.

I guess I'm wanting to know if there's anything else I should be looking for or anything else I can do to diagnose the problem. If I take the car in what should I have them look for? Any good mechanics in the Denver/North Colorado area?

Pics are a screenshot of OBD-II app showing battery voltage readings, the first on the interstate, the second after the drive at idle.

I've had a very similar problem that's developed over the weekend...im leaning towards alternator but need time to go have it tested. I don't seem to have ANY take off after the engine is warm and been run for awhile..press the gas all I want, and nothing really happens until around 3.5-4k RPM. My lights are dimming, and the idle is rough, especially if lights are on and sitting at an intersection. So far, mine hasn't stalled, but it's dipped really low a few times and then picks itself back up...i'll let you know what I find out on mine - having alt tested Wed or Thurs after work.

Thanks for the reply, it definitely sounds like we are having the same issue. Mine will usually pick itself up when the idle goes really low, but a few times it hasn't and it stalled. Definitely let me know what you find out, I hope we can both get them going strong again.

No problem...I've already checked and double checked grounds...just so i reserve the "i told you so" for later (haha), i'm betting on voltage reg in alt. for us both. What drives me insane is how rough the idle is...sitting at a light feeling as if im on a quad or bike...i can see my mirrors vibrate and feel it through the seat.

Yep, that and when the light turns green knowing that you can put the pedal to the floor and the car will sputter along until 4,000 rpm.

Shank, have you had a chance to get yours checked out yet? Work has been INSANE this week as we're running like crazy and i'm preparing to leave town for a couple of weeks. Aiming to have mine checked tomorrow, but was just curious if you'd done anything yet?

I'm in the same boat, been too busy this week to get anything done with it yet.

Compression loss?
 

Do the cars have a problem hot starting? Long drives warm up everything, it expands, and then you have a poor idle because of low compression.

I've noticed a slightly longer crank before start at times, but never any trouble starting but it seems like it could be a little slower to start when hot. Would that explain voltage fluctuation and headlights dimming though?

Any auto parts store should be able to test your alternator. If the motor is running so rough at idle, to the point of stall, I would think that it's not making enough power to keep the alternator happy.

Fuel pump.

No symptoms until 20-30 minutes of driving on the highway and then suddenly power problems up to and including shutting off is classic fuel pump flickering on the edge of failure.

Voltage sounds normal for a too-low RPM idle struggling to stay alive. If you have the fuel system pressure tested at a shop, it will still probably pass, because they can't get enough fuel pump load with it just idling there. It has to get more load to start heating up and start failing.

I've personally not experienced any warm start issues with mine.. it always starts fine, it's just the issue with the lights flickering when coming to a stop and the idle dropping low and then picks itself back up, but still rough - it only does it for a moment, but it's consistent; once the engine is good and warmed up from driving for awhile, it does it without fail. I've been planning to have both alt and batt tested at Advance Auto; again, just short on time this week.

As far as fuel pump consideration goes, wouldn't I notice a lack of power at higher RPM rather than just a sluggish bottom end or no?

Yes, generally, that is true. You didn't note if you experienced any problems up high once the symptoms start, only mentioned the difficulty with power and stalling.

If you are seeing normal(ish) power up high and all the significant problems are down low, then I would immediately shift my guess to a compression problem. That 20-30 minute delay seems odd though, as your engine hits operating temperature and maximum expansion well before that. You can test that one by just letting the car idle warm without moving and see if the idle problem slowly becomes worse as it warms up. Not just coolant, because the entire block does have to heat up too.

I'll give that a try. I'll also see if I can get it on video @ night when you can clearly see what's going on with the lights when it happens - it's so weird - they don't just dim down for a second and then brighten back up, they kind of flicker -dim-normal-dim normal - dim - and then normalize. By the sounds of it, my idle issues aren't near as bad as Shanks, but it's def not normal. Thanks for all of the advice and suggestions of where to look!

And just to clarify plugs, wires, coils, air filter were replaced less than an oil change ago - MAF and TB and ESS cleaned Saturday (none of them appeared to be dirty), and also checked all of my grounds as well as terminals -even cleaned them though they didn't really "need" it.

If the dimming is happening with an idle reduction, then don't go chasing that. If it's happening with the fans kicking on, and a more stable but low idle, then don't go chasing that. If they are dimming without any idle change or other major electrical load change (headlights, brake lights, power steering when stopped, and fans are the big ones), then yes, the alternator might have a problem.

may be onto something...It doesn't occur when i'm just sitting at the light; it's always when I'm braking and almost completely stopped - if you can imagine coming to a stop, foot still on the brake - the lights start their show around 3mph or so.. once i'm actually stopped, the lights dance once or twice more, idle dips, picks back up..still a little rough but lights and a/c fan run normally. On initial take off, there's an off-idle hesitation till ~3,500-4k rpm and then she runs like a champ.

Ah! More detailed information is always good.

Two more pathways of possibility opened up with that:

1) a vacuum leak. using the brakes has a direct impact on the vacuum 'usage' via the brake booster. This can (but not always) cause idle trouble if the vacuum change is one that hasn't already been corrected for by the fuel trims (because vacuum leaks cause AFR issues, which then get 'learned' by the ECU in the form of fuel trims to account for them). If this is a vacuum leak causing a fuel trimming problem while you are on the brakes, then the fuel trims may remain in an subpar state until you get more RPM out of the engine. One way to try to test for this is to bring the RPMs up to 4-5k while stopped before driving away, and engage the clutch with the RPMs up there. If that hesitation never appears, then it would suggest a low RPM AFR problem, which in turn would support a vacuum leak, which are always most impactful at low RPM. You can also check the commanded vs actual AFRs, the fuel trims, and the MAF readings if you have the OBD2 access. The combination is usually very telling if there is a vacuum leak problem.

2) An electrical connection problem. The high electrical demand for the headlights (you haven't mentioned anything about this without headlight usage?), brake lights (present), and fans (common when at low speed, as the car tries to keep radiator air flow) could be straining the connection's capacity to supply electrical power to the rest of the car. What system exactly is the one that is left out in the cold and low on power can certainly vary between cars. Any one car tends to have the same system order that starts to fail, but that certainly isn't a consistent order. I don't know exactly why this occurs. Two of the high electrical demand systems that can start manifesting problems here that could cause this would be the fuel system (injectors and fuel pump) and front O2 heater. A fuel system voltage drop from supply problems would result in dropping fuel pressure and/or fuel injector firing instability. Each can cause this. A front O2 heater voltage problem could result in the heater element cooling down, resulting in an inaccurate AFR reading, causing the ECU to make wrong decisions. Those repaired battery connections might not be repaired enough, or have corroded in the time since you did the work.

And to think in my last post I almost said "it's almost like the ECU is having trouble figuring out how to regulate electrical systems during the transition from higher RPM to idle" haha I thought that wouldn't make sense but I guess it kind of does. Going to double double check my connections tomorrow and also check out vacuum lines. Thanks again for the help!

Well fellas, here's what I've got. Battery and starter tested fine - alt not detecting load whatever that means. Recommended all connections were checked, so did that and got tested again. Same results.

You should look at your coils.
 

Our coils are typically only good for about 30k miles. They fail because of the excessive heat they produce. If you have no power until 4k rpms and it gets worse after the engine is hot you most likely have a failed coil. They can fail without throwing a check engine light. What is happening is that the spark finds an easier way to ground through the coil than clearing the spark plug gap. You can check for a burn spot on the backs of the coils but its not guaranteed to be there of they're failed. I would look into the coil upgrade kit (offered by black halo racing, I think). It moves the coils and adds a heat dissipation plate as well as upgrading the coils themselves.

When I bought my 05 in May 2015, I had issues with it stalling if it was hot outside. I changed my plugs and wires and it still did the stalling. I decided to check the A/C refrigerant levels, and she was low. Once I filled her up I didn't have the stall issue. This happened a few times on my old Prelude, so I gave it a shot and it worked. I also had idle issues because my clutch was going. I replaced it and most of the idle rattle went away. Final step was new coils, battery, and battery terminals. I drove her around for about an hour and I don't have any issues. She starts almost instantly now, it doesn't matter if it is a cold or hard start. I hope this helps some.

So does this mean the alt needs to be changed ? Im having the same problems my positive cable lead rusted and close to snapping im waiting for the new part to come in but believe thats not just the cause to my stalling when coming to a light its like I get close to the light start to idle and she fails but i give her a little gas and shes back and running any tips?

Hello guys,

I am having a similar problem and I am starting to get really out of possibilities.

The car is a 03 MT 6 port. The owner messed the car up with a petit induction system and used such a venturi ebay wirl thing just in front of the MAF sensor which obviously completely screwed the MAF readings. He killed his stock engine with it. then got a reman, it also died in less than 10k. Both engines where is such a bad state that I could not make 1 good engines of the 2 faield ones.

In the end I made a fresh engine for him, new rotors, new rotor housings and used side housings which where still mint. the engine runs 2 piece Ianetti ceramics and side seals are newly cut.

He put the engine in and fitted again this petit induction system with this wirl plate. the car was undriveable, afrs richer than my wideband could measure. Then I put the stock airbox with a new filter back. the airpump and cat are removed from the car, as is the VFAD

now the car drove fine but top end was a bit weak. after some investigation the APV motor was broken, it pulled current but didnt rotate. After replacing it the car drives perfectly and pulls around 220 g/s or air.

The issue that remained was the idle. cold idle is fine but once warm it starts after some time to go up and down and eventually stalls. it always starts fine warm and cold but warm it dies within one minute after start.

I removed the intake, no vacuum leaks could be located, I tried 3 MAF's, no change, I replaced the throttle body, no change. SSV is not stuck and moves freely

the car pulls around 13-15 in Hg vacuum at warm idle 750 rpm, the mixture reads lambda 1 (measured with wideband), airflow is 4-5g/s.

I do not really know what else I can do. I tried the procedure to reset the ecu and re-learn the idle but it doesnt do anything. What caught my eye too is that long term fuel trim is at -22%. is this normal?

mixture at WOT high rpm is IMO pretty rich for a NA engine around 10.7-11:1 AFR

If anyone has some other ideas..

Sounds like you are pretty familiar with these cars. But you said you reset the ecu? Does he have a proper tune on the car?

Maybe a failing fuel pump?

I'm thinking that is my problem, but I replaced the factory one not even 2 years ago with an aftermarket one from Advance Auto.

My issue is very similar to the O.P.'s, but is very sporadic.
I haven't done proper troubleshooting yet due to time constraints.
It will run fine on the highway, but after it's warmed up and driven on city streets in stop & go traffic, and usually after idling at a light, in a carwash, or a drivethru line, upon trying to accelerate it will suddenly start hesitating, popping, and stalling for several minutes, sometimes dying.
I will feather the throttle to keep it running, and then it will start running fine like nothing ever happened.

I'm planning on swapping the pump assembly, unless I find something else, and I will post an update on any results.