Severe hesitation at LOW rpm + bad mpg
 

Hey all, ik this forum isn't as active as it used to be but I'm hoping someone can help me out.

I've been struggling with exceptionally low mpg for quite a while now. No matter how I drive, city or highway, granny shifting or bouncing off the limiter, I cannot break 16mpg. BHR v2 coils, new wires and plugs approximately 10k miles ago. Both o2 sensors just a couple months ago. Always 93 octane w/ Idemitsu premix .5oz/gal + sohn. However, I've become increasingly more aware of another issue which I'm hoping is related.

When the engine is cold, and especially if the ambient temp is cold, the car falls flat on its face FROM IDLE TO 4K, at which point it's like a lightswitch flips and the car immediately comes to life. Any amount of throttle from big toe to WOT presents this issue. Video of the issue, while not as severe as it sometimes is, linked here. I posted this on Facebook and reddit and literally everyone said SSV. Problem is, from my research, that's only an issue PAST 4k. Plus, I reached in with a long pair of pliers to move the SSV actuator arm and it moves flawlessly. Vid of that linked here. Lastly, I checked my 10k mile spark plugs and they looked awful.
https://cimg2.ibsrv.net/gimg/www.rx8...e4df099810.jpg

Does anyone have any idea what could be going on? The only things I can think of at this point are the solenoid that controls the SSV, the MAF, a vacuum leak, or maybe an injector issue. I'd be eternally grateful for any wisdom you can share!

Are there any codes? Vac leak and solenoid issues should be diagnosable via OBD. On a warm idle you should have 0 furl trimd. If they're not 0, report back with numbers.
Those plugs look overfueled. What's the history of the car and engine? Any mods?

No codes. Forgot to mention that. That's why this is parricularly frustrating! No real mods, just sohn adapter, BHR v2 coils & catted midpipe, exoticspeed exhaust. I am running motul 6100 synergie+ 10w-40 semi-synthetic. Trans oil changed maybe 8-10k miles ago, diff oil changed last week. No tune or anything that would intentionally affect fueling, but I agree that the plugs look way overfueled. I've owned the car for the last 12k miles, got it from a collector completely stock. 60,500 on the clock currently. I have no reason to suspect low compression, it hot starts perfectly every time and feels great in the upper rev range.

Hope that helps!

Has the engine been out? It's easy to mix up primary and secondary injector wiring causing similar issues.

Clues should be in the idle fuel trims.

I have never had it out, and I don't have any reason to suspect it ever was. My OBDII reader grew legs a while back and I've been dragging my feet on replacing it as I want to get a proper tablet scanner but can't afford one 😅 I'll hit up some of the guys in my local group, someone is bound to have one.

Short of that, is there any way to get anywhere by simple visual inspection?

You could check if the MAF is clean and if there is oil in the intake, and check for obvious vacuum leaks, loose hoses, etc.

I've tried cleaning the MAF several times, ordered one from amazon to rule it out as the issue. No oil in the intake. And I also have a hand vacuum pump on order to test for leaks.

Anywhere in particular you'd recommend I test vacuum from?

I'm not sure a vacuum pump will held with an intake leak, you need the fuel trim data from the computer to tell you if it's getting unmetered air.

Gotcha. I need it anyway to test the intake solenoids behind the manifold.

I did happen to notice that my MAF felt a little loose when I pulled it out. I then noticed the O ring was super loose and looked like it had been pinched when it was reinstalled. The screw holes were also stripped out causing it to wiggle around in its hole. Fingers crossed that that could be part of it🤞🏻

Could easily be, at idle the car only pulls 5g/sec of air, so even tiny leaks could translate to a big % of unmetered air. At higher airflow, a tiny leak becomes less of a factor and the is able to compensate.

It idles fine for the most part, only an issue when im on the gas... Although there have been a few times where I swear the idle was bouncing around the tiniest amount and once or twice when it gave me a noticeable hiccup. But I'll give it a shot tomorrow when the sensor comes in and keep you posted when I'm able to get my fuel trims read.

16 mpg for mixed city/hwy driving is around what I get, so just for that part, it wouldn't alarm me.
But then, I probably drive a little more aggressively than most.

I'm on my 4th fuel pump in 11 years.
I'd speculate because of premixing, they usually last about 3 years.
I had the last one replaced by the rebuilder who did mine.
He bought a cheap one off eBay and it went bad almost immediately.

If you haven't had your fuel ring recall, you can get it replaced then.
Maybe a stealership would install your store-bought pump, but I wouldn't bet on it.

At this point, look at it as a preventive maintenance item if it doesn't fix your problem.

It's worth a shot, imo.

Also, if you don't have a proper RX8 fuel ring tool, you'll want to get one, especially if you have a new fuel ring.
Those rental ones are a PITA & tear up the tabs on the ring.

I have the BHR tool, worth every penny.


https://cimg5.ibsrv.net/gimg/www.rx8...e87effc717.jpg

Yeah I definitely don't think I drive it hard enough for mileage that low. Can't get away with it unfortunately due to traffic in my area always being awful.

Just to play devils advocate, wouldn't the fuel pump be more of an issue at high rpm rather than low? I'll check into the recall. I'm pretty sure mine either had it done before I got it or it didn't apply to mine.

My non-highway fuel consumption driving would be around 13-15mpg, so not getting more than 16mpg is pretty normal.
Rotaries are thirsty, that's why I don't ever-day drive it....

Be that as it may, I still feel like there's something not right, MPG related or not 🤷‍♂️ the car is borderline dangerous to drive getting started from a stop. Watch this video to see how long it takes the car to pick up from a stop. That's with my foot to the floor.

Internet car mechanic is already hard to play as it is. Without OBD2 data on a modern car, it's really just a crapshoot of what it can be.

Get the OBD2 reader before you throw parts at it.

Roger that. Let me know if there's anything other than warm idle fuel trims I should check. I have a buddy with a Snap On scanner, I'll try to meet up w him this coming weekend. Will check back in as soon as I can.

Sooo the MAF made no difference. And to make matters worse, I broke my clutch foot and am in a boot for the next month, so I can't take it to my buddy's place to use his scanner to check fuel trims. Once my foot is back in good shape I'll get over there and let you know what I find.

Hey so I had my buddy who used to have a 8 come over and help me out today, digging down to the solenoids behind the UIM to test those for proper function. While doing that, he raised a good question. Is there any chance this could be injector related? If so, what type of injector fault would cause this issue? The finer points of EFI aren't my strong suit. We ran out of time so I didn't get all the way to getting the solenoids out. Will be back in touch once I do that.

It's hard to confirm or discard any theory without OBD data. While it *could* be injector related, but injector staging is based on load not rpm, so it wouldn't have a clean cutover at 4000

Gotcha. I talked to my buddy with the nice snap on scanner, he said I can use it whenever. So as soon as my broken left foot (literally) is better and I can drive a manual again I'll take it up to him. You want fuel trims at idle, and what with the injectors, if anything? And anything else?

I'll also be sure to get a photo of my injector wiring, somebody (maybe you?) said theres a small chance my injectors may have been reconnected wrong if anyone was ever in there for any reason. And of course will report back if I find anything going on with the solenoids once I'm back in there. Hoping it will be warm enough tomorrow to get to it.

On a warm idle you'll want airflow g/sec, short term and long term fuel trim and rpm.

I -may- have found my issue. There appears to be a gap at the top of my SSV, as though it isn't closing completely. This would be in line with my suspicion that the SSV wasn't closed when it should be. Does anyone know if that gap belongs there?

Otherwise the SSV moves smoothly and holds vacuum. Please tell me this is it!
https://cimg7.ibsrv.net/gimg/www.rx8...1bdd29a509.jpg

It could be your VFAD. If you take your bumper off, it's behind it. It takes in air at lower RPMS from one gate, then when you go higher, it takes it in from a much shorter gate, and therefore more power. Maybe it's faulty. Its regulated by a microswitch thats mounted behind the stock air intake- that could be faulty too. Do you have an aftermarket air intake?