Spontaneous surging RX7

[wtr]

Evening, farkers.

A mate of mine owns an auto FC3(RX-7), and has been experiencing strange spontaneous surges lately. It seems though when he shifts to neutral, she just revs up and fluctuate between 3.5 - 4rpm. when he tapped the gas gently thinking it might be the throttle cable playing up, she steps up revs even more and continues to fluctuate. So far the only solution is to restart her, and the problem will be gone until next time when it manifests again. Normal idling was tuned by a shop to sit at 1.5rpm, which I think is a bit high...

She remains a stock except for a custom radiator, although the air flow meter has been slightly positioned on an angle due to piping. The boost sensor has been unplugged to avoid fuel cut.

With the cooler sitting above the throttle body, he's reluctant to take it off. I've had a look around for loose/disconnected vacuum hoses and found nothing but a hose with a filter of some sort unplugged, the hose neither sucks or blows. See circled area.

She also has a tendency to flood herself just before shut down. with all these symptons playing up, could it be the ECU along with a few loose vacuum hoses?

He's due to go oversea soon in 7 days and won't be back for 7 months, and is desperate to know what's wrong so he can at least leave with an answer.

Your valued inputs will be much appreciated.

 

[Mattyp]

u could jump on www.ausrotary.com and ask there, i have an FC rx7 myself, but dunno what that thing is in the pic my car hasnt got it...similar position to where my fuel filter is though....
flooding could be caused by shitty spark plugs....
take the cooler off, its 4 bolts and 2 hose clamps, takes 2 seconds, have a look around for any leaks....

 

[donthucktoflat]

should idle at around 900-1000rpm

take it to somebody who knows rotaries

 

[jjperko]

Total novice here but it could still be the throttle cable. I had the same problem in a 944, turns out the throttle lever on the engine was a bit sticky resulting in a varied idle. Experts don't shoot me down as I know nothing about rotaries but I had the same symptoms and thats what it turned out to be.

 

[BM Epic]

Mate
I suspect it will have something to do with a vacuum hose that either goes to the gearbox(auto),it sounds as though it will be an air leak!
Or it will be a hose located at the back of the engine!
Either way it sounds like the surge is a vacuum leak!
Good luck!

 

[Yowie]

Check your vac hose to your brake master cylinder, common on this model its a funny sort of a step down for some reason, but having said that never worked on a auto and the neutral thing is a bit strange vac hose to trans maybe?. Also 1500 isn't too out of the ordinary if your seals are getting a bit long in the tooth.

 

[daever]

check your idler pulleys, exhaust cam gear angles, crank position sensor, valve springs, piston rings, end bearings, plenum for cracks, ports, head gasket, rockers and finally your supercharger inlet.

 

[wtr]

Mate
I suspect it will have something to do with a vacuum hose that either goes to the gearbox(auto),it sounds as though it will be an air leak!
Or it will be a hose located at the back of the engine!
Either way it sounds like the surge is a vacuum leak!
Good luck!

Hey mate, I too think that an air leak is very plausible. But to spontaneously rev up to 3.5rpm only when idling in neutral, wouldn't the throttle blade be opened even to the slightest degree?

 

[lebronmtb]

check your idler pulleys, exhaust cam gear angles, crank position sensor, valve springs, piston rings, end bearings, plenum for cracks, ports, head gasket, rockers and finally your supercharger inlet.

I've got a pretty good feeling his car doesnt have half of that stuff seeing as its a rotary,

Also to the person who said it should idle at 900-1000, don't rotors idle high, or do they just rev high?

 

[BM Epic]

Hey mate, I too think that an air leak is very plausible. But to spontaneously rev up to 3.5rpm only when idling in neutral, wouldn't the throttle blade be opened even to the slightest degree?

You mean venturi,
Now i am not 100% sure on this,the answer is no,because vacuum will make the distributor advance,hence the revving,even on an old carb engine,it wont manually open the butterfly or venturi.
It changes the timing,which will change the revs!
But,then again i am not sure,what i am sure of is that a nearly identical thing happened when rebuilding a mates engine,that it ended up being a vacuum hose having a split in it.
Try the simple solutions first before trying anything complex and difficult,checking these hoses is very simple and wont cost anything to check!
Again,good luck!

 

[daever]

so when she "revs up" after you let off the throttle, does it go down to idle then creep? or does it just not dip below said rev's after you throttle off?

It's not your throttle cable; it would not correct itself when you restarted. Plus you would see and feel a bit of slack in the line.

if it creeps I'd put my money on an electrical fault, most likley a fucked AFM telling the ecu your sucking in lots of air. Your engine compensates so it doesn't lean out and go boom by shooting in more fuel. this would make it rev higher, spool up your turbos and suck in more air and creep. that would also cause it to flood alot and run rich as hell. What colour is the exhaust gas? If its black then you are running rich.

Also, try plugging your boost sensor back in. yoink it when you get a proper fuel cut defender.


check your O2 sensor, In your exhaust manifold if its there.

but its more likely along the lines of a sensor/afm issue.

 

[wtr]

You don't even have to tap the throttle when in neutral to bring the revs up, she does it for you. We've determined that the cable is relevant because pulling the pedal back with his foot when she does that changes nothing. The thing is, she doesn't stay at a specific rev, but rather fluctuates by 0.5rpm. I still reckon it might had something to do with the vacuum hoses.

As with the O2 sensor, he's only just changed it last week. The boost sensor was unplugged because he's a tight arse. The AFM might play a part in here, could do with a clean too. But the pipings are all secured so leakage before the throttle is unlikely, unless there's a hidden crack. The only time she smokes a bit is upon start up, because she tends to flood herself just before shutting down, which could be the AFM's fault or a bad tune. Creep wise, nil.

Thanks for the speedy feedbacks tonight though, much appreciated.

 

[donthucktoflat]

I've got a pretty good feeling his car doesnt have half of that stuff seeing as its a rotary,

Also to the person who said it should idle at 900-1000, don't rotors idle high, or do they just rev high?

you seriously didnt get the joke?

no, rotors idle at a regular idle normally.. they rev high.

 

[mtb1611]

Could be a faulty spring in the throttle actuation assembly. Had this happen on a car of mine a few years back.

 

[lebronmtb]

you seriously didnt get the joke?

no, rotors idle at a regular idle normally.. they rev high.

Do you mean regular compared to a piston engine, I honestly have no idea what an rx7 should idle at but I would have thought higher, also your original post didnt really indicate that it was a joke

 

[slip]

Could also be intermittent inlet manifold leak. Check all vac lines that connect after the throttle body especially, as they can provide air unhindered by the throttle body. Air bypass system (it's called different things on different cars - the system that controls airflow around the throttle when close to regulate idle) could be playing up also.

Taking the intercooler off is very quick and easy. If you can replace this with some piping while doing diagnosis it will make everything a lot quicker and easier.

Look for vac leaks with things like WD40, Start Ya Bastard etc that you can spray onto lines, look for bubbling or idle speed changes. Also just bend lines and look for little cracks.

 

[rollie87]

seriously, just make a thread about it on ausrotor, although chances are if you do a search there it will have already been covered

the amount of times people come onto ausrotor and make a new thread about something that has already been covered is mind boggling!

 

['Ross]

the amount of times people come onto ausrotor and make a new thread about something that has already been covered is mind boggling!

Reminds me of a certain mountainbiking website People often come on and make a thread about 'Bender', 'How to make my STP singlespeed', and 'what colour to custom paint my STP'. Funny that.

 

[rollie87]

hahahahaha, except on ausrotary.com its along the lines of.....

"ZOMG, did you know mazda made a kingswood with a rotor in it??"

"ZOMG, did you see these pics of the new rx3" - the photos in question are always someones shitty photoshop *rolls eyes*

or

"ZOMG, have you guys seem the new rx7!" again, is nearly always a photoshop, or just some concept car that mazda is working on that kind of resembles a rx7/8

 

[floody]

I'd agree its sucking air somewhere OR a vacuum related issue of some sort. Revs will go up and down as it tries to compensate with fuel. Worse in neutral as its not under load, so a fluctuation in vacuum will be more noticeable.

Plug in whatever sensor it was you've removed; check all air pressure/mass/speed metering devices are working, and go over all the vac hoses.

Hmmm...check the hoses going to the pollution canister, I had two reversed on mine and it was pumping/sucking air unmetered direct from the intake manifold; a vac feed to a diaphragm on the charcoal canister and a return for fumes to the intake were reversed. As a consequence I had a high idle and some strange overfuelling issues.