Stuck at a certain rpm
 

Hi miata lovers,
Two days now my car have a big problem i can t find a answer to!

Without any kind of warning my 91' 1.6 miata started to get stuck at a certain rpm and can t go up further. It happen between 2'000 and 3'000rpm (seems not exactly the same rpm each time) and almost constantly althrough there is some few times the car function perfectly fine for a few minutes. At this precise rpm the car can t take one rpm more (reminds me a limiter or a motorcycle antiwheel slippage) and start shaking a bit (like a bycilinder or a car with cylinders out of work type of rumble). When i put the car in neutral it revs perfectly without any hesitations.

i checked for a clogged cat (i have a normal air flow at the end of the exhaust to) and for plugs wires that seems to work well (i have 5'000miles "new" plugs, fuel filter and injectors and cat). My maf seems clean.

my afr is very good all long as well as my o2 sensor values (even when the problem happen).

Could you please tell me if it could be transmission related (i get a lot of clutch slippage) because in neutral i dont have any prob or if my problem happens only when i'm in load for other reasons?

i would like to thanks you for your help, i'm a bit isolated in europe when it comes to miata and if i can enjoy this pleasure it s largely because of you!

Is your car on the stock computer or do you have an aftermarket ECU?

Just because it seems like you have normal exhaust flow at idle, doesn't mean the converter isn't clogged. Pull out the pre cat O2 and try seeing if that changes anything. Reducing the restriction should slightly improve the concern if it is the cat. The converter could be partially clogged where when you put a load on the engine, it is forcing broken cat material against the exit side of the converter.

it will also help to let everyone know what type of ECU you are running.

Hi, sorry for the late i wasn t at home.
i have stock ecu with a tdr card (idle valve is recirculated via a check valve to prevent leaks). For the last 4'000miles i checked my afr that was perfectly regular and very nice (11.5-12@wot, 14.6 @idle+ cruise).
i found out that the seal of the idle valvle on the intake manifold wasn t good (my mistske it was during the change of the injectors) i also find out that the silicone hose of the integrated wastegate was damaged.
Now the rpm blockage seems to have disapear (i had strangely had it one or two time). The engine is back to a good afr when running...the only change is that when i release the gaz at low load and low rpm, the afr go from 14-15 to 11 and then lean (before it was directly from 14-15 to lean)... the other big problem is that my afr at idle is at 14.7 (like before) for a few second and then it slowly decrease to 10-11 (so, harley style, low and iregulate idle).

does somebody can help me?

Yes. Don't try to run a turbo car on the stock ECU.

ok thx for the advice. I dont have choice for many reasons. I already made 5000km with this config (only mountain pass) and i m sure i ll do a lot more when i ll found the solution that is, i m sure, not related with my stock ecu + tdr config. Are the idle valves prone to fail?

Stock ECUs are designed for NA setups. Just cause you drove X miles on the setup is irrelevant. We cant help you if you just give us your AFRs. We need more information .Did you even try the test I suggested to check for a clogged converter?

no, i'll love to do this test, but it s for me not possible to do a single mile with that noise and without a cat (strict laws there that explain stock ecu too).
based on what i read and the way the car was running, i still think i can have a reliable setup with stock ecu (how FM could sell these if it was not fontional). I perfectly understand that stand alones are THE way to go and i know that i much limited than you all, i m ok with it.

The fact that only the idle is affected make me think to idle valve trouble...what do you think... i change the seal between it and the manifoild but does a bad idle valve could cause it?

:facepalm:

Listen to me, an idle valve will not cause a car to not drive over a certain RPM under load. Under load means the car is moving and the engine is performing work. Just cause you rev the engine in neutral is basically free spinning the engine. It isn't trying to move the vehicle thus no load.

You need to perform the test I suggested to check for a clogged converter. You dont even have to drive it far, just down the road and back. If you have a clogged converter, and the O2 sensor before the converter is removed, your car should be able to accelerate more due to less back pressure. If you don't want to do that buy a back pressure gauge and install where the O2 sensor is. That will show you how much pressure is building. When the exhaust cant get all gasses out of the system it builds and builds causing the engine to run poor. An engine is basically a giant air pump. It pulls air in then expels it. If it can't expel the air then it's efficiency is limited thus the limit in RPM range.

You're using the stock ECU with a piggy back adapter. The RPM limit should not change. Do you have any data logging capability with the TDR thing? I'm not familiar with that unit so if you can post the link so I can see what it is I could help you out more.

don't waste your time on this guy. just move on to people who actually listen

I think my explaination of the problem were bad too...
he will explain it better than me : "As soon as I get on the gas, the car behaves almost normally, but is reading very rich. Under cruise/normal load I read 10.5-11.5, and when engine braking it still stays around 12, doesnt lean out as it normally would. The last oddity is when I go under heavier load, the AFR goes to about where it should, around 13. Idle is also reading 10-11 when normally it is 14-14.5"... that s exactly what s happen with me!

What happens if you disconnect the TDR card? Does your idle stabilize at 14.7 or is it still rich? I would suspect the piggyback is somehow interfering with the normal idle.

On your words i did it and it s exactly the same (note that nor the tdr nor the red injectors did this problem at install)... the problem started at the EXACT time i started to have this kind of low rpm limitation (this limit seems at the exact instant the turbo would normally start spinning)... it disapeared when i changed the idle valve seals but today it started again (rpm limitation) to complement my riche idle...

Are you reading the AFR from your own wideband or the stock lambda sensor? If not, you might want to check the value that the stock lambda sensor is giving during idle.

i read it from my afr and my o2 sensor. Values are perfectly correlated if it could help (DAMN RICH).

realise i can resume my question like that... is this something that is prone to faillure (i couldn t be intrinsequely due to my setup wich proved to be ok for long) and could cause both my two symptoms that are : permanent rough/rich idle AND temporare rpm blockings (it s related for sure i felt them comming at the same second).
I check ALL my tubings that are all perfectly fit (checked the turbo screw wich is as new).
i also wondering if a failling hose between the turbo and the integrated wategate could induce dommage or deregulate something that provoke my symptoms (for me only a compression leak on side and a full close exhaust side wastegate clapet)? Or at the opposite is something related-to or induced-by my symptoms could lead a wastegate hose to implose.

why do all your posts read like spam emails?