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Old 03-16-2014, 10:38 PM   #1
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Default Full throttle randomly feels like the brakes were applied, not sure why?

So when I'm driving my 2001 Miata (stock ECU/engine/intake/injectors) and at full throttle, usually from 2800-5700 rpm ish, the car will suddenly stop accellerating as though I'd hit the brakes.

When it happens, the exhaust note somehow sounds hollow. I'll then let off the gas and it'll be fine.

This only happens at full throttle, so I'm not sure why.

Judging by the age and miliage (149k) of the vehicle, I'm thinking it could be:

Fuel pump going out: Doesn't seem likely, my last pump went out and the car just wouldn't start.

Injectors issue: Not sure why or what.

VVT failure: Given the rpm, it could be this instead of a MBT related issue.

Spark blowout: New coils, wire and plugs 4k miles ago. Seems unlikely.

Engine blown: Not sure how this would feel like that, but given the amount of redline/full throttle use (pretty much any time, all the time) and milage, could be. Car has been running lean for a while as I wait for an ECU.

So, not sure. I need to keep driving, so I'll see if it gets worse, but I'd really appreciate help with fixing my daily driver.
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Old 03-17-2014, 02:25 AM   #2
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At 150k, I'd start at the pump. It's the easiest to get to and the cheapest of the bunch. Fuel pumps fail funny. Sometimes it's just dead one morning, sometimes they cause fun drivability issues that make you pull out your hair chasing the ignition system. Sometimes they die in the shop while you're getting an oil change.

I'd also double check everything on the coils wires and plugs, since you touched them last. It's also dead easy to do a leakdown test with the plugs out, which will answer your questions about blown motor.
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Old 03-19-2014, 02:03 PM   #3
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My fuel pump would cause the mixture to go lean under acceleration but would cruise fine. Sound familiar?
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Old 03-19-2014, 02:23 PM   #4
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Sounds about right. Ordered fuel pump yesterday, should be in tomorrow. Will post results.
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Old 03-19-2014, 02:30 PM   #5
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Quote:
Originally Posted by sixshooter View Post
My fuel pump would cause the mixture to go lean under acceleration but would cruise fine. Sound familiar?
I had the same failure mode. I'd get a wicked stumble on acceleration that I improperly diagnosed as the FPR going out. Fuel pump fixed it.

I've got a used 190HP somewhere in the garage if you want to give it a go, shoot me a PM. Nevermind, sounds like you've got one on the way
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Old 03-19-2014, 02:36 PM   #6
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Thanks for the offer, but denso + filter from rock auto is here tomorrow.
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Old 03-20-2014, 08:03 PM   #7
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Damn. Still happening with a new pump.

Sock on the old one was ripped. Maybe it killed the new fuel filter?

I need to check my plugs.

Ecu just came in, tempted to throw on the wideband and MS3E to see what's going on but don't want to change things and make troubleshooting worse.

Any ideas?
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Old 03-20-2014, 08:16 PM   #8
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Verify pressure? It's a little harder to do on an NB because it does not have the convenient hose clamp connections for the fuel lines like on an NA but it can be done.

I've got access to a couple spare NB in-tank regulators if you need one.
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Old 03-20-2014, 08:30 PM   #9
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Would the ms3 have a read on fuel pressure?
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Old 03-20-2014, 08:33 PM   #10
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There is no stock fuel pressure sensor so unless you add an aftermarket one, I'd say no. :(
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Old 03-20-2014, 10:40 PM   #11
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What would be the symptoms of a failing in tank regulator? If you think it's a good thing to replace, I'll definitely buy one off you!
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Old 03-21-2014, 11:30 AM   #12
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I'd imagine it would not be able to keep up with demand and go lean under accel, maybe getting lazy or something? New filter and pump leaves regulator and injectors. I'd be happy to loan you a stock reg for the sake of troubleshooting this, can't guarantee it'll solve anything though.

You have a wideband for that MS, right? Install it and see what happens when you go WOT and get the stumble. All it will cost you is a bung and maybe $20 if you don't have access to a welder.
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Old 03-21-2014, 02:46 PM   #13
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Yeah,I have a wb, was thinking of installing the lot to get all the readings but it could make troubleshooting a nightmare.

I'll get that regulator from you though?
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Old 03-22-2014, 02:46 AM   #14
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Just checked the spark plugs. They look good. Perhaps a touch lean, but definitely nothing that should be causing spark blowout.

I'm stumped. Up next would be maybe a new fuel filter? The one on there is only about 2k miles old, though, but I do run down to E on the gas and the fuel pump had a torn sock, so I'm not sure. Fuel injectors, maybe? Fuel pressure regulator? This is my daily, so I'm anxious about it all.
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Old 03-23-2014, 05:33 AM   #15
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Alright, this is interesting.

Checked codes, have misses and inconsistent cam/crank signal.

Replaced cam sensor, idles much better, still locks up while accelerating.

So I pull the crank position sensor, and I'm trying to get the new one gapped properly...

And all the four nubs are a different length from the sensor. I've read that means the trigger wheel is broken. That I did not expect.

Will the crank sensor work fine with some scoring (plastic scratching) from the trigger wheel?
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Old 03-23-2014, 01:44 PM   #16
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Trigger wheel isn't plastic.
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Old 03-23-2014, 01:52 PM   #17
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I believe Erin is saying the trigger wheel cut into the crank sensor.

That's... a problem. Do you have access to another trigger wheel and sensor? I know the sensors are not exactly cheap.
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Old 03-23-2014, 03:33 PM   #18
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Hmm. It's hall effect VR sensor, which works by having the trigger wheel teeth disrupt a magnetic field. If it hasn't broken through the plastic I'd try it with a different wheel and see if it doesn't work. If the scratches are superficial, it should still work. There might be a problem if enough plastic has been damaged to affect measuring the proper gap.

Now the question is why that happened. Was the single bolt holding the sensor loose, allowing it to move and make contact with the wheel? Or do you have a bearing problem allowing the front of the crank to move around?

If you're ready to go MS3, you might just want to get the 1.6l Protege 36-1 wheel to replace the stock four tooth. Not on VVT.

Last edited by TheScaryOne; 03-25-2014 at 07:34 PM. Reason: Corrected by Joe Perez
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Old 03-23-2014, 03:42 PM   #19
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Quote:
Originally Posted by sixshooter View Post
Trigger wheel isn't plastic.
I'm... well aware of that. What an odd comment.
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Old 03-23-2014, 04:37 PM   #20
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It's actually a VR sensor with a built in conditioner, not a hall sensor, which is an important distinction in this case.

The sensor is unlikely to be damaged by having a 1-2mm gouge in it from the wheel. The electronics are located back within it some distance, the visible tip is just steel.

That said, your problem certainly sounds electronic rather than mechanical, and the NB cam sensor is notorious, with the crank sensor slightly less so.
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