1.8NB - old FMII - 'judder/shudder' when releasing the gaspedal - Miata Turbo Forum - Boost cars, acquire cats.

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Old 07-08-2011, 08:34 AM   #1
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Default 1.8NB - old FMII - 'judder/shudder' when releasing the gaspedal

Darn it, I really can't explain my problem any better in the title. Blame it I'm dutch, lol.

Now I have this '98 MX-5 (yes, a '98 NB!) with an old FMII turbo kit. You know, the intake manifold with 4 extra injectors, dual fed fuel rail, Link piggyback ECU, GT2554r turbo, intercooler, 2.5" exhaust, blabla, 9psi.

Anyway it drives great but I'm always wondering; When I'm doing some spirited miles on a twisty road, I'd like to be able to release the gas pedal like I was able on a non-turbo miata.

How can I explain this... You're driving kinda fast, keeping the miata in the 4500-7000rpm range throughout the corners. I'd like to be able to loosen my throttle in front of a corner, and not shift into a higher gear because a turn is coming up. (So mostly I'm in like 5000+ rpm here)

In my older, non-turbo miata, this wasn't a problem, but when I do this in my current car, it seems the turbo doesn't use it's blowoff valve and so the car 'judders', it doesn't feel nice/right and it makes me not want to lift the gas pedal and shift up, but that does not make sense in front of a corner

It doesn't happen when I really slowly release the gas pedal, but that never works in practice. (Not when you drive kinda aggressively anyway) When I shift into higher gear, all is well and the blowoff does it's thing.

So is this normal on a turbo car? I had a 6psi Greddy'd miata too, and that didn't 'judder' when I released the gas pedal...
Or is my blowoff valve not working correctly?

Hopefully I can make myself clear, like I said, English ain't my native language, lol.

Sorry for being such a n00b!

Oh yeah, here's a link to a movie of a miata that's doing what I would like to be able to, if you listen closely and look at the tach, you can see a little bit of judder/stutter too, but this is probably normal, can't remember.
http://vimeo.com/8525217

Last edited by prxprx; 07-08-2011 at 11:59 AM.
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Old 07-08-2011, 11:51 AM   #2
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Driveline shudder? I'd check your engine mounts first. If their toast, you'll get that. Does the shifter torque side to side when this condition occurs?

For the sake of this thread, when I think of "stutter" I think of a engine misfire or interrpution in power delivery. For "shudder" I mean a vibration or shake from the car or driveline...like having a high-grip clutch from a start causing the car to lurch/jump/shake.
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Old 07-08-2011, 11:58 AM   #3
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Hmm it might be driveline shudder, yes. I do think the shifter moves when this happens, but it might be because the car just shakes a bit.

I'll change the title, I surely don't mean a misfire.

And yes, it feels kinda like releasing the clutch too early/fast (FM Happy meal Stage 1)

For the sake of whatever, here's a picture of the engine bay. This was still with the old spark plugs + leads and without some of the later added heat wrap and stuff, but /care.

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Old 07-08-2011, 12:10 PM   #4
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Go into your idle settings and reduce the "max vacuum" value by 10 to 15%

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Old 07-08-2011, 12:16 PM   #5
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I only have the piggyback Link ECU so can't really edit that, or am I missing something? (Only have the manual here, will check the car when I'm at home later)
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Old 07-08-2011, 12:30 PM   #6
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ahh, link - not sure how that works

Edit: from link tuning manual.

Problem: The idle speed drops or the engine stalls when coming to a stop.
Solution: When the ECU registers high vacuum followed by the * and the n, Z18 is used to
elevate the idle speed to keep the engine running. Raise Z18 a point or two until the idle speed
hangs at ~1100 RPM for a second or two when coming to a stop. Make sure to raise Z19 as
well if necessary. Also check that your IAC duty cycle when idling is not too high.
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Old 07-08-2011, 01:01 PM   #7
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Thanks for looking into it, but it's not that the engine is having a difficult time, it's just that the whole car shudders a bit when I quickly release the throttle at 4000+ rpm.
If I were to shift up at that point, nothing happens. If I decide to let rapidly let go on the throttle, the car shakes.
Darn it, hard to explain!!

The problem they describe, is not the problem I'm having.

I'll try to make a movie with me driving my car in the weekend, that surely will clear things up.
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Old 07-08-2011, 01:34 PM   #8
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So decel in gear makes the car shudder. I think I know what you're getting at because I had a mild shudder like that after going to MS. I dont know **** about link, fuel settings, decel settings?
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Old 07-08-2011, 01:55 PM   #9
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Yes, that's what happens. It feels a bit the same as when you are dropping the clutch a bit too fast. (ie when shifting done without a rev match)

I've bodged up a movie with some bits and pieces I had laying around. This was when we just finished the car so don't hate my driving, lol. Car wasn't tuned yet so was slower than it is now, but the effect is kinda present. It isn't as pronounced as I feel it is, but you might be able to notice the effect.

http://www.youtube.com/watch?v=3BzqRLpOjPk
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Old 07-08-2011, 03:51 PM   #10
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That's a mighty clean engine bay, nice!

Did you install new engine mounts or have upgraded ones? If not, at least get the 2004/2005 Mazdaspeed MX5/MSM/Roadster Turbo engine mounts. If you do a search for threads started by me, I have a whole thread on engine mounts with part numbers.
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Old 07-08-2011, 03:58 PM   #11
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Thanks! You should have seen it when we got it, was completely stock and incredibly dirty.

As for the upgraded mounts; Nope, no new engine mounts... So that could be cause one? Any other options out there? (A friend of mine has a 300+ rwhp miata without upgraded mounts and I can't remember this phenomena when sitting next to him, but then again, he didn't really ever ligt of the gas.

Of topic, this is my car right now; Yes, wheels are too big and heavy, we stupid Europeans.



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Old 07-08-2011, 04:13 PM   #12
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Engine bay looks very clean... good job ***.
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Old 07-08-2011, 04:16 PM   #13
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Does it feel almost as if it doesn't know if its in boost or vacumme. Might have had that same problem. I have MSN type mounts but on mine before I tore it apart it had a slight hesitation feeling going from cruise throttle to slight less cruise. Kinda herky jerky feeling.
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Old 07-08-2011, 04:17 PM   #14
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Well, being on MT.net I'll take that as a compliment.
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Old 07-08-2011, 04:19 PM   #15
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Quote:
Originally Posted by prxprx View Post
Well, being on MT.net I'll take that as a compliment.
don't fight it... you're going to like your stay
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Old 07-08-2011, 04:27 PM   #16
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@Zombie: Well, I always thought that when I rapidly let go of the throttle, the blowoff valve would do it's thing, like the whoosh you get when dumping the clutch and shifting into a higher gear.... It feels like mine isn't doing that. (although the boost on the gauge does drop, I think)

It feels like when all the boost is there and you let go of the throttle, it can't release the power so... the car sort of 'chokes', like it can't put down it's food, lol. Does this make sense? (But it doesn't misfire or anything, just judders/shudders/herky jerky like in the movie I posted.)
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Old 07-08-2011, 07:02 PM   #17
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Do the obvious. The BOV is easily heard. When you get off the throttle with boost, the BOV should do its thing immediately. If it is not, then fix the BOV. Hell, how much is a BOV from siliconeintakes.com anyway? $40? That's like 1 euro, right?
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Old 07-08-2011, 07:21 PM   #18
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You know what, I'll make a better movie to show the exact symptom this weekend... It might be the bov itself, or maybe the motor mounts are toast... who knows... I know the bov works when shifting, I can hear it loud and clear, lol. So it probably works as I let go of the throttle too, because it just should. I might just remember wrong.

Maybe the spring inside the bov is wrong...

I'll get back once I've got the movie.

$40 is indeed like 1 euro, but our dutch customs want like 25% of every purchase and shipping is as high as like... 2 euro, so in total it'll be 3,25 euro's.
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Old 07-08-2011, 08:56 PM   #19
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OK, let's see . . . you can hear the BOV when shifting. You can't hear it (apparently) when you just let go of the throttle.

Makes absolutely no sense. The BOV sees boost on one side and (with the throttle closed) vacuum on the other. It should respond appropriately. It has NOTHING to do with shifting. It either blows when you get off the throttle or it doesn't.

Why are you making this so hard?
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Old 07-09-2011, 02:15 AM   #20
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I don't think the OP is lifting 100% at higher RPMs when geting "the problem"...I'm pretty sure he is referring to when he lifts just a little to adjust the car/keep from hitting redline/not change gears when in the twisties similar to how the person in the above posted video is adjusting the throttle in higher RPMs. In his case, when he does it, the car is shuddering for some reason. That help you out hornetball?

OP- Have you ever adjust the tightness of your BOV? Is it adjustable?
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