Repeatable, instantaneous hiccup ~3600 rpm when MAP > 130 kPa - Miata Turbo Forum - Boost cars, acquire cats.

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Old 10-30-2014, 10:36 AM   #1
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Default Repeatable, instantaneous hiccup ~3600 rpm when MAP > 130 kPa


I'm trying to "l2t" as you folk often say, and I've been stumped. This is not a LINK specific query, so please don't bury me in that poor, neglected subforum. I am interested in the engine components which could cause the behavior described.

My car has a wicked stumble which consistently causes the car to lurch as if I'm hitting a spark/fuel cut for 1 cycle of the engine. It is an instantaneous affair; the lurch is less deceleration and more re-engagement of the driveline as the backlash is absorbed. I have not been able to find any other documentation of such an issue - most "stumbles" I've read about occur over a range of rpm.

Past the lurch, it will run to redline with no issue. It actually seems to gain power after the lurch, mimicking the behavior which some report of the stock ECU's transition from open to closed loop fuel management. However, my standalone LINK is set up with a wideband and uses closed loop all the way. It will also accelerate smoothly if the MAP is kept below the threshold.

I'd love to think that it was a simple electrical fault, such as plug wires, ignitor, etc., but it has an almost mechanical consistency of the failure mode. The lurch only appears if the MAP is above ~130 kPa and hits at an rpm between 3500-3700. It doesn't matter if I'm WOT or 3/4 throttle, it lurches all the same.

I've tried removing as many variables as possible from the equation, including turning off all boost management and running the ~6 psi spring wastegate only. No change. I've tried adding more fuel locally - no change.

Pictured is the data log of one such incident. As can be seen, nothing spikes but the AFR. The rpm wiggles up and down a bit as well. However, those look like side effects, not the causes. Ignition, fuel, MAP, TPS, everything remains changing at the same rate before and after the lurch.

I know the LINK is an outdated ECU, but the fault seems to be an engine issue, as the instructions the ECU is giving (or says it's giving) look OK. What could cause such a consistent issue?

Thanks for your help.

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Old 10-30-2014, 04:05 PM   #2
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What are your plugs gapped at? Lean misfire is the only thing that pops to mind if it's a mechanical issue.

Or could be a noise issue, like loosing sync. Try cleaning all of the grounds?
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Old 10-30-2014, 07:32 PM   #3
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Plugs are new and gapped to 0.027 in.

I'll try cleaning the grounds.

On the way to school today, the car suddenly threw a fit - the engine would die if I pushed the throttle more than 0.5 in, and there was no torque to get off the line from a stop. Then after 2 minutes of limping through downtown the engine wouldn't even stay running. After starting it would instantly die.

I turned the power off and on again (cycling the ECU) and it purred like normal. I drove to school and back (60 miles) without another problem.
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Old 10-30-2014, 07:39 PM   #4
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Didn't Ed have an issue like this?
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Old 10-31-2014, 02:38 PM   #5
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Ken Hill had this to say on a similar problem:

This problem has cropped up on a few cars I have tuned. Take at look at the INJ% screen while driving. Does the stutter occure at 22% duty cycle? If so this could be caused by mis-wired fuel injectors.

Rod H got my point, make sure each output from the ECU is going to the correct injector. (i.e. output for cylinder #1 goes to the fuel injector in cylinder #1.) It is possible to have the outputs crossed and the car will run fairly well, but it will stuble when changing over from sequential to batch, 22%.
FM2 Link stutter/missfire - MX-5 Miata Forum

I would love to think I didn't screw up the wiring when I remade the PO's injector harness, but there's the possibility that it was wrong in the first place - this stutter has been present all along. The fact that my stutter does indeed occur at INJ ~22% seems to confirm that this has a good chance of being the issue.

I'll check it out when I can. 5:00 come quickly.

EDIT: Confirmed, PO wired sequential injection incorrectly. This explains so many problems, I can't believe I lived with it for so long.

Last edited by Wowbagger the I.P.; 10-31-2014 at 08:34 PM.
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