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1.6 spec miata down on power

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#1
mbican

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Run out of ideas, could use some suggestions. 1990 1.6 down on power, flames out of exhaust when getting off throttle. Checked everything I can think of.,

No fault codes.

New plugs and wires,plugs looked okay maybe a little lean.

New fuel filter and fuel pressure good

Compression around 185 all four

Timing belt marks good, belt two years old

New O2

Timing set at 14 BTC

New fuel pump

Injectors serviced by Advanced last winter

Funny thing when checking timing when blipping the throttle mark would go way retarded before advancing

Car ran poorly all summer except for one day at Putnam Park when it ran strong did a 121.98, no afterfire from exhaust, next time out same issues, any ideas would be appreciated.



#2
chris haldeman

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Verify proper operation of the tps sensor
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#3
mbican

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Checked TPS okay

Idle / middle / wot 



#4
Steve Scheifler

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So, as usual in these situations the list is long and most often the solution is found by swapping out parts one at a time. The flame thrower symptom suggests excess air in the system as much as excess fuel but that could just be a leak at the head pipe or something. The fact that the main symptoms went away for a session is probably meaningful but ultimately it’s still likely easiest to find someone willing to supply you with parts for swapping. Some things are easier to swap than others so you may start there even if they aren’t necessarily the most likely. I’d probably start at the TPS (actual swap not just measure), AFM, coil, ECU. Injectors ate less easy and probably less likely but you may eventually get there. For example there may be one that drips or squirts a stream rather than spray a mist, but you should probably see that on the plug.

You show a new O2 (sensor) but not AFRs. You mean the stock one? It doesn’t really play any role under race conditions but you might be closer to an answer already if you had a proper wideband installed.
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#5
Bench Racer

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Is your fuel pressure gauge where you may view when driving on track? If not extend hose and mount the gauge on the cowl outside the windshield.


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#6
Brandon

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Funny thing when checking timing when blipping the throttle mark would go way retarded before advancing

 

Does the 1.6 have a crank sensor? Could this be endemic of the rubber portions of the harmonic balancer being rotted and allowing unwanted motion between actual crank rotation and read from the CPS?


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#7
Steve Scheifler

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No crank sensor.
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#8
Steve Scheifler

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mbican, you are practically a neighbor of SaulSpeedwell, he may know someone with a pile of known good spares, and even hook you up with a racer-friendly dyno if needed. Or take a couple days and come down here, I’ve got all the spares and dyno time you want just for the asking.
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#9
mbican

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No fuel pressure gauge readings under load, will get hose into view gauge, replaced fuel pump because it was probably old. Had spare ECU, kept old one that had bad baro sensor. Swapped out AFM with used one, checked coil readings were good but got a new one. AFR is on my list to get next. TPS one of few things not swapped will try next. Pulled and checked coolant temp sensor hot and cold readings ok. Funny when pulling temp sensor no coolant came out but appeared to be enough to cover element.

Thanks for the offer Steve, may take you up on that when I've pulled all my hair out.



#10
Steve Scheifler

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Which coolant temp sensor, front or rear?

I assume you have confirmed at the actual throttle body that you are always getting true WOT and nothing’s binding or hanging. What about idle? Any IAC valve issues? That usually fails such that hot idle is fast with little or no impact under load, but for all I know there are other failure modes.
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#11
Mark

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Brandon might be on to something. Check that the harmonic balancer isn't spinning on the crank or has shifted. Check the piston tdc using a probe against the marks on the balancer to start with.


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#12
mbican

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Which coolant temp sensor, front or rear?

I assume you have confirmed at the actual throttle body that you are always getting true WOT and nothing’s binding or hanging. What about idle? Any IAC valve issues? That usually fails such that hot idle is fast with little or no impact under load, but for all I know there are other failure modes.

Coolant sensor in rear, checked for WOT at TPS several times, Idles good.



#13
mbican

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Brandon might be on to something. Check that the harmonic balancer isn't spinning on the crank or has shifted. Check the piston tdc using a probe against the marks on the balancer to start with.

It is a short nose crank engine, was spare block I got with car. Had it rebuilt and installed in car two years ago after engine in car was smoked. Checked pulley is tight and mark is correct to TDC.



#14
Steve Scheifler

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Coolant sensor in rear, checked for WOT at TPS several times, Idles good.


Yeah, I saw that but wasn’t sure if you meant that you looked at the actual throttle butterfly as someone else worked the pedal, getting full without any hitch and not over-throwing. Weird sh!t can happen.
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#15
Tom Hampton

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No one has mentioned the Cam Angle Sensor, yet. Given the timing weirdness....could be a loose wire or a failing sensor putting out a weak signal pulse.

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#16
Steve Scheifler

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I don’t think his description of timing when blipping the throttle in neutral is unusual. Not to say the cam sensor couldn’t still be the problem. They tend to get sloppy at idle but I’ve not seen one go intermittent. Still, as you say it and the wiring should be on the short list to check.
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#17
Tom Hampton

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I can't say I've ever watched the timing while blipping the throttle....most probably, because of the arm length required to do both in a one-man shop. 


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#18
Tom Hampton

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Also check the igniter. Its basically just a darlington transitor. If it starts to go it won't pull much current through the coils, and you'll get a weak spark which could result in unburned fuel/air mixture in the exhaust.
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-tch
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#19
OctaneNation

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Slipped cam timing?


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#20
Steve Scheifler

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Slipped cam timing?


OP: “...Timing belt marks good...”
And it wouldn’t be intermittent. The statement that it inexplicably ran a session without symptoms is still perhaps the most interesting.
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