17 replies to this topic

[RazerX]

Ok, I am cracking open the FSM but I thought I would poll the members for a possible direction.

 

Engine is warm and at operating temps, applying a little throttle (think of just going to 1500 RPMs), the engine first gasps and then proceeds to 'surge' with rising and falling RPMs with no change in throttle position.  OBD is throwing no codes...  

 

Guessing TPS or AFM...

 

 

[RazerX]

Video...  https://vimeo.com/224419567

[RazerX]

Just found an old post where Jim stated it was the Idle Air Control Motor.   I have an extra one... I'll swap...

[RazerX]

Okay, i swapped the throttle body and thus the IAC motor but the problem remains.  It idles fine but as soon as you give it some gas it surges. 

 

thoughts? 

[Johnny D]

From the old thread, "You have a dirty or bad idle air control motor"

http://mazdaracers.c...rottle-surging/

 

J~

[Steve Scheifler]

Or two of them since he already did a swap. But not impossible.

Have you checked for vacuum leaks? Are all the hoses connected correctly?

I was thinking a misadjusted throttle position switch which seems to cause those symptoms on a 1.6 as I recall. You might see what an OBD scanner shows for TPS % at zero and full throttle. That may not point out a problem but it's something to check.

[FTodaro]

You need to clean out the throttle body including the AICV, use something that is safe for throttle body, also swap out the sensor on the side of the throttle body the "throttle position sensor",

 

I do not know if the 99 has the same issue as the VVT maybe Mr. Haldeman can comment, but i had a bad switch on the clutch pedal something to do with the cruse control.

 

To test that, try pushing in the clutch while it idles, if it quits that is your problem. I cut off the plastic stem of the switch so it thinks the clutch is depressed.

 

FT

[RazerX]

So, I swapped the entire throttle body from a known working car.  Thereby swapping the TPS and IACV at the same time.  And the problem remained exactly the same.  

 

I will look at the vacuum lines but it idles and warms up fine, which a vacuum leaks usually messes with.  It could be the harness, but that is a new harness with about one year one it, as the 99 have a separate harness for the injectors, coil and the sensors up front.. 

 

I have the clutch delete for start up but I can try jumping in the car and trying it.   

 

So, i guess I go look at the vacuum lines.  Odd it was running fine at the last event and just stated doing this in the garage.

[Steve Scheifler]

When our NA did that in gear and moving it was TPS adustment, but I get that you swapped it already. Perhaps the ECU is misreading it?

Whatever it is may have zero impact when racing.

[RazerX]

Ok, 

 

So with the car idling, i removed the connector to the IACV, car dies.  Put it back and fire it up and runs.  

 

I check the OBD (using Torque on my phone w/ a bluetooth reader), it has a manifold throttle position field that is green, usually indicated a metric that is measured.  It is showing 0% throttle, I add throttle (still surges) and it still shows zero throttle.  I now pull the connector to the TPS while it is idling, the car stumbles, but then resumes idle.  I now give it throttle and the surging is gone. ????  Does that make sense?  I now try to pull codes from the OBD and it says there are no codes even though I pulled the TPS connector.  Those that make sense?

 

I am guessing now it is in the harness or connector.  Because i have had two different trottlebodies(TPS) with the same problem and OBD is showing throttle % zero.  If that is a reasonable assumption how is best to test the harness?  Run a continuity test from the plug to the ECU?  Accessing the ECU is a major biatch in a 99.  

[RazerX]

Okay, so I traced the TPS wires to the ECU and they have continuity. 

 

What is weird, is right after I fire the car up (hot or cold) it runs fine for about 10 - 20 seconds, then the surging cuts in.  It feels electrical as there is a clear rhythm and when the idle drops it like something 'cut it off' as opposed to starving for fuel.  I can start throwing sensors at it (Crank, Cam) and then a different coil pack.  

 

I tried stomping on it still at 4k it was cutting out.  

[Bench Racer]

Following info come from an issue with my 1.6......  When my engine was cold it idled and ran at other fixed RPM points fine. With warm engine when I would get to IIRC fixed point of 2,200 RPM the engine would surge back and forth between 2,200 RPM and 1,500 RPM. I checked everything out electrical, vacuum leaks, whet ever, no improvement. The engine at the time had a new thermosensor at rear of head which sends outputs to ECU.

 

When I followed TEN & GRD shorted with engine warm, no surging. Below is a different issue, but still took some ECU inputs out of the game.

 

"However, when the DC (Diagnostic Connector) TEN & GND contacts are shorted together for Self Diagnosis and troubleshooting the ECU control loop is opened.� With the control loop open, the ECU ignores inputs from Ignition rpm, Temperature and Throttle Position sensors and sends a fixed signal to the ISCV."

 

Even though I had electrically tested the TPS and it showed within test specifications IIRC with the engine warm I unplugged the TPS and no surging from 2,200 RPM TO 1,500 RPM. Bought a new TPS sensor, issue solved. Don't know if this supports a 1999 or not.

[Erik Hardy]

I would tend to agree with your time (heat dependencies) that it is an electrical issue somewhere. Have you tried testing all suspect connectors for continuity while bending the connector in multiple directions? If there is an internal wire break within the strands, whatever strands are left over heat given a small duty cycle. Outside of connectors, all sensors, wires, ignition coils etc. are all heat dependent if they have an internal short/failure within. 

[callumhay]

https://forum.miata....ad.php?t=559876

This is an interesting thread on miata.net discussing clutch switch inputs with the ecu. I'm not an expert and this may not be applicable but if there is some malfunction in the clutch switch(es) the car will vary idle speed because the car thinks it should idle at the wrong time. Even something like a bad ground or the switch breaking or binding could do this. I'm not sure how this pertains in a car with the switch bypassed for start up. These are just some thoughts that might help. I did a Google search on this and there are threads on it elsewhere discussing the clutch switches a fair bit.
Good luck
Cal

[RazerX]

Thanks everyone.  I think i found the problem...  Spark Plugs.  Yes the plugs,  I had go through what most had suggested, even messing each wire and each hose to try an exacerbate the problem.  Having no logical place to left to proceed.  I had settled in on a methodical replace and test routine.  I have extra sensors, MAF, ECU, Coil packs, etc.  During that process i swapped out plugs that had been in for one day, with those that came about of the car with several weekends on them.  Whoola, surge was gone.  Yes, i had previously removed and re-seated the plug wires to ensure they were properly connected so I am fairly sure it was not the wires.   

 

Now that is runs, the "evil" plugs looks fine and all i can do is hypothosize how a plug can cause what I was observing.  I am interested in what others think as well.

 

But at least the car is packed and ready for the track tomorrow! 

[RazerX]

So the car ran fine all weekend and even replace the plugs with another set and no issues.  The offending plugs look fine but must have some internal issue.  

 

As always, it is often the simplest cause and resolution...  But hey what is 2 days in 106+ degree weather working on your race car if not building a 'love' relationship....

[Ron Alan]

Put the old plugs back in and see how it runs!!! Not a believer this was the issue...

[Bob Noaker]

we have a similar problem, wired the clutch switch together for a starting issue, start issue fixed, idle issue remains.