7 replies to this topic

[ECOBRAP]

Hi all,

 

Been having this issue for a while now, used to happen every once in a while but now happens all the time, looking for some help.

 

Symptom:

 

Driving along, come to a stop, put the car in neutral or clutch in. Rpms drop more than they should, to about 300rpm, car tries to correct itself and gets back up to 1400rpm, then back to 300rpm etc, and sometimes (like in the video) will eventually stall on its own.

 

I can stop it from doing this by caressing the throttle pedal a few times, and eventually I will be able to let off the gas and the car idles normally at 850rpm.

 

In the video, you will see the car idling normally at 850rpm, I tug on the throttle cable a bit then let go, and the weird idle starts, and the car stalls 20 seconds later. I could have stabilized the idle by caressing the throttle cable or gas pedal a few times, but didn't in the video.

 

Diagnostic suggestions? TPS? AFM? Idle Valve? Throttle Stop? Throttle Body? Vacuum Leak?

 

 

Best,

Matt

[mdavis]

Could be vacuum leak somewhere.  Get some water and spray it on some of the lines around the throttle body and see if they bubble at all.

 

Could be bad AFM too.  Might swap a good known one.

[Bench Racer]

Matt, first things first. If your racing SCCA your not gaining all the power you could without ambient air intake and wrapping the snorkel between the AFM and the throttle body. SCCA rules legal.

 

Is your idle issue occurring with engine cold or hot or both?

 

http://www.miata.net/garage/isc.html

 

"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.  This should produce a stable idle value for troubleshooting & fault isolation."

[davew]

Before getting too crazy with theory and diagnostic crap. Go back to basics. Start by removing the rice rocket red vacuum lines. They are notorius to have leaks at the connections. Put standard black hose of the correct size on the car and you may fix it. Second, remove your tweeked AFM and install a NON-MODIFIED unit. Unless the AFM is properly modified it can cause these types of problems. Then go after the idle speed setting, you have 2. The base idle is the tiny screw with jam nut on the throttle body. That should be set at about 800 rpm with the bleed screw closed. Then adjust the bleed screw to 950rpm. The engine must be fully warm to properly set these.

 

My guess would be the AFM. I stopped adjusting AFM years ago and do all my power adjustments from the fuel pressure regulator. This allows the car to run properly at idle and cold start.

 

Dave

[steveracer]

Could be vacuum leak somewhere.  Get some water and spray it on some of the lines around the throttle body and see if they bubble at all.

 

Could be bad AFM too.  Might swap a good known one.

"VACUUM" leaks won't bubble...

[ECOBRAP]

1. Used carb cleaner and no vacuum leaks despite the much hated rice rocket lines lol, new lines on order just to be safe.

 

2. Bench, new motor just went in, more important to have it working right before I spend time wrapping the tubing. I believe at the end of last year I may have had this issue on track, with the car almost stalling while diving into heavy braking zones, almost making me lose the back end of the car. This leads me to suspect butterfly valve is closing more than it should. But don't worry I have my tape ready to go and will only take an hour to do it right.

 

Update: I believe the throttle stop screw has disappeared? There is an adjustable bolt that stops on the valve, but the threaded hole in picture below has nothing in it? If I had to guess the bolt that IS in the picture is just the first line of defense for snapping off throttle, then after it has dampened/slowed down the released throttle, then the missing screw sets the resting point for the butterfly valve and idle? Surprised that it is missing altogether, probably why I didn't catch it the first time around? Can someone confirm that there is supposed to be a second bolt/screw in that hole?

 

Best,

Matt

 

[Bench Racer]

Yes, there should be a female Allen head set screw inserted from the front side with a locking nut. On the back side the U shaped throttle shaft bracket stops against set screw. After install and adjust the accelerator cable should be slightly slack. Get this correct and adjust idle speed per FSM.

 

When you get to removing the left headlight stuff and wrapping the snorkel tube I'd suggest using the stuff like below from Pegasus or similar with woven silica and a aluminized outer layer. Check your local race shops. For my 2 cents, the smooth aluminum tape will not do the required job.

 

https://www.pegasusa...m/2016/109.pdf    < Crap, that didn't work, go to Pegasus Racing catalog on line to page 109.

[ECOBRAP]

Thanks I'll put one in tonight and see if it corrects all of the symptoms, would seem to explain all of them so fingers crossed.

 

Yes I understand that tube deals with convective and radiant temps so I got the good stuff.

 

Best,

Matt