'92 1.6 starts and idles (rough) but dies when given throttle

[Joamon]

Hey all, I could use a bit of help.

I have a 1992 Miata that has about 150k on it. It's been well maintained for the most part (previous to me).

Shortly after I took ownership, I replaced the water pump, coolant lines, timing belt, etc. plus gave it a tune up.

About a two months ago, it started developing some intermittent hesitation during acceleration. At the time, I didn't have the time to pay attention to it, and after about two weeks I took it out and it died on the road... At the time, it would start and idle, but as soon as I went to hit the accelerator, it would die.

I started with the easy stuff, like the fuel filter, which I replaced. I have also pulled the fuel pump to check the primary particulate screen, and it seems to be fine. The fuel is new, and just in case I also put some water treatment into the half tank of new fuel which I recently put in.

Additionally, the MAF seems to be ok, although I don't know how to test for sure. I've used a can of MAF cleaner to ensure that it at LEAST isn't a dirty sensor.

I've also replaced the PCV valve and checked for obvious vacuum leaks... everything seems to be tight.

So, here she is still... She'll start, idle (a bit rough at this point) and die when I hit the throttle. Incidentally, when I used the MAF cleaner, I manually actuated the throttle, and it would rev up like it should.... That would lead me to believe that it is a fuel delivery issue, but I am at a loss as to where to go from here.

Thoughts? Suggestions? possible solutions?

Thanks
Robin

[Joamon]

Update... I've been working on it a little over the last few days, and now the stalling is inconsistent. It will idle and rev fine at times, and then - all of a sudden - it will lose any ability to rev with the actuation of the throttle. For a moment, I had thought the issue had been fixed because I had found a loose ground to the battery, I fixed the ground, and it seemed to run great for a few minutes. I could hold the engine at 2000 rpm, and also put a load on the engine by dis-engaging the clutch while in gear with the e-brake on. Now, it's back to the same thing.

I just observed, what I believe to be the issue. It is tied to the Fan turning on and off. I am assuming this means that there is an issue in the ISC system with either the air valve or the ISC valve.

Again, thoughts?

[— kung fu jesus —]

Have to ask this: have you checked for codes yet?

Next, check the intake tract. Look for loose connections, holes, cracks. Next, if the tract is stock, remove and inspect the accordion portion. Articulate it, look for cracks in the ridges and valleys of it. After that, inspect the portion of the tract that sits directly above the alternator pulley to see if it rubbed a hole in it. Check the "J" tube that connects the intake tract to the throttle body on the side.

After that, inspect and/or replace the vacuum lines. Check the nipple on the back of the intake manifold to see if it is cracked or missing. You can also check for vacuum leaks by spraying short bursts of ether near the intake tract, manifold and vacuum lines. The engine will race a little if the ether is ingested. Please be careful, ether is extremely flammable.

Check the ISC by unplugging the electrical connector. It is on the right side of the throttle body (looking at it standing in front of the car). Unplugged, the car will idle high but the car should drive fine.

Outside shot, but free, check your timing to see if it is correct.

Check and clean all the ground points in the engine bay.


After that, use a multi tester to see if the throttle position sensor is reading correctly. More on that here.

After that, I would check to see if your O2 sensor is OK. You may need to replace it.

Good luck! If you can, post a video if you get stuck so we can hear the symptoms.

[wannafbody]

The flapper door AFM might be bad

[Joamon]

I Haven't checked for codes, mainly because I don't have a tester to perform the diagnosis. I've checked for vacuum leaks and checked the entire intake. I've also used the MAF cleaner to double check for any leaks I couldn't sea or hear... nadda. Grounding points are all good at this point and timing is on.

Next for tomorrow... I'll check with the local auto parts store to see if they have a compatible tester for checking the codes. I'll also disconnect the bypass hose and clean the ISC - as i understand it, it can get sticky. I will also disconnect the ISC to see if that makes a difference....

I'll also check the TPS and O2 sensor and update when I have more answers (or lack there of). LOL

Thanks

[Joamon]

The flapper door AFM might be bad

Thanks... that was one of the first things I checked... It tested out ok... I almost wish it HAD been that, at least then it'd be running and driving again... sigh. LOL

[— kung fu jesus —]

I Haven't checked for codes, mainly because I don't have a tester to perform the diagnosis. I've checked for vacuum leaks and checked the entire intake. I've also used the MAF cleaner to double check for any leaks I couldn't sea or hear... nadda. Grounding points are all good at this point and timing is on.

Next for tomorrow... I'll check with the local auto parts store to see if they have a compatible tester for checking the codes. I'll also disconnect the bypass hose and clean the ISC - as i understand it, it can get sticky. I will also disconnect the ISC to see if that makes a difference....

I'll also check the TPS and O2 sensor and update when I have more answers (or lack there of). LOL

Thanks

You don't need a reader to get the codes.

Diagnostic port > paperclip > TEN/GND > look at the CEL light on the instrument cluster. Similar to this: https://www.youtube.com/watch?v=r3SZZIIgm0k

[Joamon]

You don't need a reader to get the codes.

Diagnostic port > paperclip > TEN/GND > look at the CEL light on the instrument cluster. Similar to this: https://www.youtube.com/watch?v=r3SZZIIgm0k

Excellent... I had thought I'd read something about this, but I couldn't find anything when I searched yesterday.

Thanks

[Joamon]

ok, everyone... thank you for all your help. She's up and running again. Turns out the ISC needed to be cleaned. I pulled it off the throttle body and cleaned the carbon out of it. I also took the air valve off the intake manifold and cleaned it up too. It seems to have done the job...

So thank you all again for the help and info... You guys rock!

[Joamon]

Incidentally, here is an update.

So, My girl ran well after the initial work done to it, but then started running "off" again the next day. The idle is varying a lot, from around 800 to upwards of 1750 or so.

I shorted GND to pin 10 in my diagnostic box, and followed the directions here: [URL="http://www.miata.net/garage/isc.html"] and cam to the conclusion that it's my ISCV. I got an error code of 34.

I'll be looking around for a used ISV from here on out, and if i try one, I may look at trying to repair the old one with the directions near the end of the article.

Just figured I'd give one last update.

Thanks again.

[— kung fu jesus —]

Brand new, they are ~$400. Hit up one of our forum sponsors, like Project G in LA, they should have one used.