Stop me before spending parts on a stock miata!
 

This is a cold start issue, I could take a video, but it'd be pretty boring.

I've got a 1990 I'm flipping. Only major issue is that it cranks for 10-15 seconds when cold with a bunch of throttle feathering to make it start. After it's warm (5-10 minutes of idling), it'll spring to life with less than a second of cranking.

compression is 165-175 on all four, stock everything, brand new timing, verified a million times, set to 11 degrees. It's done it with two different coil packs, two different sets of NGK blue plug wires. It does have irridium plugs from the PO, those will be swapped for stock NGKs soon, but I doubt its the problem, read below.

When I jump the fuel pump in the diagnostic box, it doesn't seem to help. However, when I clamp the return line with some vice grips, it dramatically helps reduce the cranking time down to 3-5 seconds. I'm guessing something in the fueling department.

So what's the thought, fuel filter, regulator, or pump? Thanks.

Stock ECU? Can you swap in a known good one for a test (do you still have the one from your smurf?). I'm pretty sure the ECU performs the "choke" function by adding injector PW just like our MS's.

Not sure how the ECU could fail to cause this but otherwise still control the car OK. Maybe it's missing an input needed to determine that it should choke. Air temp sensor is in the AFM and water temp sensor on back of head. Perhaps verify those at the ECU plug with a multimeter.

Yes stock everything but plugs. Ecu and Afm are from a different car.

I would assume the coolant sensor in the back of the head would go bad before an ecu.

I can check the sensor, im sure there's a resistance number floating around on the internet.

Could I just be compensating for a bad sensor with the crimped return line?

Fuel filter usually only causes problems under load, like acceleration, not at idle. Good idea to replace it, but I sincerely doubt that it is at fault.

Im going with regulator, have you removed its vac tube and looked for signs of fuel?

Its probably flooding after its turned off, well, not flooding but with raw fuel which becomes super rich vapour, hence needing to give it air to get it to kick.

Dann

I'd say, if it's a regulator, you'd have bigger problems, like stalling, no/low power, etc.

Have you checked he AFM, it is the mechanical type, right?
Anyone mess with the settings? Has the top cover been removed?

Or, it could be just a sticky flapper issue. I'd give it a decent cleaning first.

Curly says he needs to feather the throttle, not floor it.

Right...

yeah, it's just a little extra air, I'm not flooring it. And I'm not 100% sure that's even helping at all. With my experience with 4 carb motorcycles, it's always helpful to simultaneously floor it and crank the starter.

I'm assuming this is all poor cold start enrichment issues. Either part of the fuel system is on it's way out, and clamping the return line helps give it a few more psi, or one of the temperature sensors is on the way out and it thinks it's warmer than it is. That would give it less fuel, and again clamping the return line gives it the extra fuel it needs.

The AFM has not been opened in it's life, but again, it's from a different '90.

Josh, is there any way someone could look through the AFM while you're cranking to verify it is actually moving?

Also, I'm sure it wouldn't hurt to check the pot the flapper is connected to with a multimeter to see if the 23 year old contacts are still good - especially in the first 25% of its range.
(Think of it as a bad TPS that is worn in the range it works in 90 per cent of the time)

I've verified the flapper is moving. I can plug in a spare AFM (which has been messed with) to double check though.

So I did end up taking some videos for y'all. Both were taken without touching the gas pedal.

Return line clamped:

https://www.miataturbo.net/attachmen...ine=1380138588

And I get this:



If I IMMEDIATELY remove the vice grips, this is what I get:



Also, I checked my coolant sensor at the ECU.

This is taken a few seconds after start up.



And once warm (really warm):

https://www.miataturbo.net/attachmen...ine=1380138588

As you can see it's dead on for the FSM's 2.5v cold and .4v warm range.

I also got a hold of the previous owner and he said the fuel filter was replaced when he bought it three years ago. Once I get a fuel pressure gauge I'm checking the pressure, all signs are pointing towards the pump if you ask me.

If the pump can supply enough pressure clamped, wouldnt the reg clamp it if it were working correctly? If you open the throttle the reg clamps it harder.

Dann

Any way you could repeat the test by removing the vacuum line to the regulator, instead of clamping the return line?

So in other words clamp the vacuum line to the regulator?

I guess removing it would be easier and more accurate. We need to see "no vacuum" at the regulator.

This test would eliminate problems with/associated with the regulator, in a very rough way.
(We do know car starts with the return line clamped, that's our base here).

Ah, I see. I'll remove the line, clamp the intake leak, and try it once it's cool. I just warmed it up picking up my nephew from school.

So if it starts well with the regulator vacuum line removed, it's a regulator issue, and if it still starts poorly, it's the fuel pump?

Not necessarily... remember I said "in a very rough way"? The diaphragm inside the regulator could be torn, too. Or, the spring holding the diaphragm could be rusted out.

I once had to deal with a TBI Blazer with the same symptoms. They run on 12 psi IIRC. The spring inside the regulator had rusted, and the weak spring was not able to create the necessary pressure.

The more I think about it, the more it sounds like you could just rule out the pump.
That's why I am focusing on the FPR, and focusing on that thing from thousands of miles away is kinda hard, btw.

Can you get something like this?
Adjustable FPR | eBay

I mean, it's only $23.50, has a gauge, and those things work.

Mine is from ebay, too:

https://www.miataturbo.net/attachmen...ine=1380146663

Same long cranking issue as before with the regulator vented to atmosphere, and the intake manifold side plugged.

Behaves like before with clamped return and unplugged FPR?
Try another ECU, it sounds like its not getting any cold start enrichment.

I'll try another ECU next. I'm bringing it to a friends to get rid of the horrible Delrin motor mount. I'll see if I have time to try a different AFM before then as well, however the current one was working wonderfully in another car. The same car I'll be trying the ECU from in fact.

Interestingly enough, my issues are with a '90 AFM and I believe a '93 ECU.
'90 AFM and '90 ECU test will happen Friday.

Is the consensus that a bad fuel pump would be giving me hesitation issues in the rest of the range as well?