Tedd Thompson

I'm trying to get up to speed on this 91 Miata I play with and my knowledge on automotive electrics gets less and less as I near the 1990's. This is starting to bug me a little as I'm running out of ideas.

I have developed a low RPM miss that can be heard at idle( rough idle) and felt if I putt putt around a parking lot at low speed (kinda when It is lugged). I thought it was a matter of a bad plug which is common with a NA6 Miata's so I changed them again, still same issue . Changed the wires again thought maybe I gotten a bad wire on the last purchase, same issue. Cleaned the throttle body no difference. Doesn't sound like a valve at the exhaust but I haven't I ran a compression check yet but the upper end is only about 6 months old and I would be surprised if it was a valve. To me that leaves maybe ejectors or perhaps the coil pack.

How does one check ejectors and how is a coil pack measured. Or does someone have another direction I should be looking?... Kinda lost in the fifties..... Tedd

Vintage Chief

Let's hope you don't have ejectors but instead injectors - as in Fuel Injectors (FI). I'm not an expert, I turn my own wrenches on my F-250 diesel (which is fuel injected) and I've replaced my fuel injectors on the left bank at 135K miles and the right bank at 145K miles.

Regarding checking the injectors. I own two OBD2 scanners - nothing expensive or professional. If you had an OBD2 scanner you'd plug your scanner into the OBD2 port (generally under the dash) and then you would read your DTC (Diagnostic Trouble Codes). A FI going bad or which has already gone bad would produce a DTC code demonstrating which FI is bad.

A low RPM miss can often be the result of a bad FI, but there are also other possibilities, but you should still have a miss at high RPM - it may not be quite as apparent as the RPM's are much higher - but again, you would still generally have a miss at higher RPM.

Other possibilities are: Fuel Injection Control Module (FICM), Electronic Control Module (ECM), various sensors on your vehicle such as Injection Control Pressure (ICP) sensor. I have absolutely no idea how a 91 Miata is put together - none, zero, nada. Without an OBD2 scanner is really a tough job, but an OBD2 scanner which will pull up your DTC codes is the way to quickly find the issue. You can often go to a local shop, or even the Miata dealer and just pay them to run an OBD2 scan for you and produce the DTC codes. You will of course have to pay them for this service. There might be the possibility you can find a friend who owns one?

Vintage Chief

Check this out>>> https://www.700r4transmissionhq.com/...s-mazda-miata/

You'll most likely see the guy's video, but in case you missed it:

Tedd Thompson

Vintage Chief Norm, thanks for the quick reply. Miata didn't come out with OBD2 till 94 the early models have the antiquated OBD1 built in to the system but I'm a bit ignorant on how to read the information it has a viable ,

And you are correct injectors, James Bond has the ejectors.... Tedd

Ancient Iron

Is it a coil on plug ignition??? Might have a bad coil.

Tedd Thompson

Coils (4) are at the rear of the engine and each one is attached to the plug wire and plug, no conventional old world distributor. Is there a way to test the coil pack with a multi miter? And what readings would I expect to see.. Thanks Tedd.

Koda

https://www.2carpros.com/questions/1...zda-miata-coil

I don't know if that helps.

Tedd Thompson

I had to give up and have a mechanic friend take a look at it. The main issue was a vacuum leak at the intake manifold, gasket took a dump (separated). Felt Pro gasket which was a paper compost was replaced with a compressible metal type. I'm not blaming Felt Pro because I rely don't know but the metal sure looked more substantial. Back on the road again, smiles on my face.... Tedd