96 nNA8 high idle

[lonesailor]

I have just finished replacing my engine after a refresh, new rings, bearings, clutch, engine mounts' an engine balance, a bit of weight off the flywheel new valve guides and a cylinder head service by the machine shop. It started first go and apart from a bit of lifter tick ran well . Because I had it in my backyard shed I had to wait for my neighbour to return as I had to pull a section of fence down to get it out through his place. That happened this morning and I took it for it's first road test. After a kilometer or two gentle driving the idle speed went from the 850rpm or so when warm to around 15-1600 rpm hot. The throttle is closing completely and I have checked for intake air leaks and found none. I am assuming that the (factory) ECU thinks that the engine is cold, or perhaps that the AC is on and is increasing the air and/or the fuel flow. I haven't yet been able to check the thermosensor connection until the engine cools and I can remove the coils to get access. (I am going to convert to COPs for my next project, because I am well and truly OVER trying to do anything in that crammed space behind the engine). I have tried switching the AC off and on, it doesn't change the idle speed. Has anybody any other ideas about may be causing my problem?

[CeramicND]

Doesn't the NA8 have an idle.control valve on the throttle body? Could be leaking or sticky. NA6 has 2 valves, 1 on the inlet manifold and 1 on the throttle body. I didn't diagnose the fault, I simply removed the idle control system and blanked all the associated coolant lines as my car is a track toy. My engine was also reconditioned and my car doesn't take long to warm up and hold an idle.

Sent from my SM-A315G using Tapatalk

[93_Clubman]

NA Idle Problems miata.net wiki:
http://wiki.miata.net/tiki-index.php?pa ... 20Problems
Couple of NA8 ('95 & '96) high idle threads from miata.net:
https://forum.miata.net/vb/showthread.php?t=515909
https://forum.miata.net/vb/showthread.php?t=546117

[lonesailor]

I have solved that problem, It turns out that when I had reconnected the broken wires to the thermoswitch which tells the ECU how much fuel to give a cold engine, I had got the polarity wrong. That solved, engine runs great. Next problem was a bit harder, all fine until I shifted into reverse and the meter fuse blew disabling the oil pressure gauge and tachometer, and the rear lights. Must be a short in the reversing lights or switch, no problem I can live without them , jack up the car again crawl underneath ,see a couple of connections leading to (I think) the reverse switch on the gearbox. I disconnected them - still happening -bugger. Two days later the loom s behind the engine partially un and rewrapped searching for shorts to no avail and 10 more blown fuses after "finding" the problem and 2 more episodes of jacking the car up and crawling underneath ( No fun for someone at 73 and overweight) finally realize that there was two sets of wires there, and I had disconnected the wrong ones. Cut then other one and its all good, apart from now having a radio and a temperature gauge not working presumably after all that messing about under the dash ( no fun either) Anybody tried removing the dash? I really should sort things out under there, its a mess, was before I got the car, and I've made it worse

[manga_blue]

One of the two switches on the gearbox is the neutral switch, which is one of several inputs that the ECU uses to determine idle speed

[lonesailor]

To finish off this topic, I should update on my saga to finally sort it out. I became convinced that my problem stemmed from the wiring to the thermo sensor behind the head( on mine at least) I downloaded MX5 manuals only to find that they were for the US delivered(Built?) cars, which had a lot more ,mainly safety and comfort additions than ours. There was a circuit diagram which was difficult to follow and was a lot more complex than our cars. I tried several things with some, partial but short lived, success and eventually gave up and just pulled the plug on the IAC. this worked reasonably well except on first startup from cold. After some time struggling with this I worked out that the IAC valve must be sticking, I removed it, found that complete disassembly was impractical, so cleaned it as best I could with metho, (the best I had available at the time). Some slight improvement ensued but soon the plug came off again. Another week passed and a few K's before I needed to remove the intake again to get at a small coolant leak, so I pulled the IAC valve off again and this time used the best part of a can of carburetor and throttle body cleaner and compressed air To clean it as best as I could without dismantling, and am pleased to report that, for now at least, the problem is solved

[greenMachine]

Fingers crossed for you.

I have a set of circuit breaker fuses I use in these situations, quicker, easier and cheaper than throwing fuses at the problem. They weren't cheap, but cheaper that buying fuses by the 100 . Unfortunately, they are standard blade size, don't know if you can get them in 'mini' blade but now that I am thinking about that maybe one of those 'add a circuit' extenders would work, something else to check out and fill in the lockdown hours I guess ... .