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92 Miata ignition relay problem?

I have a '92 Miata, generally in VG condition, @153,000 miles, had it for 11 years, very few problems, but I don’t put a lot of miles on it. Recently, on a very hot Sunday afternoon (heat index @ 105) I drove it a half-dozen miles to the store, just to keep it’s juices flowing. On my return, at the end of the freeway exit ramp, the engine just shut off. It started right up, but after about 100 feet I heard a sort of clicking sound and it shut off again. I called AAA for a tow, but they told me it would be 3 hours wait (!!). I thought maybe that was a mistake and they’d give me an update. But after 20 minutes and I hadn’t heard back from them, I tried starting the car again, it started right up. I was less than 2 miles from my house and didn’t want to sit in the sun for 3 hours w/ no drinking water. This time the car got most of the way home before it did the same trick. By giving it a short cooldown time twice more I was able to make it home. Later, I was able to have it towed to a mechanic who has always been reliable in the past. He had it for 3 days but couldn’t get it to repeat the problem. He said my thesis that it was a relay overheating was plausible, but that it could be many things and the only way to find which one was to drive it till it failed completely. Well, I don’t want to have it break down in the middle of nowhere, so . . . anybody have any better ideas (preferably someone who’s had the same problem)?

I used to have this problem on a Merkur fuel pump relay. I could swap relays with an identical relay that powers the wipers (I think) and the car would always start right up. I got tired of rescuing my wife and hot wired the fuel pump across the relay. I couldn’t figure out why it was overheating, just that it did.

Next time it happens feel the relays to see if one is getting hot and swap it out.

Given that this car has a crankshaft position sensor and this is exactly what happens when these things are failing, that is a real possibility. They are rather expensive for this car. Check for spark next time it quits. No spark and I’d think the CPS is the problem.

The traditional way to find intermittent thermal problems is with a heat gun. But I’ve never applied that to an automobile.

The method you could use is to direct the heat gun at the relay panel, with the cover removed (if it has one). Be careful not to overheat anything. Use an IR thermometer to monitor the temperatures and keep them below, say, 150ºF. But if the fuse panel is under the dash, that may be very difficult to impossible.

You might try tapping the relays and see if that has any effect.

Thinking about it, relays tend not to be heat sensitive components. A bad connection is more likely. Try wiggling the wires going into the fuse panel.

edit: I like Mustangman’s suggestions better.

I will also agree with the advice about the CPS sensor causing the issue. It could also be the fuel pump relay or possibly something else within the ignition system. You stated you heard something like a relay clicking so that is a little concerning. This could possibly mean that there is a loose wire connection to a critical relay and just cleaning up the connection will solve the issue. If there is a fuel delivery problem then having a can of starter fluid should show that up by spraying a small amount into the intake when the engine won’t start up. Tapping on suspected electrical items with a screwdriver handle should show up a loose connection. By leaving the car out in the sun on another hot day may help bring out the problem again while you are at home.

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I like that suggestion! Separates fuel problems from spark problems!

Concur with the advice above, the primary suspects are either faulty fuel pump relay, faulty fuel pump , or crank position sensor. If I had that problem I’d measure the voltage at the fuel pump connector. If it was low , indicating a voltage drop somewhere between the battery and the fuel pump, I’d replace the fuel pump relay on a flyer. If the battery terminals and posts haven’t been cleaned and de-corroded recently, I’d start with that first.

Thanks for all the suggestions. You’ve given me some good leads to follow!