"Weird" Idle Issue; Possible Symptoms and Troubleshooting Steps to Remedy
 

1993 RX-7 R1, completely stock minus an additional ground from the battery to the body.

1. From cold start the car idles perfectly.
2. During normal operation the car idles perfectly.

However, if I drive the car for a normal "extended drive (20+ minutes) park it for an hour and then start it up the car idles, bounces a little bit and then stalls.
Potential stalling / signal of about to stall goes away immediately after a few minutes of driving.

Technical details: car is based in Arizona, ~9,000 miles, all services up to date

Any thoughts on troubleshooting steps to get to the root cause and eliminate this phenomenon would be much appreciated.

Thanks!

Where abouts in AZ?

Do you pop the hood open after you do an extended drive? My car also used to start/idle a little funky after heat soaking when parked. Now I just pop the hood for a few minutes after any sort of drive to keep the intercooler/IAT sensor from heat soaking too much.

Scottsdale

1st thing I'd probably do is change the 02 sensor, plugs and wires. I'd also look for leaky or unattached hoses around the intake and rat's nest.
9k miles total? Has the car sat for extended periods?

Thank you! I did not consider replacing the O2 sensor. However I did consider cleaning the idle control, cleaning reflowing injectors, and adding additional grounds as next steps.

Original owner of 30 years kept the car as a time capsule / garage queen in a temperature controlled environment. I'm the new owner and intend to drive it 😃

Advanced maintenance steps were done; fluids, filters, belts, plugs, grounds, rubbers replaced where necessary, vacuum lines & hoses inspected / tested in detail etc. Car runs near perfectly besides the above "intermittent" / "weird" issue.

https://cimg5.ibsrv.net/gimg/www.rx7...df3a910a5c.jpg

https://cimg4.ibsrv.net/gimg/www.rx7...f20444107c.jpg

Wow.

Maybe try swapping out the TPS or at least testing it. There has been an issue that the TPS will test fine, then go out of range when the engine gets hot.

Not only open the hood, but park a big big box fan in front of the car and accelerate the cool down process.

Heat soak can continue for hours.

Are you getting any codes* at all?

One of the FPR's functions on this car seems to be to deal with vapour lock on hot starts.

[Consult Page F-105 of the USDM FSM]

You might want to know if the solenoid [PRC] that controls the FPR is functioning.

* codes of interest may be:

Codes 12 & 18: TPS low and high range
Codes 20 & 37 Metering Oil Pump functions
Code 25 - Pressure Regulator Control


[Keep in mind that you may have a bad tank of gas. The stock ECU may be only able to handle a narrow range of octane variance.]

Pull and clean the ISC, that's the first step in any bouncing idle situation. A sticking ISC will do a world of weird crap. I have posted an overview of that process a ton of times, dig around and you'll find it. Not a hard process to do.

Dale

I’m in Gilbert. I’ve got a 15k mile car that runs perfectly, so we could do a quick parts swap on any questioned one.

Note that the paper-ish ISC gasket is now NLA. You might be able to replace with a later model metal one. In any event be mindful of that.

My Power FC was hunting at idle a long time. I would wow between boost and rpm at idle about 15 times until it adjusted to the non-spec leak.

Turned out I had a minor vacuum hose broken and leaking (The one to the oil injectors.)

So even a single vacuum leak can cause issues.

A smoke test can find leaks you don't know you have.

But digging around in such an ancient engine bay can result in cascading problems as old solenoid nipples crack and rigid vacuum hoses split.

Also expecting to drive a low mileage car regularly with 30 year-old seals in the engine that has sat for so long,.....well watch diligently for coolant leaks into your exhaust stream or mixed with your oil

(and/or oil in your coolant.)

In my fantasy of finding and owning a very low mileage car, I include having the motor stripped down and all the perishable seals replaced.

The point being to preserve the good condition of all the metal surfaces of the engine and avoid having to replace those if there is a catastrophic seal failure.

That way you can drive around in your beautiful minty car for the next 65,000 miles with (more) peace of mind.

Did you recently purchase the car from bat

that would be awesome. I'll PM you shortly and hopefully we can connect!

Yes I found that content. It's on the troubleshooting roadmap. I'll be back in town next week and I'll go through the step recommended here. Thank you for putting it together!

Wow, super clean car, I would definitely do the O2 sensor and don't discount the fuel pump or Pump amplifier either. If those heat up they can create a lot of weird issues like this, resistance in circuits can cause drop fuel pressure. May want to pull the fuel pump out and check the condition of the pump and the bulkhead connector to see if that has been warm at any point. Even if its low miles, corrosion can set in pretty easily especially if it was stored with very little fuel in the tank. Depending on if you plan on keeping the start completely stock for value, I would even consider just popping in a new fuel pump like a Walbro 255 from a reputable seller as a peace of mind. They run about $125 IIRC, I just did mine earlier this year.

Ultimately I added two additional grounds which eliminated the issue entirely. The car has two "aftermarket "grounds.

1. Body x Battery
2. Firewall to Motor / Trans

Not sure whether or not to declare these mods as must have reliability mods, but having witness the direct cause and effect on the added stability form the additional grounds I wouldn't hesitate to recommend this mod to the community.

Car runs flawlessly now! Time to hit the road! :dubs:​​​​​​​