FC S5 funny idling issues
 

Hey guys,

Just picked up a 91 FC n/a and I love it, but it has some strange issues at idle.

The rpms will pulse from about 1000-1200 every few seconds and it sort of feels like it's misfiring. However, once the car is given even a little bit of gas, that goes away. Zero problems when driving, and sometimes after driving for a while the problem won't happen when I'm at a light. It may start again later, though. Today when I started the car up, it didn't go to high idle until a few seconds after coming to life. I'm thinking something is stuck, but I'd like your opinions on it.

If I adjust the idle down to 800 or so, the CEL comes on and goes away immediately when pressing the pedal. Did I adjust it in the wrong space? (I used the flathead screw with a spring inside it to adjust)

I hopefully can rule out bad compression, as the car starts immediately in the morning. Faster than any other car I've driven. And once it's warmed up, shutting it off and restarting it 5 minutes later (a test I found in a buyers guide for checking compression) is even easier. No white smoke even on startup, and the coolant has zero oil in it. Car pulls nicely too.

Does anyone have an idea of what could cause this, or are there other tests I should do? I've already ordered ngk plugs and wires just for piece of mind at the very least. I'll upload a video when I can. Thanks everyone!

Here is a video I took of it warming up, and it behaves like this on the road as well whenever I'm stopped and don't have my foot on the pedal. The AC is not on either.

Video is asking the viewer to sign in.

Weird, I set it to public. Maybe it's because I used the mobile link. Here is another link that hopefully works:

The TPS adjustment screw is to be adjusted only after the car is completely warmed up (20 minute drive should suffice). The Green/Red wire of the sensor should read 1 volt w/key to on. The timing advances at about 1200 rpm so that could be contributing to your problem. Also, it's important to know if this problem of yours occurs when the engine is cold or warm or both.

I only touched the screw after the car was warm and driven about an hour. That's when I got the CEL which went away with throttle. Was it the correct screw? It sort of worked, though the throttle still liked to 'stick' open if I touched it.

This problem happens when the car is warm too but is less noticeable and sometimes the car will idle just fine, around 950 RPMS (a little high). However, this is less common than having it pulsate.

You refered to this screw as the TPS adjustment screw which is probably not what I wanted. I just wanted to adjust so that the throttle doesn't hang open, which is what I thought was happening. However the pulsing makes this sound like a sensor issue, or plan old bad ignition--though in that case I don't think the idle would ever be fine, and the problem would probably be noticeable when giving the car gas.

I'll fetch my multimeter at some point to check the wires.

The car is also staying at slightly over half on the temp gauge so I have ordered an OEM thermostst and gasket from Mazda. If this is messing with the idle in any way, I'm not sure, but it's probably a good idea to keep things a bit cooler anyway.

Thanks for your help.

The sticking could be related to the throttle cable.

You can check error codes by grounding the Orange wire in the Green check connector near the lead coil followed by turning the key to the on position.

The gauge on an S5 would read close to the 1/2 way mark (gauge really only has but two positions, good or overheated). S4 gauge is more sensitive than the S5 but still isn't very accurate.

I'd like to check error codes but the light only goes on when the car idles low. It goes away if I touch the pedal, so I'm guessing it is erased when the car is shut off. Does this sound like a fault with the TPS seeing as how it's triggering a CEL? I'm going to check for a vacuum leak today as that could also trigger a light.

I've noticed the problem happens less when I rev match into second before coming to a stop. The car still idles around 1000 though, but that's where I have it adjusted.

What would prevent the car from doing 3k at startup, by the way? A sticking throttle? I had to hold it there myself last night as it didn't go automatically. Im going to go clean the throttle body and check a few things out. Hopefully someone will know what my problem is from the video!

I stated how to manually check for error codes. Codes are usually stored.

The 3000k rev on cold start is temp related (doesn't do it all the time).

Vacuum leak.

I'm going to go check for a vacuum leak and see if any codes are stored. Also going to ground the initial set coupler when i fiddle with the TPS and the BAC after warming the car up.

Here's a picture of my gauge when warm, all the pictures I have seen of an s5 gauge cluster are lower than this so I was worried. I understand it's a total dummy gauge, but still. It gets to this point very quickly, maybe after 3 minutes of idling from cold.

Also, the fast idle wouldn't trigger last night after parking it for 10 hours and starting it at night. The car was cold as can be, so I'm going to check everything to do with the throttle now.
https://cimg1.ibsrv.net/gimg/www.rx7...0a39922401.jpg
Thanks for your suggestions guys. Hope to sort this out soon!

Again, the 3k rev is temp related. It won't do it if the engine is too cold because it's not healthy for the engine. Just because the car is being cold started does not necessarily mean it will go though that process.

Oh, I thought you meant it won't do it if the engine is still warm. My bad!

Can't find my multimeter but I'm bringing some wire out with me to ground the coupler.

I fixed it!

I used a multimeter to set the TPS to 1k ohms at closed throttle. I replaced the thermostat which was the super wrong part, and I grounded the initial set coupler to the negative terminal, setting the idle to 750. I'll update the thread if the issue comes back. Thanks for your help, guys!