Idle/throttle tip-in troubleshooting
 

Hi folks,

Okay, time for me to start digging into idle/throttle issues on my car. It's a '93 Touring with working A/C. Only 15,000 miles on the engine and chassis. Vacuum lines and check valves changed around 14,000 miles. Injectors were also cleaned and balanced at that time and the fuel filter was replaced.

Symptoms:

This one has happened since I got the car:

1. Cold start tries to go to 3000 RPM, but it kind of hunts between 2500 and 3000. Not a regular/cyclical oscillation. Just doesn't hold steady to 3000 RPM. If I blip throttle to cancel the fast idle, it drops and doesn't really catch well--stalls out.

These are somewhat more recent symptoms:

2. With A/C on, at a stoplight, with engine warmed up, sometimes idle oscillates around its target value in a pattern that seems to be increasing in delta to the point where I expect the engine will stall. I end up adding throttle to compensate.

3. I stopped in a parking lot after a hill climb. I popped the hood (it was HOT), killed the engine and the fans stayed on. Coolant temps were fine. After about 5 to 10 minutes, we started out again. A/C was still on, car barely wanted to move in first gear, as though the engine was flooded. It cleared up once I was in second gear though.

4. Throttle tip-in has recently gotten a bit herky-jerky in every gear, regardless of engine-temps. Used to be pretty-darned smooth. Just feels like it's not feeding quite enough fuel immediately when cracking throttle. It's not a huge stumble or anything--it just isn't smooth.

So, from what I've read so far, we have a TPS, ISC, a bypass valve and a dashpot. I've never had a car with a dashpot before. Most of these symptoms look like they could be a combination of TPS and ISC, with the dashpot only being a factor when the throttle plate is closing, which I don't have any symptoms about right now. Does that sound about right? I figure I can verify adjustment of the TPS/ISC/bypass and hope that they don't need replacement?

Any thoughts are appreciated, thank you.

I would start with removing the ISC and cleaning it. If your car is pretty untouched/low mileage you probably have the original paper ISC gasket. Get the new metal gasket from Mazda, scrape off the paper one and use the metal one.

With the ISC off, spray the insides with carb cleaner and get any black junk out of it. Use 2 jumper wires to hook up the 2 electrical posts to 12v and ground and tap the ground wire to pop the solenoid back and forth. Hit up with some WD-40 to lubricate it and try that out.

Get the ISC happy first, that will affect a lot of things.

Dale

Thanks Dale, I'll have to pull that apart and take a look at it, hopefully this weekend. Well, I'd better get the ISC gasket first--I'm sure the TB has never been apart on this car, so yeah, paper gasket will still be there.

Is this the right gasket? Seems to have the right flange shape but I thought the ACV was some other part of the system.

https://www.atkinsrotary.com/store/9...A1-20-661.html

Is the car still standard ECU? I agree with Dale, and check the idle air control valve, as that can gum up and give some grief.
However I'd also suggest (if you're running the stock ECU) to try and pull any codes from it using the diagnostic connector/jumper method.

That's the right gasket. They're calling it a Bypass Air Control gasket which is what the 2nd gen RX-7's idle speed control motor was called. But, I recognize it in the pic, that's definitely it.

Dale

Thank you again, Dale. And SA3R, yes, still the stock ECU. I've never pulled codes on the car, I may jsut do that for curiosity, even.

Okay, so, back to investigating my idle issue. Here's an update on the symptoms:

1. Cold-start fast idle doesn't hit 3000, more like 2500 with a bit of hunting. Not a big deal. I would probably just go PowerFC some day and kill the fast idle cam anyhow, but I figured I'd report the symptom as it may be relevant. If I blip the throttle to get it to drop down to 1500 RPM, generally the idle won't catch and the engine will stall.

2. Car hesitates on throttle tip-in when cold. Seems like it goes lean when I press the throttle.

3. From warm starts (like, go to a restaurant and come out an hour later), with AC on and compressor engaged, idle hunts around target value. It oscillates with increasing amplitude until the engine stalls. Seems like a lean condition to me with overcompensation acting on a delayed signal. It probably starts when the A/C clutch goes on and load jumps--whatever delay is experienced sends me into this oscillation. One could say the same thing about symptom #2 I suppose--feels like there's a signal delay.

I have done the following:

1. Cleaned the ground from throttle body to firewall and added another ground from battery to driver-side shock tower.
2. Removed and cleaned ISC (it was pretty minty) and verified that it responds to current (popped it in and out, didn't have anything available to drive the stepper). Replaced it along with new metal gasket.
3. Verified that TPS values were in the proper range as described on this page, and that there were no dead spots throughout its range. I didn't attempt to adjust the TPS at all.
4. Removed the dashpot and verified rod operation (didn't attempt to adjust it).

The car drives great, by the way, once it's been driven for like ten minutes from either cold or warm start. It's weird. I took it by Yoshi at Neptune Speed for some advice, but by the time I got there the car was driving buttery smooth, no 3000 rpm bucking, no bad throttle tip-in, etc. He suggested that with the age of these cars, problems like these are hard to tune out without something like a PowerFC. I get that, but would love to get it perfect in stock form if at all possible. Yoshi probably thought I was being a bit nutty, considering the kind of work he does all the time LOL. BTW, there were some sweet FDs and FCs up on the lifts at Neptune Speed. Would love to have the keys to that garage. :D

Thanks for any further help!

Possible it could be something with the coolant temps sensor for the ECU. If the ECU isn't getting a good signal from it the car will run poorly when cold. I think you can use a meter to check the voltage from that wire to the ECU, would be the easiest way to check that. Cold engines need more fuel.

Check your clutch switch, a bad clutch switch can do a lot of weird idle things. There are 2 clutch switches, only one goes to the ECU.

Dale

What happens when you cold start with it in gear? The accelerated warm up system doesn't work in that situation.

I have a suspicion that the 93 models had a rough tune from the factory with a stock ECU during cold running, and that the 94 and 95 models had an updated tune that at least improves (but not always solves) these issues.

I would love to see your impression from swapping in a 94 or 95 stock ECU.

Thank you, Dale. I have the ECU pinout and I see water temp and clutch switch. Do you know what the response graph for the water temp sensor? Or can you tell me what the voltages should be at cold and hot?


I haven't tried that scenario. I'll give it a shot next time.

Are the 94 and 95 ECUs 100% interchangeable with the 93? I'd totally give it a shot just out of curiosity if I could source a newer ECU for cheap, or if someone could loan me one.

Y'all might not believe this. The problem seems to have been a missing bolt on the airbox cover. I haven't looked at the airbox since I had a shop change the vacuum lines out for me way back in 2016. Looks like they left one of the airbox bolts out. I never noticed the missing bolt until I was taking a picture for another member and both he and I noticed the missing bolt in the photo.

I can only presume that this means that the airbox wasn't properly sealed, but I'm not sure how this would have affected air metering or any ISC controls at the throttle body. It's not like there's a MAP all the way up at the air filter, but with all the goofy vacuum lines in the stock FD, something wasn't liking it.

The car drives fine again, so that's great! Thanks for all your help with the troubleshooting, sometimes in the end, it's the stupidest thing. Guess I can't be lazy and let others work on my car anymore. :wallbash:

Weird.