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Idle hunting / surging 1200-1700 rpm - need help troubleshooting

Old 05-11-17, 05:49 PM
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Angry Idle hunting / surging 1200-1700 rpm - need help troubleshooting

Video:

This started happening out of blue. Started the car up. Idled and drove normally. Got on the highway. Got off the highway and noticed the idle hunting / surging non stop between 1200-1700.

I searched and the usual causes are a vacuum leak and ISC problems.

To troubleshoot the vacuum leak, I filled an almost empty Simple Green spray bottle with water and sprayed every vacuum connection I could find while the engine was running. I thoroughly sprayed the rats nest with the UIM on and the car running. Didn't notice a change in idle. I then decided to rinse off the engine and hosed it down while the car was still running. No change in idle. I don't think it's a vacuum leak based on this.

I then tried to troubleshoot the ISC. Started the car from cold. Manually moved the wax rod so it wouldn't increase the idle speed too much. Once the coolant hit about 50*C and the idle speed started decreasing below 1700 RPM, I noticed that the idle hunting started. I disconnected the ISC and the car idled the same.

Anyone have ideas on what to do next?


I haven't turned a wrench on this car in 5-6 years and don't remember how it works anymore
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Old 05-11-17, 07:02 PM
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Check your clutch pedal switch. Spring in it probably broke.


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Old 05-11-17, 08:47 PM
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I'm going with adjust TPS
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Old 05-12-17, 02:15 PM
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Originally Posted by Fritz Flynn View Post
I'm going with adjust TPS
Forgot to mention that I already checked the TPS voltage readings on the PFC Commander and they were in range.
It could be that the readings in the PFC are not entirely accurate but I doubt it.
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Old 05-12-17, 04:33 PM
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I had a similar engine surge when rebuilding my engine (not the same circumstances as you I know), but when I went to install the primary injectors one of the rubber grommets fell off causing a vacuum leak. The soap bubble check didn't catch it, because of the location, but a smoke test did.
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Old 05-12-17, 07:25 PM
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Originally Posted by Mrmatt3465 View Post
Check your clutch pedal switch. Spring in it probably broke.


Matt
Checked. Spring is present and accounted for and looks springy
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Old 05-12-17, 07:26 PM
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Originally Posted by briansfd View Post
I had a similar engine surge when rebuilding my engine (not the same circumstances as you I know), but when I went to install the primary injectors one of the rubber grommets fell off causing a vacuum leak. The soap bubble check didn't catch it, because of the location, but a smoke test did.
how does one do a smoke test?
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Old 05-12-17, 11:56 PM
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Generally speaking you just introduce smoke into the vacuum/manifold system with the car off. You should be able to rent a smoke machine from auto zone/advanced auto, or if you know someone who smokes cigars/vapes that would work too.
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Old 05-13-17, 09:42 AM
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Originally Posted by briansfd View Post
or if you know someone who smokes cigars/vapes that would work too.
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Old 05-13-17, 03:37 PM
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hey if it works, it works hahah
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Old 06-10-17, 06:44 PM
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Just find someone with a WRX, they most likely vape.
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Old 07-05-17, 12:05 AM
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Finally got around to spending some time researching this again.

Checked TPS settings and they are in spec.
ISC tested fine using the FSM procedure of testing ohms.
Plugged / un-plugged ISC and no change.
Unplugged vacuum hoses to see what effect that had on the surging idle and found that the symptoms were different. Resulted in a slightly higher idle and you could hear air escaping so I don't think this was a vacuum leak.

Swapped to the stock ECU to rule out that the Power FC was bad. The car continued to surge / hunt at idle. Slightly differently but a surge / hunt just the same.

With the stock ECU, I realized that I could checked codes so I jumpered the diagnostic and took some video. Still deciphering the video. I think it was either a 28 or 38. Which I think is either EGR and AWS. Had never checked codes before so that was interesting. Rather primitive.

Wish me luck
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Old 07-12-17, 12:21 PM
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I would still be skeptical of the ISC, if you have another one try swapping

Might not be a bad idea to check your throttle cable tension. I fixed one before where the cable was too tight not letting the throttle close all the way and it was fighting to control the idle.

Once I loosened the cable a little, it worked great. If this is the case, be sure to adjust your TPS accordingly. I've found the throttle body / tps has a huge effect on the way the car idles.
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Old 07-12-17, 12:54 PM
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OK, idle speed is caused by air getting around the throttle plates to the engine. That's it. The trick is to find how that air is getting in.

I would remove and clean out the ISC with carb cleaner. It's possible it's got some junk in it that's sticking it open. Clean out with carb cleaner, get 2 jumper wires, hook them to the 2 electrical terminals of the ISC, and connect them to 12v and ground. Tap the wire a few times to work the ISC solenoid back and forth. Hit the inside with WD-40 to lube things up a bit too.

It's also possible the AWS is trying to stick open or something if you still have that.

Also, you can have the engine running with the throttle body elbow off. You can use your thumb to plug the hole in the throttle body that leads to the ISC to rule that out and you can plug the hose going to the AWS to rule that out. Also, if the throttle is stuck open, like the thermowax is stuck or jammed up, that can also be an issue.

Remember, it's just finding where that extra air is coming in. VERY LITTLE air is needed for idle so it can be a pretty small thing.

Oh, since unplugging the ISC doesn't change things, it's most likely not some other input to the ECU. The ECU can only set idle speed with the ISC, that's it. If it's unplugged, that's not it.

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Old 07-12-17, 11:34 PM
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^Good ideas. Hadn't thought of them. Will do that this weekend.

The thermowax thing looked like it behaved oddly. Then again, it's been so long since I've been under the hood of the FD that everything looked odd.

If anyone has pics of the thermowax rod thing, please post in open and closed positions.

Thanks.
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Old 08-08-17, 12:53 AM
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New / previously unnoticed symptom - if I try to hold the RPMs at say 3,000, the revs will fluctuate / surge just like they do at idle. I'm thinking it is the TPS after all. Going to order one.
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Old 08-08-17, 11:32 PM
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Neat bit of trivia - there are 3 unsold, new TPS sensors in all of the US. 3 months back order for more.
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Old 08-09-17, 11:38 PM
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My money is still on vacuum leak. A big one.
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Old 08-11-17, 11:42 AM
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Following up on post #2 and your reply in post #6:

Is there any difference in idle behavior between running in neutral with clutch engaged and running in (any) gear with clutch disengaged?

Even if spring is not broken, the internal contacts of the switch might be corroded, causing open contacts when the switch should be closed. Or the contacts could (maybe) be stuck closed regardless of the switch position.

The switch is connected to both terminal 1Q of the PCME and to the cruise control unit, on a blue/orange wire. (See sheet B-1d of Section Z in the manual.)

You could make a resistance check between the clutch switch connector terminals using an ohmmeter. When the clutch is engaged, there should be continuity between those terminals; when the pedal is depressed, you should see an open circuit.

My experience with my '90 Miata was that clutch switch faults play hell with the idle behavior.

Last edited by wstrohm; 08-11-17 at 11:45 AM.
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Old 08-12-17, 08:58 PM
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GA

I had same problem with surging idle, turned out to be leaking lower intake manifold gasket. A clue for that is when under boost, the same leak blows out rich fuel-air and you can smell the raw gas sometimes.
Spray carb cleaner etc. around lower intake when idling to see if it has an effect. Replacing the lower intake gasket is a PITA, but not much choice.
I agree with DaleClark: hunt down the vacuum leak.
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Old 08-13-17, 10:13 AM
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^^^ that is actually quite brilliant. I had the same idle surging issue but have mostly cured it by adjusting the idle screws. However, i have noticed recently that I am running quite lean under boost since the idle surge began. A damaged LIM gasket and losing gas during boost would make sense in this case.
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Old 08-13-17, 11:20 AM
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I had same problem with surging idle, turned out to be leaking lower intake manifold gasket.
When our '94 had a leaking LIM gasket (fiberboard OEM) the idle had increased in speed, but did not oscillate. I replaced it with the later steel gasket and the problem went away.
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Old 10-16-17, 04:01 PM
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So the new TPS sensor didn't fix it. Going to go look for a used ISC to swap out.
Tested the clutch switch - no problems
Went through all the TB settings - all good.

Fun times...

Last edited by gracer7-rx7; 10-16-17 at 04:05 PM.
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Old 10-16-17, 07:33 PM
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Originally Posted by gracer7-rx7 View Post
So the new TPS sensor didn't fix it. Going to go look for a used ISC to swap out.
Tested the clutch switch - no problems
Went through all the TB settings - all good.

Fun times...
I know it was mentioned above, but did you check if the throttle cable is sticking, or compromised? My car's pressure reservoir was loose and caused the throttle cable to stick, causing idle issues / hunting.
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Old 10-17-17, 09:39 AM
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Originally Posted by moehler View Post
I know it was mentioned above, but did you check if the throttle cable is sticking, or compromised? My car's pressure reservoir was loose and caused the throttle cable to stick, causing idle issues / hunting.
Yep. I actually replaced the cable when I did the engine a while back. It operates smoothly.
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