Random loss of power and idle stumble
 

I've got a few issues that crop up randomly and a few culprits. I'd like to see if I can avoid just doing repairs until one works, though it looks like that may be the only possible route.

These symptoms are random. I have not been able to pinpoint how and why they happen, though occasionally if I dont rev high enough on a downshift to second, and the engine torques, it may start the issue for a few minutes. Or, if I push hard in 1st every once in a while.

Usually, the car will start fine, and after a minute, the idle will droop to 500rpm for a few seconds and climb back up. If i give it throttle and let go, it will do the same. Throttle "blips" will almost stall the car. This issue also happens when rolling up to lights. Once back to 800, ill have to hold the throttle for a good second for the revs to start climbing. When driving, it feels like its running on 3 cyl, super slow and sluggish.

It usually fixes itself after a while (usually when warmed up a bit more) where I get a normal steady idle and full power back, but will STILL come back for a couple of minutes even when warm.


New plugs and wires. Sanded and put dielectric grease on the engine ground. No codes.

Here are the culprits

- Being so random, it leads me to think it's electrical. But, if i push the car hard through a few gears, it sometimes fixes it. Wouldn't coil packs fail MORE up top, ruling them out? Do coil packs fail consistently, or randomly like this.

- The crank pulley has a slight wobble to it, so it may have something to do with that? But why so random then? The CAS is gapped correctly to the teeth.

- Some other ground

- Engine temp sensor or IAC valve. But, would the temp sensor have that large of an effect on performance and randomly too?


ANY inclination to one culprit, or reasons why some may not be the problem would be tons of help!

Took her out today since it was 55 with the top down, and NO issue whatsoever.

Makes me think its gotta be electrical or a sensor if its temperature dependent.

Coil packs should fail more up top, but they can behave strangely sometimes. They can be weak and drop intermittently.

IAC valves can get sticky or clogged with carbon deposits.

Vacuum leaks can cause bad metering of the fuel because the air leaks in behind the MAF and the fueling isn't compensated accordingly.

A crank pulley wobble is a problem. It gets worse over time. Setting or checking the initial timing can be troublesome because the timing marks move. Factory base timing is set at 10 degrees but these cars respond well to bumping that up to 15 or so by getting a little more peppy. But you won't likely be able to do anything accurate with the timing if the pulley is separating.

Thanks for the inputs

I dont believe its a vaccum leak due to how random it is.

Could coil packs be affected by temperature (cold=run worse)? If so its possible, and i have been seeing alot of the 99-00 coil packs having issues.

I replaced the crank pulley with a new true one and there was still a slight wobble which leads me to believe its a keyway issue, which I can fix. But, how would this "wobble" act so randomly, and seemingly temp affected?

My problem is i can't replicate this issue on command, so really doesnt point me to any direction.

I'd continue to look into vacuum leaks and air passage blockages, only because it sounds "more" like those. Your profile says NA, but since you're posting to the NB section, I'd also check the following:

1. Clean the MAF sensor (probably not the culprit, but it takes 30 seconds).

2. Clean the EGR valve on the back of the IM - notoriously get coked up on NBs.

3. Clean the top half of the Intake Manifold, there's a small hole for EGR that also gets coked up really badly. Once the TB is removed and the top half of the manifold, you'll see a small hole to the upper right, about 4-5mm in diameter, about 1-2" down the throat. That gets really badly plugged. Just spray that down to clean out the carbon. You'll be surprised how much gunk is in there. It tends to clear up a lot of problems with general drivability.

Just a few things to mention on the IM cleaning:
1. take care not to bend the intake manifold metal gasket, so you can re-use it.
2. Don't overtighten the 6 IM bolts. They're soft and you can possibly strip the heads off.
3. Remember to put all the vacuum and harnesses back on properly. :-)

NA is my primary miata, this is my fixer upper.
I've already cleaned the maf and the egr hole on the intake manifold. Did the propane test on the main hoses I could see and didn't see anything affect.

Didn't clean the egr on the back of the Im. Is that connecting to the hole on the im?

Yes, that's right. The hole in the IM is actually the more common cause then the EGR crudding up on NBs.