6 replies to this topic

[Tom Hampton]

My engine idle went to hell last weekend (dropping to 250rpm...or dying outright). I had to heel-toe just to keep it from dying on the way home. I'm hoping its just a vacuum leak (but, I'm worried it might not be that simple). I figured I'd go buy some bulk vacuum hose, and caps and replace things one by one until I found the leak (or don't). There was no noticable change in behavior off-idle. So, I haven't lost all hope.

Does anyone know the inside diameters required for all hoses (not including the brake assist lines). 3, 4, 5mm?

These will just be temporary. If I find the leak, I'm just going to order a new set of hoses from MSM. OTOH, if its something more sinister there might not be much point in a new set of hoses.

1992, 1.6L

[Qik Nip]

Tom:
Before I started replacing hoses wholesale, I'd get someone to sit in the car (to keep it running) while you start looking for the vacuum leak. From the sound of it, I'd bet on a hose that has come completely off (brake booster?) - or a PVC valve that's gone totally south.
Rick

[Tom Hampton]

Rick-

Makes sense. I haven't had a chance to even lift the hood yet.

I live a small ways from the local parts store, but I drive past it on my way home from work. So, I was just going to pick up the bulk bits on my way home to save myself a trip in the morning. Can't hurt to have a few feet of each size spare hose in the parts bin, anyway...right?

But, it certainly makes sense to look for gross faults (disconnected hoses, broken PCV, etc) before looking for harder to find stuff. I have a vacuum guage, too...so, I should be able to see if vacuum is the likely culprit at all.

[LarryKing]

How old is the battery, and are you certain your charging system is working correctly?

[Tom Hampton]

Battery was new in 2010. Car turned over fine (not sluggish or weak) all day Monday, even at 5am when it was 32F.

I normally keep it on a trickle charger, but I forgot to put it back on after I got it back from paint (thanksgiving week). When I pulled it out of the shop last week the battery was very low (4 weeks without a charge), and I barely got it started without needing a jump. After 5 minutes @ off-idle, I had to shut it off for a minute. Restart was strong....

So, based on those two observations, that I think the battery and charging system is working fine.

At least up until Monday. I suppose the alternator or regulator could have failed during the day. Drive home was ~2 hours in the dark, so I was using headlights. They were full bright: no dimming over time, or dimming at normal idle rpm (850...not the faulty 250 which isn't really fair for the charging system).

I think all that suggests its okay. But, its a good point and I'll check it anyway, to be sure.

[Tom Hampton]

While searching over on miata.net I ran into a reference to a "hose techniques" silicone hose kit. According to the HT kit documentation the kit includes:

3.5 mm 8 feet
4.0 mm 7 feet
6.0 mm 11 feet
8.0 mm 3 feet
10.0 mm 10 feet

Suggested Uses - Suggested Hose I.D. Sizes*
Intake Manifold (small fittings) - 03.5mm
Boost Controllers (Greddy, HKS & other) - 03.5mm, 04.mm or 06.0mm
Windshield Washer Bottle - 04.0mm
Intake Manifold (small fittings) - 06.0mm
Radiator Overflow Hose - 06.0mm or 08.0mm
Hood Hold Up Rod - 06.0mm or 08.0mm
Intake Pipe (large fittings) - 08.0mm (includes aftermarket intakes & some radiator overflow)
Misc. covers of hard lines, etc. - 10.0mm (refer to Slit ‘n Groom™ Technique in instructions)

Obviously all of these are not VACUUM. But, I'm guessing 10 feet of 3.5, 4, 6, and 8 will cover it, and then some.

3.5mm ~ 5/32" (will be loose, and may need a clamp) or 1/8" which may be tight
4mm ~ 5/32" (should be perfect fit)
6mm ~ 1/4" (near perfect fit) or 7/32" for a "snug" fit)
8mm ~ 5/16 (should be perfect fit)
10mm ~ 3/8" (snug fit)

[Tom Hampton]

Turns out it was a crack in the air inlet tube that goes from the afm to the throttle body. The crack was in the corregated section.