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Greg Smith
Greg Smith HalfDork
5/8/18 11:24 a.m.

This weekend, wes about to return home from an event. 

My NC Miata has ~165,000 miles and no recent issues. I have have a slight clatter (which I thought was a loose heat shield) at idle, but goes away under load or with AC on. 

Fired it up to come home and - charge light is on. Grr. 

OK, plug in the OBD & check using Torque - 11.8v. not charging. Everything else seemed OK (I did not pop the hood, because at this point, I was flashing back to old Triumph TR7 days of trying to get home on battery power) - headlights on - 11.5v, car still runs. Off - back up to 11.8. So I have some juice to work with. 

Get a couple miles closer to home and now it's Christmas tree time - CEL / ABS / TCS all come on. I check the CEL - which says low coolant temp, check coolant temp, reading 150. I press on. At this point, I'm thinking bed sensor reading due to low voltage. 

When I get home about 15 minutes later, doing ~45 almost the whole time, i'm down to 11.6v. No missing or  other issues. I park in front of the garage, and it starts steaming. Crap. 

I do what I should have *miles* ago and pop the hood. yeah, it's steaming and hissing at the coolant expansion tank

And my serpentine belt is there, no visible damage, but off the pulleys... and hey, look! The nonmoving idler pulley is laying on the tray at the bottom of the engine bay. That's unexpected. 

(more to follow, pics to follow)

wlkelley3 UltraDork
5/8/18 11:31 a.m.

Kinda sounds like the alternator locked up. Possible bearing seized. That would explain no charge. When it threw the belt would explain the overheating. Check compression for engine damage. 

Greg Smith
Greg Smith HalfDork
5/8/18 12:03 p.m.

I rotate the coolant tank overflow host upwards and it makes a neat smokestack. (sorry, no pics... I was seriously flustered)

Yeah, belt-driven water pump. Grab IR thermometer. Most of the engine metal that I can hit is mid-high 200's. Exhaust side (may have been the exhaust manifold itself) was over 300. 

It seems that the idler pulley bolt broke. On this engine it goes past the front cover (slotted for some unknown reason) into the block. The bolt will not grab as is and I'm thinking there's a great chance the bolt's broken off in the block. (and who knows how the gaskets have held up to the additional temps!!)

The idler pulley is the one pressing the top of the belt down. The engine front cover has a slot to the left of the bolt. 

After taking some stuff out of the way, the remainder of the bolt threads into the block easily for a turn and a half, then tightens further with a wrench... block threads seem OK. (Where is the rest of the bolt??? Left on the road somewhere? How did it fail this way?) The threads that are present are covered with crud because the front cover is slotted to the left of the idler pulley. Why? Probably less expensive to cast it that way instead of machining another hole, I guess. I'd say it's a crappy design, except it *did* take 165k miles to fail. 

The great news is that this appears to be just a tapped 'ear' on the block - nothing going into the engine internals.


At this point, I will have parts (new serpentine, tensioner, idler pulley (with replacement bolt... from the pictures, it appears that my bolt at 1.75" long is at least 1/2" short of what it should be...) and air filter, because it's due and I'm there anyway) tomorrow. 

I'm planning to drain & change the oil and check for any coolant indications. 

And probably drain & change the coolant as well. 

Is there anything else I should do before firing things back up & checking compression, etc?





jfryjfry HalfDork
5/8/18 12:03 p.m.

Similar to the above but it seems that your idler pulley broke. Hopefully it didn’t overheat bad enough to damage anything.  New pulley, charge the battery and give it a shot. 



simultaneous post.   I would check the oil and coolant but only drain if you see contamination.  And if you see contamination you won’t stop at draining.  You’ll be pulling the head at a minimum most likely :)

if the bolt broke (maybe It was overtightened) you’ll need to extract and replace. 

Greg Smith
Greg Smith HalfDork
5/8/18 12:05 p.m.

Thanks for the comment!

The water pump, AC compressor, tensioner pulley, and alternator all spin easily. So does the idler pulley. The only thing that doesn't at the moment is the pulley on the crank... which is expected :)

The remainder of the idler pulley bolt is... gone.  since the bolt has a plastic piece around the threads so the assembly (bolt, front washer, pulley, rear spacer, 'keeper' plastic) can go in as a unit without losing parts, it dropped as an assembly and I have it all. 

As I said, the current one threads right in with appropriate 'grab' if I take the pulley off it and use the threads that were previously exposed via the slot in the front cover. So it doesn't look like I need to locknut the back or helicoil, and I don't have to extract the existing bolt. The exact source of the failure is really a mystery to me here. 



Greg Smith
Greg Smith HalfDork
5/8/18 12:13 p.m.

Oil is close enough to 'due' that i see this as a preventive measure to try to get out anything that cooked a bit. And the coolant is old enough that changing it is cheap prevention/ 

Maybe this would be an appropriate time to run some seafoam through it? I dunno...


rslifkin SuperDork
5/8/18 12:19 p.m.

Honestly, if you got the thing above 250 and without the water pump running (so no coolant circulation), there's a very high chance it's toast.  You might have gotten lucky, but I'd be pretty surprised if the head gasket is intact, head isn't warped, etc.  

Greg Smith
Greg Smith HalfDork
5/8/18 12:29 p.m.

Kind of what I expected to hear. We shall see.........


codrus GRM+ Memberand UltraDork
5/8/18 1:17 p.m.

I'd suggest a leakdown test before running it.  You checked oil for coolant, right?


Greg Smith
Greg Smith HalfDork
5/8/18 2:29 p.m.

none noted yet (dipstick check for normalcy)

I will see when I drain all the oil to a fresh bucket just to be sure. That's planned for tomorrow evening. I might be able to do that part tonight. 



dherr HalfDork
5/8/18 2:46 p.m.

Sorry to hear that Greg, the idler pulley bolt breaking is indeed odd. The NC's are known for cracked expansion bottles causing them to overheat, but this is strange. Hopefully the head is not warped and you can put it back together.  Have Andrew swap it for a 2.5 if it is indeed warped wink

On a plane to Seattle or I would try to stop over and help you with the diagnosis.


Greg Smith
Greg Smith HalfDork
5/8/18 5:17 p.m.

No worries Dave. I need to get a compression tester anyway... Parts will be here tomorrow & I will look into things tomorrow evening. 

I dod go back out to the scene of the carnage and 

Will have to check on the expansion bottle. It was spitting coolant from the overflow / cap, I don't think it was leaking elsewhere, but it may be compromised. 

As I had mentioned before... the remainder of the bolt threaded into the tapped hole and felt fine, but I tried it again, and as i tightened it in all the way, lo and behold, the rest of the bolt started twisting its way out the back. So, I stuck a magnet to it so I could save the part:


and was able to extract it easily from there. It still amazes me that the bolt threads were essentially undamaged, and I guess that must be a stress point on the hardware. 

Bolt removed:







dean1484 GRM+ Memberand MegaDork
5/8/18 6:06 p.m.

I would not wast money on fluids until you know it is not toasted 

Greg Smith
Greg Smith HalfDork
5/9/18 11:16 a.m.

battery charged, parts in hand. Will try the following:

 - install new tensioner pulley, idler pulley, serpentine belt per Mazda specs for bolt torque & belt routing / install procedure
 - pull fuse for fuel pump
 - disconnect coil packs, pull all 4 spark plugs
 - temp install battery, MAF/AFM (currently both are pulled for access)
 - do compression check on all cylinders in sequence  (throttle at WOT, ~5 seconds cranking for each cylinder) 

If that looks good, replace plugs / coil packs / fuel pump fuse. drain oil & check for coolant (using a new container / then maybe pour into empty, clean washer fluid jug to check for coolant on the bottom?).

 - If oil uncontaminated, replace filter, refill oil. (It's just about due for a change anyway)
 - top off coolant or drain & refill. (I'm inclined just to drain & refill)
 - test run at idle in driveway. Verify charge, temp monitoring OK, expansion tank holding pressure. 
 - test run on road

Any other thoughts? Am I missing some critical step here? Any "good to do but not critical" steps?

This is my daily driver, sometimes up to 1000 miles/week, so I want to make sure I'm getting the details right. Experimentation is for the cars I play with. 


Greg Smith
Greg Smith HalfDork
5/9/18 11:24 a.m.

In reply to Greg Smith :

This is the install guide I was going with:



codrus GRM+ Memberand UltraDork
5/9/18 12:30 p.m.
Greg Smith said:

 - do compression check on all cylinders in sequence  (throttle at WOT, ~5 seconds cranking for each cylinder) 


Any other thoughts? Am I missing some critical step here? Any "good to do but not critical" steps?

Do you have a leakdown tester?  IME, leakdown is better at finding iffy-but-not-completely-blown head gaskets than a regular compression test is.


Greg Smith
Greg Smith HalfDork
5/9/18 1:19 p.m.

I don't. Would something like this be sufficient? https://www.harborfreight.com/cylinder-leak-down-tester-62595.html

or do I need something better? I'm not familiar with leakdown testing, but I do have a decent air compressor and could see about doing this if it's worthwhile.

$40 is an easier bite than $125 (less AAA discount) but I could also get a NAPA one on the way home. 


Greg Smith
Greg Smith HalfDork
5/9/18 6:24 p.m.

Well... it's decided for me. I skipped HF, and Napa would have to order one in for tomorrow. 

So tonight's testing is compression only. 

Brotus7 HalfDork
5/9/18 7:54 p.m.

Hoping it isn't toast.

Regarding failure mode: I think it was actually a lost of torque that caused it.  Bolts are strong in tension and weak in bending without preload.  The bolt preload is high enough that bending load from the belt on the pulley will prevent the bolt from experiencing the bending signature tension and compressive loading.

Without bolt preload, the bolt shank is subject to tension and compression from bending, and some degree of cyclic load.  Can you take some close up pictures of the bolt chunk where it broke?

Interesting related read: 

Mark Bailey, MSc, PEng, PE


Figures 6 and 7 show broken right side wheel studs. These are the portions of the studs that were in the wheel hub. The parts of the stud that broke off, along with the nuts, were never found. The flat fractures with “beach marks” (fine curved parallel lines resembling an aerial view of a beach) on the fracture surface are typical of right-side stud fractures, and are classic examples of what engineers refer to as reversed bending fatigue

Figure 6. Fatigue fractures on studs from a motor home right-side wheel after 2900 miles. Fatigue fractures are usually flat and transverse to the stud axis.

Figure 7. Beach marks on the fracture surface are clear signs that this stud from the right rear of a van failed from fatigue. Failure was less than 121 days since installation.

Greg Smith
Greg Smith HalfDork
5/9/18 9:22 p.m.

 - install new tensioner pulley 2 bolts - spec is 14-19 lb-ft, tightened to 20
 - install new idler pulley 1 bolt - spec 15-22 lb-ft, tightened to 22
 - install serpentine belt per Mazda specs
 - pull fuse for fuel pump (fuse 32 per http://www.autogenius.info/mazda-mx-5-2006-fuse-box-diagram/)
 - disconnect coil packs, pull all 4 spark plugs
 - install battery, MAF/AFM, air filter
 - top off coolant with 50/50 mix 

 - do compression check on all cylinders in sequence  (throttle at WOT, ~5 seconds cranking for each cylinder)

Here goes... #1. full throttle, turn key... nothing. No crank at all. 

Right. I'm an idiot. This is a manual and I don't have the clutch in. Full throttle, full clutch, crank 6 soul-draining seconds... and walk arount to the engine bay. 160. hm...
Check the belt. OK, 6 seconds of cranking didnt take it off the pulleys...

#2. still nervous, but the engine is spinning freely. 157.5

#3. 160. 

#4. 167.5 Woot! This is as good as I could possibly have hoped so far...

and now setback time. 

I install the $4 plug... OK.
Then #3. But it doesn't go into my socket properly. Once installed, half of the rubber insert is misaligned in the socket. Where's the rest? Around the #4 plug, naturally. I can loosen that and remove it with a magnet. 

Search the garage and 2 other spark plug sockets, but the wrong size. Grr. 

I will not be deterred. Time to superglue the insert back in place.

And plugs in, engine back togetrher, initial run is a bit rough, and the TCS light has come on. Temp is up to 200 degrees before I shut things down and add more coolant... the expansion tank gave a pretty nice whoosh when i opened the coolant cap, so it seems to be holding pressure. 

Second start seems good. I think the initial start was back to factory default tune and it's learning once more. 

 - test run at idle in driveway. Verify charge, temp monitoring OK, expansion tank holding pressure. (Complete)

Time to pack things in for tonight. 

I turned it around for easier oil drain tomorrow. 

Drain oil, check for coolant
replace filter, refill oil.
Drain & refill coolant.
 - test run on road


Greg Smith
Greg Smith HalfDork
5/9/18 9:26 p.m.

I'll see what I can do re: the bolt pix. I do have both pieces.

grover GRM+ Memberand Reader
5/9/18 9:53 p.m.

In reply to Greg Smith :

Pretty good result so far.  Hoping for the best. 

Greg Smith
Greg Smith HalfDork
5/10/18 12:17 a.m.

In reply to Brotus7 :

High res pic of the broken bolt (left) and bolt end (right):  https://1drv.ms/u/s!AsG6a72my_O7gtEna_EfbRpqmOUF0A

It's weird... the thread kind of tore away which is why the broken bolt was able to push the broken end through the back side of the hole. 

Pulleys & belt installed:

Spark Plug (all 4 looked like this - seems right for an iridium plug: https://1drv.ms/u/s!AsG6a72my_O7gtEqLDRmWnUu65Mcng



AWSX1686 GRM+ Memberand Dork
5/10/18 3:58 p.m.

Sounds hopeful so far!

AWSX1686 GRM+ Memberand Dork
5/10/18 3:59 p.m.
dherr said: Have Andrew swap it for a 2.5 if it is indeed warped wink

Way ahead of you. ;) Though firstly we hope it;s unnecessary, was going to say sell it to me cheap and I'll put in a 2.5 for myself...

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