It's the rings/pistons isn't it? :(

[XMX]

Hi all,

So our little 1.6 NA has been running increasingly rough over time, it always had the usual HLA ticking which I've been experimenting with different oils to cure - so far Penrite 5w-30 synth has been a winner. It's never a constant - comes and goes depending on unknown variables, not related to temp/pressure/time.

We fitted a newish exhaust recently, stock pattern/diameter but with a hotdog resonator, and it amplified the spluttering from the tailpipe to the point you can hear it chugging at idle, there is now some black soot (unburnt fuel?) on the exhaust tip.

On the advice of AutoOne, I tried a Penrite 10w-50 with some Morey's oil stabiliser - ticking is worse now, chugging the same. On retrospect it might be too thick but given high km's I wasn't sure...

I'd tried a compression test earlier in the month but without removing all plugs and without holding the throttle open - tried again today doing so and got:
1 - 183psi
2 - 209psi
3 - 76 (!) psi -> increased to 108/99 psi after putting oil in spark plug hole
4 - 192psi

Engine has done ~360,000kms, I was hoping that it might be stuck valves given the HLA noise but I'm going to guess it's rings?

Oil changes have been pretty regular most of it's life from the service history, last interval was unusually long at ~350,000 to 364,000 kms, but almost all highway driving at 100km/h, 80kms per day.

So is it rebuild time?
I've just noticed a slight hesitation today when accelerating, presumably these things go downhill fast when they occur?

Thanks in advance...

[smy0003]

Just replace the engine. A 1.6 can be had for very very cheap.
Just swap it over for a good condition replacement and keep enjoying yourself

Sent from my LG-H815

[XMX]

Suppose that would work if I could find one here in WA...

[93_Clubman]

Compression test aside for the moment, is 'chugging' at hot & cold idle or just cold? If at cold idle, but goes away once warm, then it could be the coolant temp sensor the ECU relies on:

My experience with CTS has been poor cold idle (running lean when cold), but I note manga_blue has previously mentioned running rich, so it seems it can depend on how they fault, ie not rich enough during cold idle, or too rich during hot idle.

Re compression test, was it done like following (including removing all plugs):

Long story short the test needs to be done at operating temp, with a jumpstart to ensure the starter is going full power and with the throttle wide open to get all the air in you can. If it wasnt done like that it doesnt count.

viewtopic.php?f=29&t=59832

If the motor is stuffed, might be a good time to drop a 1.8 in.

[XMX]

The chugging idle doesn't seem to depend on operating temp but then again varies in severity... I know there is a test you can do with hot water and a multimeter so I suppose it's worth a shot. That said one buggered cylinder would also ruin idle I guess.

The hesitation is like a really bad chugging misfire but then goes away after 1500-2000 rpms as the revs climb, and I've noticed it's worse if you try and accelerate in too high of a gear/change up too early.
Tried gapping the plug in the low compression cylinder down to 0.8 and cleaned any oil off it that may have been there from adding oil to the cylinder to raise compression - problem hasn't gone away.

As far as the compression test goes, I'd read Dan's thread which made me go back and try it again hoping the results would change. The only thing I did different was not hooking the battery up to another car, but it's pretty new and when I went back to cylinder 1 to see if the result was different due to draining the battery the compression was the same if not a few psi higher.

Is it possible 10w-50 + oil stabiliser is too thick? Had previously tried some LiquiMoly tappet noise additive at the end of the last oil fill for a few days before I drained it for this - can't say it made any difference to the noise or running.

I should mention that the idle is a bit 'shaky' yet seems to register ~850rpm, not sure if that means anything.

Thanks for all your help, much appreciated!

[Red_Bullet]

Contact Mazda for a warranty claim.

[StanTheMan]

Contact Mazda for a warranty claim.

OMG that's gold!!!!! LOL

[manga_blue]

I'd actually say that compression was pretty good, especially considering age, except for a burnt out valve in no3. That would be what's causing all the missing.

[93_Clubman]

Is it possible 10w-50 + oil stabiliser is too thick?

Re HLA noise & 10w-50: http://www.mx5cartalk.com/forum/viewtopic.php?t=32831
A number of forumites have used Nulon Lifter Free successfully for HLA noise.

[Ross]

I'd actually say that compression was pretty good, especially considering age, except for a burnt out valve in no3. That would be what's causing all the missing.

Exactly the same symptoms with our other NA6 (400k Km), - exhaust valve on No3.
Remove & replace with a newer 1.6L and job done. or have the head refreshed.

[manga_blue]

Cooling around the exhaust ports on number three is a bit dodgy because of Mazda's half-assed attempt to convert the cooling system from an east-west to north-south configuration. Number 3 exhaust valves are usually the first to fail. Others won't be far behind.

It shouldn't take a good workshop more than a few minutes to diagnose a compression problem like this.

Assuming that it is in fact a stuffed valve then you have two options: recondition the head or replace the engine.

If you decide to get the head reconditioned then factor these into your budget:
1. Remove, transport the head to/from a recon shop and then refit it ~ $400, assuming you get a local workshop to do that for you.
2. Clean, test, dismantle and reassemble the head ~ $200
3. Recut all valves and seats, probably replace the odd valve or seat, at 360k probably replace all valve guides, replace all seals ~ $25 per valve or $400
4. Replace the timing belt and front oil seal ~ $200 (mad if you don't do this while everyting involved is already dismantled)
5. Valve regrind gasket set ~ $150
6. Fresh filter and fluids ~ $100
7. Maybe new set of HLAs ~ $250
8. Surprises ~ $250

So for around $2,000 you get a known head that will perform as well as a new one. Your block looks very good for its age. A B6 block that has had regular oil changes with reasonable quality synthetic can be good for 500k before new rings and bearings, otherwise 300k on well managed dino oil.

The labour for replacing the whole engine is pretty similar to that for replacing only the head. You should still figure in a few hundred for fluids, filter and surprises. For that you get an unknown engine that may or may not perform well and may last another 200k or maybe only 20k.

[Red_Bullet]

^^^That's an excellent informative reply, way more useful than my useless one liner, sorry about that.

[StanTheMan]

^^^That's an excellent informative reply, way more useful than my useless one liner,

But it was soooo phunny

[hks_kansei]

Also, dump the oil and flush out that stabiliser sh*t.

All it does it thicken the oil and make it take longer to flow where it's needed.
Sure, it looks like it makes an engine have oil pressure, but that's because it's essentially trying to pump honey throught it.


It's the sort of crap you use on an engine you already know is stuffed and just want to buy another couple of months before a rebuild

[XMX]

Thanks lads for all the input.

A burnt valve would produce similar low compression results, but then it shouldn't have jumped up when oil was added down the plug hole I thought...

Changing the head would be ok, can do the labour myself easily enough and can get a reasonable deal on parts, but would hate to recon the head to find it's the rings.

Short of another compression test is there any other way to know if it's valves or head? Would escaped soot/markings be visible with the camshaft moved out of the way?

PS. Asking Mazda for a warranty claim would be amusing, we have full service history from day one including a 1991 photo of the original owner in a turtleneck in the dealership with his new MX5 haha, itill has the dealership stickers on it!