Diagnosis of a top end problem
Discussion
The never ending saga continues... 
Background -
Sunday we had a really good run out, so the engine had had a thorough workout, nice and hot etc. I dont abuse it, but it had certainly been up the upper reaches of the tacho a few times (as it should).
- Car is an early 1.8 Mk1, so fixed timing etc, with the CAS at the end of the exhaust camshaft.
- Engines previously been a strong, smooth engine given its 100k miles (as you expect with these cars.
- Timing belt and all associated bits are less than 10k miles old.
- Car seems to have good oil pressure, its not doing anything different to what it has in the past (I have the 'real' gauge).
- Plenty of good oil in the car, and not burning any.
- Still pulling well, no noticeable loss in power.
The cars picked up a really horrendous noise. Had a few sets of ears looking at it and the consensus its coming from the head, on the exhaust side.
Its rotational, (not stuck tappets, I know what they sound like!), and sounds like a collapsed bearing or something. At idle, or under a really light load (top at 35mph) its not too bad. As you rev it it definitely sounds rotational as you can hear it going in and out of balance as its speed changes.
I'm trying to think how the cams are supported. Obviously there will be a number of oil bearings along there length, but what supports them at the end?
Other ideas bandied about were a damaged cam lobe, damaged valve spring or follower.. I'm thinking possibly an oil blockage might have caused one of the camsahft bearings to run dry?, or a bearing shell has disintegrated.
Obviously my first port of call is to whip the cam cover off but I cant do that till tonight.
Any thoughts?
Background -
Sunday we had a really good run out, so the engine had had a thorough workout, nice and hot etc. I dont abuse it, but it had certainly been up the upper reaches of the tacho a few times (as it should).
- Car is an early 1.8 Mk1, so fixed timing etc, with the CAS at the end of the exhaust camshaft.
- Engines previously been a strong, smooth engine given its 100k miles (as you expect with these cars.
- Timing belt and all associated bits are less than 10k miles old.
- Car seems to have good oil pressure, its not doing anything different to what it has in the past (I have the 'real' gauge).
- Plenty of good oil in the car, and not burning any.
- Still pulling well, no noticeable loss in power.
The cars picked up a really horrendous noise. Had a few sets of ears looking at it and the consensus its coming from the head, on the exhaust side.
Its rotational, (not stuck tappets, I know what they sound like!), and sounds like a collapsed bearing or something. At idle, or under a really light load (top at 35mph) its not too bad. As you rev it it definitely sounds rotational as you can hear it going in and out of balance as its speed changes.
I'm trying to think how the cams are supported. Obviously there will be a number of oil bearings along there length, but what supports them at the end?
Other ideas bandied about were a damaged cam lobe, damaged valve spring or follower.. I'm thinking possibly an oil blockage might have caused one of the camsahft bearings to run dry?, or a bearing shell has disintegrated.
Obviously my first port of call is to whip the cam cover off but I cant do that till tonight.
Any thoughts?
Edited by snotrag on Thursday 8th April 09:43
snotrag said:
...the consensus its coming from the head, on the exhaust side.
Assuming you're right about the source area and that it's not a leak at the exhaust manifold, then something in the valve train sounds most likely. If there was a knackered cam lobe or a broken spring I think you'd notice a drop off in performance or at least a sense that the engine was running unevenly, so I think your idea of a cam bearing on its way out could be correct. Does the cam position sensor ever wear out? Pass!Edited by snotrag on Thursday 8th April 09:43
MX-5 Lazza said:
There are plenty of things to suspect before cam-bearings (in fact I can't remember ever hearing of a cam-bearing failure on an MX5).
Exactly, im well aware of this which is why I'm so scuppered!I got it up on stands last night, got the plastics and strut brace out the way, and had another listen. Part of the problem is that because I've got a noisy induction kit, and a very noisy exhaust, its really hard to pinpoint any sounds.
When looking from the top, its ocming from the exhaust side of the cam cover... but when underneath the car you can hear it quite loudly.
I've been thinking about the clutch change we did the other week... although it doesnt sound like its coming from the area, its an obvious route to follow. But what could be loose? The flywheel was remvoed but refitted, loctited and torqued up. The clutch plates also properly fitted, checked and double checked. loctite and torque wrench used everywhere its needed, and two of us on the job checking it all. And I'd done 250+ miles at least before there was any noise - it was running beautifully after we''d done the clutch. Course the problem is that its a bloody big job to whip the gearbox off again to check it all.
But then to counter that... it definitely sounds like its coming from the top end...
i didnt get any further last night as I've spent so many hours working on the car the past few weeks I was getting arsey and i had the foresight to walk away before I started throwing tools about. I'll wait till the weekend and have a proper, logical look at it.
I'm properly baffled.
Edited by snotrag on Friday 9th April 10:12
inman999 said:
Definitely not a leak from the exhaust manifold?.
Its had a new exhaust gasket fitted within the last week, torqued up in the correct sequence, heat cycled, re-torqued up etc etc... and its not the right kind of noise anyhow. Its a high pitch, metallic rattle or rubbing noise. Definitely something loose or worn.
MX-5 Lazza said:
Water pump? I know you said top end but I'm sure you know how sounds travel around in that engine bay. You could try using the short broom-handle trick to listen to different areas of the engine to see if you could narrow it down a bit...
Waterpump is less than a year old. I've had the end of a long screwdriver on various bits of the car as a stethoscope but its not proving fruitful this time!
Munter said:
Mine made a lot of clattery noise when the timing belt tensioner failed. But it wasn't much more than HLA noise...just consistent. I know you said yours were new 10k ago but might be worth a check before you start pulling cams etc.
Thats a good call that... I can certainly see how that would 'fit' with the noise I'm getting. I'll have to get the camcover off anyway but then i guess i'll be able to see down to the tensioner or whether the belts slack.
Spent today looking at this.
Had a number of people looking at and listening to the car.
We've all had the same thoughts... It definitely sounded like there was noise coming from the top end... but then it also sounds very loud once underneath the back of the engine.
Checked all the ancillaries. Then took camcover off - it all looks fine. Clean, shiny valvetrain, clean oil, all tight and non-rattly. Belt fine, tensioners fine, etc etc.
So - even though it doesnt sound like its coming from the clutch area - logic would dictate its the next thing to investigate.
Coinsidering pulling the engine and gearbox out as one as its actually probably easier then removing the box from beneath.
So at least a week without a car again. Getting sick of this now - my car has spent more time on jackstands in the past two months than off them - literally.
If the engine is coming out - then again, I want to do it properly.
What jobs should I do while its out? Wworth replacing engine mounts? I'll also take the chance to give the engine bay and engine a really good clean up.
Had a number of people looking at and listening to the car.
We've all had the same thoughts... It definitely sounded like there was noise coming from the top end... but then it also sounds very loud once underneath the back of the engine.
Checked all the ancillaries. Then took camcover off - it all looks fine. Clean, shiny valvetrain, clean oil, all tight and non-rattly. Belt fine, tensioners fine, etc etc.
So - even though it doesnt sound like its coming from the clutch area - logic would dictate its the next thing to investigate.
Coinsidering pulling the engine and gearbox out as one as its actually probably easier then removing the box from beneath.
So at least a week without a car again. Getting sick of this now - my car has spent more time on jackstands in the past two months than off them - literally.
If the engine is coming out - then again, I want to do it properly.
What jobs should I do while its out? Wworth replacing engine mounts? I'll also take the chance to give the engine bay and engine a really good clean up.
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