Hero to Zero in an Hour

[ukkid35]

Thanks to all those who have helped me with advice and even parts recently

Sadly my top end rebuild didn't fare too well

Everything went great for an hour, and then I lost one cylinder

I assumed it was electrical, as there were no untoward symptoms, no noises, smoke or anything unpleasant

But after nursing the car home and checking the compression I found...

260 - 270 in seven cylinders - and zero in one

I did struggle with timing the engine, so presumably I weakened something which eventually gave way

Anyone want to guess what I might find when I pull the heads?

It will take me at least 40 hours before I can get the heads off, so you have plenty of time to offer suggestions

The engine is an AJP8 with more valve overlap than will ever be allowed ever again, ever

Edited by ukkid35 on Sunday 23 September 19:55

[227bhp]

I've plucked out the salient points:

ukkid35 said:

I did struggle with timing the engine, so presumably I weakened something which eventually gave way

What exactly does this mean?

ukkid35 said:

Anyone want to guess what I might find when I pull the heads?

Not really, pull it apart and report back.

ukkid35 said:

more valve overlap than will ever be allowed ever again, ever

What exactly does this mean?

[Turn7]

Did you do wet and dry Comp tests?

[227bhp]

Turn7 said:

Did you do wet and dry Comp tests?

With zero compression?

[Turn7]

227bhp said:

Turn7 said:

Did you do wet and dry Comp tests?

With zero compression?

Good point !

[mtrehy]

leak down test.

mind you it's academic really as it has to come apart.

[ukkid35]

227bhp said:

What exactly does this mean?

It means the chance of piston valve contact when installing the cam chain is significantly higher than for most engines

Especially for me as a rank amateur home mechanic

What I don't understand is how it sealed, and the failed quietly without a massive engine failure

[Boosted LS1]

A rocker or valvetrain broken part.

[mtrehy]

"Thanks to all those who have helped me with advice and even parts recently

Sadly my top end rebuild didn't fare too well

Everything went great for an hour, and then I lost one cylinder

I assumed it was electrical, as there were no untoward symptoms, no noises, smoke or anything unpleasant

But after nursing the car home and checking the compression I found...

260 - 270 in seven cylinders - and zero in one

I did struggle with timing the engine, so presumably I weakened something which eventually gave way

Anyone want to guess what I might find when I pull the heads?

It will take me at least 40 hours before I can get the heads off, so you have plenty of time to offer suggestions

The engine is an AJP8 with more valve overlap than will ever be allowed ever again, ever"




then it would have been knackered immediately and wouldn't have got progressively worse....

Edited by mtrehy on Sunday 23 September 20:31

Edited by mtrehy on Sunday 23 September 20:32

[mtrehy]

I don't know the engine but cam cover off and have a look - should tell you something.

[ukkid35]

mtrehy said:

I don't know the engine but cam cover off and have a look - should tell you something.

Absolutely will do

But I'm struggling to see what could keep a valve open

Perhaps a shim has broken free, but surely that would stop it opening so far rather than closing

Perhaps there's debris stopping the valve closing

That seems a bit weird though, but perhaps the least destructive

If a valve is bent, how did that happen without debris?

[Mignon]

I sincerely hope you didn't find 260/270 psi in any cylinders as that's diesel territory.

[Mignon]

A story from a long time ago springs to mind. I was called in to act as independent expert in a dispute between a member of the landed gentry and the garage that had fitted new cam belts to his Lancia flat 4 engine. It ran fine for a bit or so he thought and then ground to a spluttery halt. He took it to a Lancia specialist and they called me as it needed a forensic stripdown to find out definitively what had happened as court proceedings were looming. It didn't take long. The cam belts were timed in not to the TDC mark on the flywheel but to the ignition timing mark at some 30 degrees BTDC. The inlet valves had been kissing the piston crowns without actually showing any symptoms until eventually a rocker snapped. How relevant this is to your own story we will find out in due course I imagine.

[stevieturbo]

Mignon said:

I sincerely hope you didn't find 260/270 psi in any cylinders as that's diesel territory.

Yup, no way either the gauge is correct, or the test correct.

But....zero does tend to mean bent valves, or certainly valves no longer sealing for whatever reason. Although if it was a timing issue, you'd expect symptoms on more than one cylinder.

[Little Pete]

stevieturbo said:

Yup, no way either the gauge is correct, or the test correct.

But....zero does tend to mean bent valves, or certainly valves no longer sealing for whatever reason. Although if it was a timing issue, you'd expect symptoms on more than one cylinder.

Agree with stevieturbo on the timing.
It could just be a broken rocker on an inlet valve. The valve would be closed on the inlet stroke giving you a low reading.

[Mignon]

Little Pete said:

Agree with stevieturbo on the timing.
It could just be a broken rocker on an inlet valve.

Vanishingly unlikely since it's OHC with buckets.

My compression ratio article suggests cranking pressures for different specs of engine.

https://web.archive.org/web/20110903073621/http://...

Google says the AJP V8 is 10.5:1 and 290 degree cams which is definitely rallyish. Interpolating my chart between the 10:1 and 11:1 rows would indicate 180 psi or so, maybe a tad lower so probably the OP has got his numbers mixed up and he meant 160/170 psi rather than 260/270.

With that much cam duration it would seem a bit more CR would not go amiss. My rule of thumb for nice engines was you aim for 200 psi cranking pressure on that chart. 11:1 or even a bit more is indicated with that much cam.

[PeterBurgess]

We have seen 240/250 psi on an MG twin cam 16v in a Maestro when the inlet cam was one tooth advanced, bags of bottom end grunt no top end. Ran sweetly once cam timing sorted. I suppose lucky the valves didn't hit! If OP psi correct maybe cam one tooth out? I know it doesn't explain why only one cylinder down but may explain high psi if gauge correct.
Peter

[ukkid35]

I have seven pics that look like this



Last time I did a compression test, the figures were about 230, but that was with old valves and without freshly cut seats

The same gauge gives about 160 on my Suprex engine

The test was done on a warm engine, all plugs out, fuel pump off, and full throttle, about five revs on the starter motor

I didn't bother taking the eighth photo, I hardly bothered looking at the gauge, you could hear there was no compression

Could the valve be stuck in the guide?

I can't hear any debris, I'll put a camera in the spark plug hole later

I did drive the car about 40 miles and it seemed to be running perfectly, I didn't take it above 4k, but it was performing very well, and idling very smoothly

I should add that the heads have been skimmed three times, the first was brutal, but the last two were fairly subtle, perhaps as much as 1.0mm has been removed in total - could that explain the compression figures?

Edited by ukkid35 on Monday 24th September 07:57

[ukkid35]

PeterBurgess said:

We have seen 240/250 psi on an MG twin cam 16v in a Maestro when the inlet cam was one tooth advanced, bags of bottom end grunt no top end. Ran sweetly once cam timing sorted. I suppose lucky the valves didn't hit! If OP psi correct maybe cam one tooth out? I know it doesn't explain why only one cylinder down but may explain high psi if gauge correct.
Peter

The engine is SOHC and the cam timing spec is inlet bucket 1.5mm lower than exhaust when rocking at TDC

I am certain the cam timing is correct, and have checked it repeatedly, but installing the chain and getting there was unpleasant

[ukkid35]

Mignon said:

My compression ratio article suggests cranking pressures for different specs of engine.

https://web.archive.org/web/20110903073621/http://...

Having now read you Compression Test advice, I can see that I followed it mostly

I didn't pull the ignition circuit relay - will do next time

I couldn't see the gauge as I was running the starter - perhaps an assistant would help

I said earlier I ran it for about 5 revs, but I meant to say 5 compression cycles