Misfire and low compression

[Rutty]

The Vitesse started misfiring today and down on power. After checking the usual suspects made no difference, I have done a compression test and only get 85 psi on 3 and 4 the other 4 cylinders come in between 160 and 185.

I guessing the head gasket between 3 and 4 has gone but does anyone have any other suggestions before I strip it down.

[glang]

Try a retorque? Long shot and although you can repeat the comp test with the manifolds removed the rockers would have to go back on... 

[Steve AKA vitessesteve]

Can you see oil in the water or water in the oil.  Have you done both wet and dry compression tests? A squirt of oil the plug hole will temporally reduce/eliminate leakage past the rings.

In any case if it were my car I would want a look inside.  Do you or a friend have inspection camera you could poke in the spark plug holes?

[Nick Jones]

On 29/09/2024 at 16:17, Rutty said:

I have done a compression test and only get 85 psi on 3 and 4

Yep, afraid your diagnosis is likely correct - blown through the fire-ring between 3 & 4. 
 

Is it a mk 1 engine? They are much more prone to this due to the weedy head studs….

[thescrapman]

It’s a Mk1 engine.

[Rutty]

Yes it’s a Mk1 engine, The coolant has no oil in it, 
I redid the compression test on a cold engine this time with and with out oil in the bore see below the second reading is with oil in the bore.

1, 155 163

2 158 162

3 50  53

4 50. 53

5 125  132

6 165. 170

I am no expert but I would say that’s pretty conclusive it’s the head gasket blowing between 3 and 4. It also makes a loud whooshing noise when testing 3 and 4.

The other cylinders there is about a 4 to 7 psi increase with oil in the bore, I assume that’s within tolerance.

Number 5 is a bit low compared to the others but as the but as it doesn’t change much when wet I am thinking more likely leaking valve's.

As I have to take the head off I was intended to get it fully checked over and hardened valve seats fitted.

Any thoughts on my diagnosis.

[Dannyb]

Spot on I reckon.  You could do a leakdown test and prove air entering one cylinder is coming out of the other.

Danny

[thescrapman]

Not wishing to be a doommonger, my original head had a big crack between 2 cylinders, that caused the HG failure.

Inspect very carefullly when it comes off. 
 

[glang]

Retorque the head, it'll be alright🤞🤔

[Rutty]

I finally finished the decorating so was alowed so time in the garage today. I have removed the head gasket and as suspected it's blowen between 3 and 4 also a leak test on 5 showed the exhaust valve is leaking which tyes up with the lower compression result in 5. 

The head will be going off to be serviced, while that is away I need to clean up the block and piston tops what is the best way to do this.  I will need a new head gasket am I right in thinking the Payen copper gasket is the one to choose, plus am I right in thinking the head nuts need to be replaced aswell. 

Thanks Mark.

[Dannyb]

I found Scotch Brite pad are best.

Danny

[Nick Jones]

Scrape (don’t gouge/scratch!) with Stanley knife blade or similar and then scotchbrite as suggested.

Yes to new nuts. On the 3/8” studs the threads are absolutely marginal for the torque. Strongly recommend buying the flanged head nuts intended for the A-series (sold by MiniSpares et al). If still using washers they MUST be hardened ones.

[glang]

How tight were the nuts when you undid them? And yes I suppose best to use copper gasket rather than composite as the latter seem to be an unknown quantity...

[Rutty]

10 hours ago, glang said:

How tight were the nuts when you undid them?

The nuts where not overly tight but manual says 45lb.ft and I would say that’s what they felt like.