Compression

[Mattz]

Hi everyone. Real scary time with my z3m. Took it in for a shim check service. Was told that they couldn’t carry this out as they didn’t have the tooling. This took them a week to find out. They changed spark plugs and all the fluids.
After 2 days of having the car back it dropped oil on the garage floor. I took it back and they cleaned the oil off and left Car on idle to find the leak. Number 5 spark plug blew out of block taking the thread with it.
I’ve got them to repair this today and I’ve now picked it up.
They did a compression test and they gave me a sheet with these figures.
Cyl 1 81 psi
Cyl 6 84
Cyl 2 86
Cyl 5 81
Cyl 3 84
Cyl 4 83
These are all missing a 1 at the start aren’t they ?
Guy is saying it’s either a cracked head or head gasket. Any one cast any light ?

 

[t-tony]

Hi everyone. Real scary time with my z3m. Took it in for a shim check service. Was told that they couldn’t carry this out as they didn’t have the tooling. This took them a week to find out. They changed spark plugs and all the fluids.
After 2 days of having the car back it dropped oil on the garage floor. I took it back and they cleaned the oil off and left Car on idle to find the leak. Number 5 spark plug blew out of block taking the thread with it.
I’ve got them to repair this today and I’ve now picked it up.
They did a compression test and they gave me a sheet with these figures.
Cyl 1 81 psi
Cyl 6 84
Cyl 2 86
Cyl 5 81
Cyl 3 84
Cyl 4 83
These are all missing a 1 at the start aren’t they ?
Guy is saying it’s either a cracked head or head gasket. Any one cast any light ?

I doubt they're missing a 1 Matt. They're relatively low but all pretty even, I'm not an expert re ///M models, maybe they're down a little due to needing the valve shim service? Can't see why they would suspect a head gasket fault or cracked head from those figures. Hopefully an ///M man will come and add some comment soon for you mate.
@Mnbrennan , @Lee , @swamper , @Nodzed

Tony.

 

[Pingu]

I would say that there is something seriously wrong with those numbers.

There are no special tools needed to check the shim gap. Did they not want to start the test incase they needed to adjust the gap?

I've never known such a low compression, yet so equal. If something is cracked or leaking, I would expect the rates to be lower around the fault, not everywhere.

You should have two sets of numbers, one set for the dry test and one set for the wet test.

Did they do the test with the throttle closed? That would explain the low results, but would not be a good advert for their competency. A compression test should be done with the fuel pump disconnected and the throttle fully open.

My exhaust manifold is off at the moment, but if the weather's ok, I'll do a compression test in the morning.

 

[Mnbrennan]

The good news is that they are all pretty similar. If there was a HG problem I'd expect that 1 or 2 cylinders would be down.

The numbers are low and therefore I suspect one of two things

1) an incompetent tester or
2) that the valve clearances are way out.

You should get the valve clearances sorted anyway, these engines are sensitive to valve clearance and if they're out it would cause a lack of power

 

[Mattz]

Hi everyone, thanks for your answers. I’ve rescued the car now which I’m glad about but I think the mechanic just was out of depth.
I had the car dynoed after the initial service ( before the oil leak) and it made 310 bhp. This would mean that it’s fine power wise ?
The reason he mentioned the head gasket was because of the oil leak. It wasn’t leaking oil before I took it to him though !
A forum member recommended reddish motorsport in Bristol so I’m booked in there for a full 3 hour check and then a shim service just after Christmas. Hopefully they can sort it out.

 

[Jack Ratt]

I'd suspect the valve seats and valve stem seals rather than valve clearances. At compression the valves are shut, and given that BMW 6 cylinder engines bottom ends are bomb proof you would look elsewhere for the leaks and valve seats and stems are the most likely culprits.
I've just had my cylinder head skimmed and pressure tested and I had to have the valve seats recut and had new valve stems fitted. I removed my cylinder head and took it to an engine specialist in Cornwall and it only cost me £205 including VAT, however I did supply the valve stem seals myself as I'd already bought a top end gasket set for the rebuild.

 

[Mattz]

I'd suspect the valve seats and valve stem seals rather than valve clearances. At compression the valves are shut, and given that BMW 6 cylinder engines bottom ends are bomb proof you would look elsewhere for the leaks and valve seats and stems are the most likely culprits.
I've just had my cylinder head skimmed and pressure tested and I had to have the valve seats recut and had new valve stems fitted. I removed my cylinder head and took it to an engine specialist in Cornwall and it only cost me £205 including VAT, however I did supply the valve stem seals myself as I'd already bought a top end gasket set for the rebuild.

Hi Jack. If that was the case then the dyno would have shown low power though ?

 

[Jack Ratt]

Hi Jack. If that was the case then the dyno would have shown low power though ?

I wrote my post before you posted your last post with the dyno results. Those figure look good so I think your spot on about questioning the mechanics capabilities.

 

[Dino D]

Just make sure you only take an M car to a place that works on M’s regularly as they are different, especially these ‘proper’ M cars (the new stuff is just bashed up turbo diesels lol!)

 

[Pingu]

I considered the valve shims as the problem last night, but decided it wasn't them. Your dyno test has proved it.

There is no way an engine with only 85psi pressure could produce 310bhp, unless you were using methanol.

 

[t-tony]

If would think it's likely that the comp. test wasn't carried out correctly in the first place, the fact that all the readings are relatively close to each other and that it produced 310 bhp on a dyno would support this too.

Tony.

 

[Jack Ratt]

I'd be more concerned about the engine blowing out a spark plug. I reckon they're telling porkies
I'd wager they did the compression test and didn't screw the spark plug all the way in. They then said they let it tick over ??? If they revved the engine it could blow an unsecured plug out, it wouldn't on tick over.

I've just seen a YouTube vid of a compression test on an S50 engined M3 and it made 175 - 185psi so @Mattz is spot on with his comment about the readings being 100psi light

 

[Pingu]

I'd be more concerned about the engine blowing out a spark plug. I reckon they're telling porkies
I'd wager they did the compression test and didn't screw the spark plug all the way in. They then said they let it tick over ??? If they revved the engine it could blow an unsecured plug out, it wouldn't on tick over.

I've just seen a YouTube vid of a compression test on an S50 engined M3 and it made 175 - 185psi so @Mattz is spot on with his comment about the readings being 100psi light

Quite right on the pressures.

The engine's compression ratio is 11.3:1. 11.3 x 14.5psi = 166psi. You always get a higher reading as the engine tries to pump more air into the tester on each stroke.

 

[Mnbrennan]

I'd be more concerned about the engine blowing out a spark plug. I reckon they're telling porkies
I'd wager they did the compression test and didn't screw the spark plug all the way in. They then said they let it tick over ??? If they revved the engine it could blow an unsecured plug out, it wouldn't on tick over.

I've just seen a YouTube vid of a compression test on an S50 engined M3 and it made 175 - 185psi so @Mattz is spot on with his comment about the readings being 100psi light

Agreed re the compression test

As for the spark plug, I think it's quite common for them to come loose and in particular on cylinder 5. I saw a video on FB of a ticking engine, some said very specifically it would be a loose spark plug on cylinder 5,and that was the case! Talk about a crystal ball!

 

[bessieblue]

I Think i'd get a second opinion before doing any futher work

 

[Mattz]

Hi and thanks again everyone. I’ve emailed the garage about the flaky figures and he hasn’t responded. Surprise surprise. I’ve given the muppet £500 for the work prior to all this being reported after I took it back in the second time. I’m getting reddish to do a full health check and shim service so I’ll take things from there. If they find anything at all wrong I’m off to court.

 

[t-tony]

Good luck Matt, I hope you get to the bottom of this.

Tony.

 

[Mnbrennan]

Hi and thanks again everyone. I’ve emailed the garage about the flaky figures and he hasn’t responded. Surprise surprise. I’ve given the muppet £500 for the work prior to all this being reported after I took it back in the second time. I’m getting reddish to do a full health check and shim service so I’ll take things from there. If they find anything at all wrong I’m off to court.

They sound like muppets to be honest