Compression problems

[v8alligator]

Hi all.

Am after some advice on this one as I am not sure of the next step.

A few weeks back my 3.5 efi developed a miss fire on 2 cylinders (5 & 7)

On removing the plugs shore enough 5 and 7 were as black as anything compared to the others. At the time I did a compression test and the results were this:

1 - 135
2 - 138
3 - 133
4 - 142
5 - 78
6 - 141
7 - 67
8 - 140

I thought this was conclusive so stripped the heads.

There was no damage \ wear in the bores and no signs of a head gasket failure.

I took the heads to be skimmed and tested. The results showed cylinders 5 & 7 had 2 burnt out exhaust valves.
4 days later the heads where back. Skimmed, all valves recut and 2 valves replaced. :)

I have rebuilt the engine today and I now have a more serious issue.

The engine is breathing very heavily but appears to be only a few cylinders as its like a buff out the oil filler and not a continous haze

Compression test results as off today:

1 - 207
2 - 206.5
3 - 206
4 - 203
5 - 183.6
6 - 199.5
7 - 33
8 - 197

As you will spot. Cylinder 7 has a massive issue. But also what a massive diffrence having the valves recut has made.

Am really after some pointers as to what I should do next. A heavy breathing engine can only be rings right? But then I would expect to have seen damage in the bores?

Could it be a bad head gasket? This time round I have used tin rather then composite, could this be the issue?

Any advice greatfully recieved.

Bryan.

[DaveEFI]

I'd be inclined to check there is still some clearance at the rocker to the new exhaust valve in No 7. The hydraulic tappets can only compensate so much, especially with a thinner gasket.

[v8alligator]

Hi Dave.

Thanks.. suggesting this is lifter preeload?

I know my heads have been heavily machined and I allready have shims under the pedstalls on the rockers.

Would a open valve give heavy breathing?

Thanks

Bryan

[unstable load]

Would a open valve give heavy breathing?

An open valve would prevent build-up of compression in the affected cylinder.
I would take a good look at cylinders 5-8 if I were you, there is something wrong on that side. All things being equal, you should have figures that are within 5% across the lot.

[Darkspeed]

Do a leak down test -

Could be the rings are stuck in the groves
Hot test or cold test on the compression?
Lifters sticking
Rockers sticking
Rocker shaft bent

Heavy breathing will usually point to worn rings and maybe the increase in compression has just made things worse.

The leak down could let you know where the gas is escaping on 7

What do the cam lobes look like ?

Are you torquing up the outer bolts or just nipping up

[DEVONMAN]

Sorry Darkspeed didn't read your post before I sent this

My guess is that the compression rings on no 7 are broke or gone soft or the head gasket at no 7 is leaking slightly into the valley area.

When the exhaust valve was repaired in no 7, the rings may have
deteriorated even more due to the now increased pressure.

Have you installed the 4 outer bolts on the cylinder heads? If over tightened this can lead to head gasket leaks into the valley area.

[DaveEFI]

I'd not expect those outer bolts to have an effect at all on a cranking compression test? IIRC, they are more likely to cause problems at high revs and load?

An easy test as regards tappet clearance is to force an old feeler gauge between rocker and valve with the engine idling Use an old one as it will get bent. Find one which does this with the known good ones - probably about 5 thou. If you can't insert it to the suspect one, or it is noticeably tighter etc you might have the answer. Worth given it a go before dismantling.

I've only ever used this method to find a noisy hydraulic tappet, though.

You could also remove the rocker arms and do a visual check on valve stem heights with a straight edge.

[DEVONMAN]

I believe the concern here is the heavy breathing out of the oil filler.
I don't see how tight tappets would cause this.

[Darkspeed]

I'd not expect those outer bolts to have an effect at all on a cranking compression test? IIRC, they are more likely to cause problems at high revs and load?

My thinking, and probably over thinking is that perhaps the head and block were distorted slightly by the outer bolts at some stage. The heads are now flat but the block may not be and torquing up of the outer bolts making things worse and leaking into the valley due to the heads not pulling down correctly.

For me it would be a valley gasket off and a leak down test.

Andrew

[DaveEFI]

I'd be guessing the low compression and 'heavy breathing' may be two different problems. And check the easy things before dismantling.

Perhaps the pistons were are polished clean when the heads were off?

[v8alligator]

Hi All,

Thanks for all the comments.

I have done some leak down testing today... here are the results

With no rocker gear installed:
1 - 70%
2 - 70%
3 - 90%
4 - 70%
5 - 50%
6 - 70%
7 - 5%
8 - 55%

With all the rocker gear installed but without shims / preload not set:

1 - 35%
2 - 15%
3 - 65%
4 - 40%
5 - 30%
6 - 25%
7 - 5%
8 - 45%

With all the rocker gear installed with shims / correct preload set:

1 - 80%
2 - 80%
3 - 100%
4 - 70%
5 - 60%
6 - 80%
7 - 5%
8 - 55%



Testing with the rocker gear off was done to eliminates valve leaking. They have just been machined so would not expect this to be a problem.

Test done with the rocker gear on but not shimmed has shown the valves are all being held open and the cylinders are leaking

Once I had shimmed the rocker gear to the correct preload (2.2mm of shims where needed due to heavy machining to the heads) the cylinder efficient is not bad but would hoped for the result to have been more even.

Tomorrow night I am going to remove the heads and take a look at the bores and if needed will pull number 7 piston out.

To answer some of the questions.

Compression test was done with a cold engine.
The 4 outer bolts have never been installed so am only using 10 bolts
All the lifters are moving freely
No bend push rods
Rocker shafts are okay
Cam lobes are all good with very minimal signs of wear

The engine has only covered 850 miles in the last 6 years since being built. when I did the heads a few weeks back the cylinders are still showing honing marks and there is no signs of damage in the bores. I would expect to see heavy scoring in a bore if it was a broken ring?

Bryan

[JP.]

Did you run in the engine properly to break in the piston rings. Pedal to the metal the first 300 miles.... or where you just easy on the throttle...

Had have my Ford 400 (6.6 litre ) heads done a month ago with new valve seats and new valves and it took a good run before the engine did run properly.

[v8alligator]

Hi JP,

The engine is in my comp motor so is often driven very hard.

Not convinced this is a broken ring issues. I would have expected to see bore damage and or broken pieces

I am going to repeat the compression and leak down test tomorrow with the valley off to see if I can identify head gasket or piston rings leaking.

[Triumph-V8]

What you will need is a systematically searching for leaking
instead of working on the most likely failures.

For that I would make a sparkplug that can be connected to
compressed air.
Connect and apply pressure about 5 bars and be aware that the piston will
move to BDC with some power.
Than you have all the time in the world to look where the air
is leaking out.
Be aware that there can be a position where a valve is opened.
That rocker must be removed.


If it comes out of the air breather, sorry its piston/bore related
and the worst that could happen.

If the water bubbles its the head gasket or head crack

and when the air pops out of the
inlet or exhaust manifold its valve related.

When you have the culprit post it and we will find a solution to cure that.

[DaveEFI]

The very low figure for No7 after those tests would suggest valve or head or gasket etc problems - unless there is a hole in the piston. I've never seen a compression that low due to piston/bores even on a very worn engine.

One easy way to check the head is to remove it and put it on its side and fill the ports with paraffin. There should be no leaks into the combustion chamber even overnight.