Compression test 6.9

[koan]

I've been putting off doing this in case I found something nasty. It's now done.

Pressures in PSI dry/wet (warm engine)

1 125/130, 2 80/95,   3 120/120, 4 165/175

5 160/170, 6 160/170, 7 155/170, 8 160/170

It says to me cylinders 5 to 8 are OK with pressures around 170 PSI.

Not at all good are 1 and 3 at about 120 PSI, and 2 at 80 PSI is bad.

Any thoughts on this, is it just a coincidence that the two low cylinders are next to the really low cylinder? There doesn't appear to be any bubbling in the radiator expansion tank.

The difference between dry and wet on all cylinders rings etc. are OK, this was the thing I was worried about.

koan

[WGB]

1. When you say wet engine Koan - did you put oil down the bores to make sure any faulty rings sealed.

2. What are you using as your cylinder numbering sequence - is it the standard M-B one with 1-2-3-4 right hand bank and 5-6-7-8 left hand bank.

3. My guess is a that if the rings are OK and there are no headgasket problems you have three dodgy exhaust valves but if it is running OK it will run for a lot longer yet.

Bill

[koan]

1. When you say wet engine Koan - did you put oil down the bores to make sure any faulty rings sealed.

Yes, wet with oil.

2. What are you using as your cylinder numbering sequence - is it the standard M-B one with 1-2-3-4 right hand bank and 5-6-7-8 left hand bank.

Yes, the right hand bank being on the left looking at the engine from the front.

3. My guess is a that if the rings are OK and there are no headgasket problems you have three dodgy exhaust valves but if it is running OK it will run for a lot longer yet.

You don't think its a bit odd, one bank good and three out of four on the other are stuffed?

It started to miss when hot at idle and just above around Christmas. I just put new plugs and leads on but not driven it yet so I don't know if that's fixed.

All will be revealed when i take the heads off, which might be a while away given what else I have on my plate at the moment.

koan

[Brian Crump]

It might  be worth running the test a 2nd time - I did a comp test on a ###### (can't admit it without a drink) and got 175-175-90-175 even with oil squirted down the sparkplug hole. After a short sharp drive the compression came up to 175 all round. Conclusion? Maybe the rings had lined up the gaps on the initial test.
Was this a dud test? No: the test was done in 1989 and the motor is still running strong.
Regards,
BC

[13B]

with those numbers there should be an imbalance in the engine at idle that no matter how much you fiddle with the idle speed/mixture or ignition timing it just doesn't go away.

One of my previous 450s had a weak cylinder which due to its low compression would foul the spark plug up regularly... less compression means that cylinder fires less hot which gives rise to plug fouling.  Install a hotter (numerically lower) plug... if you are using BP6ES, put in a BP5ES on that cylinder.

Or if you have the $ then just fix it I suppose.

My experience with Wankels is that brand new they make about 150PSI, once they've done 100,000km they read 100-120PSI, and the even run alright at 80-90PSI, with hotter plugs.  Once they drop below about 75 PSI they don't have enough compression at cranking speed to start them - so its rebuild time.  You're probably wondering why I'm saying all this, but its because a cylinder will run dependably for a long time with low compression, if you can prevent the spark plug from fouling.

I.

[WGB]

You don't think its a bit odd, one bank good and three out of four on the other are stuffed?


koan

I would still expect a head gasket/crack in the head to show in the cooling system long before a significant compression drop.

Other lateral ideas - Hydraulic followers jammed open (I guess they usually fail by deflating and becoming noisy) or injection problems causing lean mixture to those cylinders and subsequently toasting valves.

The right hand bank is also at the tail end of a stretched chain and subsequently timing will be more affected on the RH Bank than the LH one but that wouldn't explain why Cylinder 4 is so good.

Sympathies go out to you but I am sure when you are finished it will be better than new.

Bill

[koan]

It might  be worth running the test a 2nd time - I did a comp test on a ######

Common problem on ###### engines Being the ever optimistic person that I am I'll test it again after some driving but I wont get my hopes up.

with those numbers there should be an imbalance in the engine at idle that no matter how much you fiddle with the idle speed/mixture or ignition timing it just doesn't go away.

It used to run pretty smooth at a 500 RPM idle, not perfect but better than I would expect for a thirty year old engine. Since Christmas It's good cold but rough when hot, I couldn't find any particular cylinder at fault by removing plug leads one by one so there is an imbalance if not a full miss. Maybe the compression problem started at Christmas.

I do use  BP5ES plugs rather than the 6es just because it is an old engine. Just replaced them, the old plugs all looked the same, light grey with no soot, oil or baked on deposits, not much different from the new ones.

I would still expect a head gasket/crack in the head to show in the cooling system long before a significant compression drop.

Is it possible for a leak between cylinders not to leak into the water jacket?

The compression figures are consistent with leaks from 2 into 1 and 3 but the chances of two leaks developing simultaneously is remote. I've not ever had any overheating either.

Other lateral ideas - Hydraulic followers jammed open (I guess they usually fail by deflating and becoming noisy)

I replaced the rockers about 18 months ago and did the shim selection procedure with a dial gauge which was a pain, I was satisfied they were right in the end. There is noisy lifter in the bad bank that leaks down if the cam lobe happens to stop on it over night, it makes a lot of noise till it pumps up. I have 16  new hydraulic adjusters to put in and i lashed out (ha!) and bought the test tool that does away with the dial gauge.

The valve gear has always been noisy to my thinking, I thought it just needed adjustment but it didn't make much difference.

New timing chain too about 18 months ago, timing marks are spot on - after retarding the right bank one tooth which I assume was advanced a tooth at some stage to compensate for the stretched chain.

My plan is to find someone with the tester for exhaust gas in the cooling system, fits on in place of the  radiator cap and contains liquid that changes colour if there is exhaust gas there. Drive it a bit and do a quick compression test on the bad cylinders. Have a look at the vale gear to make sure nothing is wrong there. Maybe put some compressed air into 2 on the compression stroke (both valves closed) and see if i can hear anything through 1 and 3s plug holes.

Probably time to start collecting the required gaskets.

I'm not stressed about it, I'd just rather not do it right now, but probably will because i enjoy working on cars and the house painting can wait...

Thanks everyone, keep the ideas coming.

koan

[CraigS]

Is it possible for a leak between cylinders not to leak into the water jacket?

No. This is exactly what one of mine had, on the left bank between 5 & 6. It is a known weak point, but yours is the opposite side - albeit at the front.

[WGB]

I have never seen a 6.9 engine manual and do not know if the cylinders are "siamesed" or whether there is a water jacket between them.

I have seen other cars where the gasket has blown between cylinders and there was no leakage of oil , water or gas into the cooling system but they were all 4 cylinders and ran very crappily with two snafu cylinders but a V8 is different.

Is it possible that your valve clearance adjustment was too tight and the valves are being held open when hot?

It also seems a bit strange that it runs smoothly when cold but rough when warm - ?mixture or Auxillary air Valve problem.

Bill

[CraigS]

I tried to find the photo of my gasket, but it has disappeared, but to look at it, you would hardly know it was blown - just some discolouration around the water jacket and between the cylinders 5 & 6. This was sufficient to allow water into the engine. You might also check the torque of the head. They are not supposed to need torqueing down, but they often have loose head bolts.

[koan]

Dd a bit of investigation into the low compression today.

First, none of the garages I frequent have the coolant exhaust gas test gadget.

Someone said they tested for a  blown head gasket by putting compressed air into each cylinder at TDC via the spark plug hole and seeing if the water level in the radiator or tank rose. Did this on the three down cylinders with no visible change to the water level. This test along with no bubbles when running just about confirms it's not a leak into the water jacket.

Next removed the rocker cover and used the go/no go valve adjustment gauge (see here) on the bad cylinders, they are all within spec so it's not as simple as a valve not closing.

What I was able to find was air flow out of the exhaust tail pipe at TDC of all three of the cylinders suggesting exhaust valves not sealing very well.

koan

[WGB]

Next removed the rocker cover and used the go/no go valve adjustment gauge (see here) on the bad cylinders, they are all within spec so it's not as simple as a valve not closing.

What I was able to find was air flow out of the exhaust tail pipe at TDC of all three of the cylinders suggesting exhaust valves not sealing very well.

koan

OOps - sounds like a head off job to sort out - sympathies go out to you.

Can worn valve guides cause the exhaust valves to seat unevenly?

Bill

[koan]

OOps - sounds like a head off job to sort out - sympathies go out to you.

Can worn valve guides cause the exhaust valves to seat unevenly?

Don't know if guides can cause the sort of problem my engine has.

But that's the sort of thing I'm worried about, if its just a valve job OK, but if it needs new guides and valves or even seats In the bad positions I would want to do the lot,  both banks and it could get expensive very quickly.

koan

[WGB]

I would hope the worst would be valves and guides and for that reason alone it would be better to do earlier rather than later so that the seats are preserved as much as possible.

Bill

[koan]

Collected new head and inlet gaskets today, still need to organise exhaust gaskets.

I'll start work on it soon, that's in the next month or so.

koan