firing on only 7 out of 8 cylinders SL450 R107

[RaceDiagnostics]

Went through leads 1 to 7 and there was a very percievable drop in rpm when pulled off. On cylinnder 8, only a virtually impercievable change.

Checked leads, ok, and pulled out the plug, it looks ok, see below.

Next I went in search of a colourtune plug I bought 25 yeas ago but couldn't find it.

I wonder if it could be being starved of fuel, I'm not sure how the fuel injection works on the 450SL, any suggestions?

I'll try to get hold of a new colourtune plug tomorrow and see if there is a flame.

 

[RaceDiagnostics]

Got hold of a colourtune plug today, checked out the suspect cylinder 8, however it looked good at idle and when reved. I then rechecked the rpm inpact of disconnecting cylinder 8 spark plug lead vs all the other, it deffinately has only a minimal impact vs the others, perhaps its a compression issue as fueling looks ok.

This is a video of it at idle.

http://racediagnostics.com/cms/uploads/images/misc/cylinder 8.wmv

This video shows cylinder 7 in comparison, fist at idle and then when reved, iteresting to see it lean until the fueling catches up an returns to the correct lambda.

http://racediagnostics.com/cms/uploads/images/misc/cylinder 7.wmv

 

[Bellow]

Unless it's losing compression past the valves, I would expect the plug to be oily if compression was being lost past the rings. Plug looks clean, almost weak. Can you swap injectors to try and move to another cylinder, which would point to blocked injector?

 

[RaceDiagnostics]

I've never removed one before, and am a little worried about damaging them in the process. How delicate are they?

 

[LTD]

Have you swapped plugs ?

 

[RaceDiagnostics]

Put a new plug in tonight, no impact.

 

[RaceDiagnostics]

Just bought one of these off ebay so I can see the rpm impact more accurately DIGITAL RED LASER TACHOMETER NO CONTACT RPM TEST COUNT on eBay (end time 17-May-10 23:10:38 BST)

 

[Bellow]

If you have doubts about the compression, then a compression check is in order. For the cylinder to be as dead as you imply, the compression must be close to zero. So an adhoc test with your thumb over the plug hole should suffice. If it blows your thumb into your forehead, then assume enough compression and swap injectors. They won't be so frail as you'll damage them, no matter what type.
That plug looks like it's been firing, just firing a weak mixture. Unless the other plugs are the same colour, rather than significantly darker, I'd still put weak mixture top of the list for now.

 

[mykulmerc]

Don't worry about it - you've still got 3 more cylinders than I have

 

[RaceDiagnostics]

I got a chance to do some more testing on this tonight with the digital tacho, interesting to see how much push is coming from each cylinders.

Tick over rmp 720rpm

Cylinder 1 when disconnected, rpm dropped by 95rpm ~ 14%
Cylinder 2 when disconnected, rpm dropped by 90rpm ~ 13.5%
Cylinder 3 when disconnected, rpm dropped by 110rpm ~ 16.5%
Cylinder 4 when disconnected, rpm dropped by 80rpm ~ 12%
Cylinder 5 when disconnected, rpm dropped by 70rpm ~ 11.5%
Cylinder 6 when disconnected, rpm dropped by 95rpm ~ 14%
Cylinder 7 when disconnected, rpm dropped by 80rpm ~ 12%
Cylinder 8 when disconnected, rpm dropped by 50rpm ~ 7.5%

So although cylinder 8 is pushing it only at half the rate of the best cylinders. The plan for tomorrow is to have a look at the compression on each cylinder.

 

[iscaboy]

1/ getting a good spark at high compression is MUCH harder than getting a good spark at tickover, or out of the engine.

2/ the plug you show isn't right, you've got deposits, inc on the central insulator, this is acceptable if the plug has several thousand miles on it though.

3/ have you checked the HT leads? Even integral leads and caps (no swap) can be checked with a multimeter and a megger.

4/ have you run a compression test (screw in tester only please) or pulled air filter / silencer to listen for a chuffing valve?

5/ have you got an infrared laser thermometer, if you have measure the exhaust manifold temps, they should all be the same, near as dammit

6/ for about 40 quid now you can buy USB camera snakes, they will fit inside a spark plug hole and show you the inside of the cylinder / top of piston / cylinder walls, and show things like scores from broken rings.
http://www.maplin.co.uk/Module.aspx?ModuleNo=257206&C=Froogle&U=257206&T=Module

7/ run it too hot lately, or messed with the lube oil (head gasket or stuck ring)

HTH etc

short of this you're getting high tech, calibrating fuel injectors etc etc.

 

[RaceDiagnostics]

Bad news, I went through all of the cylinders today with the compression gauge, here are the results.

CYL-1 5.5 bar
CYL-2 5 bar
CYL-3 5 bar
CYL-4 4 bar
CYL-5 4 bar
CYL-6 4 bar
CYL-7 4 bar
CYL-8 ZERO bar

Is their any option other than a total rebuild?

 

[Number_Cruncher]

All of the figures seem to be very low? Was the engine warm? Was the throttle held fully open while cranking?

However, you've found that No 8 is significantly different from the others, indicating a fault.

The next step is a cylinder leakage test.

This will help to identify if the problem is in the valves, or in the piston and bore.

My guess will be that you'll find a valve has burnt out.

 

[RaceDiagnostics]

All the sparkplugs were out but the throttle was closed, I totally forgot to hold it open.

How do you do a "cylinder leakage test."

 

[iscaboy]

Bad news, I went through all of the cylinders today with the compression gauge, here are the results.

CYL-1 5.5 bar
CYL-2 5 bar
CYL-3 5 bar
CYL-4 4 bar
CYL-5 4 bar
CYL-6 4 bar
CYL-7 4 bar
CYL-8 ZERO bar

Is their any option other than a total rebuild?

get an oil can, make sure it is filled with CLEAN motor oil

put 1 or 2 squirts of oil into #8

DO NOT PUT THE PLUG BACK IN!!!

(do not put the plug back in until the engine has been cranked for at least 60 seconds, then turn the engine with the plug leads removed, you want to avoid hydraulic-ing the block)

Re-do the compression test.

If the compression is considerably higher, then your problem is piston / rings / bore, if it doesn't, your problem is valves...

You don't need the throttle open to do a comparative compression test, which is what you're doing, comparing all 8 cylinders....

Now sit down.

You could be lucky, and have a stuck valve, and double your luck, and have a stuck valve that hasn't struck the piston, so it's a head off job.

You could be unlucky, and have a piston / ring / bore problem, in which case it is engine out.

You could be very unlucky, and have a holed piston, bent valve and knackered head, in which case you want to start looking for a "short" refurb engine block.

However, what you do know is that you don't have good news, you don't have a cheap fix, and you don't have a fast fix.

Spend the next week pondering phrases like "good money after bad" before you spend another penny on it.

 

[Number_Cruncher]

All the sparkplugs were out but the throttle was closed, I totally forgot to hold it open.

How do you do a "cylinder leakage test."

Set that cylinder to TDC on its firing stroke, lock the crank in place, and connect an airline to the plug hole.

Then, you check to see where the air leaks out from - from the inlet manifold, and it's an inlet valve problem - from the exhaust pipe, and it's an exhaust valve problem - from the oil filler cap, and it's a piston / bore problem - from the coolant reservoir, and it's a head gasket problem, etc, etc.

>>You don't need the throttle open to do a comparative compression test, which is what you're doing, comparing all 8 cylinders....

I agree. But, if those figures were obtained with the throttle open, the OP would have much bigger problems than just No 8. The OP has since said the throttle was closed, which explains the low readings.

 

[RaceDiagnostics]

OK, I'll give it a try later.

A small bit of extra info, I took the pressure gauge off the hose and put my finger over the end, it puffed but not hard enough for me not to be able to keep it sealed with my finger.

 

[iscaboy]

OK, I'll give it a try later.

A small bit of extra info, I took the pressure gauge off the hose and put my finger over the end, it puffed but not hard enough for me not to be able to keep it sealed with my finger.

tells you all you need to know...

at a compression ratio of 8:1 you should get 8 x 15 = 120 psi

at standard car 14mm plug has an area of 0.239 (approx) square inches, so 0.239 x 120 = 28.68 pounds

a man (who works with his hands) can *just about* hold that, for a short time, but you have to press HARD to seal 120 psi, and you get a big dent in your finger

good news is you don't have a hole in the piston

 

[RaceDiagnostics]

OK after a couple of squirts of oil, cylinder 8 showed up 2bar, I guess that means its the piston rings.



Whats the downside of running it as it is for the time being.

 

[iscaboy]

OK after a couple of squirts of oil, cylinder 8 showed up 2bar, I guess that means its the piston rings.



Whats the downside of running it as it is for the time being.

If you catch a broken / stuck ring quickly.... you just fit a new set of rings...

if you run it knowingly you start knowingly doing damage.

which could include

resleeving the block, or reboring 1 cylinder and throwing the engine out of balance, or reboring all 8, and maybe even throwing a (con)rod and buggering the crank.

fact is you should at this stage start thinking complete engine strip and refurb, you might as well, you'll be doing 90% of the labour just to change 1 set of rings...

if your left shoe wears out, your right shoe is worn, so you'll never trust the other 7 until they're checked....