Engine Compression questions - running high compression + engine misfire

[KLP 92]

I have a 560 SEC with 102k on the clock.

I had the head rebuilt on it 6-12 months ago, 500 miles ago precisely due to a few running issues.

Well i've developed a misfire in the car (yet again!!) so decided to have the compression checked on it. The reading as as follows, from the front of the car to the back:

Left side:

205 PSI
205PSI
205PSI
205PSI

Right side:

205PSI
225PSI
235PSI
215PSI

The readings are in PSI and have been checked using 2 seperately gauges and performed when the car is hot and been driven all day.

Now i'm concerned about the right hand bank being so high in compression, especially the 235 and 225 reading. Is this a problem or not?

Also would this be a cause for the misfire or do i need to look elsewhere? So far the car has had a new:

Fuel distributor
Rotor Arm
Distributor cap
Ignition coil
EZL ignitin control module
CIS KE control relay
Spark plugs
ignition leads
Fuel injectors.

not sure what else it could be! I have been getting a ticking tappet noise form the right side of the engine, diagnosed as a tappet (even though a new one was installed 6 months ago).

Just curious to see what you chaps make of the compression ranges, as i took the car back to George Fraser who said that the figures were fine and wouldn't be giving me the running problems.

Any help/advice wopuld be greatly appreciated.

 

[BlackC55]

I would have thought it was a bit high also.

Strange one that

 

[KLP 92]

I would have thought it was a bit high also.

Strange one that

Is it a problem being that high?

 

[jaymanek]

Ive found it very difficult to get these engines not to misfire too... My 500sl ran perfect for around six months after changing plugs, dizzie, leads, cap and having a tune up... it has since started misfiring again at idle, only minor though.

My 560 is the same (again only slight) , idle is not a happy place for these engines!

 

[KLP 92]

Ive found it very difficult to get these engines not to misfire too... My 500sl ran perfect for around six months after changing plugs, dizzie, leads, cap and having a tune up... it has since started misfiring again at idle, only minor though.

My 560 is the same (again only slight) , idle is not a happy place for these engines!

I'm sick and tired of it now! I've changed everything and it still comes back

Problem is they don't get driven enough. I pulled it out of the gargae after 3 weeks and its misfiring again. I@ve checked everything again but no joy.

I was so fed up that i went to a salvage auction to buy a 1990 R129 500SL and transplant the engine over. Guide price on the car was £1500, CAT D, but ended up going for £3500, too steep especially when i'm stripping it.

How do you find the 560 in terms of performance? I don't think they're quick for what they are either.

 

[jaymanek]

is your misfire at idle only?

i find it the same as the 500, infact the 500 is probably a tad quicker due to it being in a slightly lighter body...

the sound is great, i love overtaking in it, but i would never ever race anyone off the lights!

 

[Howard]

Navs ,

I thought with the AMG cams, yours was making about 350ish bhp ...

and it doesn't feel fast ?

 

[jaymanek]

yes but bhp is high up the rev range, these cars dont feel quick off the mark due to their size and weight... Once on the go you feel like you could outrun anything....

 

[KLP 92]

Navs ,

I thought with the AMG cams, yours was making about 350ish bhp ...

and it doesn't feel fast ?

Never really been wowed by it to be honest. The car has never really felt like its running 100%. I presumed it would be the engine bedding in after rebuild, but hasn't improved.

May be due to the fact i've owned some quick cars over the years, and the stock 300BHP 560 is a 7 sec 0-60mph car.

Now what i really want is a salvage SL60 (cheap )

 

[jaymanek]

youve only done 500 miles though since the rebuild?

 

[Dieselman]

235 psi does sound high for the compression but I wouldn't worry about the 215psi.

The tappet noise could be connected if it's an exhaust valve not opening fully so holding air in the cylinder.

I doubt this would cause the misfire though, I would suspect fouling plugs or injectors, remembering that you have high lift cams the idle will always be more susceptible to problems.

Have you tried injector cleaner and a good blow out? After that I would go for ignition being the problem.

Buy a cheap flash tester to measure the power of the HT sparks, you might be surprised.

 

[KLP 92]

is your misfire at idle only?

i find it the same as the 500, infact the 500 is probably a tad quicker due to it being in a slightly lighter body...

the sound is great, i love overtaking in it, but i would never ever race anyone off the lights!

The lumpy idle is a trait of the 560s, they all do it. Its widely acknowledged that the 500 is alot smoother to drive than the 560.

I think the same was true about my 500SL as well. I don't even race anone on the roll, don't want to be embarrassed, all show no go!

My misfire is present at 1200 revs and 1600 revs. Well its more noticable at these points, but the car is hesitant throughout the rev range and is drinking like a fish!

 

[KLP 92]

235 psi does sound high for the compression but I wouldn't worry about the 215psi.

The tappet noise could be connected if it's an exhaust valve not opening fully so holding air in the cylinder.

I doubt this would cause the misfire though, I would suspect fouling plugs or injectors, remembering that you have high lift cams the idle will always be more susceptible to problems.

Have you tried injector cleaner and a good blow out? After that I would go for ignition being the problem.

Buy a cheap flash tester to measure the power of the HT sparks, you might be surprised.

Spark plugs are all light brown in colour, even though there is a smell of unburnt fuel from the exhaust. There are no fuel leaks and the CO has been checked and reset to 1.5%.

Injectors are a year old, i've already taken it for some serious blasts today, no effect, if anything its worse.

The car has new leads, plugs, coil, and ignition control module! Distributor cap and rotor arm has been checked.

 

[Dieselman]

If it's bing a bit jouicy it sounds like either and air leak or the mixture has gone rich.

Start with the basics. I can't stress a flas tester enough..only costs a few pounds and will chech the HT side is ok.
After that perform a vacuum test and spray around the manifold joints with plus gas or similar to see if it makes a difference, next is injectors and fuel mixture.

 

[KLP 92]

youve only done 500 miles though since the rebuild?

Maybe 750 at a push

It comes out when the suns out, and even then it gets town use. Most of the mileage was to and back from Woking, to have the interior done!

 

[KLP 92]

If it's bing a bit jouicy it sounds like either and air leak or the mixture has gone rich.

Start with the basics. I can't stress a flas tester enough..only costs a few pounds and will chech the HT side is ok.
After that perform a vacuum test and spray around the manifold joints with plus gas or similar to see if it makes a difference, next is injectors and fuel mixture.

The car is booked in at George Fraser's at 7:30am tomorrow (he's more eager than i am, and want to personally diagnose the fault)

I'll pinch his flash tester and check. I've checked the mixture, its set on 1.5%CO, checked by 2 machines. So mixture is apparently correct, even though it smells a bit.

I've played with the mixture, running it rich and lean, no change to the misfire.

 

[Ian B Walker]

Nav, I have said it from day one. I am sure you have a valve or valve spring issue. Naturally I am open to correction

 

[Dieselman]

If there was a valve issue wouldn't that tend to create low compression as opposed to high.

Of course I am open to correction on this.

 

[Dieselman]

I'll pinch his flash tester and check. I've checked the mixture, its set on 1.5%CO, checked by 2 machines. So mixture is apparently correct, even though it smells a bit.
.

Bet it's got high HC reading.

Co reading is a measure of how much fuel is in the mixture, HC reading is how well it burns the mixture.

Try revving up and checking CO at higher revs. If lower suspect vacuum leak causing lean idle, which has been corrected so it thens runs rich with throttle open.

 

[Ian B Walker]

If there was a valve issue wouldn't that tend to create low compression as opposed to high.

Of course I am open to correction on this.

You are right, however I was referring to a conversation Nav and I had about 5 months ago. Apologies for assuming all knew about the conversation

Forget the compression pressures for a moment and refering to the tapping sound, to me it would indicate that either the valve was bent and not closing, the valve spring was bouncing at a certain rpm, a valve seat is burnt or maybe the rocker arm has bent