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Old 17-05-2009, 08:55 PM   #1
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454 irratic idle ???

Hi all,
Trying to help a mate out with a 454 which is running a little bit missy at idle when warmed up.

Done a compression test and getting 10 to 11 bar across the board except one cylinder which is down on about 8.5 bar. Could anyone tell me what the spec. should be and the permissible variation between cylinders.

Also are there any common top end faults i.e. lifters or valves etc?

Or are there any other devices on these engines which may make them run a little erratic when at running temp maybe a cold start excess fuel valve that could stick open or such like?

Any info would be appreciated.
Cheers, Si.
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Old 17-05-2009, 09:50 PM   #2
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idle probs

start with new plugs,leads and fit new cap and rotor
if still not right check for any air leaks around the inlet manifold and vac pipes use length of fuel pipe ,one end in your ear then wave the other end all around pipes and manifold listen for rushing air or whistle sounds.
if no leaks found remove plugs again keeping them in order and compare them this might narrow down your search area.
this last test can make you jump now and then do not attempt if you have a pace maker.if you'r scared get a pair of plastic plug lead pliers but i just use any pliers. loosen all plug leads slightly first then with the engine running remove 1 lead and listen for the drop in revs put lead back on check all cylinders like this the cylinders with the least rpm drop are down on power or compression.
if the cylinder has been missing due to spark rellated probs the compresion can often show low due to fuel washing the rings clean and people often start pulling things apart before ensuring all spark/fuel ratio are correct first.
hope this is of help.
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Old 19-05-2009, 07:31 AM   #3
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Quote:
Originally Posted by cosworth mirage View Post
start with new plugs,leads and fit new cap and rotor
if still not right check for any air leaks around the inlet manifold and vac pipes use length of fuel pipe ,one end in your ear then wave the other end all around pipes and manifold listen for rushing air or whistle sounds.
if no leaks found remove plugs again keeping them in order and compare them this might narrow down your search area.
this last test can make you jump now and then do not attempt if you have a pace maker.if you'r scared get a pair of plastic plug lead pliers but i just use any pliers. loosen all plug leads slightly first then with the engine running remove 1 lead and listen for the drop in revs put lead back on check all cylinders like this the cylinders with the least rpm drop are down on power or compression.
if the cylinder has been missing due to spark rellated probs the compresion can often show low due to fuel washing the rings clean and people often start pulling things apart before ensuring all spark/fuel ratio are correct first.
hope this is of help.
Thanks for that, should of mentioned has had plugs, leads and cap. Also on compression test squirted oil down bore to seal rings and made no difference so thinking probably top end problem if anything. Still not sure wether compression low enough on single cylinder to be the cause of this, will try your other sugestion though.

Thanks again.
Simon
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Old 19-05-2009, 09:08 AM   #4
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I wouldn't think low comp on one cylinder would cause irratic idle.

Much more likely to be fuel / Ign, or maybe some sort of air leak on the induction side.

Slightly uneven idle yes, but not irratic.

8.5 bar is circa 125 psi, not low enough make it run horrid. 11 bar is 160 psi, which sounds ok, depending on engine spec.

What model of 454 is it?
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Old 19-05-2009, 12:44 PM   #5
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Do a leak down comp test with oiled cylinders to see whether a valve is letting by.

If it is then I would say number 5 exhaust valve. Is that definitive enough for you

My 502 suffers from reversion which starts out as slight pitting to valve face followed by complete meltdown of valve when it gets hot. If you're running the same sort of cam overlap you could have a similar problem.

As JF pointed out elsewhere blipping throttle before turn off can cause the sort of reversion which sucks a tad of saltwater back to sit on the valve face. Minor rust spot forms and then fuel ignition causes a massive increase in temperature locally on the valve face which in turn leads to catastrophic failure of the valve later.

Other than that you need to know what 454 it is to understand whether you have roller lifters etc
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Old 19-05-2009, 03:08 PM   #6
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Poss Cause

Quote:
Originally Posted by simon View Post
Thanks for that, should of mentioned has had plugs, leads and cap. Also on compression test squirted oil down bore to seal rings and made no difference so thinking probably top end problem if anything. Still not sure wether compression low enough on single cylinder to be the cause of this, will try your other sugestion though.

Thanks again.
Simon
I had a erratic missfire on a four cylinder Mercruiser,I replaced just about every thing ,in the end cleaned every terminal on the engine wiring loom-cured!!!!!!!!!!!!!!!!!!!!!as this cost nothing it might be worth checking
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Old 19-05-2009, 05:19 PM   #7
simon
 
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Country: uk
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Interests: Anything on the water
Boat name: Bad Medicine
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Cruising area: Solent to Weymouth

Join Date: May 2007
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Posts: 83
Quote:
Originally Posted by Jon Fuller View Post
I wouldn't think low comp on one cylinder would cause irratic idle.

Much more likely to be fuel / Ign, or maybe some sort of air leak on the induction side.

Slightly uneven idle yes, but not irratic.

8.5 bar is circa 125 psi, not low enough make it run horrid. 11 bar is 160 psi, which sounds ok, depending on engine spec.

What model of 454 is it?
Just my bad choice of wording there, uneven probably describes what he is getting better. It is only very minor and the guy is comparing it against how smoothly his previous 5.7 ran at idle. He could even be expecting too much but having noticed the compression readings after a brief look it has raised a little concern there.

I will find out what model 454 it is.

Thanks to you all so far for your ideas, gives us a bit to be getting on with. Any other inpsirations would much appreciated.
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