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Old 02-14-2003, 05:06 PM
Posts: n/a
Question 73 450SL Red Hot Exaust Manifolds

I bought a 73 450SL about 8 Month ago and have been battling to locate what is causing the exhaust manifolds to glow red hot after going down the hwy at 3000 RPMS for 5 minutes, pull over to the side of the road in the dark and see the exhaust manifolds glow where they connect to the exhaust pipe.

What have I done so far:
-Replaced the exhaust system with factory original
-This car doesn't have cat. converters
-Changed out points to electronic ignition, plug wires plugs and new coil
-Replaced the water pump/thermostat and tested flow rate of radiator.(was running hotter than normal)
-Changed out the computer circuit card for the Fuel injection.
-Changed out the Manifold Pressure Sensor -Adjusted fuel pressure
-Checked timing phycial marks and with a light to spec. 5ATDC (I can advance it up to 20BTDC and it decreases the red hot glow the closer I get to 20BTDC The car runs smooth but exhaust manifolds overheat!! This is a tough one, no prior history on the car. Any suggestions? Thanks in advance!

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Old 02-14-2003, 05:11 PM
engatwork's Avatar
Join Date: May 2000
Location: Soperton, Ga. USA
Posts: 11,234
Other than glowing red hot does it do any physical damage to the anything? How does it run?
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Old 02-14-2003, 05:21 PM
Posts: n/a
Thanks for the reply,
Runs fine, sight miss that goes away after time and I think that is because the valves have got overheated in the porcess and worped. Pulled the plugs again after a heat spell to check for possible clues and seem to be normal. The more I advance the timing from 5ATDC to 15BTDC I can here the valves (metalic noise increases). Also by doing this, advancing the timing is the only thing that I have found so far to decrease the heat and glowing. Everything else I have done has no change.
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Old 02-14-2003, 06:33 PM
Registered User
Join Date: Jan 2003
Location: Matthews, NC
Posts: 1,355
The 450 should have mechanical and vacuum advance in the dist. At 3000 rpm, the timming should be maybe 30 to 40 degrees.
To test, set base timming to 0 deg. Remove any vacuum lines from the dist. Then increase rpm to about 2000 rpm. The timming should increase 10 deg or so. I don't know exactly how much because I don't have the specs on that engine. If it doesn't increase the mechenical advance in the dist is frozen or rusted.
Next reconnect the vacuum line or lines to the dist and again increase rpm to 2000 rpm or so. The timming should now be 10 to 20 deg higer than it was on the first test. In other words now you should have mech and vac advances together. If test 2 failed then the vacuum advance is bad or the vacuum hoses are bad or miss-routed.
I think you will find both bad. Thats about the only thing that can cause the exhaust to get that hot.
Look into this and let us know what you find.
84 500 SEL (307,xxx miles)
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Old 02-15-2003, 12:13 PM
M D Nugent
Posts: n/a
Another possibility

Doesn't the exhaust temperature go up as the air-fuel mixture becomes leaner? If so - and if the manifolds don't typically get red hot on these cars - it could be that your mixture's too lean.
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Old 02-15-2003, 12:27 PM
Registered User
Join Date: Jan 2003
Location: Matthews, NC
Posts: 1,355

This is true, and a very good point. However Teammerritt indicated that the car ran fine and that the problem seemed to get better if he advanced the timming. If it were running lean, advancing the timming would have made the car have much less power, and spark knock would be terrible.
If the vacuum and/or mechanical advance are not working, the car will seem to run fine, it will however run hotter and use much more gas. The exhaust gases will still be burning when they exit the engine, thus the exhaust manifolds will see a blow tourch effect.
84 500 SEL (307,xxx miles)
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Old 02-15-2003, 12:45 PM
M D Nugent
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Thumbs up

Thanks, Paul - that's how I learn!
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Old 02-15-2003, 05:55 PM
Posts: n/a
Thanks Guys for your input!! I will take this and run with it to see where it leads. I have checked the advancement with and without vacuum and appeared to be o.k. pulled the distributor and cleaned/lubed it and cleaned the trigger points, but will revisit that issue. I think we are close. Will report back on findings.
Would the Trigger points have something to do with this or would it just run rough.
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Old 02-15-2003, 06:00 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Does your car have an AIR pump?
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 02-16-2003, 02:59 PM
rdanz's Avatar
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Join Date: Sep 2002
Posts: 1,158
I don't know if this relates but a few years ago a friend of mine adjusted his valves wrong and the manifolds glowed red.
If you don't have adjustable valves it mite be a worn cam or something in the valve train. Just a guess
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Old 02-16-2003, 08:08 PM
hill's Avatar
Registered User
Join Date: Feb 2001
Location: Northern Calif Sacramento
Posts: 736
Check the cam timing.

Have the cam timing checked and the harmonic balancer for slippage.
Happy Benzing
Darryl, Hill
2005 SL55 AMG Kleemanized
1984 500 SEC
1967 W113 California Coupe
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