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73 450SL engine misfire & vapor lock
I've got a 73 450SL that starts great when cold, but misfires significantly until it warms up. Once it is warmed up, it runs ok, but with greatly reduced power. If I stop for more than 5 minutes, it will not start until it cools down. I have been able to "coax" it back to starting when I furiously pump the gas pedal while trying to start it. I've replaced the plugs, wire, rotor, and points. I just checked the point gap and reset it to nominal. I've replace most of the vacuum lines as well. I don't believe this is electrical, but maybe an injection issue or perhaps intake manifold issue. I'm kind of at a loss as to where to go next without tearing the whole top end off. Any suggestions would be greatly appreciated. Thanks.
Steve |
#2
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fuel filter/fuel pump - status
blowing smoke - what color? Plug tips - what color? Timing set ? valve lash ? injectors - status - new/old/refurbished?
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Ron 2015 Porsche Cayman - Elizabeth 2011 Porsche Cayman - Bond,James Bond Sadly MERCEDESLESS - ALways LOOKING ! 99 E320 THE Queen Mary - SOLD 62 220b - Dolly - Finally my Finny! Sadly SOLD 72 450SL, Pearl-SOLD 16 F350 6.7 Diesel -THOR 19 BMW X5 - Heaven on Wheels 14 38HP John Deere 3038E Tractor -Mean Green 84 300SD, Benjamin -SOLD 71 220 - W115-Libby ( my first love) -SOLD 73 280 - W114 "Organspende" Rest in Peace 81 380 SL - Rest in Peace |
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Vapor lock is from metal fuel lines. If you have any, change them to rubber, and that will go away.
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Could be running very lean which might explain why it just about starts from cold with extra fuel being dumped by the CSV.
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Actually, it sounds too rich. The reason I say this is because it'll start up hot but if it sits (IE: Thermo-time switch turns the CSV on during cranking) too much fuel is put into the fairly warm engine for a start.
Check your trigger points. Running poorly when cold is usually a sign that trigger (NOT breaker/ignition) points are gunked up. Clean them with something NON ABRASIVE. Fix vacuum leaks also, I bet you have a lot on an engine that old. Main culprits would be the MAP sensor hose, door locks, transmission modulator and the brake booster line.
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Current: 2021 Charger Scat Pack Widebody "Sinabee" 2018 Durango R/T Previous: 1972 280SE 4.5 2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi" 1999 Chrysler 300M - RIP @ 221k |
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got to agree with tomguy, sounds like its too rich. if it starts fine cold then acts badly warm but you can get it to start when you pump the pedal, its probably seeing the pumping pedal as WOT and the engine computer is programmed to shut of the injectors if you crank with the pedal fully depressed (specifically to cure flooded engines!)
check everything tomguy suggests, I would also check your TPS and make sure its reading all of the way off at idle. pull a couple of plugs and see if they look sooty black or not. if they are and everything else checks out put a spark tester in line and make sure you are getting a 15Kv to 20Kv spark. it could be that your ballasts are breaking down and your spark isnt strong enough to burn all of the gas and you are also "flooding" it from incomplete combustion (this is a possibility as the coil gets weaker as it get hotter which would also fit your symptoms) cheers barri
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61 Austin mini 67 Lotus 7 74 450sl 76 Cadillac 8.2l (501 ci) some new cars megasquirt conversion on: djet 74 450sl http://www.mercdjetmegasquirt.britautorepair.com/ cis 76 450sl http://www.merccismegasquirt.britautorepair.com/ the best view is always from the point of no return |
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Quote:
I no longer have the problem. Most significant thing I did, was to adjust the fuel rail pressure. Mine was at about 27 psig and I bumped it up to 30. It goes to 32 psig when running, presumably because of back pressure in the return line. I now need to lean out the engine using the MPS, but without O2 meter, I have not tried yet. Only other problem I have partially fixed, is the Aux Air Valve which also affects idle. It sticks when cold and engine idles too fast. I freed it up and it now seems OK once engine is hot. But the idle screw is still only 1/2 turn open and there is air passing through the bypass - Rubber hose is new and flexible and I can squeeze it and hear the air flowing through. Maybe I will have to resort to adding an orifice. At the moment, I start up with idle at 1400rpm and when hot it drops to 500 rpm which is a bit low and causes some stumbling. If I set it higher, the 1500 rpm cut-out kicks in and I get surging/cycling until engine warms up. I probably need a new AAV, but new ones are expensive!
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