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1976 450slc hard start warm engine
03-12-2011, 08:37 PM
Post: #1
1976 450slc hard start warm engine
1976 450slc Mercedes 4.5L, 36,000 miles. I have replaced fuel pump, fuel filter, fuel accumulator, cold start valve, new gas, new coil, new plugs, new air filter. Car starts fine when cold, but after warmup it won't start again until it is cold. i.e. sitting overnight. I was told there is a fuel pump relay located inside the car above the fuse box behind the passenger side glove box but I have not been able to locate it. Could that be my real problem here? If not, what else shall I do to diagnose this problem?
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03-12-2011, 10:25 PM
Post: #2
RE: 1976 450slc hard start warm engine
Lots of things "Could" be the problem and you can spend a lot of time and money just changing parts hoping to hit the right one. I like to locate the problem or at least narrow it down. When the car is hot, what is missing? Is it getting gas? Does it have spark to the plugs? Once you have determined what is missing you have narrowed the search in half. From there you trace back until you find out "Why" that item is missing.

Please let us know what you find and we can then give you better advice.
Thanks, Garner

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03-13-2011, 12:42 PM (This post was last modified: 03-13-2011 01:34 PM by Rupe.)
Post: #3
RE: 1976 450slc hard start warm engine
Can't remember if this car has CIS type injection or L-Jetronic (Bosch) injection but I can throw you a couple of pointers.

If it's the CIS type there's a rather large rubber boot between the air filter and the intake flow meter. If the boot is not tight (air tight, as in cracked, split, clamps loose, mice chewing holes, etc) it can cause you all kinds of grief. Coughing from the intake during hard starting often causes issues here when you are not looking.

If it's the Bosch type (electronic) you may be looking at something easy like a temp sensor going too rich for a hot restart. Most of those had two sensors. One is for the temp and varies the mixture and the other is for the cold start injector. (not to be confused with the temp sensor for the dash gauge) They are usually mounted near the thermostat housing and have those special plugs with the small bail wire holding them in place. You may be able to narrow things down for a "too rich condition" by removing the wire from the extra injector mounted on the intake.

Edited to add: Removing the wires from the temp sensors will make it go full rich and jumping the harness plugs with a paper clip will make it go full lean. Obviously you can confirm sensor readings with an ohm meter and a pan of hot water and there should be a variation but it's been too many years and I don't recall the specs. Since these sensors do go bad it may be worth the investment to just get it over with if the budget allows.

Either way, let us know how you make out.
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