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Old 02-11-2001, 06:18 AM
Registered User
Join Date: Feb 2001
Location: Washington, D.C.
Posts: 10
1977 450sel

I purchased this car in November, and from the beginning it was a rough cold start. But after it had warmed up it would start fine. However, the problem has gotten worse the temperature doesn't seem to make a difference in the starting. Also it has now started to idle roughly at stops, the rpm's drop as does the oil pressure. The car doesn't die at stops but I think it is leading to that. I have started to check into the fuel injection system. K-Jetronic. I was wondering if anyone had some input as to the problem area or general knowledge of this system. Thanks. P.S. this is my first car and I love it!
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Old 02-11-2001, 10:25 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Quite a simple system, virtually no electronics, a balance of airflow pressure and fuel pressure controls a verticle piston in the fuel distributer. As the piston rises and falls it exposes more or less of a thin slit alowing more or less fuel.

The pistons verticle movement is opposed by a pressure that is a partial of the original, steady, system pressure. This "control" pressure (also called warm-up pressure) is created by allowing a metered secondary flow to back-up behind a controlled leak called the control/warm-up pressure regulator.

Your starting/warm-up problems inherently revolve around this device's proper operation. If you could measure the two pressures you would find system pressures around 70psi (the actual value is less important than its stability). The control pressure is temperature dependent and starts at around 30psi (in Florida) and grows as the motor warms up. If you could monitor this pressure the important thing is the range of movement. It should grow to around 40-45psi.

This pressure also is varied under load for acceleration enrichment. This is accomplished by vacuum working on a diaphram in the control pressure regulator. Your car will have a 5-7psi decrease in pressure (richer mixture) from idle vacuum to full load vacuum.

The problems you are experiencing could warrant pressure testing, but only after tune-up specs are in line. The V8s are very sensitive to ignition timing. Give it another five degrees over spec and be sure the advance mechanisms are working as they often cause the timing to be set to low. The timing spec was given at idle with the vacuum lines connected to the vacuum advance/retard. It was also expected that vacuum be in the retard line (the instructions say A/C off - with A/C on the retard is shutdown for idle compensation - if this isn't happening or if the retard chamber doesn't work the timing will look right and be ten degrees retarded). I always check the idle timing with the vacuum disconnected and connected. If I get no difference with vacuum then I set the timing ten degrees higher initially.

Be sure you have no vacuum leaks. Testing for cylinder balance should be done to indicate injector, spark plug, or other single cylinder issues.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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