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Saab Model 99 Bulletin Board
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Re: 1978 99 problems with fuse for fuel pump
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Posted by Gary Stottler [Email] (#1463) [Profile/Gallery] (more from Gary Stottler) on Tue, 8 Apr 2025 12:34:17 Share Post by Email
In Reply to: Re: 1978 99 problems with fuse for fuel pump, Thevsaabman, Tue, 8 Apr 2025 10:11:23
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A couple of additional thoughts-

1) I still think the old fuel is an issue that will make it difficult to troubleshoot other problems. It's not an issue with Octane, but rather that as fuel ages, the light hydrocarbons evaporate first and the remaining heavier HCs are harder to vaporize and ignite. So that by itself will tend to make the engine run "leaner" because the fuel is not vaporizing properly.

2) that said, measuring the pressures is the fundamental tool for diagnosing the CIS injection system. Usually the line pressure is not an issue unless the return line to the tank is plugged or restricted. But the control pressure is the key element for mixture. If that is stuck rich then you could run better cold and worse warm. But best to put the pressure gauges on and look at that both cold and hot.

3) The cold start injector gets 12V either directly from the starter during cranking or from the vacuum impulse contact during running and is grounded through the thermo-time switch. So it only activates until the thermo-time switch opens, then there's no ground and the CS injector should not fire. The 12V line to the vacuum impulse contact is a common problem because there's no fuse on that 12V line, so if it gets shorted things burn/melt. So I'd look carefully at that line from the source to the vacuum impulse contact. You can see if the cold start injector is getting the proper power signal by using a "noid light" or a basic 12V test light.

I hope that helps!

Best,

Gary 


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Gary Stottler
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