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If AM start is fine, check valve isn't your problem.
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Posted by RayF (more from RayF) on Wed, 23 Mar 2011 21:35:16 Share Post by Email
In Reply to: extended cranking to get car started 93 Turbo 900, DanClearwater FL, Wed, 23 Mar 2011 05:36:17
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If it starts fine first thing in the morning as you say, when if the check valve were leaking down, it absolutely would have leaked down all the way, then a leaking check valve is not the reason your car cranks a long time to start when it's hot.

You may have a leaking-down check valve, in fact probably do with any older 900, but if your first start in the morning is fine, then look elsewhere.

Can't remember what NTC stands for but Saabnut is maybe on the right track with that suggestion, it reports engine temp to computer and that affects starting.

Also, I hate to raise the idea, but have you checked the engine's compression?

Make sure you have a good quality battery and you should be able to live with the long crank.

As Landjet points out, ignition on DOESN'T run the fuel pump on these cars (only happens when CRANKING or when engine is running), but you can make up a jumper that will, tapping into the skinny violet wire from the ECU to the fuel pump relay, which, if you tap into it and put in a pushbutton switch that goes to ground, will let you run the fuel pump at will. I have one C900 set up that way and it cuts startup cranking way down if I push the button first before cranking.

The reason the jumper helps is that when cranking, the starter motor draws so much current there's hardly any left to run the fuel pump to put pressure back into a fuel rail depressurized by a leaking check valve. Pre-pressurize it with a quick push of the button after turning the ignition on but before turning the key to start, and the car starts up sooner.

For anyone who might want to try the grounding trick to trigger the fuel pump, the ECU is behind the carpet panel ahead of passenger's right ankle. The violet wire I refer to is the only one that color out of the ECU, and you can tap into it right after the connector out of the ECU, or over at its other end near the relays, on a bracket above and ahead of the ECU. I used a crimp-on 1/4" spade lug connector. It squeezes on with a pair of pliers and cuts thru the insulation to make contact with the wire, without cutting it. Then I made my run to my switch, and back to a ground, with some zip cord lamp wire. You could use speaker wire as well. 1/4" push on to the violet wire, just wrap the return around any good screw to a body ground. You could use a doorbell or something more elegant as your pushbutton.

It only works when the ignition is on, but you definitely want only a momemtary contact switch for this, not something that's always on. Saab's fuel pump setup makes sure the pump doesn't continue to run if the engine stalls or dies, say after a crash when maybe a fuel line gets torn open; thus the fuel pump won't keep running to pour more gasoline onto the inferno that kills you.

Unlike many models of cars, the C900 does NOT have a few-seconds-long pulse of fuel-pump-on-at-ignition-on designed into its ECU. It instead relies on the fuel pressure check valve to keep the pressure there from the last time the car ran. But that feature fails on these with age, giving the embarassing result that your maybe otherwise great-running C900 cranks and cranks on startup before springing into perfect running life.



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