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Help Please -Misfire

1079 Views 8 Replies 5 Participants Last post by  earthworm
I have tried using the search feature but it does not seem to have helped me. So PLEASE can anybody offer any ideas

I have a 1997 900 which has been marvellous except for a missfire which has cropped up on occasion over the last 18 months and is now almost continuous.
My son mainly uses the car so please excuse me if I am a little vague on the symptoms. The local non specialist garage has looked at it a number of times but not been able to permanently cure it. We have now taken it to a garage which specialises in Saab, and they are unable to find what causes the missfire. I have found when I drive it, the miss is when you are on a constant throttle and that if I accelerate hard there is no problem, but when I go back to a steady throttle opening which probably giving me between 50 to 60 MPH the miss is back. The garage have investigated the electrics and not come up with anything but I would be grateful for any suggestions as it is becoming very expensive - its been in the garage for 3 days:cry:.

The mechanic who is looking at the car is suggesting checking the crank case sensor but to do this will break it when it is taken out. Anybody know what this sensor does and can it cause a miss fire?

Could it be fueling? A while ago, my son put in a tank of BP's High Performance Petrol, (Optimax?) in the hope it would help the misfire, but found that the car was worse on it.

Any suggestions would be most gratefully received.

Simon M
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Here is some data on the crankshaft position sensor:

"Information on the position and speed of the crankshaft is sent from the crankshaft position sensor to pin 16 of the control module. The sensor is connected to ground via pin 43 of the control module.

Mounted on the crankshaft is a slotted ring with 58 ribs. Of inductive type, the sensor is mounted on the wall of the engine crankcase. The distance between the slotted ring and the sensor is 0.4 - 1.3 mm and it is not adjustable.

The sensor acts as a small alternator and generates a sine-wave a.c. voltage. The voltage and frequency signals from the crankshaft position sensor vary with engine speed. Sensor input at idling speed is 5-10V a c., 870 Hz, and at 2500 rpm it is 15-20V a C., 2400 Hz. The frequency is of interest to the control module as it is directly proportional to engine speed. By measuring the frequency of this current, the control module can determine engine rpm.

Two ribs are missing on the slotted ring after the 58th rib.

When the first rib passes the sensor, the control module knows that the crankshaft is 117°before top dead centre (BTDC) for cylinder No. 1. By measuring the two missing ribs the control module can determine the position of the crankshaft.

The control module mainly uses engine speed and the crankshaft position as a basis for calculating ignition timing, injection time, injection duration and for idle speed control.

Fuel injection is cut off when engine speed exceeds 6400 rpm.

As soon as the control module receives signals from the crankshaft position sensor it grounds the fuel pump relay, causing it to operate.

Should the sensor fail to work or in the event of a break in the circuit (no continuity), the engine will not start.

The sensor's resistance is 860 ±90 ohms."

If replaced, try and located the Bosch sensor (p/n 9177221), its listed as $45 USD while the other one (no idea who makes that one) as $130 USD. Hope this information helps.
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