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Discussion Starter #1
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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Is this a turbo or non turbo model. I would swap out the spark plugs first. If this is a turbo car, it might be the direct ignition cassette.
 

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Discussion Starter #3
She is a non turbo.
I am sure that they will have swaped the plugs they have been taking parts off a similar model ( distibutor cap, leads etc) and trying them to see if it makes any diference but no luck so far. I will check if they have done the plugs on Monday morning. I am told that she had the wrong cap etc but changing them has still not been the answer.

Thanks.

Simon M
 

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As this is an intermitant fault, I would take the car to an Independent that has a rolling road, who will be able to duplicate the conditions under load, when the problem arises.

Changing/swapping parts to see if a difference is made is an expensive and un-necessary way of finding the problem, although lucrative for the garage.


HTH
 

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Having read the symptoms(more carefully than usual), I'd say there is nothing wrong with the electrical end of things. But, even then, the spark plugs should be cleaned and re-gapped as necessary..

The car seems to be running a bit too lean during light,partially open throttle..

Check and clean as necessary the MAF and the throttle body, the fuel pressure and its regulator and , of course, the vacuum lines..
 

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Discussion Starter #6
Thanks for the ideas guys:cheesy:

When the garage ring me later this morning to give me a price for a new crank case sensor I will make the suggestions that you have raised. Hopefully one of these may solve the problem otherwise I supose the only route to go down is the crankcase sensor. Any one know if the signal from it can be tested with out replacing it? I am told that to take it out will break it but I wonder if a signal can be picked up from the lead?

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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Discussion Starter #8
The problem has been solved at last:cheesy::cheesy:.

Whilst moving a vacume pipe the mechanic found that the engine ran normally.
The pipe was attached to the wireing harnes with a cable tie. The vacume pipe being O.K he opened up the wireing harnes and found that there was a problem with a wire going to the injectors, re made the joint or reconected the wire and now she is going well once again.

Thanks for you help guys.

Simon M
 

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Do you mean to say that this vacuum pipe wore that one injector wire's insulation enough to cause a "mild" but erratic short...Maybe someone with good intentions ,installed a tie-wire to clean up under-hood clutter, but instead created the problem, and NOT the factory...

Unbelievable !!
And I must be more careful...
 
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