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My 1998 saab 900 turbo has been dying while driving. The issue only happens when it is very hot outside. After running the engine for 10-15 minutes my cars tachometer will jump up and down sporadically for a few seconds before hovering around 1000 rpm and finally dropping to 0 and killing the engine. The actual rpms don’t sound like they are fluctuating, just the tach. The car is able to start right back up and drive for a little before dying again. I just replaced: battery, crankshaft position sensor, fuel pump, fuel filter, spark plugs (Ngk saab specific), and transmission. If anyone can help me, you will be a lifesaver.

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    If it's a 98 it should have an OBDII port, have you gotten any codes? – GdD Jul 31 '18 at 12:28
  • Why'd you replace the transmission @D.Dris? It wouldn't seem to have anything to do with the engine quitting, are there other issues? – GdD Jul 31 '18 at 12:33
  • Short of some pattern failure miracle you’re going to have to do some electrical testing. You’d need wiring diagrams and patience to hunt down this problem. Did this start before or after the work and did you replace the parts to try and fix this? When the tach acts up does it reflect in scan data? – Ben Jul 31 '18 at 21:39
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    No codes. transmission was replaced for separate issues but I mentioned it because plenty of previous forums I have found seem to like to suggest it for similar problems. – D.Dris Aug 1 '18 at 1:11
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Trionic (5, I think) is very sensitive to vacuum leaks, unmetered air getting to the combustion chamber, that can feel like this. Go buy about 15 feet of 4mm ID vacuum hose and replace all of that you can find and go from there. After this long, they get old and rot, even with low miles.

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