Piston Slap: Bogging Saab Viggen Keeps Austin Weird
Longtime TTAC Commentator KeepAustinWeird writes:
Hi Sajeev, I’ve got a 2000 SAAB Viggen with 113K miles. Recently, I’ve noticed that after I’ve been driving the car for about 20 minutes and the car is good and warmed up, the motor totally bogs until I can get to 3,000 RPM, at which point the car takes off. This is particularly annoying (unsafe) pulling away from full stop, where I either have to gun the motor or crawl thru 1st and part of 2nd before the RPMs rise enough for the boost to kick in. Wondering if this is a MAF (which I’ve cleaned) issue, a drive by wire sensor issue, or a dying turbo issue?
That’s a tough one to armchair from the Internet, unless you spend hours on SAAB forums. That said, I suspect several trouble spots for late model turbocharged applications: fuel starvation, Engine Computer trouble code, exhaust/turbo leak or a vacuum leak. I don’t see a turbocharger failing this early, unless oil changes and abuse are a problem. Then again, abuse a Viggen mid-corner and you’ll be kissing a whole lot of Austin hill country. And have far more pressing issues than engine bog.
You cleaned the MAF (and I’ll assume you checked for trouble codes) so my money’s on a vacuum leak. Do the easy stuff first: a complete visual on the rubber/plastic/whatever lines, especially around the fuel pressure regulator, if applicable). Check every hose that starts from the intake and snakes every which way, and, if needed, get under the car to spot look for cracked, split or blown hoses specific to the turbo/intercooler. Most modern vacuum systems are more than just rubber hose, so if a forum doesn’t give enough diagnostic info, punt and go to a SAAB wrenchhead.
Another possible concern: the lack of fuel. When did you last change the fuel filter? If it’s been over 30,000 miles, that’s too long. If that doesn’t help, check fuel pressure while driving: rent a long-hose fuel pressure gauge from a parts store, connect at the testing location (usually the fuel rail) latch (but don’t close) the hood, drive and compare the gauge’s numbers to factory specs and DIY for free. If the pressure is below spec at idle or drops under full throttle, the pressure regulator or fuel pump need to be tested.
Good luck with all that.
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