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Discussion Starter · #1 ·
The Problem:

Only after sustained high speed runs (ie. 100+mph for about half an hour), the car has begun to stumble during idle. This clears up after about 2 minutes of idling. During normal city and legal highway driving problem does not occur. What gives?

Fixes that did not work:

DI Cassette replaced less than 5000 miles ago.
New plugs. (I use the BCPR7ES given how hard I run the car)
Injector cleaner.
Cleaned out tank.. Ran to almost empty, refill and then again.

Engine Mods:

K&N Filter Panel
DIY intake funnel
B235R (Viggen) Intercooler
Forge FMDV004 Atmo Ram Dump Valve
Saab SS Cat Back
Throttle body transition casting port & polish
Cylinder head port and polish
Hose Tech SVH
Maptun Stage I ECU
Joe P MBC XZ [series installed]
Swedish Dynamics Kevlar Clutch
Thermostat Hack

 

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its probably a faulty DI cassete....even though its only 5000 miles old. I had the same problem. They are supposed to last 80 or 100 thousand miles but i got mine replaced at 40 thousand. Im sure u just got unlucky and had a messed up one. I dont know much about spark plugs, but i would check if anyone else has them and has had the problem. good luck, will
 

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HNNLIC
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Hmm i don't know about a faulty DI. Usually a problem with the DI would cause stumbling expecially at higher RPMs, not just during idle. A good way to check would be remove the DI and plug some extra plugs into it, grab a friend to start the car, and see if all the plugs fire (Or take it to a dealer). It sounds like it may be something else though...probably with one of your mods. Lemme think about it some more. :roll:
 

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well i remember before i my car died form a faulty DI cassete, it always idled a little rough in the morning. I would let it warm up for 5 minutes then come outside and leave, and a lot of the time right when i started to back up the car almost stalled and then regained power. And i know for a fact it wasnt my stick driving. After my car died and the DI cassete replaced it was perfectly fine. It has not idled roughly at all.
 

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HNNLIC
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Ahh okay. Then i guess it would be best to at least check the DI for failure. I thought the DI would usually either work, or not... I didn't know a faulty DI could work sometimes.
 

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Discussion Starter · #6 ·
Definitely not a bad DI. Bad DI would fail DURING high RPM and any idling failure would occur regardless of how the car was driven just immediately before.

Traced it to a fueling issue.

When on boost, turbos typically run rich. Rich fueling on boost allows higher charge pressure and reduces detonation due to lean fueling and cools the intake charge. The rich a/f state also builds carbon residue on the O2 sensor. To compensate, once off boost the ECU cuts back fuel to a lean state in order to increase exhaust temp and burn off deposits on the O2 sensor.

In my case, the O2 sensor is finally going kaput and sending an I'm dirty signal back. This explains why problem only occurs after high speed runs and goes away after a few minutes of idle. Validated by watching the A/F closely. Stumble = lean reading.
 

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Good detective work slaab...and thanks for the knowladge!!!

question...what octane level fuel do you usualy use?
 

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Discussion Starter · #8 ·
I usually run 93 Octane. Higher octane has an increased flashpoint and therefore allows higher boost levels.
 

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my DI failed when i was backing into a parking spot......and i had not been driving hard at all....
 

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i was going to suggest checking your vacuum lines...I had a similar problem and looked at the lines running out of the base of the throttle body to find that one of the lines cracked just under a hose clamp.

Leads to a pump-like device that apparently has something to do with the phony boost gauge, cause that was acting pretty wakey at the time.

I replaced all of my vacuum lines with red silicone type from Saab Savior, and she runs like a champ now.

Replaced the fuel filter, cassette, cleaned the K&N, and installed new plugs before I figured out where the problem was.

Something to think about doing if your car is aged....
 

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Car stumbling

I notice sometime when I start my Saab 30 mins after its been fully warmed up the engine stumbles and then regains power. It sounds like its going to shut off for a few seconds but then comes back. What could be the fault of this? Bad DI Cassette? I don't think its ever been changed, my 9-5 has about 106,000 kms on it. Thank You.
 

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jm58, it's highly probable the DI's going bad. I'm on my second DI at 130,000 km. The first was changed at around 70,000, then I started worrying around 100,000+ so I got another (the third) DI, put that one in, and the old one sits in the trunk as the take-me-home spare..
 
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