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How to diagnose random stalls

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How to diagnose random stalls

Post by amblerman » Sat Mar 02, 2019 1:00 pm

Hi Folks

My 1999 s70 non-turbo 5sp (125K) has been acting strange lately.

Every now and then (once a week on average.. sometimes more... sometimes less), it will exhibit the symptoms of a failing throttle body. For example it will drop to a low idle and the pop back up to normal.
Some times teh RPMs will just drop to 0 and the car stalls. The car will start right be back up and then act totally normal.

Sometimes it will be idling fine and then there will be a hiccup as if a few of the coils were grounded out for a second and the whole engine stumbles for about .3 seconds and then returns to normal.

the ONLY codes that have been thrown in my 1.5 years of ownership have been an occasional P0420 which is the "Hey, your CAT might be failing or it might not be.. who knows" code.

I already have the Xmodex throttle body on my car from the previous owner. I have gone through their diagnostic flow diagram, but because my car hasn't thrown any codes when it stumbles or acts weird, the flow charts points to a "Might be your MAF" result.

I don't know how old my MAF or O2 sensors are but I'd rather spent time/money on figuring things out instead of spending money on random parts.

Does anyone have suggestions on how to pinpoint what is causing my car to act this way? I have a basic OBDII reader but it cannot read live data. And this brings me to a general question I have.

Can a fancier OBDII reader that can read live data help me here or do I need a VIDA system. It's not clear to me what a VIDA system can catch vs a more comprehensive OBDII reader.

If anyone has suggestions on what to test or look for, I'd greatly appreciate it.

Summary of symptoms.
99% of time car runs and idles fine.
Occasionally it will......
  • hunt at idle. RMSs drop to around 500... car sounds funny... needle them bumps up to 900.. sounds better.
  • Be idling find and will stumble for about .3 seconds.. Then idle fine.
  • while driving at highway speeds, I will get a stumble at around 2000-2500 RPMs
  • After putting clutch in, RPMs will drop to 0
I know this list sounds just like a failing ETB but I thought the Xmodex units didn't fail in this way. Am I wrong about that?



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Re: How to diagnose random stalls

Post by abscate » Sat Mar 02, 2019 1:16 pm

Probably worth monitoring the fuel pressure while running to see if you can catch the parameters during the stall event. I’m coming to PHL for Pittcon and could lend you my AUTEL that week, it is a logging scanner

Very similar behavior to my low speed, icIng conditions which I’m convinced iced up my Throttle body
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Re: How to diagnose random stalls

Post by wizechatmgr » Sat Mar 02, 2019 5:31 pm

I actually was able to reproduce what abscate speaks of this past week. 3 journeys of less than 15 minutes in plenty of snow while going at low speed <=30 mph. Once brought onto the highway after sitting it ran great (likely the engine heat thawed things while sitting...)
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