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Old 03-03-2014, 12:46 PM   #1
ELM
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Default Need help diagnosing temporary losses of power

Car: 2004 Bonneville, 3.8

Symptoms: Car starts normally. If left to idle, it is fine for a minute or so, then idles poorly, then stalls. Or, if I start it and immediately drive off, it runs fine for a few seconds then temporarily loses power for a few seconds but then returns to normal without actually stalling. It does not run rough like it has a miss -- it just has no power which, to me, feels more like a fuel flow or air flow issue.

What I have already checked without luck:
  • the air filter is clean
  • the mass air flow screen is clear
  • changed fuel filter
  • ran a fuel pressure test and the results were within spec
  • checked DTC codes: no codes and no pending codes

Any suggestions as to what to try next would be very much appreciated.

Thanks
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Old 03-03-2014, 01:48 PM   #2
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Welcome to the Forum Ed.
The (IAC) intake air control, controls idle and may need cleaning or replacement.
I would try to clean it first by removing it from the throttle body, and use a throttle body cleaner spray to the ports and IAC spring end to remove carbon.
Get it clean, and re-install it...& see if that helps restore idle and driveability.
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Old 03-04-2014, 11:43 AM   #3
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In addition to what was already posted, when car runs bad at idle...shut off, disconnect MAF, and restart the car...if it runs noticeably better, might need a MAF....
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Old 03-09-2014, 05:55 PM   #4
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Thanks for the suggestions. The weather has finally improved enough for me to able to try them.

Soft Ride: I tired cleaning the IAC Valve but it did the same thing. Because the valve didn't look very good even with cleaning, I went ahead and replaced it with a new one but the car acted the same way. I read up a bit on what IAC valves do and how they work and it certainly seemed like a top candidate for the cause. But, it seems to be something else.

Tech II: I tried stopping the engine and disconnecting the MAF Sensor. Unfortunately, the results were inclusive. If I stop the engine when it begins to run rough and the re-start it, it runs fine whether or not I have disconnected the wires. I think my next step is to pick up a can of MAF Sensor cleaner and see if that helps. Would you agree?
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Old 03-09-2014, 11:52 PM   #5
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I know this may be far off as my engine is a Northstar V8 where this is a common issue, but I would check for a vacuum leak - I had idling issues (less severe than yours though) and it turns out all the intake manifold bolts were finger loose. When I tightened them the problem disappeared.

I spent a lot of money diagnosing my idling issue but the last thing I thought of was a vacuum leak!
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Old 03-10-2014, 09:19 AM   #6
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I know this may be far off as my engine is a Northstar V8 where this is a common issue, but I would check for a vacuum leak - I had idling issues (less severe than yours though) and it turns out all the intake manifold bolts were finger loose. When I tightened them the problem disappeared.

I spent a lot of money diagnosing my idling issue but the last thing I thought of was a vacuum leak!

A vacuum leak on any engine can cause problems at idle.
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Old 03-10-2014, 09:44 AM   #7
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A vacuum leak on any engine can cause problems at idle.
That'* very true, only reason I mentioned the type of engine is that the leak probably won't be as easy to find as mine (as the intake manifold bolts being loose is a somewhat common issue with Northstars) but it'* worth checking those bolts anyway on the 3.8 and then looking for other loose bolts and vacuum leaks in hoses, etc.
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Old 03-10-2014, 01:28 PM   #8
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agree with all diagnosis above, but I had exact same issue with v8 chev. turned out to be bad ground connections on motor that grounds computer components, note: no computer codes were showing either
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Old 03-10-2014, 01:54 PM   #9
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agree with all diagnosis above, but I had exact same issue with v8 chev. turned out to be bad ground connections on motor that grounds computer components, note: no computer codes were showing either
How did you find that?
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Old 03-10-2014, 01:59 PM   #10
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trial and error, even though they may look ok, disconnect them and clean them. see if it fixes problem. GM never seems to ground the equipment enough to last.
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