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

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.

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....

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?

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.

That's 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's worth checking those bolts anyway on the 3.8 and then looking for other loose bolts and vacuum leaks in hoses, etc.

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?

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.