Stumbling under load, only intermittantly (*sigh*)

I have a 2000 Chevrolet Venture, 3.4L motor. Here is the latest in a string of problems:

The van started hesitating under load. It starts fine, idles terrific. It cruises fine. The "Check Engine" light is not on. But when attempting to accelerate from a stop, or when climbing a hill, the motor feels like it wants to fall on it's face. The problem is that it is intermittant. After trying in vain to troubleshoot, I finally broke and took the car to the local GM dealer. There are no codes. The only thing they could do was connect to the fuel rail and read the pressures. They also took the van for a drive. But of course, as my luck would have it, they couldn't recreate the problem. The van was running flawless that morning. It does seem to get worse towards the end of a "trip".

I've already checked for arcing spark plug wires (engine running at night) and I've cleaned the MAF sensor with electronic cleaner. No difference. The problem is slowly getting worse, but it's still not throwing a code; and of course, every time I try to troubleshoot, it's running perfect again.

The wife and kids are taking a long trip next weekend, and I'm nervous. Please help!!

When it starts stumbling again unplug the MAF sensor and see if there is any change. The MAF sensor could be going bad. When you umplug the MAF sensor it will throw a code. Do you have a scanner?

OK, I will try that, thanks.
No, I don't have a scanner. The local auto parts store is willing to check codes for free with their scanner (in hopes that you purchase repair parts from them, of course!)

If you use their scanner start your car and take a look at the MAF values. Also checkout the Throttle Position Sensor. Put your foot on the gas and see if the sensor responds OK.

I see that you are in Butler. I am in Pittsburgh.

Thank you, Venturenot.
I stopped into Advance Auto, and they told me that their scanner can't read realtime data. They said it can only read a code once it has been thrown. So that's why I took it to the dealer (Mike Kelly Automotive). He basically told me the same thing, then charged me $40 to test the fuel pressure rail. Nothing.

Is the gas mileage OK, maybe O2 sensor?

Gas mileage seems OK. 18, which I what I usually get when running OK. The funny thing is, when the MAF sensor died before (almost 3 years ago), I was getting 23! It ran like crap and had no power, but I guess it was running too lean.
http://www.automotiveforums.com/vbulletin/showthread.php?t=956732

Good luck to you, I'll keep checking your post for an update. A Scanner is really worth the investment, I picked mine up on ebay for a decent price and has saved me 10 times what I paid for it. If you are ever in Bethel Park check out Bonneau's Auto Repair, they are very good with these vans.

Still haven't figured this out.

Question, please -- just thinking out loud:

When it begins acting up, is there an aerosol spray or something that I can use to dowse components while the engine is running, and force it to stumble? I.e., if spraying [xx] on coil pack 2 or on a vacuum line makes the engine stumble, I just found my problem?

You can search for vacuum leaks by letting the engine idle and spraying the hoses with starter gas (whatever the correct name in english is). If the engine revs up then you've found your leak.

Regarding the coils it might work to spray them with cooling spray (whatever the correct name in English is), that's a common way to troubleshoot electronics.

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Something you could also try is running a bottle of STP total system cleaner on a full tank, or similar product. That solved a problem for me when the EGR valve would stuck open occasionally.

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Thanks sverker! I'll give those suggestions a try later this week and report back.

Well, I tried the Ether (starting fluid) trick on the vacuum lines, but that didn't create any surging or stumbling. I also tried a couple other sprays on the electric components, such as Cooling Spray and Wire Dryer, but that also revealed nothing. So I dumped two bottles of the STP "Complete Fuel System Cleaner" into a full gas tank, and now we'll wait and see what that does.

Did you get any improvement?

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Hi sverker! Thanks for checking-in.

It did seem to help some, it still stutters and stumbles, but not as severe. It's maybe 2/3 as bad as it was. Since then, I have dropped a few more cans in, but they did not help any further.

If I knew what components that additive cleaned, perhaps that could help me narrow it down. I would think that additive would NOT have any effect on the MAF sensor.

Ehm, you are supposed to only use one bottle on a full tank and not more often than 6000 km. Otherwise you may have things in the fuel system dissolving...

The cleaner goes the same way as the fuel so no, it would not get to the MAF sensor. You can clean that one with electronic cleaner spray but don't touch the wires.

I also think it could make sense in your case to check if the trottle body and egr valve are dirty and if so give it a good go with carborator cleaner spray.

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If STP helped, try putting some seafoam into a vacuum line- that will clean it out.