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Bonneville GXP/ Northstar Powered Cars Discuss your Bonneville GXP and/or any other Northstar powered Olds or Cadillac... Including the 3.5L Twin Cam V6 (Short Star ) 4.0L and 4.6L Northstar V8's. Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 07-10-2007, 01:16 PM   #1
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Default stumble on start up, finally hits good and then runs fine

My 3.5 has been acting squirrely lately.

Sometimes when I start it, it will turn over, then stumble, sputter and eventually die. You can keep it running if you give it a little gas, albeit running poorly. If you give the throttle a quick stab it will rev up and then run strong after that, but i dont want to have to do that.

After it dies, it usually takes one more start attempt and it will catch and run fine, no other driveability problems. It has seemed a little lacking in performance, but I think that might be due to extreme heatsoak. Its been in the 90s pushing 100 here lately. Also extremely humid, as Missouri is.

I replaced the FPR, and thought that had fixed the problem, but a couple starts later it started acting up again.

Currently as a troubleshooting measure, Ive been driving around with my MAF disconnected. I have so far 10 or better starts under my belt with no issues at all. Im going to leave the MAF disconnected a while longer and see if the problems continues to be gone.

I have no codes to work with that I didnt induce myself, and Im not in the mood to start shotgunning parts, especially since parts for the 3.5 are more expensive than your average "we put this engine in like 15 cars" engine.

Thoughts? Ideas? Thanks guys
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Old 07-10-2007, 03:41 PM   #2
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Dirty MAF? (have you tried cleaning it?)

I've had symptoms like that with a bad HO2S1....have the O2 sensors been replaced in 140,xxx miles? Do you have a scan tool to check voltage/output?

Clogged fuel filter?

that'* all off the top of my head right now...
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Old 07-10-2007, 03:52 PM   #3
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I recently cleaned the MAF a few weeks ago. I dont think I broke anything, but I guess its possible. You would think it would throw a code if something was wrong.

It just stumbled again a few minutes ago with the MAF disconnected, so I think that rules it out as a possibility.

Too my knowledge that 02 sensors have never been replaced, but the car has only been mine since about 105,000 miles. I wouldnt think an 02 could case a starting condition I get an occasional code for the sensor after my cat, but I know it doesnt affect starting. Plus my mileage is still pretty good. Just got 27mpg last time I filled up.

Fuel filter was replaced early this Spring I believe. My fuel pressure is still pretty good, my cheapo gauge reads 40 or a tad more when primed, idles at 35ish and comes up to the low 40s when revved or when the FPR is disconnected. Im not sure what its supposed to be though.

Part of what makes this so difficult to troubleshoot is the fact its so irregular. Im just wanting to figure out what it is, before it leaves me sitting alongside the road somewhere. Especially since im driving out to Oklahoma soon.
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Old 07-10-2007, 04:16 PM   #4
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Yeah, probably not the MAF.
Does it do it after it sits awhile? Injector leaking down?
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Old 07-10-2007, 04:28 PM   #5
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How long should the fuel pressure hold? When I prime the pump, then turn the ignition back off it loses pressure pretty quickly. It will drop from 40 to 30 in just a couple minutes.

If the injectors arent holding, would the spark plugs be carboned up?

Ive heard theres also a valve in the fuel pump that can go bad and cause fuel pressure to drop.
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Old 07-12-2007, 03:17 PM   #6
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Being that you say it finally runs ok, I wonder if the Short Star engines are affected by cold carbon knock as well?. Do you get any kind of a 'rattle" when it first fires or try to give it torque? If your injectors are leaking, that could certainly cause some buildup.
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Old 07-13-2007, 02:47 AM   #7
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Quote:
Originally Posted by GXP Venom
Being that you say it finally runs ok, I wonder if the Short Star engines are affected by cold carbon knock as well?. Do you get any kind of a 'rattle" when it first fires or try to give it torque? If your injectors are leaking, that could certainly cause some buildup.
Ive never noticed anything like that at all.

Something Ive observed lately, especially today as I drove it several hundred miles today and started it many times, is that if I turn the key to the ON position, and pause for just a second before cranking it, it will start just fine. I have always started the car by simply turning the key straight to the START position as fast as my hand will go, like almost everyone else does. But if I take that slight pause, I dont seem to have an ounce of trouble with it.

That just makes me think even more that its fuel related. I plan on pulling the plugs to take a look at them when I get some time. The car is still running fine, and today I decided to see what sort of highway mileage it was capable of. So I filled up until the handle clicked off, reset the trip meter, drove exactly 51 miles, then filled up again and it clicked off at 1.406 gallons....for those of you bad with numbers thats 36mpg....thirty-six, 3-6....I was in awe. Im not sure if it was a fluke, or if I didnt do something right. But I found that to be awesome. Had the cruise set at 65mph and I dont recall downshifting once, plus I had the defrost on. Thats the best mileage ive ever seen out of this car.
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Old 07-13-2007, 10:38 AM   #8
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I just wonder, if it is just you being a little quick on the start trigger.Todays cars have become so computerized. I always let mine do it'* "warm up", or "boot up" or whatever it'* doing while the gauges dance around back and forth when the key is first turned on. Maybe it'* confused.
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Old 07-13-2007, 01:34 PM   #9
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That makes sense I guess, but Im not doing anything different than I have since I bought it, unless my hands have just gotten quicker somehow.

How long should the car hold fuel pressure? It drops pressure down pretty fast if you leave a gauge hooked up too it.
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Old 07-13-2007, 01:48 PM   #10
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I wouldnt think that a problem, modern systems use a loop back to the tank so I dont think it lasts long after the ignition system is off. I would think the motor wouldnt smooth out at all if your injectors were leaking. Does the gauge spike when the key is on but the engine not started?
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