Random Stumble on Startup?

11888 Views 9 Replies 3 Participants Last post by Robby, Nov 6, 2015

jblackburn

Discussion Starter · Nov 6, 2015

Background: manifold and everything attached to it was replaced recently for PCV issue. I think it comes with the throttle body, EVAP valve, and possibly injectors.

Since then, completely randomly, I've had a bit of a stumble on startup - either first thing in the morning or sometimes after a hot soak. 90% of the time, it'll crank over and fire up normally. Yesterday was particularly bad - it stumbled to life and then RPM hunted around 1500 up and down for 5 seconds or so. Then it acted like nothing happened and the idle settled down to 1000 after 15 seconds.

Car runs great otherwise, and no noises or anything else to indicate a vacuum leak. I checked the throttle body (new) for oil buildup, checked for vacuum leaks at the intake manifold to head ports (nothing), and then checked the coil pack/plugs. Nothing wrong with anything there.

The more I think about it, the more I think it's the purge valve or the check valve right after it (also new). Hot starts and near full tanks of gas seem to really be the only correlation to getting the stumble regularly - it has happened on cold startups after sitting for a number of days only 2-3x.

Thoughts anyone?

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ChevyGuy

· #2 · Nov 6, 2015

The closest I've had to that is a bad fuel pressure regulator in my old car. The diaphragm ruptured, causing gas to go into the manifold via a pressure sensing line. I had to start the car as if it was flooded (because it was). The leak was small enough it didn't bother things once the engine was running.

But I'm sure there's probably a few things that can cause that.

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jblackburn

Discussion Starter · #3 · Nov 6, 2015 (Edited)

I did notice the fuel line was pulsing a little bit at idle as each injector fired, but I figured that was probably normal for a 4-cylinder. There's a little damper in the middle of the line before it goes into the fuel rail, probably to smooth out those pulses.

If the FPR diaphragm ruptures, it usually sends gas up a vacuum line, right?

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Robby

· #4 · Nov 6, 2015

I believe my 12 eco behaves in similar fashion but knowing open loop startups, cold or hot, are operating off a fixed program, I've never given it much thought.
As you indicate, it kinda hunts around for several seconds but I contend it sorts itself out once it starts to get useful EGT (O2) information.

One of those things I notice, but as long as it is only moments after startup and driveability are not affected I write it off as this machines nature.
As a guess....most likely a result of the overich mixture that is part of a startup...be it a cold or hot restart......once the ECM starts to lean it out based on O2, it calms down.

My normal AM start includes letting it run long enouph to attain base cold idle (about 900 or less) before putting it reverse.
During this period, I can actually hear the engine smooth and rpm's rise as it leans.....then in another second or two the idle starts to drop to below 1000.....once stabilized, In gear we go.

This is, for the most part, repeated anytime the car is parked over an hour.

Anyways, I think it is of no concern and can't think of a thing, short of re-writing a program, that could change a thing.

If this occurs at operating temperature though, while operating in closed loop, there would be a different story.

Rob

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ChevyGuy

· #5 · Nov 6, 2015

jblackburn said:
If the FPR diaphragm ruptures, it usually sends gas up a vacuum line, right?

Yes. In small amounts, it just gets sucked into the engine and burnt without trouble. But the problems show up when the engine pulls the least amount of air - during starting. But I'm sure the same could be said of any other kind of leak.

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jblackburn

Discussion Starter · #6 · Nov 6, 2015

Robby said:
I believe my 12 eco behaves in similar fashion but knowing open loop startups, cold or hot, are operating off a fixed program, I've never given it much thought.
As you indicate, it kinda hunts around for several seconds but I contend it sorts itself out once it starts to get useful EGT (O2) information.

One of those things I notice, but as long as it is only moments after startup and driveability are not affected I write it off as this machines nature.
As a guess....most likely a result of the overich mixture that is part of a startup...be it a cold or hot restart......once the ECM starts to lean it out based on O2, it calms down.

My normal AM start includes letting it run long enouph to attain base cold idle (about 900 or less) before putting it reverse.
During this period, I can actually hear the engine smooth and rpm's rise as it leans.....then in another second or two the idle starts to drop to below 1000.....once stabilized, In gear we go.

This is, for the most part, repeated anytime the car is parked over an hour.

Anyways, I think it is of no concern and can't think of a thing, short of re-writing a program, that could change a thing.

If this occurs at operating temperature though, while operating in closed loop, there would be a different story.

Rob

I've had it take a few cranks longer to start sometimes than normal, but I've never had it stumble to life on 2 cylinders before the others get lit off like it has been doing. The RPM hunting - quite noticeable as the needle moved up and down on the gauge - would also be new for my car. If it does it again, I'll try to shoot a video.

Once warmed up, you can sometimes hear it move the idle a little bit down around 700 RPM, but it's not noticeable on the tach.

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ChevyGuy

· #7 · Nov 6, 2015

You may have to let the problem develop - if it does.

In my case, the problem couldn't be found. Not even the mechanic could find it. But it became evident when the diaphragm went out completely. The car stalled right after a fill up with fuel running out the throttle body. Which explained another problem I was having. The air intake line from the air box to the TB disintegrated at the bottom where it attached to the TB. I thought it was old age. It was from being exposed to gas running out the TB (where the vacuum line from the regulator connected.)

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Robby

· #8 · Nov 6, 2015

Sorry....I interpet stumble as just that......your stumble then is my startup misfire mis-fire.......not long enough to set a mis-fire code though.

More often then not a plug or two showing signs of a 'damp short'......the rich startup mix, combined with porous porcelin surrounding the firing tip, shorts the plug from tip to shell. Don't know why it becomes porous....but it does.
As the mixture leans inconjunction with the hurricane that occurs with each intake moment, the porcelin dries out and the plug returns to normal.

Got a set of removed plugs laying around?.....try em.

Rob

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jblackburn

Discussion Starter · #9 · Nov 6, 2015

Robby said:
Sorry....I interpet stumble as just that......your stumble then is my startup misfire mis-fire.......not long enough to set a mis-fire code though.

More often then not a plug or two showing signs of a 'damp short'......the rich startup mix, combined with porous porcelin surrounding the firing tip, shorts the plug from tip to shell. Don't know why it becomes porous....but it does.
As the mixture leans inconjunction with the hurricane that occurs with each intake moment, the porcelin dries out and the plug returns to normal.

Got a set of removed plugs laying around?.....try em.

Rob

Hmm...will do. Shortly before it went into the dealer, I installed a 2nd set of iridium plugs. I think my old coppers are about worn out.

I did check them this morning though, but wasn't looking too closely - they all looked like I'd expect em to.

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Robby

· #10 · Nov 6, 2015

Really can't see anything on the white porcelin.....one of those things you change to get them out of the equasion in an attempt to minimize brain damage.

Rob

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