Rough idle when first started

gmcman

Original poster
Member
Dec 12, 2011
4,656
Recently I've noticed the engine has a long starting process, cranks but needs gas pedal depressed to get engine to finally fire, mainly after refueling.

Also, when first started, at most times long after it has been refueled, it will stumble then pick up again, pretty much on a 5 sec interval for about 30 seconds.

I believe it's either a fuel filter or other evap issue.

I listened near the tank and the pump sounds normal. I did notice that after the engine was started, after about 10 seconds, I can see the bottom of the gas tank get sucked upwards. When the vacuum releases, the tank settles down and the idle appears normal.

I don't know if the tank getting sucked in from the evap is normal.

I may have a weak coil pack, but wanted to try the fuel first.

Is that a normal evap sequence to draw that much vacuum at startup?
 

budwich

Member
Jun 16, 2013
2,027
kanata
I believe that you should be able to monitor the evap pressure which might give you your answer.
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
I hate shotgunning but try replacing the gas cap. It has both a high pressure vent and a vacuum relief. Or leave the cap loose and see if things improve. You'll get a code for it but at least it shouldn't effect driveability and will relieve the vacuum.
 

Redbeard

Member
Jan 26, 2013
3,466
Question: When you top off your tank do you stop filling when the first time the nozzle "clicks" off? The '04 I had - the mrs. noted when we first purchased it that I liked to fill it to the top. And regularly it took a bit extra to restart the TB. The mrs. would stop filling when it first clicked off. Guess what, she was doing it better that way. Then infrequently it would take a bit extra time to start it. So I learned not to try and put every drop that might fit in both the tank and the filler hose. I assumed with trying to top it off some of the evap system just didn't like it at all.
 

gmcman

Original poster
Member
Dec 12, 2011
4,656
My main issue is a good 6-8 second crank after filling with fuel, after that it's an occasional stumble after starting. I do stop after the first click, maybe hit it one more time but I don't fill the neck up.

Once running, it runs like a top, good power, smooth, just the engine start.

On some mornings it will stall while warming up.

I have been seeing a few misfires on Cyl 6 and 3 after startup, could be the coil packs.

Now what could be the issue, is a recurring P0440 code and it could be the vent valve in the rear. I did have a broken wire at the harness that I fixed but the copper wire didn't seem to be a nice, clean, copper color, more like a litz wire....but not exactly,

That could be the issue and I will check today, as well as trying to start with the gas cap loose.

When I get the no start after fueling, there are no codes, just acts like there's no fuel.....TB is also clean.

The long start and stumble happened abruptly 2 weeks ago, nothing that started to show it's face over time. Always started and ran perfect, then one day it didn't start right up.
 

mrrsm

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Oct 22, 2015
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gmcman

Original poster
Member
Dec 12, 2011
4,656
I was able to get a video of the hard start after fueling. @Mooseman , I left the gas cap tightened for this to hopefully replicate the symptom.

I warmed the engine before I left for work, started perfect, idled perfect, drove 30 miles with zero issues.

The video starts at the fuel island, engine running, then I re-started 3 times with no problems.

I then paused the video and fueled up, added 15.9 Gal and stopped at the 1st click, then added a couple cents to round off because I'm old school like that.

Then with no sign of firing, it cranked for a long time then coughed a few times, then I cranked again and it started to run rough then smoothed out.

Only thing I did was fill it with fuel.

 

Blckshdw

Moderator
Nov 20, 2011
10,665
Tampa Bay Area, FL
Something I would try, just for the sake of nothing to lose, would be not filling the tank completely and see what happens. See if the issue is caused simply by fuel entering the tank, or if it's caused by the volume of fuel in the tank after a fill. :twocents:
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
I agree. Topping off the tank is bad news. It can fill it so much that raw fuel can enter the charcoal canister.

However, doing a bit of research, your issue may be caused by a faulty purge valve, the one at the motor. This guy had the exact same issue as yours and replacing the purge valve fixed it.

 

TJBaker57

Member
Aug 16, 2015
2,897
Colorado
recurring P0440 code and it could be the vent valve in the rear

The vent valve is a normally open valve, allowing the fuel tank vapors to be cleaned through the charcoal canister and escape. It only closes when the PCM decides all the conditions are correct for running an EVAP test. One of those conditions is that the tank is to be NOT full.
 

gmcman

Original poster
Member
Dec 12, 2011
4,656
Interesting. I replaced all those components about 2 years ago, vent valve, charcoal canister, purge solenoid...the fuel pump will kick off a couple times during fill so my thinking is the vapors cannot escape. I did also change the filler neck and that could also be the issue.

Still baffled on why it would not fire when restarted after fueling. Either flooded or just no fuel.
 

TJBaker57

Member
Aug 16, 2015
2,897
Colorado
baffled on why it would not fire when restarted after fueling. Either flooded or just no fuel.

I'm with Mooseman here, test that purge valve. If it is not fully sealed as it should be you could be pulling raw fuel into the canister and a lot of fuel vapor into the engine thereby flooding it.
 

TJBaker57

Member
Aug 16, 2015
2,897
Colorado
You could have a look at fuel tank pressure (vacuum) using Torque Pro. It is one of the "[GM]" PIDs. That might indicate if the purge valve isn't fully closing.

The tank pressure PID ## should be 114E. I noted in my current version of Torque Pro the PID number is incorrect, listed as "F432". No idea when or why that was changed but it is wrong.

The EVAP PID, "1170" shows the percentage of purge valve activation. So if one sees the tank vacuum building when the purge valve remains at 0% then the purge valve is leaking.
 
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budwich

Member
Jun 16, 2013
2,027
kanata
Of course, if you just filled your tank (and the cap was removed), you should see little to no pressure from the pid.... and then you should see it remain relatively small with little to no pressure for a while.
 

TJBaker57

Member
Aug 16, 2015
2,897
Colorado
I captured an EVAP test today! About a minute after startup one can see the vent solenoid close and the purge valve modulating around the upper 30s to upper 50s (%). The tank vacuum as inches H2O is seen to rise abruptly then the PCM closes off the purge valve sealing the fuel system. The PCM monitors the decay of the vacuum. The PCM then opens the vent valve, dumping the vacuum from the test and operates the purge valve at about 55% for about 2 minutes with the vent open. Then a second test is seen where the system is again sealed off but at a lower vacuum level.

Screenshot_20211027-194938.png
 

gmcman

Original poster
Member
Dec 12, 2011
4,656
Thanks for the graphs @TJBaker57 I really want to understand why everything is happening, that's the love of diagnosing these issues. I agree it points to the purge solenoid and/or gas cap. In the end, knowing exactly what's going on is also a good learning experience.

What had me scratching my head, was when I began to crank the engine after fueling, there was zero sign of firing, like no fuel or spark was present. So this problem happens immediately and what could cause this was what had me stumped.

Does the TB close just a hair more to draw a higher vacuum from the evap?

Maybe the vacuum is still very strong with the TB plate in the same resting position and it doesn't take but a moment to draw the fuel vapor or maybe liquid from the evap line, thus flooding the engine.

Could the fuel vapors be forced from the tank upon refueling, into the TB if the purge valve is stuck open? This seems very likely which would give the intake a very rich mixture upon first cranking.

@Blckshdw I will try fueling 3/4 and see what happens if I don't replace parts beforehand.

@Mooseman I have started my engine and let it warm up before leaving work the last 4 (working) days, each time after I came back after 10 min, the engine has stalled.

Yesterday I did the same but loosened the gas cap before I started the engine, it remained running and only a slight stumble when I tightened the cap.

This morning I did the same, but when I returned after 10 min, the engine was still running, I tightened the gas cap and it immediately stalled.

Never thought a gas cap would do that so abruptly.

For the record, I do stop fueling after the 1st click, I don't force the filler neck full. Worst case, a couple extra ounces to round off the $ value.
 

TJBaker57

Member
Aug 16, 2015
2,897
Colorado
I really want to understand why everything is happening, that's the love of diagnosing these issues.

Agreed. I think I enjoy the diagnostics more than the actual work to fix things!

I agree it points to the purge solenoid and/or gas cap. In the end, knowing exactly what's going on is also a good learning experience.

Also agree here but unfortunately I have attained an age where I frequently don't remember what I've learned!!

Could the fuel vapors be forced from the tank upon refueling, into the TB if the purge valve is stuck open?

I didn't post this yesterday but I did notice an unexpected tank pressure during the refueling. I left the app active and recording during refueling. Key OFF of course, but the PCM will remain active as long as the app is talking to it. During the refuel I see the tank pressure go to a positive pressure. With the open vent I would not expect to see this, unless my canister is a bit clogged which is entirely possible if not likely since it is original and I admit to being a "topper" at the pump.

I intend to do a few more recordings to see if this is a normal occurrence at least for my vehicle. If it happened with a vehicle whose purge valve was either stuck open or leaking it just might push vapors/fuel at least into the lines if not clear through to the intake.

That said, it still seems odd that your engine would not at least show some attempt at firing.

Screenshot_20211028-075922.png
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
Could the fuel vapors be forced from the tank upon refueling, into the TB if the purge valve is stuck open? This seems very likely which would give the intake a very rich mixture upon first cranking.
That is quite possible, especially if it's liquid fuel being pushed from the canister.

One thing you could try is disconnect the vacuum line to the purge valve and plug the line from the intake. You could also see if there is fuel in the line or watch for fuel being pushed out while fueling. This would prove two things, your purge valve is bad and you have raw fuel in the canister. I had fuel in the canister of the Avalanche because the overflow was broken in the tank and it always filled to the top so it drew fuel into the canister. But because my purge valve was working, I didn't have the starting/stalling issues. During that fiasco, I first replaced the canister, the vent valve and filter and then finally the tank since the overflow valve is inside the tank.
 

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