Bogging and sputtering when cold - LS1TECH



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Bogging and sputtering when cold

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Old 03-17-2017, 12:11 AM   #1
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Default Bogging and sputtering when cold

So as the weather has been warming up here into the 70s, I've noticed that my car won't idle smooth when cold and occasionally stumbles or sputters like it's about to run out of gas when taking off. Upon cold startup, the idle keeps dipping down to 700 when it should hold steady around 850-900. As I drive off, it will fall to 500 RPM every time I press the clutch pedal while coasting and sometimes if I'm approaching a stop it will also bog and sputter at around 300 RPM like it doesn't have the power to pull from a dead stop. By this time the engine temp is usually 170F. I then usually have to rev it a bit and it recovers. All of this disappears by the time the engine has fully warmed up.

IDK if this is related but the car has always surged a bit in 2nd and 3rd gear at part throttle between 1200-1400 rpm. It's a gentle jerking motion, kinda like a novice driver letting the clutch out too fast.

The car is stock except for a lid. Recently got a tune-up. New NGK TR5IX plugs, MSD 8.5mm wires, throttle body was cleaned, MAF was cleaned, K&N filter is clean, and new Denso O2s. Scanned the PCM, no DTCs set either. I did a fuel pressure test and that looked good too. 55 psi key on and held steady key off. 60 psi going down the road. I was hoping to monitor the fuel pressure during one of the bog and sputter episodes but it didn't happen today, probably because it was a little colder. I also scanned it yesterday using the OBDlink app and O2 sensor activity looked good, even during the uneven idle.

What else should I be looking at? IAC valve? Despite the tune-up, fuel economy is total **** too. 17 mpg with 50/50 highway and city driving.

Last edited by Jeep_junkie; 03-19-2017 at 01:48 AM.
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Old 03-17-2017, 06:16 AM   #2
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What do the fuel trims look like? When is the last time the intake bolts were torqued? Sounds like a vacuum leak, the computer can't adjust the cold mixture. Another issue can be rusted fuel lines inside the lines clogging the injector screens. This will cause the symtoms you describe.
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Old 03-17-2017, 10:43 AM   #3
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The intake bolts were loose when I got the car again few months ago so I tightened then up. I'll pull the injectors out and check for debris in the screens. I didn't check the fuel trims this time but they looked good when I installed the new O2s. 2% and 5% IIRC. I'll check again today.
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Old 03-17-2017, 01:22 PM   #4
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When's the last time you changed the pcv valve and inspected the pcv hose system.

Also I hate k&n air filters for our vehicles. They can slime up the mass air flow sensor and cause the problems you are describing
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Old 03-17-2017, 04:18 PM   #5
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Originally Posted by chrysler kid View Post
When's the last time you changed the pcv valve and inspected the pcv hose system.

Also I hate k&n air filters for our vehicles. They can slime up the mass air flow sensor and cause the problems you are describing
Only a month or so back. No leaks in the pcv system. The valve works as it should.

The filter isn't over oiled or dirty. I did clean the MAF too. It had been soiled from blow by coming through the throttle body. Cleaned out the catch can and it's been nice and dry since.
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Old 03-19-2017, 01:47 AM   #6
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Well, I haven't had a chance to log any driving yet, but it hasn't acted up again since. I did however monitor fuel pressure under wide open throttle and it dipped to 50 PSI during WOT and down 45 PSI coasting to a stop. Also, I misread the gauge earlier, it actually reaches a max of 60 PSI with the motor running. Will edit the original post to reflect this.
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Old 03-19-2017, 11:44 AM   #7
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Well there you go, low fuel pressure will cause all kinds of troubles.
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Old 03-22-2017, 02:46 AM   #8
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Originally Posted by RockinWs6 View Post
Well there you go, low fuel pressure will cause all kinds of troubles.
So I checked pressure as per the FSM. It dropped nearly 10 PSI in 10 minutes with the key off. Definitely leaking fuel somewhere. Now to figure out if it's the injectors or something at the other end.
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Old Yesterday, 12:10 PM   #9
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Quote:
Originally Posted by Jeep_junkie View Post
Only a month or so back. No leaks in the pcv system. The valve works as it should.

The filter isn't over oiled or dirty. I did clean the MAF too. It had been soiled from blow by coming through the throttle body. Cleaned out the catch can and it's been nice and dry since.

So, you're telling me, the foreward most point on your intake system is being fouled because at some point in your engines run cycle it is blowing oil out of the cylinders, that oil is escaping the suction stoke of the valve, overcoming all of that negative pressure, going past your throttle body, and oiling your mass air flow sensor?

Your mass air flow sensor is being fouled by air coming in from the intake
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Old Yesterday, 01:35 PM   #10
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Originally Posted by chrysler kid View Post
So, you're telling me, the foreward most point on your intake system is being fouled because at some point in your engines run cycle it is blowing oil out of the cylinders, that oil is escaping the suction stoke of the valve, overcoming all of that negative pressure, going past your throttle body, and oiling your mass air flow sensor?

Your mass air flow sensor is being fouled by air coming in from the intake
Well yeah I too wondered how oil from the intake could reach the MAF considering air is flowing in the opposite direction, but when I first took off the bellows from the throttle body there was engine oil puddled at the bottom of the blade and in the folds of the bellows. I cleaned it up and also emptied out the catch can. Haven't seen it return since. The filter is definitely not over oiled. Feels dry to the touch but I'm going clean it in any case since there's a significant amount of dirt only on the half of it exposed to incoming air flow. Or should I just drop in a paper filter?

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Old Yesterday, 05:40 PM   #11
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Originally Posted by Jeep_junkie View Post
So I checked pressure as per the FSM. It dropped nearly 10 PSI in 10 minutes with the key off. Definitely leaking fuel somewhere. Now to figure out if it's the injectors or something at the other end.
Well after going back to the FSM, everything seems to be pointing at a leaky FPR, which the FSM says requires a whole new in tank assembly. Before I throw expensive parts at it, what else could I do to fix this?
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