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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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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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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 03-24-2017, 12:10 PM   #9
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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 03-24-2017, 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 03-24-2017, 05:40 PM   #11
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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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Old 04-04-2017, 01:23 AM   #12
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Originally Posted by RockinWs6 View Post
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.
I finally got around to logging it. Nothing wild in here. It's all part throttle driving around town and some idle. Do the fuel trims look OK to you? What should I be looking for?
Attached Files
File Type: xls OBDLink CSV Log.xls (327.0 KB, 8 views)
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Old 04-04-2017, 07:03 AM   #13
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I don't do any tuning so I'm not sure about the tables.

It sounds like a cold misfire. I always run copper plugs because the iridium and platinum plugs have a really fine tip and usually result in a loss of torque in midrange power. You did not gap the plugs, correct? Gapping a platinum or iridium can cause a misfire
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Old 04-04-2017, 02:10 PM   #14
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You already posted above your fuel pressure is low....replace the assembly so your fuel pressure is normal and go from there. You can't just guess at things hoping you'll fix it???? I'm not sure what else you want? Of course its going to run all over the place with the fuel assembly needing replacement.
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Old 04-04-2017, 02:29 PM   #15
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You already posted above your fuel pressure is low....replace the assembly so your fuel pressure is normal and go from there. You can't just guess at things hoping you'll fix it???? I'm not sure what else you want? Of course its going to run all over the place with the fuel assembly needing replacement.
Exactly. I'm trying to not guess and instead figure out exactly why it's doing what it's doing, i.e. bogging and sputtering sometimes but always consuming too much fuel. I'm hoping somebody knowledgeable can look at the logs and tell me if the excessive consumption is from overfueling due to a constant lean condition.

And while we're at it, is there a write up anywhere on disassembling the fuel pump module? I want to replace only those parts of it that are defective. Nothing else.
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Old 04-04-2017, 03:08 PM   #16
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You replace the entire assembly and if I were you I'd buy a good one if you don't want issues down the road. Now with your attitude its very hard for me to continue to help you. The reason you replace the entire assembly there are too many parts that are worn that need replacing in there. Your fuel pressure is low, the check valve is leaking, the regulator is probably toasted. These fuel assembles are submerged in fuel 24/7 how long do you think they last? There is a sponsor that sells all the parts but with your limited experience you may make more trouble than you have already. Good luck
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Old 04-04-2017, 07:57 PM   #17
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You replace the entire assembly and if I were you I'd buy a good one if you don't want issues down the road. Now with your attitude its very hard for me to continue to help you. The reason you replace the entire assembly there are too many parts that are worn that need replacing in there. Your fuel pressure is low, the check valve is leaking, the regulator is probably toasted. These fuel assembles are submerged in fuel 24/7 how long do you think they last? There is a sponsor that sells all the parts but with your limited experience you may make more trouble than you have already. Good luck
Man I appreciate your help but I asked the community for assistance in diagnosing a problem, not for a lecture on my "attitude". I don't see what I did to offend you, but the whole point of coming here and doing this myself, and not being pressurized by holier than thou mechanics at a shop to splurge on parts ***** nilly (like you're doing to me given your previous response), is to learn how the system works and if the symptoms could be related. If you're tired of my desire to ask questions, you're welcome to ignore them, but don't come in here knocking my "limited experience" and potential to "make trouble". You neither know me, nor do you have any idea of my experience or abilities.

Now would somebody please have a look at this log I posted?
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Old 04-04-2017, 09:30 PM   #18
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Fuel pressure test results seemed good. Fuel pump complete assembly for a 99-02 plastic tank is around $100, for my 98 it's more like $368. Fuel pump failure would have much more severe symptoms, it wouldn't just be at a particular rpm it would be a constant issue.

I'm looking at your table and your tps% are really sporadic. Line 466 on your table seems strange, you're getting a high flow reading from the mass air flow sensor, low Flow reading from the map sensor, and a zero value from the tps.

So you have a possible vacuum leak, a slow mass air flow sensor, or a bad throttle position sensor. Your readings on the Tps never show over 70%

I mean I can look at the table but there's nothing to compare it to. Unless you took your buddies car with the same mods on the same drive for the logging there's not much to check the table against
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Old 04-04-2017, 10:29 PM   #19
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Fuel pressure test results seemed good. Fuel pump complete assembly for a 99-02 plastic tank is around $100, for my 98 it's more like $368. Fuel pump failure would have much more severe symptoms, it wouldn't just be at a particular rpm it would be a constant issue.
It's just about within spec as the FSM describes the test procedure but it did drop to 45 psi at WOT and stayed there until I turned it off. Is that normal. Also, when i was testing fuel pressure with the key on engine off, the fuel pump wouldn't re-prime every time if I turned the key on and off a few times.

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I'm looking at your table and your tps% are really sporadic. Line 466 on your table seems strange, you're getting a high flow reading from the mass air flow sensor, low Flow reading from the map sensor, and a zero value from the tps.
It could have something to do with the sampling rate of the app I'm using. it's based on movement detected by the GPS. I'll see if I can get it to sample at a faster and constant rate.

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So you have a possible vacuum leak, a slow mass air flow sensor, or a bad throttle position sensor. Your readings on the Tps never show over 70%
I was driving around town, so I never got the chance to stomp on it. It doesn't have any of the drivability symptoms of a bad TPS.
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Old 04-05-2017, 06:21 AM   #20
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Yeah your fuel pressure needs to be a constant 58psi. Amy deviation from that number means the pump is failing.


It's not accurate to tell if the car is running lean by looking at factory oxygen sensor readings. To.view that kind of data you would need a wideband 02 sensor. If we could view the data log and see errors the car would have already triggered a check engine light. So we can already consider those results acceptable, but like I said, the factory 02 sensors are not accurate to determine an air fuel ratio
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