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Old 06-13-2017, 08:15 PM   #1
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Unhappy Misfire/Roughness under 3k RPM, Camshaft sensor?

As far as I know this car is stock, it's an 03 SVTF. It has an issue where it either misfires or jerks under 3k RPM but really bad at 1500-2000. It seems to misfire at idle too. I almost never get a code, period. I did get a cylinder 1 misfire code ONCE. I did datalogging on the way to work using AutoScan XL. It showed something I wasn't expecting, the camshaft position error PID showed ranges from 25 degrees to 78 degrees and bounced all over the place. I can't imagine this is actually happening as I would imagine I would be having a lot more issues such as knocking/valve damage. I also noticed one of the O2 sensors was all over the place in readings with no real decent explanation but I do not know how they should act in the first place. Above 3k RPM it seems to run fine although it doesn't seem nearly as agile as it was when I got it.

What I had replaced beforehand:

Spark plugs, Wires, cleaned MAF, fixed vacuum leak, new CV axle (happening prior to this).

Any thoughts would be greatly appreciated! What should I be monitoring for PID's in AutoScan to help narrow this down, seems bogged down trying to access a ton of PIDs so the refresh rate is bad.

Edit: If anyone wants to take a look at the log file I'll be glad to post it.



Last edited by laser411; 06-13-2017 at 08:23 PM.
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Old 06-14-2017, 12:14 AM   #2
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Take a look at the timing belt, they need replaced more often on these cars then most.

O2 sensor is constantly trying to get perfect air/fuel, so the jumping around is normal. What mileage are you at?

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Old 06-14-2017, 06:45 AM   #3
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I'm at 200k. I just figured out the PID says VCT2, clearly we only have one VCT so I have no idea where it's getting those numbers from, maybe same computer for the TI-VCTs? Anyway, going to monitor it again but this time monitoring VCT1. What other parameters should I be monitoring? Again this issue only happens under 3k RPM, rarely above 2.2K and pretty frequently in the 1500-2k range.

What I am monitoring this time through:

VSS
RPM
Load
MAF
Fuel pressure (If truly supported, it shows up but who knows)
IAC duty cycle
VCT duty cycle
Short and long term fuel trims (We only have bank 1 right? my understanding is there is 1 bank per engine side)
Spark advance
VCT1 error %
Coolant temp
Intake temp
O2 Sensors 1 and 2

Last edited by laser411; 06-14-2017 at 07:08 AM.
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Old 06-14-2017, 08:57 PM   #4
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Alright little more backstory and the results of the datalog too.

Backstory: When this problem started it was VERY rare to happen. Over time it has gotten worse and affects a wider range of RPMs (Used to be ONLY at about 1500-1600). It feels like it loses power completely for a split second and almost feels like it yanks the car back a bit. It only happens when on the gas and bucks harder the more you are on the gas. At idle it just seems like an intermittent misfire, like one ever couple seconds. I almost never get an engine light, I did get a cylinder 1 misfire code just once and bank 1 lean once but I think the lean code was a vacuum leak. Above 3k RPM this issue either doesn't occur or the fact that the engine is spinning so fast it happens too quickly to feel and gets absorbed by the flywheel.

So, what I learned from the datalogging:

VSS = Fine, no artifacts

RPM = Bucks/Stutters happen too quickly to register in logs

MAF = Seems consistently reading correctly

Fuel pressure = Shows in KPA and I forgot the reading but it was pretty consistent.

IAC duty cycle = I have an issue that predates the current one and that is when I shift to neutral or hold in the clutch coming up to a light, the engine almost dies (sometimes does). When this occurred the IAC duty cycle maxed out at 100% so I am assuming I need to clean it or replace it.

Short and long term fuel trims = Didn't keep an eye on short term but for long term, the trip to work was from 9-13% and on the way back was like 24%-28%. I have NO idea why, nothing changed as far as I know but it bucked less on the way home.

VCT1 error %never went over 3 degrees and was typically 0-2, I assume that's good

Coolant temp = Normal

Intake temp = Normal and without artifacts

O2 Sensors 1 and 2 = All over the place (Only exception is during idle) and I honestly don't know how they should act or how to read it.

I am convinced it's not timing related though, I feel like it's either going to be a sensor, mechanical issue (not timing), or it's a bad/shorted wire. Anything spring to mind guys? Any help weeding things out would be great!
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Old 06-15-2017, 08:48 PM   #5
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Ok, so a bit more information. I am now convinced it is not mechanical, I did a reset learned behaviors using forscan (something I thought was automatically done when resetting the codes using my scan tool) and the problem almost completely went away for a while. It would happen VERY rarely, and shortly after I got a code. Bank 1 too lean. With everything else, what is this most likely to be?

Also, while idling my long term fuel trim is pegged at +25%,is that normal?
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Old 06-16-2017, 11:38 AM   #6
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I drove my car with those symptoms for 3 years. I just recently replaced the Intake Manifold gaskets, and the Exhaust Manifold gasket. It's smooth as butter now.

On mine the Intake manifold coupling clamps would slide off creating an air leak.
I also replaced the manifold gaskets which were little rubber circles between the plastic intake manifold and the head.
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Old 06-16-2017, 03:17 PM   #7
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A few things:

1) O2S1 is the upstream sensor. This is the sensor that the computer uses to determine weather the engine is running rich or lean. Normal operation of this sensor is to bounce between .1V and 1V at idle. Under heavy acceleration you will see it hang out towards the 1V mark. You shouldn't see it stick around .1V for very long. O2S2 is simply the catalyst monitor that the computer uses to make sure that the catalytic converter is doing is job properly. You can pretty much ignore that PID all together.

2) Try unplugging your MAF sensor. This will default the computer into a pre-programmed fuel delivery mode. If you are running better than before, you may have a damaged or faulty MAF sensor. If it runs worse with it unplugged, plug your MAF back in and continue diagnostics.

3) As funtruk stated, it is common for the stock intake boots to develop leaks. This allows unmeasured air into the engine and causes a lean condition. If the O2 sensor see's this lean condition for too long, it will set your check engine light and the Bank1 lean code that you earlier described. There are aftermarket silicone intake boots that seal better and last longer. If you have to replace them, I highly recommend the silicone ones.

4) Your +25% long term fuel trim is the computer adding as much fuel as it possibly can to compensate for a lean condition. I would start looking at those intake boots, vacuum lines, or any other possible spot for unmeasured air to be entering the engine.
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Old 06-16-2017, 06:46 PM   #8
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Any recommendations for the least painful/expensive way to locate this leak?

P.S. Thank you so much for your replies and great ideas!
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Old 06-17-2017, 08:37 AM   #9
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Smoke machine is about the only way. I tried all the other youtube methods of finding a vacuum leak, they didn't work.

Over the course of 3 years I just started replacing parts. Every sensor on the car. New coil & plug wires. New coil pigtail connector. All new vacuum lines. on and on. Nothing really cured it completely. Finally, I was wrapping my header and just decided to replace the gaskets and that's where my leak was.

It's a frustrating and difficult miss to diagnose and fix. But the car drives like a completely different vehicle when you don't have to manipulate the throttle around town to keep it running smooth.

Good Luck finding yours.
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Old 06-17-2017, 04:45 PM   #10
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I'll find someone who vapes and have them blow in the break booster line lol.

Btw what SHOULD my vacuum pressure be? My gauge shows it at 10 in/hg at idle which seems pretty low but it was steady as a rock. The first 2 seconds it hit like 25 or 30 during that brief high idle

Last edited by laser411; 06-17-2017 at 04:48 PM.
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