Rough Idle HELP!

[02YukDenXL]

It feels like it's missing more than at just idle. My throttle is fly by wire, not cable. I don't think I have a iac. I'm gonna look into the tps once I have a chance.

I have a coworker who has an obdII USB connector and software for a laptop. I'm gonna try and plot fuel trims and rpm and see if anything jumps out at me. Those two 30-minute fuel trim videos were amazing.

 

[about20ninj45]

It feels like it's missing more than at just idle. My throttle is fly by wire, not cable. I don't think I have a iac. I'm gonna look into the tps once I have a chance.

I have a coworker who has an obdII USB connector and software for a laptop. I'm gonna try and plot fuel trims and rpm and see if anything jumps out at me. Those two 30-minute fuel trim videos were amazing.

You do have an internal iac and its not serviceable, its built into the throttle body, hence is why i said replace it. Keep us updated throughout your diagnosis that way we can chime and and provide as much help as we can. It would also if you could get me a video (with audio) of when its idling rough.

 

[VikingTrad3r]

wow, those videos are amazing for the average hacker like me! going to get a scanner! watched them over lunch.

definately check out the fuel trims ! should be able to dial in what your issue is just by that. at least dial it back to a couple things. the tps can very very easily be tested with a cheap multimeter. i have done the propane test on my 1987 vette to find a hose that was loose and had a vac leak. now i know wtf i was doing

seriously i love those vids!!!

 

[SLCHOE]

JUST NOTICED YOU NEVER CHECKED YOUR FUEL PRESSURE YOURSELF! SCREW THAT MECHANIC. CHECK IT YOURSELF FIRST BEFORE YOU DO ANY OF MY MENTIONED TESTS.

Short and long term fuel trims banks 1 & 2 at idle

Your LTFT is high but just barely not high enough to set a P0171 or P0174 codes. Your engine is running lean. So basically your engine was designed to run at a certain fuel volume and the injectors at a certain pulse width at any given RPM to achieve 14.7:1 stoichiometric efficiency. The +/- % values on LTFT is indicating how much (and how much % of normal) fuel correction is needed to maintain 14.7:1 stoich. +% is adding fuel while -% is taking fuel away (via pulse with modulation of the fuel injectors on both scenarios).

Your engine is adding 8-10% more fuel to make it run normal. So it is running lean initially before the ECM corrects for it.

 

[SLCHOE]

O2 sensor voltages at idle. Both videos were taken after I drove for about 45 minutes this morning.

Your downstream sensors are very lean. If you watch they hover around 100 mV to almost 200 mV. That is very lean.

 

[SLCHOE]

While your LTFT is a bit lean, your upstream O2 seems to be switching just fine as it should. What doesn't look right are your downstream O2 readings. They are way too lean based on the upstream readings.

I suspect low fuel pressure (fuel pump going weak) .

First check the fuel pressure. This is easily done with a fuel pressure gauge since the fuel rail supplies an easy to connect to schrader fitting on the passenger side of the fuel rail. Specs is 55-62 PSI with key on and engine off. If it is below that (and seeing you already changed the filter) you need a fuel pump. If fuel pressure is ok, continue on...





One more test you can do after the Key on engine off fuel pressure test is to monitor the fuel pressure with the gauge under load. You may need a friend to help you with this one.

With the fuel pressure gauge still connected to the fuel rail, watch the pressure at idle. It should drop a few PSI lower than at KOEO but not a lot. Then powerbrake it (put your foot firmly on the brake pedal then put it in drive and bring the rpms up to about 2500 rpms. If the fuel pressure drops a good amount below the spec, the pump is bad.

 

[SLCHOE]

I don't look at the O2 sensors when diagnosing untill the last unless the code keeps comming back. Any little problem will affect the O2 readings on the scan tool ECT vacume leak,plug condition,fuel pressure.

You should always use O2's and Fuel Trims (which are basically interpretad data from O2's) to diagnose drivability issues. They are a window into the engine.

 

[02YukDenXL]

@SLCHOE Funny you say that. I bought a fuel pressure gauge today. I don't trust a word of the three mechanics I've previously brought the truck to. Did I mention the first one told me to ignore it?? (Wtf). Never going back to him.

I'll check fuel pressures and then I'll investigate the O2's. I have my professional engineering test this coming Friday (8 hour exam) so not much messing around with the truck until after that.

I'll keep you all updated and thank you to everyone who has chimed in!!

 

[SLCHOE]

Sounds like a great plan. I'll keep my eyes open for your response. Good luck on the exam. Sounds brutal.

 

[SLCHOE]

edited previous post.