Good evening everyone, heard you guys are good with trouble shooting assistance.
My 99 Jimmy SLT has the dreaded Service Engine Soon light on, And i have a P0171, And a P0174 Both banks are lean.
truck runs great on open loop mode but when using my dads HP Tuners software i can see when the PCM commands closed loop mode it stumbles and the fuel trims creep up slowly until the L1 and L2 bars fill up all the way the bar only goes up to +25.
Then it runs fine but with poor fuel mileage averaging 9 MPG not good for the wallet!
it only stumbles after it goes into closed loop for about 5 seconds until the mixture is adjusted by the computer. But in this stumbling it runs TERRIBLY with hardly any power.

What I do know is
I have checked or changed the following
-Fuel Pump because the sender was not working also the one replaced was not the original
-Fuel Filter was replaced old one had nasty gunk in it
-Catalytic Converter replaced old one was clunking and had reduced flow
-Air Filter changed with routine service
-PCV Checked and Verified working
-MAF Cleaned Verified but have a spare just in case
-No vacuum leaks most hoses replaced intake is tight
-IAC Cleaned and Verified working
-Spark plugs look OK not sooty or crusty
-No misfires or missing
-EGR Cleaned and Verified But doesn't have a gasket?

When i got the truck a year ago it had or still has the following codes
-P0147 has gone away within a month of running this thing although may still be an issue i am unaware of
-P0171 Current
-P0174 Current
-P0463 Fixed
-P0507 Fixed

What I don't know or cant test
-Fuel pressure I myself or my dad do not have a gauge for the fuel rail
-O2 sensor condition i can see that they are switching but may not indicate an issue
-Condition of the injectors
there may be more to this list such as an exhaust leak or something letting oxygen in and the sensors seeing that but i am not too sure

I don't have much money so i am trying to do this cheap but right. I do have a wrecked parts jimmy the engine and transmission are the same and are perfectly intact its a 2001 but most parts are similar, incompatible parts are few and far.

 

I have no personal experience with this but I thought I'd share what I found in my 99 Service Manual in case it might help.

I did notice that in the trouble shooting section that it asked if both P0171 and P0174 are set. One of the possible things to then check in the fuel pump pressure. To do that test is seldom very hard and the tool can often be rented at a local auto parts store.

I hope this helps.

 

Do you have a scanner capable of reading live data? We have cost effective solutions here especially if you have an android device.

George

 

My dad has HP tuners software i can see a lot of things graphs, gauges, sensor voltages, valve positions etc... just not fuel pressure. However I am still kind of learning how to use it, i do somewhat know how to use it and i know what I'm looking at. there may be a another vacuum line for the transmission i found on my parts machine ill put a picture, it may be bad on my daily driver. And driving home the other night the SES light turned off and the next day turned back on and i have noticed when I'm coasting when the light is on the instant MPG only goes up to 65 and when the light is off it goes up to 99. ultimately if needed i can force the computer to do a relearn but that's the last resort nuclear option. and the car runs great in open loop mode when first started. and the EGR does not have a gasket but it shouldn't cause that much of a vacuum leak issue would it

 

so this week the SES light has been off for about 4 days it turned off Sunday and has yet to return now im not expecting it to stay off. now it has been pretty cold here for that time fuel mileage is a little better when i lift off of the petal still stumbles some sometimes worse than others
i will have to plug the computer into it and see what its doing

 

Your lean codes indicate unmetered air after the MAF (vacuum leak, air horn, IAT, etc), insufficient fuel delivery (pump/regulator/injectors) or a bad sensor (MAF, O2, ECT) or engine computer. You need to:

Do the fuel pressure test in the sticky, especially leak down

Correct any intake/vacuum/exhaust leaks including the EGR gasket. IDK if thats enough air to cause all of this but its consistent with your problem.

Get all 4 fuel trim values at idle and 2500

Can you get the MAF value at idle and again at the full throttle 1-2 shift point?

Can you look at sensors in live data so that we can make sure that they all make sense like ECT, MAP, TPS, MAF, IAT?


George

 

Thanks for the reply george.
Well the light just came on yesterday while driving home, it has been off about 4 days or at least since Sunday. Which is a new record since owning this thing, anyways I threw a gasket on the EGR just to see what it would do and it appears that its running somewhat better. However like you said and i was planning on doing it anyways I am going to hook up the computer and go crusin' to look and verify these fuel trims. I can take a video or picture of the laptop while viewing the live data as well as everything else its all one one screen at the same time.
And what i noticed when looking at the instant fuel economy since throwing that gasket on the EGR is when coasting its going all the way up to 99 and is stable not capped out at 65 like before. Although i could have fixed the issue and the light has yet to go away however i find this highly unlikely knowing this car.
In addition to gasketing the EGR up I also checked and verified the PCV is working normal and the hose is in decent condition.
fixed a gasket on the cap that sits on top of the throttle body. intake to throttle body adapter?
Some of the last simple things I can think of right now is swapping o2 sensors and checking for exhaust leaks to see what is going on. Also as i recall i know that a p0147 error was stored when i got it. Which is the after catalyst or sensor 3 Bank 1 o2 sensor heater circuit, which still may be having issues and i just don't know it or is intermittent and the computer doesn't see it either.
I have checked most possible areas where vacuum could be leaking and couldn't find anything or fixed some issues. I have also pinched the hose off right on the engine to no difference as well as the brake booster.
I have NOT checked the EFI system, because it is beyond my understanding and I fear doing so may irreparably damage it and render it unusable I will if i have to but for right now i will not. Or until all other options have been exhausted.
I do have the Haynes book for more difficult tasks but sometimes the pictures they show aren't great.
I will have to acquire a fuel pressure tester to see what my pump is doing or if i have a leak.
One last thing o2 sensors when I changed my cat the tip of them had surface rust. I'm not sure if they are getting "Lazy" but they are definitely original and may need replacing.
I will have to try what you recommended, but for right now this is all i got for an update on my jimmy.

 

Alright fellas!
Hooked up the computer again to may car and i was decently surprised when there was a p0147 error waiting for me like i predicted. Anyways i got some screenshots of the recording for everyone.

the codes i have currently



notice the LTFT1 and LTFT2 fuel trims

i can add more screenshots of my recording if needed but this should do for now

 

thanks for the info. I’ve been under the weather but I’ll try to look at this today.

George

 

OK, a couple of things.

HP tuners implies a computer reprogram. Is this engine computer stock or has it been modified?

Capturing this data while driving vastly complicates our diagnosis, especially furl trims which we need, except for a couple of specific tests. Lets get the main one out of the way now. We need the MAF reading at the full throttle 1-2 shift point.

For everything else we need to be in park. Three sets of data.:

Engine cold, key on no start. We need all the sensor readings and the atmosperic pressure and temp at that time.

Idle, engine warm, the data above plus can you graph the O2 sensors?

Engine warm, 2500 rpm same data as idle


George