2004 Tahoe Issues Start/Idle/Running

[MRIGGS2345]

Hi there, hoping someone might have some insights into this. Sorry for the long windedness trying to make sure I include all details that may help tie it all together.


For the TL;DR people I have an intermittent rough idle and the car goes into open loop mode sometimes which causes the engine to nearly stall and/or buck. No codes, except this morning I finally got a P0118. With my other issues I'm wondering if ECT sensor makes sense or if I possibly have a PCM or other issue going on.


I have a 2004 Chevy Tahoe LT with the 5.3L Flex Fuel Vin that my wife normally drives. It has about 246,000mi I'm 2nd owner I believe.

Problems started a couple months ago she said it would idle weird sometimes. I figured it just needed a tune up so I replaced the fuel filter, plugs, and plug wires and found a bad wire in the process. That made it better for a while.

Shortly after we started getting this bucking/jerking intermittently while driving. I at the time assumed the transmission was starting to slip, but couldn't verify if it was trans or engine. This slowly got worse to where it was every time the vehicle was driven.

A couple of weeks ago she said it sounded like it was trying to start even after she let go of the key. Then the other day it wouldn't start at all, it would only crank. I made an assumption while I was out and picked up a new ignition switch and installed. No luck. Battery appeared good (not that old, full charge). Fuses and relays all tested good. I checked and have 52psi at the fuel test port which seems to be in spec for my model. At the time I believed I didn't have spark, but I don't have a good tester so that may not be 100% accurate. I changed out the crank position sensor on a recommendation from a family member. That also didn't work.

I was poking around more checking the voltage on the ignition harness and all seemed good there. Checked all my ignition grounds to the braided cable on the firewall which is apparently a known issue. Finally during one of my tests when I cranked it seemed to just barely try and catch. I cranked again while feathering the gas and it fired up idling really rough. The rough idle smoothed out after a few minutes. I was reading online about a crank position relearn. Most used a nice scanner but all I have is a scangauge II tool. I read about one method involving a specific driving procedure so I tried that.

During the entire drive I had no issues with idle, running, surging, or anything else. I thought great it must have been that crank sensor and drove it home. The next day the bucking started again at one point and then stopped again.

The morning after I had a dimly lit security light on the dash (very dim if it wasn't dark outside I wouldn't have seen it) but everything ran ok in the morning. The light went off all by itself, but the bucking started again later.
I found a video about bad PCM connection so I removed and checked/cleaned the connectors with an electronics cleaner yesterday. They looked good, I saw no issues there.

I have finally found on my scangauge the bucking is when the car goes into open loop mode. I can start in open loop mode of course and most of the time it runs ok, but whatever is affecting this seems to screw it up so badly in closed loop it reverts back to open loop sometimes and nearly stalls out. Again this is intermittent, sometimes it drives fine but usually at least 10 times in a 30 minute trip it'll switch.

I finally got a code this morning (P0118) indicating engine coolant temp sensor. My temp gauge and scangauge don't seem to indicate any issues (starts cold and warms up to just below 210-ish after a while).

I'm wondering though if all these issues can really be caused by an ECT sensor or if I have a failing PCM. Or maybe something else?

[maxwedge]

When it reverts to open loop, what are you seeing on the scanner referring to o2 sensor activity? Also what is the scanner showing for coolant temps? I assume you replaced the electrical portion of the ignition switch.

[MRIGGS2345]

Unfortunately my scanner doesn't show o2 sensor specifics, only fuel system loop status.

Whereas normally it would be open loop until the time/rpm, temp, o2 voltages make it closed loop my scanner shows open loop on start then closed as it should but then periodically it shows it switching back to open when I experience the issues. This open status only remains for less than a second to maybe 2 seconds then reverts back to closed loop.

The scanner and dash gauge seem to match for coolant temps usually in the 180-210 range when operating. I haven't hawk-eyed the readings since I only got that code this morning, but I hadn't noticed any weird readings or variation while driving.

Yes, I replaced the actual electrical switch, the key and such above are still OEM. I believe it was an oreilly auto parts standard brand #US331.

Thanks

[maxwedge]

You will need to have access and a thorough understanding of the more detailed info a full scanner will display, misfire counts, o2 sensor inputs, and a lot of other pertinent data.

[MRIGGS2345]

Any recommendations on a decently priced scanner for a shade tree mechanic? I originally bought the scangauge II years ago just mainly for a code reader and to display mpg on an older vehicle. I don't want to spend thousands, but I don't mind spending a little dough on something more useful I can use later on.

[maxwedge]

Look at the Actron series of scanners.

[MRIGGS2345]

For anyone reading this later on, I ended up replacing the engine coolant sensor that afternoon. Since that time I've had no more issues out of the vehicle, which is driven daily.

I ended up getting a better scanner as well, but will be saving it for later.

Thanks

[j cAT]

Originally Posted by MRIGGS2345

For anyone reading this later on, I ended up replacing the engine coolant sensor that afternoon. Since that time I've had no more issues out of the vehicle, which is driven daily.

I ended up getting a better scanner as well, but will be saving it for later.

Thanks

the coolant sensor does have a corroded connection to it . many have this issue. you changed the sensor and the A terminal probably has a corroded contact on the ground side . the PCM connection to the ground on the PCM..P0118 high voltage and the signal will create a erratic signal and throws the A connection on the ground side..
I have a scan tool actron CP9680 I update software on the scantool ALL the vehicles . works good and does have a signal chart to see how the O2 sensors signal and many diagnostic codes.. $135...

[MRIGGS2345]

I may have spoken a little too soon, or possibly a different issue. The surging has been fine now ever since the coolant sensor, but I am starting to notice I sometimes idle a little rougher than I remember when we got the vehicle.

I believe I've attached some excel files of both an idle monitoring and an acceleration up to speed.

I don't have anybody's scan to compare against for outliers but just my initial looking and checking what sticks out to me and I don't know if this is an issue or not:

* My long term fuel trims are -4.7% pretty much the whole time both banks.
* My o2 sensors upstream seem to not fluctuate as normally as other graphs I've seen (but not seen another 5.3L). Where others peak and valley pretty stable mine seem to stay higher or lower for longer at a time.
* My o2 sensors downstream seem to be fluctuating more than one would expect at times. I thought they were supposed to be pretty flat, but sometimes mine mimic the upstream just to a lesser extent.

I'm wondering if I have a cat issue? or maybe my scan time is too long given all the parameters? Or perhaps I just don't know enough yet to understand this?

Will do some more research as well when time permits, but hoping someone who knows these motors well can help me out too.

Thanks

[Stealthee]

When is the last time you cleaned the TB? A dirty DBW TB can cause rough idle.

[MRIGGS2345]

I believe I last cleaned it when I started having issues with the vehicle about a month ago.

I did some more digging today on the way to work and in Mode $06 I have a complete and fail result for 05 58 (Rich to lean sensor switch time (calculated)).

I'm trying to look this up to see which of the o2 sensors this relates to, but so far the only charts I have seen do not include this exact code unless my particular scanner has inverted the ID on the display section and it's actually 05 85 (B1S1)? My live data seems to show both upstream sensors having similar behavior.

Still no actual engine P codes or CEL.