Hi Scotty
Have a 2003 5.3L flex fuel Suburban automatic, 200k miles with a weird problem. Please bare with me if I miss pronounce the correct terminology or talk gibberish as I know NOTHING about engines and all information I'm about to give I've learnt within the last two weeks. Mostly watching your channel. No triggers on engine codes. Three weeks ago the engine would just shut off on the highway. The good old Chev problem. Wait a minute or two and it will start up again only to repeat die again after couple of miles. It would happen randomly and 99% of the time when engine is heated up. I fixed/replaced the following:
New MAF, new air filter, new fuel filter, added another 2guage ground cable to the firewall and engine, 4gauge to frame in front. Also removed, cleaned and checked all main ground cables, new oil pressure sensor (was faulty anyway). Since I've changed the oil pressure sensor the vehicle won't just stall randomly anymore but has no power at high engine load. Fuel pressure are within specs according to the flex fuel engine guidelines. 50psi on prime which drops to 47psi and holds, 42psi once engine started, jumps back to 50psi once I remove vacuum from fuel pressure regulator. One thing I've noticed when giving a little gas pressure would jump to 50psi but when you floor it presure would drop into high 30s.
Info that might be helpful. The o2 readings on my scanner are as follows:
At idle or low engine load (30% and below) driving the o2 STFT bank1&2 readings would fluctuate throughout the whole range of 0.1v - 0.9v AND 0% - 3%. o2 LTFT bank1&2 are stable and would fluctuate between 0.65v - 0.75v AND 0% - 2%. All tests were done in closed loop.
As soon you step on the gas the engine would sound like it's starving from fuel. Depending on how aggressive I am with the throttle a massive pop sound would come from the front as well with the following o2 readings:
o2 STFT bank1&2 would both jump into the 50% with 0.9v reading. Both o2 LTFT would jump into the 20% with a 0.85v reading. (Also noticed the timing advance would dip to -10). No engine code will be triggered during this process. At most engine would just stall.
It seems to happen only when engine is under load/driving and heated uo. If the vehicle is stationary you can rev the engine to as high as it goes. No stuttering or misfire. Vehicle idles perfect.
Again apologies for my lack of knowledge, terminology or if info provided are all over the place and not related. I thought now is a good as any time to start learning about vehicles since I have a bit of free time on my hands. Your advice would be appreciated. Thanks
UPDATE 2 - IT WORKS:
Replaced the crankshaft sensor which was done within 15min surprisingly. Turns out not to be as hard if you do a bit of preparation. However it did not fix my problem.
I turned my attention back to the camshaft sensor since my temporarily insolation tape solution method made a difference to the problem. With the open wires there was no doubt it shorted. I reckoned the sensor probably took a hit and got damaged. I replaced the sensor with a new one and now EVERYTHING works again. I've added pictures of how I fixed the wiring harness. There is literally no space to work and rather went with the heat shrink option to be on the safe side. I also found the cables to be damaged higher up as well. Looks like pure engine heat was the culprit as the plastic around the wire is brittle out of wear and tear. So I do apologise for being quick to judge and blame someone.
Thanks for the help guys 👍. Appreciated. AND Scotty keep the videos coming. I've learnt a lot from you. Specially the maintenance side of one's vehicle.
Pictures:
https://drive.google.com/folderview?id=1A-wir7kSNLQCop0l5gPTrSJokfjEK4AC
nice work. I would also zip tie the harness to something solid, so that vibration doesn't make stresspoint and break the wires.
Definitely, while fiddling around I've noticed the spiral wrap that's around the cable is all brittle and crumbles to the touch. So going to replace all of it. I found a new hobby 😀
hahah yah. Don't get too wrapped up in that.
Thanks for the link. Yes I have grounded that specific cable with a 12gauge. Apparently the ground cables on these models go bad after a while. So jumped the gun and did an entire ground system upgrade while at it.
You fuel pressure is too low. I'm fairly sure it should be priming to 55 - 62 PSI, and not dropping into the 30s.
Thank you for your reply. One can't seem to find any specs of what the exact psi should be for these models. The closest I've come was the following:
models with
- VIN: V, T & U should be 55-62 psi
- VIN: Z should be 48-54
The V, T, U, Z referred too is the 8th letter in the VIN. I have a Z which is also referred too as the flex fuel version. I've watched a couple of videos of fuel pump replacements and people were getting the same psi readings and all seems to work.
You could be right. I thought VIN 'Z' was TBI.
With the little knowledge I've gained over the last couple of weeks I've also come to the conclusion it's the fuel pump. After a ton of video's I've realised it's not a easy task. Specially for someone like me who has never done it before and do not have the proper tools. I have also noticed a lot of people have changed their fuel pumps and turned out not to fix the problem. So trying to troubleshoot other possibilities first and hopefully fix the problem. I'll cross that bridge when it's absolutely the last resort.
You are right. VIN Z is flex fuel
https://en.wikipedia.org/wiki/General_Motors_LS-based_small-block_engine#3.78_in._bore_blocks_(1999%E2%80%932007)
I take back what I said.
48-54 PSI is the correct pressure.
But it's still suspicious that it's dropping to 30PSI under throttle.
I would also check the cat. con.
Catalytic converter is also a possibility. I was on a road trip and filled up with E85 since it was the only fuel available in the area (luckily I have the flex fuel but was not aware of it at the time). I am new in America and still learning your fuel grades. My problem started halfway through the E85 fillup. Could have been the cause or just coincidence. But that brings up the cat subject. Could it have dirtied the cats? or worse be the last straw of blocking it completely. I have not done a temperature test yet but both o2 sensor 2 seems to indicate it is not the case and cats seems ok. Atleast at idle or 30% throttle and less driving around. The o2 sensors 2 read between LT 0.75v & 0.85v / 0% -3% if it moves at all. Only when driving and you floor it does the values go haywire. The system then runs extremely lean at 50%+ and jumps between 0.1v -0.9v. Basically identical to sensor 1 values
it's a flex fuel engine, so it's made for E85.
If you remove the upstream O2 sensor and the engine runs better, then you know there's a restriction.
Another test you can try is to spray starting fluid directly into the intake while gunning the throttle
Thanks will disconnect them and give it a try quick. I have tested with all o2 unplugged but not the upstream only. Will give feedback shortly
The second option is a bit difficult as the problem I experience only happens while driving. Not while the vehicle is stationary. The engine runs like a dream & purrs like a kitten when it idle's or reving while the in stationery position.
Just tested with the upstream o2's disconnected. Problem still occurs. Does this confirm cats are still ok? Also it's worth noting the problem only happens when engine reaches temperature 91c aka 194f and while driving. As soon as one gives the engine a bit of load by stepping on the gas ( around 35% and above engine load) it loses all power and sounds like fuel starvation depending on how hard you floor it.
no. not unplug. I mean remove the O2 sensor completely so there's a hole for the exhaust to escape.
Apologize for that. Miss understood your response. Great, makes sense. Will give it a try in the morning and let you know.
I have put this test on the back burner for today since I've found something else that requires attention (see update). Will get back to this test once the other task is completed
QUICK UPDATE WITH INTERESTING FIND:
After reading and gathering a ton of information over the course of the last couple of weeks and doing too many tests to mention here to illuminate possible causes two "possible problems" were stuck in the back of my mind.
First one being the fuel pump. Done a lot of tests and it tested within specs of the model vehicle I have. Although I could only test the fuel pressure while vehicle was in stationery position and the vehicle only showed problems while driving and engine under load. Due to lack of proper equipment I could not test the presure while driving. Hence it was something I could not completely illuminate.
The second thing that kept coming up was the crankshaft sensor. There are ways to test it but with where the sensor is located it is unreachable and extremely difficult to test. One has to remove the starter and by the videos I've watched it is not an easy task. My plan was to leave this part till the end like the fuel pump as getting to this sensor seems like a major task on it's own and I'll rather replace the unit at once than doing tests.
- BUT on my journey of investigating the crankshaft sensor I kept on reading crankshaft and camshaft. Even when looking for crankshaft parts I noticed two different looking units and did not pay attention to the different spellings. The words look extremely similar and for a non mechanic and untrained eye overlooked it. I initially thought it was for different engine versions hence the difference look/design. I woke up this morning started investigating the difference. Turns out it is two seperate units and I watched a couple of videos. The camshaft unit although one can't see anything and have to remove the top to get to was reachable and worth investigating before I go through all the trouble of replacing the crankshaft sensor. Refer to picture on my findings. It will speak for itself. My major problem when flooring the gas peddle while driving is solved after fixing the connector. The engine still does not have the power it use to have but I feel I'm on the right path. I went out and bought a new crankshaft sensor (Duralast brand since it's all they had) and will be replacing the unit later this afternoon when the engine has cooled down. Will keep you updated
PICTURE:
https://drive.google.com/folderview?id=1A-wir7kSNLQCop0l5gPTrSJokfjEK4AC
it looks like the wire has been tugged on, breaking the insulation. And recently, judging from the clean copper. But the conductors are still attached.
True. Looks like someone did a bad back probing job. Be it on purpose or with a blunt/wrong tool. I recently bought the vehicle. Like I've mentioned. It is difficult to get too. I squeezed my hand into the back to unplug the connection. Had to work by touch only. Maybe the person was in a bad mood or frustrated that day. Non the less it made a difference. I'll splice in new cables once I've installed the new crank sensor
I would just put gob of dielectric grease on it, wrap it in tape, and call it done. By splicing, you're just reintroducing a new failure point some where else.
Thanks for your help. I've updated the main thread and also included pictures of my fix. Since the main problem is out of the way I can get back to testing those cats.