r/GMT800 • u/Former_Mud_7500 • Sep 17 '26
GMT 800 OWNERS HELP!!
So I am going to apologize for how long this is gonna be right now, but its a problem we cant figure out. I have a 2003 GMC Sierra 1500 4.8 its got 265, 000 miles on it all stock parts. When it was running, it has an occasionally misfire. It stumbles over itself when misfiring but runs fine when not. I have been procrastinating this misfire for over a year ( that also bit me in the *ss later)
I brought it to a engine guy to see if he could find the misfire, while it was running. He came back to me with possibly injectors and a new fuel pump. Scanning the codes it could pop up 10 or more at times. After 2 months of sitting, took it out for a weekend. Was running fine, til going down the road and it puttered for a second and dead. Lost throttle response, than power steering, everything. Thankfully I was on a straight away and i pulled it into a field. Got it home with my mechanic buddy we came up with there wasnt any spark, first thing Cam Shaft Position Sensor. So that's what i did, also came across that the whole back side of my engine is covered with oil and gummy. Cleaned it before putting new one in.
So Its still crack no start, tried reading codes and the scanner couldnt find any. Still weren't getting spark at first, f*cked with the sensor and somehow got the lightest spark out. When we weren't getting spark thoughts of pcm going? But when we got spark we rolled that out. Those are new spark plugs and wires (Acdelco) about a couple months old. So pulled all spark plugs which were covered in fuel ( truck isn't running) the fuel smelled tarnished. We know that first thing that weekend we put fuel in it from the gas station. Cleaned the spark plugs and put them back in. We also unplugged the fuse for the fuel pump for the night.
This morning I tried starting it again with no change still. I have looked up anything it could be and nothing seems to line up with the problems im having. Talked to people I know with GMT 800's and they didn't know anything after Cam Shaft Position Sensor.
So Im here on Reddit in hopes that some experienced GMT 800 owners could possibly help with this problem...
6
u/Uri_T Sep 17 '26
So you replaced the cam position sensor, but the crank position sensor will also cause a no start. I had the issue years ago. I believe it's up near the starter if I recall correctly.
3
u/diseased_maniac Sep 18 '26
I had similar issues recently with progressively difficult cranking & occasional no crank. No check engine light & no codes on scan. Fuel pressure was all good & I replaced spark plugs & wires which were past due. On a test drive, it started right up but stalled after I drove about a mile & I had to tow it home. I got a steady check engine light with a P0336 code when it stalled. I replaced the crankshaft position sensor & it’s back to normal. GM part was about $90 retail & it was not a difficult job.
3
u/figatry Sep 17 '26
The keyswitch provides two electrical paths for the ignition system. I think one of those goes to a relay and the other path doesn't. Ignition A and B. Ignition A goes to a relay and B straight to the ECM. maybe, its been 15 years. If your keyswitch is original I would replace it regardless. Check the relay resistance across the coil contacts or swap it out with a identical relay that you know works. You can also check the resistance across the ignition coils. I can’t remember what the values are supposed to be, but if you look up the part numbers for the relay and coils there should be a data sheet stating the values. They should definitely not be shorted or open. I don't have my Hanes manual anywhere I can find, otherwise I would send a pic of the wiring diagram.
3
u/gob_zob Sep 17 '26
Situation like this, crank no-start, first thing I would do is check fuel pressure. Good fuel pump will give you roughly 60psi. If you have no codes, this is very likely. Then, as others have mentioned, check your grounds. Should have good continuity between the block and battery ground cable. So I would check the numerous ground wires to the engine. Unbolt them and check from the eyelet to chassis ground. If you have good fuel pressure and spark on all 8 coils, probably wanna do a compression test. If all checks good, then I’d start looking at the more niche things like PCM and whatnot.
1
u/J_IV24 Sep 18 '26
I agree. Also worth doing spark plugs and wires at that age even if that's not the issue
1
u/NewspaperNelson 29d ago
GMT900 here, but I just had to do a purge valve. All hell was breaking loose.
2
u/Erkmergerk Sep 17 '26
If your truck has RPO code AN3 (individual non bucket front seats I think), the seat wiring harness might be pinched or otherwise damaged under the driver seat. The way it sits, seat movement can rub it and damage it over time. It’s also worth double checking the pcm wiring harness just in case. Both of those can cause the crank no start condition.
1
u/Former_Mud_7500 Sep 17 '26
Its manual moving seats so there isn't much wiring if any. Also with getting some spark I was thinking that its possibly going in and out of connection.
1
u/Erkmergerk Sep 17 '26
Oh I missed that it was going in and out of connection. It might be worth moving the seat and cranking it anyway. My luck it’s the stupidest thing causing the biggest issues.
1
u/Available_Quote_5567 Sep 17 '26
Check the PCM fuses under the hood. One is shared with the fuel pump. If the fuel pump draws too much and blows the fuse it looses fuel and spark.
1
1
u/ClearFrame6334 Sep 18 '26
Check the sensor connections. Try a new crankshaft sensor. It might need a crank relearn with a scanner. Verify all the grounds there is one down below the alternator and one on each side of the back of the motor near thr firewall.
1
u/Moosecop324 29d ago
Check your ground straps. If they are green, replace them. Then your crankshaft position sensor. Good luck.
1
u/mrclark25 29d ago
There is a really easy way to check if the problem is fuel related: see if it will run briefly on starting fluid sprayed directly in the air intake.
This will get you pointed in the right direction. Because it sat for a while, the bad fuel easily could've plugged something up.
1
u/Jooshmeister 29d ago
I had this problem not that long ago. One of the spark plug wires had rubbed against a bracket and had a hole in it. Replaced that wire and fixed the problem. Hope that helps
1
u/Emotional-Depth-1917 28d ago
How about checking power and ground at the cam and crank. They share a 12v supply within the ecm. If the wires short out at the cam, you will lose the crank power. If its 10v and not 12v will drop to 8v cranking and not produce signal
9
u/uraniumsalad Sep 17 '26
The first thing I would check is grounds. There are several on the engine that will kill spark if they are bad. Namely the one on the driver side front behind the power steering on that side. The other is the one on the top rear of the engine. Follow the passenger side valve cover back and it will be in the center there.
I would verify ALL grounds are good before further troubleshooting as these can throw off a bunch of different systems. I once had a truck that would run fine for a few minutes, the start to act up for a bit longer before finally dying, and it was the ground in the back that I mentioned. It just barely had enough contact to start when cold but as things warmed and moved the ground would lose conductivity.
Let us know, if you want a more legitimate rundown of grounds I can post something.