Hello, I love my GMC 2500 4x4 but have been having issues over the last 2 months. It first started missing (engine light flashing) when I hit a bump in the road. Five times the trouble started when I passed a bridge on paved roads. **Gotta be a ground..!! That eventually turned into a hard start much like a loose battery cable. After much reading on the internet and 4 visits to mechanic shops who applied 3 different scanners, the truck finally died on the road today.

It won't start and more or less just weakly turns the starter very slowly and sporadically. Prior fixes included: Plugs and wires changed, fixed a vacuum leak, checked for intake leaks (none), cleaned the MAF sensor and scanned the O2 sensors (one is weak). Tightened and cleaned all battery cables and the ground was checked. Battery is strong and battery ends are good. Did the tug test on throttle body sensor and pedal sensor. Does not indicate symptoms of the fuel filter or the cat. converter. Scanners gave the P0300 (Random misfire) each time. Not all on one bank. Mileage is 220,000 and fuel pump was changed probably 80,000 miles ago.

More reading leads me to suspect the computer ground itself (under the battery). I understand there are more body grounds on the rear of the engine. It may be the computer itself. Any ideas appreciated...and thank you.

I suspect the misfiring and weak starting are unrelated.

Have you checked the battery voltage while off and while running?
Hard to do when she won't start, but did you prior?
Should be 12-13 volts off and 14-15 running.
That'd rule out battery and/or alt, which leaves starter.

Concerning the misfire, maybe a plug is bad?
Wire not connected firmly?
No doubt you've checked that already.
Bad coil on that cylinder maybe?

Hope you get her figured out bud.

Yes I did check the battery and it'* almost new as is the starter. New plugs and new wires all tight and working. It cranked and run like a champ when that light wasn't on. When the check eng. light was on, it had trouble starting when leads me to think that the missfire and the starting problems ARE related. A mechanic checked the coils and like I said, when the light wasn't on it ran perfectly with lots of power. Those *bumps in the road led me to suspicion a ground somewhere. Actually and oddly enough, it would react to a certain bridge on the highway and did so 4 times and 1 time over another bridge. I was beginning to think there was UFO activity in the area....haha. Also many times when I turned a corner. Sometimes when missing, I could stop, turn off the engine and restart and it would be okay. Other times, just the opposite (would miss upon restart).

Now this is really getting interesting and I'm anxious to know what it is. I'm going to check the computer ground tomorrow and do more research yet tonight.

Thanks for your reply Huckleberry....I appreciate that.

Have you checked fuel pressure? It could be a faulty fuel pump or fuel pump relay. Or even a bad ground at the fuel tank harness. It also could be a bad crank sensor or sensor wiring. Since it won't start now I would get a scanner on it and make sure it'* communication and if it is look to see if it shows any rpm while it'* cranking. Remember low fuel pressure can cause random missfires causing your code.

i would agree with both the fuel and crank pos sensor or wiring, but i dont know how the fuel would cause slow starting where the crank or crank circuit would change your timing and cause slow starting/cranking

Thanks guys, I appreciate your input and am not discounting any possibilities at this time. I will let you know when we find the problem. We all learn that way and this may be simple but it'* sure got alot of us baffled at this time. I even voiced the problem to the geniuses at the local coffee shop and they're stumped. Now THAT'* SERIOUS !!! Hah

lol. let us know what you find

UPDATE:

I had to take our Dodge, switch the trailer and load the GMC gravitorically. That'* a word I coined to which basically means I pulled the GMC to the top of a hill, parked the Dodge and trailer below with ramps in place, wife on the Dodge brakes and I rode the GMC onto the trailer. Beat the heck out of winching it on by hand with comealongs.

It'* now in the shop and the young (39) mechanic is pulling his hair out. I think he wishes I'd taken it elsewhere. Maybe one important note I missed telling is that just about 30 minutes before it died, at a speed of about 25 while slowing for a corner, I heard a noise that sounded like a cornstalk in the fan. It lasted only 2 seconds and then the pickup ran worse after that. NOW I'm thinking....timing chain??? (That could account for a relation between the hard starting and the missing issue).

We navigated around town yesterday and the wife had to keep her foot on the gas pedal when I was outside or it would have died. We tried to head for home but only got 10 miles and when it died there was no noise....just like someone turned the key off. The engine was not overheating but I thought the oil pressure gauge was dropping. This morning I tried to start it and the engine cranked normally but wouldn't start.

More tomorrow if there'* something to report.

A simple compression test or even a look under the valve cover to see if the rocker arms are moving will determine if you have internal engine damage.

1) The mechanic checked the rocker arms...they're okay. That should rule out the timing chain. He said he expected the scanner to show that the computer is dead; but it wasn't.

2) He said the Autotheft light was on and his scanner shut that off which enabled him to get it started. (It had been flashing periodically over the last 3 weeks)

3) Although it started, it still runs like C*@p (missing on alternate cylinders)

4) It gives the MAF sensor code

5) Day 2...still pulling his hair and beginning to use bad words.