Can't tell if transmission, fuel system, or something else

gasoline
transmissions

#1

I recently started having problems with my 98 GMC Sierra c1500. I’ll be driving and all the sudden it will jerk as if it’s shifted from 5th to 1st gear, but the revs don’t go up, they drop to about 600 like it is about to die and then it go back to normal revs. It only does this while I’m on the gas and very rarely at idle. I don’t know if it could be the transmission or something is blocking fuel from the engine. Please help, thanks in advance.


#2

With limited information the first thing I would guess was a clogged fuel filter. Not sure if you truck has a seperate filter or if it is integral to the pump. When were the plugs changed? Any other maintenence or symptoms? How many miles on the truck. Is your check engine light on? If it is most auto parts stores (outside of the Peoples Republic of California) will check them for free. Post the codes here.


#3

I would suspect the fuel system first as the cause of the trouble. Try changing the fuel filter and if that doesn’t help I would move to the fuel pump next.


#4

the truck is at 234000 miles. I have had the truck since about 200000. The only things i have done to the truck are replace the heater core, thermostat, and regular oil changes. I have noticed in outside temps of 90 degrees and above the transmission jerks out of 1st into 2nd but in the colder temperatures it doesn’t. The check engine light pops on and off now that this issue has started. Its throwing 7 of them…Inflator restraint instrument panel module switch circuit malfunction(B1054),Left front speed signal missing(C0226), Left front speed sensor circuit open(C0225),Engine coolant temperature sensor circuit high voltage(P0118),Fuel level sensor A circuit high input(P0436),Engine coolant temperature circuit high intermittent(P1115), and
Transmission component sliiping(P1870).


#5

I think that would be a hickup or a hard miss under load. If its not the fuel, I think you need to look at the ignition system such as plugs, wires, coil or even crank sensor, etc.


#6

I think the only one of those codes which would pertain is the transmission slipping code. You need to determine if it’s really slipping or not before starting down a parts-replacement-diagnostic path. There’s too many parts to replace, you’ll run out of money before you run out of ideas of things to replace. Suggest you get a proper diagnosis on the slipping issue first.

If you get this fast rpm drop for no reason at idle in neutral too … hmmm … well, that’s unlikely to be caused by the transmission … and those are the only codes being thrown ? hmm … … I’d be inclined to hook up a fuel pressure gauge on the fuel rail and see if the fuel pressure is dropping when that happens. Sometimes a shop will configure a fuel pressure gauge taped to the windshield so they can watch it as the truck is being driven too. Don’t try that yourself though, unless you have the necessary experience.