98 gmc k1500 5.7 manual trans will start missing during driving when the shift light comes on? Why?

10

Asked by May 26, 2014 at 05:31 PM about the 1998 GMC Sierra 1500 K1500 SL 4WD Extended Cab LB

Question type: Maintenance & Repair

I have changes the plugs, wires, cap
and rotor button, also changed tp
sensor. The truck does not miss
when just setting and idling or even
Revving it up, only during driving,
notice it will start right around 2000
rpms and if you barely keep easing
into the throttle it will miss the whole
time but when it's missing if you floor
it will straighten up and go on with
out missing. Does this ever gear. You
can unplug tps sensor and it will not
miss at all, that's why I changed the
tps sensor, but I found out when I
unplug it that it's throwing the
computer in loop mode that why it
don't miss when It's unplugged. It
runs fine with it unplugged. Also
during missing it will not throw check
engine light on. And if it does the
only thing check engine shows is  
random misfire in cylinders. That's
the only code. I've also check fuel psi
which is at 58psi during idle and
shoots to 60 and 61psi when giving it     
Throttle. Any ideals on what this
could be? Thanks

13 Answers

56,635

Have you tested the ignition coil and control module? Sometimes they break down under load. Also, check your spark plug gap. If it's too wide it'll misfire under load. One last thing that comes to mind is if you have what is known as a "spider" fuel injector assembly they are very prone to failure. This will also throw a random/ multi cylinder misfire (P0300) trouble code. HTH. -Jim

1 out of 1 people think this is helpful.
10

Sorry forgot to mention that, I've already put new ignition control module and coil on it. The plug gap is .60. Which I did when I put the new plugs in. And yes mine does have the spider injection but if it was it then it would miss during i idle or during acceleration when setting still. Not just during driving around 2000 rpms, and also it would still miss even when I unplug the TPS sensor...

1 out of 1 people think this is helpful.
56,635

Hmmmmm, good points. At idle you need less fuel. Same is true when you're just revving it. But under load you need more fuel. The injectors spray more volume into the cylinders. Same is true when in open loop operation and when flooring it. So have you checked for a vacuum leak? Hook up a vacuum gauge to any port on the intake manifold. At sea level it should hold around 21 inches steady. If it won't there's a leak. Perhaps too small to effect idle but just enough to upset the apple cart under load. Try running a bottle of fuel injector cleaner through it at your next fill up. Perhaps the fuel injectors are restricted. HTH. -Jim

10

Ok thanks I have already tried fuel injector cleaner, the best I can find. Also I have checked for vacuum leaks and can't find none, but I haven't hooked it up to a vacuum test gauge, cause I don't have one. I really don't want to get into the spider injection system unless I know 100% for sure that's the problem. I know the injection system cost around $275. But I don't want to pay that and do the change and it still has the same problem. I'm gonna try to get a vacuum test gauge and go from there. Would it be possible that it could be the crank sensor or can sensor? Does anyone know if the shift light runs off of the vehicle speed sensor? Would that be a possibility ? Thanks you all for any advice.

10

Cam sensor*** not can.... Lol that's what happens when you have auto correct...

What plugs did you use.? If you used bosch, champion bad move try autolite or a/c delco.

10

Used ac delco. The only plugs that I ever use in gm vehicles.. I would never use Bosch or champion in anything.

10

Would a o2 sensor cause me to have the problem I'm having? But if it were a o2 then it would at least throw a check engine light code, which it don't. And the only code that it does throw is random misfire in cylinders. Don't know if I've mention it, but it only seems to miss when my shift light comes on at 2000rpms.

400

I fixed this exact problem with my '98 5.7 liter by replacing the fuel filter! If you want to know why, read on. I feel bad for those who've just thrown money at this thing without analyzing anything! The prize goes to the guy who replaced his whole engine with a crate motor only to have the same misfire! First I want to be clear on the symptom because I've seen hundreds of posts about misfire which can be interpreted as many things. It started with random cylinder misfire at light throttle. Felt exactly like an ignition issue. Random miss, occasional light bucking/backfire. No codes yet. A couple days passed and then it got worse/more frequent. Code was set for pre-cat O2 saying low voltage (or lean) I do not throw parts at things, so I wasn't about to replace the cap, rotors, wires, etc until I confirmed the WHY. Besides, I can assure you that 8 wires did not randomly fail all in the same day! Also consider the OEM stuff is pretty durable and often is much better than the aftermarket junk. Internet tells me to replace the fuel spider but my engine idles fine and will throttle up no-load. Again, a bad spider shouldn't do that! Nor would a bad crank sensor, stuck valve, bad cap, loose distributor, or the hundred other things I've seen posted! Ran fuel pressure leakdown test - held pressure so fuel regulator probably ok. Have 48-52 psi -a little low but within spec. I then thought if the fuel filter is plugged, it may prevent injectors from opening because this design uses a check ball in addition to a solenoid. They need a minimum of ? to crack open. Apparently mine were just on the "edge" and randomly did not open (especially during load) Turns out to be the case. New filter and problem solved!

3 out of 3 people think this is helpful.

I have the same problem can't figure it out. I've got new plugs,wires,cap,rotor,ignition coil,and I've also ran injector cleaner through it. HELP?

400

Do you have random or steady misfire? Under load or no-load? Did you run a fuel pressure check or replace the fuel filter? If not, run a pressure test at idle, revving up, and driving (if you can) Fluctuations beyond spec can indicate other things going on. Again, mine issue was that the injectors didn't open every stroke because the fuel delivery (and pressure) was just on the ragged edge. Under load it would misfire and went from once in awhile to every time I throttled.

It's pretty steady unlaod. And hardly misfires just idling or revving. Just replaced the fuel filter still same problem.

400

What is the fuel pressure? At idle and revving?

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