Home Forums Hunting & Shooting Hunter's Campfire Low RPM miss, 93 GMC Sierra

My truck[93, 5.7] has developed a terrible miss at low RPM's that is only happening at low speeds and under about 1400 RPM's. No miss whatsoever under load[pulling trailer etc] or at higher speed/RPM. Riding out our driveway at low speeds if I touch the throttle lightly it will just about give you whiplash and the RPM needle jumps from 0-1,000 like a second hand on a clock.
I checked under the hood and noticed a worn plug wire and thought that's it. Changed them all and it's still there. Both plugs and wires were changed not too many miles back[but several years].
I plan to change plugs and rotor cap tonight.

Any other thoughts would be appreciated.

Just remember that the boxes that the plugs come in just say spark plug,they don't guarantee that they will work.

You are doing what i would do,only other thing is maybe some crap in the fuel system.

Vacuum leak?

Why do they do that?

Blonde brought a car into the dealership and complained it was running rough. Service manager said he would check it out. Brought it back a few minutes later and said it was fixed. Blonde asked what was wrong. Service manager said just crap in the fuel system. Blonde said great, how often do I have to do that?

Mine the rotor came loose and was hitting inside cap.

I replaced the fuel tank and pump about 2 years ago[doubt they changed the filter/screen though]. That would be next.

Will see if rotor is bad tonight.

Honestly, I know just enough to be dangerous auto mechanically speaking and have spent my budget on tires[for two vehicles], new side/box panels for this truck on the extended cab portion along with new shocks[and shock mounts] and a new PS hose this year.
I'sa broke....and just wanna hunt some deer.

Have you had it scanned?

Service engine light isn't on, so I guessed it wouldn't matter?

Does it have the Vortec injection system? (not sure when that came out) My 96 GMC has the same symptoms as yours and apparently the first generation Vortecs are known for it...

The later model injectors are a bolt in and run around $300 for the whole system. I'll be going that route soon.

Electric fuel pump in the tank & injectors? If so make sure the fuel pump is producing enough pressure. I had fits with that on my '98 C1500.

I believe the '93 models had the throttle body injection system. Mine did on the '92 I had. My bet is the electrical side. Cap, rotor, plugs and wires should iron this out. The throttle body looks like a carb, but has 4 injectors. Could be one of the injectors is bad, but seems as though the engine would be running worse than it is. It could also be, one of the injectors only fires when more pressure is added.

What the OP can do is while the engine is idling, pull a plug wire at a time to find the cylinder that is misfiring. Pretty easy to do, and you can find that specific cylinder that is missing. If the engine misses more, move on to the next plug. If no change, you have at least found which cylinder is not firing. Also, it may seem as though the engine is running on all 8 at higher rpm, but it is difficult to tell sometimes.

Clyde

It is pre Vortec and old throttle body.
I actually loaded my Yamaha Rhino on trailer and pulled it a couple of miles[interstate exit to exit] when this first started and it did fine, no miss whatsoever at higher RPM.
I didn't change plugs initially because they are relatively low miles, but it sits a lot[especially in summer] but I did notice some corrosion on the tip of one of the plugs.
I guess sitting does no good.

Change wires, rotor and cap. Plugs too.

Run some sea foam through it too.

I would look at the throttle position sensor. Had that happen on a 92 TBI. Easily replaced. Would also look at the idle air mixture valve. They also had an electronic EGR valve that sometimes gets a little carbon chunk keeping from closing all the way. That one definitely caused low RPM suck on my 94 GMC.

Should be a TBI motor. Have the various sensors - throttle position, engine vacuum, idle air control - measured for correct function & readings. Your ignition works fine under load, so it is something else.

that has happened several times over the years and has always triggered service engine light[not on now].

I think something like this is more likely than an ignition problem. Bad wires, plugs, cap, etc generally cause more problems when under higher load because it takes more voltage to make the spark then. Like BLG said, it is a throttle body injection system so low pressure and no pressure test port. It would also be OBD I so the computer is not very sophisticated. Not many error codes. With a good scanner though, you should be able to look at sensor values like the throttle position sensor.

Jerry

Check engine timing.

Interesting a friend has same basic issues but idles just fine. At low RPMS is fine, its up to highway speed that things go bad....

Replaced fuel filter wondering if clogged partially when fuel demand was up.

IIRC now its ignition module and coil. OBD1 says everything is fine.... LOL.

I"ll be keeping up with this thread for sure.

Throttle sensor and even something like cam sensors can do some strange things. Crank sensor supposedly is bad on a 4 wheeler here, at least its all I could test out that indicated bad... it acted up when it got up and warm, down low was fine...