k1500 350 V-8 shudders at idle

[smackdown44]

Hi all,

Truck starts no problem, idles for a couple seconds but begins to shudder at a stop while in gear. Acceleration from stop is fine after above 800 rpm's and all other speeds except around 60 mph when you want to accelerate she shudders a little then too.

I have a 98 K1500 with a 5.7 liter V-8 140K miles; new plugs, air and fuel filters, new serpentine belt, & battery.

I have treated the last 6 tanks of gas with fuel injector cleaner of various brands with no resolution to this problem.

Where might I find these o2 sensors and air mass sensors to clean and or replace?

Does this ignition system even have a distribitor cap? I don't think so.

any help would be appreciated ;-)

[MT-2500]

Originally Posted by smackdown44

Hi all,

Truck starts no problem, idles for a couple seconds but begins to shudder at a stop while in gear. Acceleration from stop is fine after above 800 rpm's and all other speeds except around 60 mph when you want to accelerate she shudders a little then too.

I have a 98 K1500 with a 5.7 liter V-8 140K miles; new plugs, air and fuel filters, new serpentine belt, & battery.

I have treated the last 6 tanks of gas with fuel injector cleaner of various brands with no resolution to this problem.

Where might I find these o2 sensors and air mass sensors to clean and or replace?

Does this ignition system even have a distribitor cap? I don't think so.

any help would be appreciated ;-)

Yes it has a cap and rotor.
Check for cracks or moisture and check camshaft retard.

Go over secandary ign good and fuel pressure.

Good AC Delco plugs ?
Good hot blue spark to all plugs?
Any check engine lights or flashing check engine light?

02 sensors are in exhaust system.
MAF sensor will be in air intake.

[tblake]

If you look at the dist cap, you will see that number 3 spark charge has to make it to the opposite side of the cap and winds around different numbers. I have seen where number 3 archs internally inside the cap to other plug wires causing a missfire much like you describe.

If the cap and rotor have not been replaced, they are probably due.

[gmguy1966]

I had one a little older, a 1987 with a 350, and it did the same thing. It wound up being a stuck EGR valve. Worth checking, but I would think it would be setting a code on the 91 if that was the case.

[jdmccright]

Rough idle in gear can be caused many things, but it seems the most common to me are the following:

1) Oxidized cap and rotor terminals and contact points. If you look at the metal "contacts" inside the cap and the edge of the rotor, you might find them encrusted with oxidation over many miles of use. Scrape them clean and blow out any residue. Also check the center button in the cap for even wear.

2) Cold engine or transmission mounts. The weather is turning colder so the rubber mounts are becoming stiffer, reducing their ability to absorb vibration. Newer mounts might help or switch to polyethylene (though those are inherently stiffer), but not much you can do about this...simple characteristic of rubber.

3) Broken engine or transmission mount. You can usually see this as severely cracked rubber or even complete separation between the two halves (you can use a prybar to lift the engine/tranny away from the frame and see if the rubber is still pliable).

Hope this helps!

[Wheeler88]

My 88' was shuddering and missing mostly at idle stops and was also somewhat noticeable on the highway while accelerating/cruising. Had me stumped for a long time, finally took it in to my friend who is certified GM master mechanic and he fixed it quickly by changing out the garbage Autozone store brand plug wires that I installed with a new set of AC Delcos. Problem solved.