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Dodgerench
Dodgerench, ASE Certified Technician
Category: Dodge
Satisfied Customers: 3385
Experience:  30+ years Dodge/Chrysler exp., ASE Master with L1 certification. Driveability/ combustion specialist
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I have a 1998 dodge 1500 pickup with a 3.9 liter v-6 gas

Customer Question

i have a 1998 dodge 1500 pickup with a 3.9 liter v-6 gas engine that has a very rough idle i initially ran the codes and it had a misfire on cylinder number 2. i pulled the spark plug on number 2 cylinder and found that the bent steel end above the electrode was burnt in half. i changed all spark plugs, distributor cap and rotor, all spark plug wires and the injector to number 2 cylinder only also the idle air control valve and it still has a very rough idle and it straightens out at higher rpm. i also changed the speed sensor at the transmission as a code was also detected on this and causing the check engine light to come on. i also cleaned the throttle body of all carbon build up. this motor only has 35000 miles on it and has been garage kept for a long period of time. idrained all of the fuel and put in good fuel.can you tell me what to do from here?
Submitted: 1 year ago.
Category: Dodge
Expert:  Dodgerench replied 1 year ago.

Hi, welcome to JustAnswer!. This is Ed.

Oooof.... and I mean oooof. You have engine damage.

For the spark plug you found to be physically deformed and have the horrible low-speed driveability you have, you almost have to have a valve hanging open. It could be a broken valve spring, a dropped valve seat or some sort of outside debris that made its way into the combustion chamber of AT LEAST that one particular hole. It's not possible to say just what happened yet, but I can't imagine a broken spring causing both the loss of driveability and the broken spark plug.

You have debris circulating.

That's going to bring us back to a dropped valve seat or something introduced to the engine from above. If there has been nothing done to the engine within thousands of miles before this event, I'd have to place odds on the dropped seat theory.

Your 3.9 uses aluminum cylinder heads, which requires the use of pressed-in hardened valve seats. Overheating of the engine is a major contributor to a dropped seat because it expands the aluminum and loosens its grip on the seat, but sometimes stuff just happens. When a seat drops out, it's immediately munched up by valve action and piston interference, which would account for your destroyed spark plug. Check your compression on at least cylinder #2 and if bad, run a test on the remainder of the engine.

Chances are good that you'll be pulling that cylinder head at the least.

Ed

Customer: replied 1 year ago.
The nber two cylinder has one hundred and ten pounds of pressure
Customer: replied 1 year ago.
i pulled a compression teston cylinder number 2 it was 110 lbs.
Expert:  Dodgerench replied 1 year ago.

I'd say that's low, but compression testers vary. What's often most important is how they compare to other cylinders using the same techniques. Did you try another cylinder?

Expert:  Dodgerench replied 1 year ago.

The burned off electrode you mentioned may be explained by detonation or simply extreme wear on the spark plug.

Expert:  Dodgerench replied 1 year ago.

Have you checked other cylinders for similar condition of the spark plugs?

Customer: replied 1 year ago.
I replaced all no damage to the others. What baffles me is it runs good at higher rpm. But it is using a immense amount of fuel.
Expert:  Dodgerench replied 1 year ago.

I'd compression test other cylinders using the same exact methods as you used for #2 before making any decisions at this point. 110 psi isn't a dead cylinder but is far from what I'd consider healthy, given typical test techniques. Check a few more and let me know how they compare to the #2 hole and we'll go from there. Cylinders that are basically hurting may show no outward signs of trouble at higher loads and RPMs but be quite rough at idle. Let's get more info.