ignition problems; misfire when hot

[87glhs]

87 GLH-S Charger; 2.2L T2.

Runs out absolutely fine when the engine is cold.

Once it heats up, it misfires under acceleration until the RPM's hit about 3000, then it smooths out. When it misfires I notice the tach starts jumping all over, and the misfiring seems to directly coincide with these sudden jumps.

I've swapped plugs, plug wires, coil, and removed the small cap (at the coil).

I'm not seeing any codes--reads "55" whenever I check it.

Problem is definitely heat-related, and ignition related (due to sudden jumps I'm seeing on the tach).

Anyone see this type of problem before?

[cordes]

If the tach is jumping around while it's doing that I would swap the HEP with a known good one and give it a test drive.

[chilort]

Any kind of modifications we should know about?

[turbovanman²]

Agreed, if the tach is jumping, try or buy another HEP. If you buy a new one, aftermarket are junk, buy a Mopar one.

[87glhs]

No, no mods. Everything is stock.

HEP--talking about the inductive pickup donut under the rotor in the distributer? I believe I have a spare from a donor.

I'll give it a try and let you know.

[cordes]

No, no mods. Everything is stock.

HEP--talking about the inductive pickup donut under the rotor in the distributer? I believe I have a spare from a donor.

I'll give it a try and let you know.

Yes, the black plate deal under the cap.

[87glhs]

Replaced HEP module with a new one, made no difference.

Checked grounding on engine--used a long, heavy gauge jumper and connected each of the various straps/points (fender, firewall, etc.) to the negative terminal of the battery, saw no change.

Definitely heat related, does it consistently now. Starts up fine cold, begins acting up after 15min or so. Tach jumps around at idle, can hear random misses. Problem more pronounced (bucking) under load up until about 3000rpm, then seems to smooth out.

?

[black86glhs]

Time to look at the vacuum harness for leaks. Also, check the map and TPS.

[turbovanman²]

If you've checked all your grounds etc, then it could be the LM took a dump, or even the PM.

[black86glhs]

If you've checked all your grounds etc, then it could be the LM took a dump, or even the PM.

This is true. I had 2 bad ones before I got my new one.

[87glhs]

That's next on the list--I have one spare for each. Will let you know--

[87glhs]

Problem found.

Ran across www.thedodgegarage.com recently and noticed their page on common 2.2L problems.

Turned out to be a loose encoder disc in the distributor. When the engine was cold, it worked marginally. Once it warmed up, the disc could be easily moved around. Swapped in a spare distributor and problem solved.

Thanks for everyone's input.