shaking at idle in reverse and forward. 1987 XJ Wagoneer

[Ecomike]

Shaking at idle only when in gear, in reverse and forward.
Does not shake in Park or Neutral.
1987 XJ Wagoneer 293K miles.

Need some ideas on what would cause it, only when in gear.

Runs like a new engine, in Park or neutral, and I can run it from idle to 4000 to 5000 rpm, and gun it, and hold it, and it runs like a new engine.
It was running, driving just fine for weeks, Once I replaced the fuel pump.
(It had been parked for year and when I got it back to running, the idler pulley smoked a Mud Dobbers nest they built on the pulley/bolt area, LOL.
But I drove it for weeks after that with out any issues.

I am seeing an RMP drift, down and back up in Park when I hold the gas peddle at say 3000 rpm. Doubt that is related.

Everything else works.

 

[RCP Phx]

Bad motor/transmission mounts would be the first suspects.

 

[Ecomike]

Bad motor/transmission mounts would be the first suspects.

The fact I can gun it in Idle from 700 rpm to 5000 rpm, back and forth with zero shaking, suggests it is not the engine mounts or timing, or fuel, or miss fires, etc and I replaced the trans mount when I replaced the transmission. Not enough miles or stress for a highway / city daily driver to be the Trans mount, IMO on this one.

Engaging the transmission at idle in Reverse or drive, is what triggers the violent shakes at idle.

It smooths out if I drive it. I always to seem to have problems no one ever had before LOL, caused by something no one ever had before.

It is almost like shifting into gear, foot on the brakes idle, is causing miss fire / shake, but with no obvious miss fire sounds. Might be time for Snap on scanner......

I spent years working on solving an overheating issue even the Muffler shops missed, It was leaking only after a long highway drive doughnut in the exhaust manifold pipe to exhaust pipe flange, where the doughnut was reused, with a scar on it by prior owners, It blew exhaust on the oil pan only after a long high way drive, and pulling off the free way to a stop light, would spike the oil and coolant temps on 100 F ambient temp days, with the AC on.

Keep the ideas coming please.

Thanks

 

[Ecomike]

Bad motor/transmission mounts would be the first suspects.

Could the torque converter being trying lock for a splint second? on/off? That would cause a stall I think?

 

[Ecomike]

The lack of any miss at idle, apx 750 rpm, in park and neutral is enough to suggest it is not the torque converter, or transmission.
The idle drops in drive, reverse or forward to about 500 rpm, and causes the shake.

Still looking for more ideas. Idle in drive, IIRC, should 700-750 rpm, not 500 rpm, so in drive or reverse the idle is going too low.

Idle air control valve fouled?

 

[Ecomike]

Bad motor/transmission mounts would be the first suspects.

Its more like a miss /shake. Need more ideas folks

 

[Bent]

I just put new injectors in the MJ, 1987. I also replaced the boots and the plugs while I was at it. The two plugs nearest the front of the engine where the rain water blows through the radiator were corroded to the point of my being amazed they were even firing. You might want to take a look at those. In my mind, it did seem to smooth things out a little bit. It was super easy, and inexpensive to do. And they were not pigtails.

Just seeing what sticks to the wall. Good luck!

 

[Ecomike]

I just put new injectors in the MJ, 1987. I also replaced the boots and the plugs while I was at it. The two plugs nearest the front of the engine where the rain water blows through the radiator were corroded to the point of my being amazed they were even firing. You might want to take a look at those. In my mind, it did seem to smooth things out a little bit. It was super easy, and inexpensive to do. And they were not pigtails.

Just seeing what sticks to the wall. Good luck!

Yesterday I wiggled the fuel injector electrical connections, and several (2-3) when wiggled changed the engine speed, and shake at idle. I recall buying new connectors years ago and planning to replace the OEM connectors, but never got around to it. It may be at least part of the problem.

 

[Ecomike]

Bad motor/transmission mounts would be the first suspects.

Bad engine mounts are easy to see with the hood up, and parking brake and brakes on as you give a little gas to it in Reverse or forward. At idle in gear, they would cause a miss, that causes a shake, IMO. Also in Park, hood up, pressing the gas should make the engine move too much with bad engine mounts IMO.

I had a ford Temp with routine engine mounts going bad, but it never caused a low idle in gear shake.

Mine is dropping to 500 rpm (should be 750 rpm) and shaking only in gear at idle. In Park or neutral it is 750 rpm with little shaking. In park from, 900 rpm to 4500 rpm, it purs and roars like a new engine.

 

[Bent]

...at least part of the problem.

In our quest for a silver bullet.