Engine shake at idle

Hi. I have a 90 5.0., and I have an idle problem I cannot resolve...

The car is an automatic. It has about 75,000 miles on it. The engine is all stock except for headers, and I removed the smog pump. The car runs strong, burns no oil and displays no peculiarities of operation except for the idle issue, which is as follows:

The engine has a slight jerk or judder that is immediately noticeable at idle. You can really feel it when you put your hand on the intake, and you can feel it in the steering wheel at idle. If you listened to the car it would sound fine and idles steady at 750 rpm and holds 18 inches of manifold vacuum.



To fix this I have tried the following:
New Motorcraft IAC
New Motorcraft TPS
New Mallory coil
New gapped plugs
New wires
New cap and rotor.

Any ideas?
 
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If it's original, i'd replace the balancer.
They dry rot and can wreak havoc, even if it's not this problem, it still needs to go.
 
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I have run the codes and there are none. I have a Pro-M processor, by the way
I was really asking to do the cylinder balance test, which is a primitive misfire detection program the stock ECU will run to see if an injector is dead or a cylinder is not contributing for whatever reason. I don’t believe the Pro-m can do this.

years back, I had a slight vibration at idke that disappeared above idle. I thought it was balancer. I ran the codes and got nothing useful. I ran the cylinder balance test and it showed cylinder 4 was dead. I had been running on 7 cylinders for a year and didn’t know it. It pulled hard and would spin the tires easy into 2nd gear....on 7 cylinders.

I ohmed out the injectors and sure as hell one was dead. Got 0 ohms wren I tested with meter. Replaced injector and all was good.

I still think you have a misfire, so you’ll need to troubleshoot the old fashioned way. Spark, fuel, compression. You can use a pressure guage on the rail and a 9v battery to trigger each injector. Prime the pump, get a static pressure reading and then trigger the injector one by one. It should drop.

you can pull a plug and lay on intake to look for spark.

conpression i would save for last, unless you suspect something like a hanging or broken valve, or a blown HG.
 
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