My 1990 isuzu trooper was driven to town when it started to heat up and miss a lot. No power on acceleration. Shop took compression test. # 1 piston 150 # 2 50 # 3-0- # 4.-0- I had the auto shop replace head gasket in my 1990 2.6 isuzu trooper. Now it smokes bad and starting to miss whats up?

what is the mileage , black smoke indicates the pistons rings may be letting oil into the cylinders , get a compression test done on the cylinders to confirm , you dont mention vehicle age , engine size or type , all this info could help diagnose the problem

the cars come with many diff. engines. and EFI systems that match it.
so cant help you.
but ill answer in pure physics.
1: lost DC 12v power to the EFI system causing spark and fuel losses.
2: lost engine compression. 1 or more cylinders.
3: lost spark or never doing a full tuneup every 60k at least.
4: bad fueling, too much or too little.
what do do is a few simple tools to discover the truth.

do you want to gue$$, or test.

next step is a compression test then a leakdown test to eliminate a mechanical problem (burnt valve etc..)

Sounds like the injector pump is defective.

probly lost compression in some of the pistons, do a compression test on all cylinders

check out the neutral saftey switch on trans, if its reporting unknown gear on scanner while moving t bar it wont know what gear to be in and give you similar problems, also has oil been leaking into switch before?

replace trottle poss.sensor

bring number1 piston to top on compression stroke.just before top dead,drop disttr in @ point rotor btn to #1 cylinder

A "leak down" test will test the cylinder effectiveness, as will a compression test. These will tell you how good the engine is.

verify the sparks plugs, fuel filter and catalytic converter. If you can replace the sparks plugs and fuel filter!!