What is causing this intermittent problem- 87 Nissan Hardbody truck 4cyl runs good one day other days black smoke billows from tailpipe smoke smells of unburnt fuel misses very badly when this happens?

QUOTED Answer"read the codes"

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---On a pre-95 vehicle, the diagnostic system is what is called OBI. Most auto parts centers will have an OBI jumper, but one can be created from a piece of wire with both ends stripped 1/4". Stop in at your local AutoZone if you don't have a repair manual and look into their HANES manuals to find out where the OBI connector is, and how to jump it to get the codes.

What you will find after jumping the OBI connector and turning the key to ON, will be a sequence of lights that will flash on the dash. You will need to count how many times it flashes, it might be like 6 flashes pause, 4 flashes pause, 6 flashes pause, 11 flashes pause, 4 flashes pause and then it will repeat the sequence.

Any other notes that you can add will help with the codes that you receive. Especially in a case where it says that the EGR system is the problem. If the engine idles awkwardly (e.g., revvs high, drops down, revvs high, drops down, ... ) it could be the Mass Air Flow sensor, and with the Mass Air Flow sensor giving a bad command to the ECM, it would make it adjust the fuel incorrectly, causing problems in the EGR system. Also check all vacuum lines and if they look dry and cracked, change them out. Rubber only lasts around 5 years and if the hoses you have are original, you could have a vacuum leak throwing off one of your sonsors. Spraying hair spray or any other aerosol around the vacuum lines will change the RPMs if there is a leak. Don not* use a highly flammable liquid like starting fluid when doing this.

Check grounds on the ECM and if the vehicle is rusted badly, consider that rusty body panels will hamper conductivity and cause intermittent problems when you hit bumps.

Get the codes.

I came back on this cause it dawned on me that this would be a Z24 engine. That would mean that there are many* vacuum lines that are crucial to correct emissions and the firing of the exhaust side spark plugs. The exhaust side spark plugs are controlled by vacuum solenoids that are located on the passenger side (LHD) inner fender. IIRC, the intake plugs are on the outside circumference of the distributor cap and the exhaust plugs are on the inside circumference of the distributor cap. Make sure that all plug wires from one side are running to the same place on the distributor cap (e.g., inside or outside).

This engine family utilizes 8 plugs for efficient burning of raw fuel when the engine is under load. If you were climbing a hill, you would be giving the engine more gas by stepping harder on the gas pedal. this unburnt fuel (because the engine is getting a request for more fuel, but isn't spinning any faster) is burned by the second set of plugs and sent back to the intake through EGR.

As for correct vamping of the vacuum lines and such, you will find the best diagrams in a Mitchell Service Manual for your vehicle... or in my second choice, a Motor manual.

If you were stepping into the "GO" pedal when it ran rich, your problem might just be bad plugs on the exhaust side or the vacuum lines running to/from the vacuum solenoids.

Check your plugs too and try a set of Platinums. If Bosch Platinums are used, DO NOT attempt to gap them, they will be gapped correctly from the factory. Attempting to gap them can crack the porcelain and cause the Platinum element to break off and fall in by the valve and valve seat, damaging the head if it gets trapped between the two.