1987 was the first and last year for the electronic feedback carburetor system on 3.9L engines. My truck is a 1988, first year for computer-controlled Throttle Body Injection (TBI) for LA engines, however, there were a number of carbureted trucks built that carried over into early 1988.
I’ve done quite a bit of work to my truck since this video was filmed. Idle is smooth down to about 550 RPM. Make sure your throttle body is 100% clean and sealed. You can buy a throttle body rebuild kit from RockAuto, but the correct kit may be incorrectly listed as an injector rebuild kit. I bought both kits to ensure I had all the gaskets to complete the job. One of the vacuum manifolds has a slightly different original gasket, unfortunately I cannot find an exact replica, but the included gasket will still work. Unsure if it makes a difference. Check the composite isolator that sits between the intake and throttle body. I had to sand mine down to remove old, hardened gasket material to create a good seal. There is also a procedure to set the idle on TBI engines. Arbitrarily adjusting the idle screw will throw off the computer and the idle will need to be reset to be 100% correct. Check your timing. Timing should be 10 degrees BTDC +-2 degrees according to my 1988 Factory Service Manual. These engines originally were equipped with a “silent” timing chain coated in vinyl. Over time the vinyl wears and creates additional clearance causing chain slop and timing jump. When I rebuilt my engine, I replaced the timing chain with a Cloyes double roller chain and added a tensioner. The 238 LA is a 90-degree odd-fire V6 and naturally runs a bit rougher. My 1988 Technical Service Bulletin manual recommends installation of a timing chain tensioner. I highly recommend purchasing an OTC Monitor 2000, 4000E, etc. and the correct Chrysler software cartridge. These scan tools are very similar to Chrysler’s DRBII scan tool for OBD1 diagnostics which our trucks support. You can check your injector spray pattern in a dark environment with a flashlight. It should look like a cone, consistent pulse. Otherwise, check your MAP, TPS, and O2 sensor.
Certain parts are challenging to find. Salvage yards are your best bet. Sometimes parts come up on eBay. Keep your eye out. Been looking for a few things myself. I spent months looking for a new fuel pump/sending unit assembly. The original was completely rusted. I’ve been trying to find a 22 gallon fuel tank, chrome grille and headlight bezels, and an 8 1/4 rear to replace the stock 7 1/4.
