First doing anything else is find 88 Balancer Unit on Bottom end. (in oil pan above the filter. can't miss it.) Find new Or find a shop can rebuild yours. Either is hard to impossible. That will kill all other plans and no point bothering to collect other data.
Balance shafts is in brass bearing and bearing likely worn out. Bad bearing here will wreck the entire engine.
87 VIN R is PnP w/o the Balancer. Find and if needed rebuild that.
VIN R for 87-92 are all PnP but some big changes over time and need data for model year of car came out of. Example: Later years may have a Timing Chain. Others like 87-88 only have the "weak" timing gears.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
could be dying cam gear etc. Many other parts are available but a worn balancer then don't bother fixing them. Any engine w/ filter in the pan have balancer. Worn balancer can wear crank shaft gear driving it.
Could try Jasper and other rebuild engines but many won't have them for a lot of cars over 20+ years.
[This message has been edited by theogre (edited 08-10-2017).]
so two options... get another intake (89-90 GA and others used same) reuse head and intake you have.
look like engine has no warranty.
(some 88-92 VIN R uses a timing Chain set not simple gears and cam is made to spin backwards to old versions. VIN U had them too but U have many differences and if you get them have to fix mounting other things to the engine. RA have picture of a long block VIN U many mounting points are different.)
[This message has been edited by theogre (edited 08-15-2017).]
Want a rebuilt engine. I believe that Jasper engines has them.
------------------ " THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite. "THE COLUSSUS" 87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H " ON THE LOOSE WITHOUT THE JUICE "