i have a 87 vette that was running rough, so i decided to do a tune up (plugs, wires, cap, and rotor) when i finished the car didnt run any better, but my oil pressuse went from 61 down to 52 and when i first start up my car the dash has no power and after 30-40 seconds it will come on but my speedometer goes crazy. then when i drive it all the gauges seem to be working except my fuel range starts bouncing down to zero and back to normal. please tell me someone has an answer, i've tried everything
Well, first thing is to learn about what you're trying to repair...
When a computer controlled engine starts acting up, a tune-up is rarely the solution. Its not that simple. When the sensors, ECM and the electrical system is "right" the thing tunes itself as it runs, so the act of changing parts usually does little except to make the parts guy richer and you poorer. Sensor failure or adjustment causes poor performance. Plugs will last for years, as do wires and rotors/caps. Good quality parts can go 100K and still run well.
First, get familiar with the wireing harness and locate the grounds. There are MANY.
This is a plastic car thats got a complicated computer system that reads resistance and low voltage in order to manage the various controls and get information.
The grounds that you are most concerned with are a bundle of 4 or 5 that are on a bolt on or near the oil filter. They are not easy to get to, but that group is the ECM, dash and injection ground.
Clean and tighten those. Look for the small ground strap in the same area. Check that as well.
Next, take the battery out and clean and repair all the POWER wires on the jumper post BEHIND the battery. Look at the wires and the fusable links. Each of those manages a different system in the cabin or the engine bay. That is where the power originates for the ECM and dash.
Before doing ANY of that, read the stored codes. The ECM will tell you what to look at and what IT is seeing. It also tells you what it was seeing yesterday and whats wrong today. You have to know how to interpret the info that the ECM will display in the form of "codes".
After reading the stored codes, then proceed with the basic maint of the harness by repairing and cleaning the grounds. Everything under the hood is managed by grounds, NOT hot wires.
Look at fuses, pull 'em and test them. Clean as needed.
90% of the issues with poor running are electrical. A test lite is helpful, a digital volt meter is useful,.
You may find that the harness has damage or has some wire-rot inside the plastic sleeve that you can't see. The harness section that runs behind the distributer is prone to damage from rot.
Often the "tune-up" disturbes these wires and trouble starts there..Try to wiggle the harness sections to see if there is any difference in how it runs at idle, if it will idle.
A REAL GM service manual is necessary to do further diagnostics. Don;t bother with the book (magazine/paper back) manual from the auto parts store...those are junk and don;t even work well under a table leg. Go to E-Bay for a used set of manuals OR do a search here to find the link for a NEW set. Used 2 book sets sell for $75 and are
priceless to the owner.
Get the trouble codes, clean & fix the grounds and post the results. Even if that does'nt fix it, you will have moved in the right direction.
