Just took my 84 vette out of the garage for the first time this year and it wouldnt run for crap. bucked and didnt want to take off. has new injectors, fuel filter, plugs, wires, all common 84 issues addressed. has a new 85 fuel pump. we checked the ignition timing a few weeks ago; took off dist cap, and brought engine around to 0 degrees and the rotor was right where it should be, between 1 and 8 like the helms specifies. we started it, set the timing with an inductive light to spec of 6 degrees. i then shut it off, unhooked the battery for a few minutes and reconnected the timing wire, then the battery per the service manual. the car idled pretty well in the garage. i took it out today, just wouldnt go. i pulled the timing advance connector, and rechecked the timing, and i could hardly see the line, it was so far off the scale, it was maybe 25 degrees ahead with the timing advance connector disconnected, after it was right on 6 degrees last time we checked it per the manual. it was as if the dist jumped a tooth. i took the cap off again, brought the engine around to where the balancer line was on zero deg and the rotor was EXACTLY where it was a couple weeks ago when we first set it to 6 deg BTDC. ive therefore determined it to be a serious electrical prob. it seems as if the computer control system is randomly ignoring the timing bypass wire circuit, and the advance is randomly functioning, then not. can a bad ground cause this? is there a particular section of the harness in 84's that is prone to shorting or burning up? would i be best off ripping out the whole ECM harness and trying to find a good used one? thanks!

 

I have never heard of an electrical problem that could cause what you describe. Was the correct EST wire disconnected in both cases? Could you have possibly left the distributor loose the first time? Are you using a dial back timing light, that wasn't properly zeroed out? What was the purpose of removing the cap and checking the rotor alignment, the first go round? Whatever else may be wrong, the distributor did not jump a tooth.

RACE ON!!!

 

What you're describing sounds more like a fuel starvation issue. Have you tested your fuel pressure yet? It should have between 9-13 psi to run correctly. I know you've stated that many of the parts are new, but that doesn't mean they're any good....particularly after sitting over a long winter.

The easiest place to hook a fuel gauge is to run a "T" fitting just past the fuel filter, as the 84 doesn't have a schrader valve.

Also, examine your injector spray as the car is idling. Are both injectors spraying a nice conical mist?

I'd be willing to wager the 64 cents in change in my right pocket that you have a fuel, not ignition, issue.

 

Fuel problems rarely cause the timing light to read differently from one session to the next.

RACE ON!!!

 

the reason for checking rotor alignment a few weeks ago was because when i bought this car 6 years ago i was told it had a new 383 engine in it, which appears true, but upon closer inspection i noticed loose plastic pieces inside the dist, and there was way too much play in the rotor. in short they put a worn out dist with 140,000 miles back into a new engine. also the gear on the bottom of the dist was very worn. so i bought a new dist and a multitude of other parts only to keep chasing this same problem for years now. a few weeks ago i pulled the dist out to be absolutely sure that the old dist hadnt ruined the cam gear. the gear on my current dist looked fine, and the engine timed up nice at 6 degrees like i said. and the rotor didnt move physically compared to the balancer mark, so no i dont think it jumped a tooth. i just wanted to be clear that i checked that. the EST wire that i disconnected was the pink and black wire that comes out of the harness right near the brake booster. and i have a craftsman timing light that isnt adjustable. thanks again for reading.

 

Ahh...you're correct as usual. So I lose my 64 cents?

Assuming the car ran correctly when parked (after it was initially timed), and it hasn't actually jumped time, I would start testing ignition components....beginning with the ignition module.

[edit]. Have you checked to see if the timing chain may have jumped a tooth?

 

The correct EST wire to disconnect before setting the timing is a tan wire with a black stripe. It has a single wire connector and is located just to the right and below center of the brake booster. I wonder what wire you have been disconnecting, or if the colors have faded. Have you been clearing the codes after each reconnect?

RACE ON!!!

 

yes thats the wire ive been disconnecting. it's a little dirty and faded. and yes i have a battery quick disconnect that i undo when i reconnect the EST, before i restart the engine. it has to be some kind of short or intermittent connection somewhere in the ECM harness. I was hoping someone else has heard of the same problem, but i guess not. i think i may have to find another harness.

 

in the electrical troubleshooting shop manual supplement that comes with the helms, page 8A-26, it mentions a Vehicle Elapsed Time sensor that "provides input to the ECM relative to the total time the engine has been running." anyone know what this is for and if it can have a negative effect on the engine if it goes bad?

 

I'm surprised to book doesn't say any more about it than that. I am reasonable sure that it is a device that causes the idle speed to be a but higher in a new 1984 car. It is also known as the cold start module. Yours would have "timed out" decades ago. It doesn't affect a thing, any more.

RACE ON!!!