[Diy_efi] Spike? on tach line.

Mark Bacon mfb9 at comcast.net
Wed Jun 2 01:10:45 GMT 2004


I have been calling everyone I know, and searching everywhere to find some
assistance with my problem. Maybe you can help me, I hope.

I have a '94 Ski Centurion boat with a 5.7liter chevy 350. I recently
converted the carb system with a TBI system ( TB, sensors, dist and coil &
#...747 ECM) from a late '80's Chevy Truck. The harness and chip were
supplied by Affordable Fuel Injection. New Vortec Heads, Cam, Timing Chain &
Sprockets, Manifold, Fuel pump, Wires, Plugs, etc. The install went
smoothly, and fired right up, but I have encountered a really goofy problem.

There is a spike on the tach, seen both digitally and analog. ie idle at 700
rpm suddenly jumps to 2400 rpm intermittenly, then back to 700 rpm. This
spike occurs every couple of seconds, regardless of the engine speed. The
resulting problem is that the ECM retards the timing when it sees the spike,
and under load results in a backfire. While watching the timing mark at any
constant speed, I can see the timing mark jumping around.

If I disconnect the timing bypass connector on the distributor, the timing
and rpm's become steady. When I replug the bypass and restart the engine, it
returns. I am not sure if what I am seeing is really a spike on the tach
line, induction of some sort, or if it may be in the ECM. And how do I get
rid of it? Could you please offer some assistance?

Thanks in advance,

Mark Bacon

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