[Diy_efi] Spike? on tach line.

Peter Jonasson jonasso at mcmaster.ca
Wed Jun 2 15:10:02 GMT 2004


Being as this is a boat installation I would check all the ground =

You may be running into a strange ground loop problem, so do not assume =
all motor connections are on a common ground. A star ground approach may
solve your problem. Connect all the grounds to a common source, usually
close to the negative of the battery. Try to avoid looping ground wires =
one point to another.=20

Question, does the fuel pump run continuously or does it fire up on =
Maybe the pulse you are seeing is an inductive kick from something else
turning on and off.

Regards... Peter=20
"There are 3 types of Engineers..."
"Those that can count... and those that can't"

-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org] =
Behalf Of Mark Bacon
Sent: June 1, 2004 9:00 PM
To: diy_efi at diy-efi.org
Subject: [Diy_efi] Spike? on tach line.


I have been calling everyone I know, and searching everywhere to find =
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 =

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

If I disconnect the timing bypass connector on the distributor, the =
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 =
rid of it? Could you please offer some assistance?

Thanks in advance,

Mark Bacon

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