[Diy_efi] Followup on 88 Corvette ECM

[Perry Harrington]

Hello all,

I wanted to followup on my Corvette troubles.  Seeing how I've spent a considerable
amount of time trying to solve my problem, I thought I'd share what I've learned
about GM ECMs.

First, I have a 94 TBI 350 chevy truck as well as the 88 Vette, this will become
relevant later.

Well, I don't drive my Vette a lot, and a while back (March) it started having the
ole injector blues.  So I swapped in a set of Ford 24lb injectors as recommended by
Vetters on the net.  The problem seemed to have cleared up, but I still had a miss,
and a dead spot just off idle.  Additionally the motor lacked power and if you
hit it hard it'd knock.

So, I replaced the TPS sensor (looked kinda ratty) for good measure, since the
dead spot seemed to be throttle related.  That didn't fix the problem.

Next I replaced all the ignition consumables.  Plugs, wires, cap, rotor, since
I hadn't done so since I bought the car in 99.

That didn't solve the problem.

I got the feeling that the fuel pump had something to do with it because what
initially caused me to park the car before March was a blown fuel pump relay
fuse.  I am told that when that happens the fuel filter and/or pump is to blame.

I'd already replaced the filter.

So I replaced the pump.  The old one looked okay, the sock was kinda grody, but
rust was evident inside the sock.  Rust was also evident in the fuel rails when
I replaced the injectors.

That didn't fix it.

Now I've replaced everthing in the fuel system but the FPR and lines.  The miss
is still there.

Now we bring the truck and the Vette together.

I decided to hook up my AutoXray to see if the MAF was working correctly and if
the ECM was doing goofy things.

I didn't have the AutoXray before March, so I hadn't scanned it yet.

There was an EGR code, but that's it.  However, while scanning through the realtime
data screen I saw something parculiar.  The Coolant temp was 1.8 degrees F, but
the gauges on the dash said 180 when the gauge said 217, the CTS said ~21F.  Now,
putting that info together with a problem I had with the truck I realized the CTS is bad.

I had the exact same failure mode with my truck.  It ran so pig rich that it ran
better on 1 injector!  I replaced the FPR diaphragm and that seemed to help a little.
Then on a whim I pulled the CTS connector and the truck started to run better.

I ohmed out the sensor and while in the engine it read 32Meg ohm.  Out of the engine
it read fine.  So I replaced the sensor with one I bought for it a long time ago,
problem solved.

Back to the Vette.  Some time ago the puller fan stopped working.  I did some research
and found that the puller is entirely ECM operated.  It turns on when you have the A/C
on, and when the motor gets warm.

So to test my theory I turned on the A/C and the puller turned on!  Wow, that problem
solved.

Now I gotta pick up a CTS for the Vette tomarrow and see what happens.

It's rather wierd that I'd have a CTS failure on 2 GM vehicles that are 10 feet from
eachother.

Anyhow, the purpose of this message is to give a little followup on my problems, so
that people will have a better understanding of failure modes.

One last thing:  Why is the GM computer so stupid as not to notice the CTS is bad?

It has a MAT sensor and CTS sensor.  If the MAT sensor is reading less than 100F, the
CTS sensor should read hotter or near equal.  If the GM programmers had thought about
that piece of logic, they would have saved me a bunch of time.

That is a CORE sensor that when failed should throw a code, not cause the engine to
run like crap.

I don't mind replacing all the stuff I have, as they are consumables and will
ensure that those are causes of trouble in the near term future.

--Perry

-- 
Perry Harrington			Data Acquisition & Instrumentation, Inc	
perry at dainst dot com					 http://www.dainst.com/

Those who would give up essential liberty to purchase a little temporary safety
deserve neither liberty or safety. Nor, are they likely to end up with either.
                             -- Benjamin Franklin

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