[Gmecm] Hard cold starting

[Jared Ryan]

I have tried working on this before but never had any success, nothing that made a real difference.

Here is what I have:

1984 Chevrolet El Camino
'87-later-style 350 GM long block, NEW in 2005
Crane 2032 roller camshaft
Edelbrock Performer TBI intake manifold
GM throttle body with 55 lb. injectors (IIRC; I have a spec sheet for them at home)
Stock 305 exhaust manifolds
Painless Wiring TBI wiring harness

It is a relatively tame engine, meant for street reliability in a small pickup.  My problem is cold starting.  Here is what happens:

When it is cold outside, and the engine has not yet been run that day, I turn the key, crank it, and the engine catches after four or five seconds, and goes to a smooth, moderately high idle (1300-1400 IIRC).  Then, after a short period of time - yesterday it was about ten seconds - it will suddenly stop running.  There is little warning; sometimes it will run rough for a second or two, but it is very abrupt.

When I turn the key, it starts back up in a second or two, but it usually does it again.  Yesterday I went through three cycles of this before it started and stayed running.

What I do not understand is that the car previously had a 305 in it, with the exact same setup with the exception of the roller camshaft and Edelbrock manifold (used a modified GM TBI manifold), and I never, ever had this problem with the 305.  Ever.  It just started up and stayed running, smoothly, like a fuel injected engine should.  Same wiring harness, same fuel pump, same fuel lines.

This behavior usually does not happen in the late spring and summer months.  Whether it happens, and how long it takes before the engine quits, seems to be highly dependent on the ambient air temperature.

What I have seen from the ALDL seems sane, especially regarding coolant and intake air temperature.

I have tried both a 1228746 ECM with a '90 Caprice 350 chip, and a 1227747 ECM with a '91 1500 Suburban 350 chip.  I reset the minimum air when changing ECMs, but the behavior is the same.

I may have a possible clue.  I tried enriching the open-loop mixture in a chip for the 1228746, and I got it so that it would - most of the time - stay running when it is cold outside.  However, this was at the expense of the exhaust smelling VERY STINKING RICH.  Like, stinging the eyes.  But that at least gives me the idea that it is somehow going lean, and that is what makes it die.

I looked at the EGR valve yesterday and it looked like it was stuck open, so I put a new one in, but there was no change.

Is there something really stupid obvious that I may have overlooked?  Something, anything, that might make it go lean and stumble and die when it is cold?  It seems like, once the combustion chambers warm up a little, it is fine, but getting it to that point can take starting it three or four times.

What I am wondering is if there is something beyond the ECM programming that would allow it to start easily when dead-cold, but then somehow make it go lean after a few seconds.

I am very, very frustrated with this.  I love the vehicle but I can't trust it to drive it on cold days.  It is not normal behavior for GM TBI, and I don't know where to turn.

Once it is warm it runs GREAT.  Smooth idle, even acceleration, 22 mpg gas mileage - just a joy to drive.  But that hardly makes up for not being able to just start it and GO first thing in the morning!

Jared