[Gmecm] Hard cold starting

Fri Jan 13 15:02:54 UTC 2012

I agree that Aaron's idea is definitely one to run down.  I had actually formulated such an idea in my mind a few days ago, but dismissed it, because I did not realize the ECM cycled the fuel-pump relay during the starting process.

I'll get out the VOM and check the voltage across the oil pressure switch connector terminals, and the switch itself.

What did you end up doing it to fix it in your situation?  Was it just the switch?  If so, perhaps the switch is taking too much pressure to close.

On Jan 13, 2012, at 8:58 AM, Rick McLeod wrote:

> I think Aaron is on to something here which I'd concur with
>  
> I had a similar problem in a conversion I did and found a intermittent oil pressure switch that fed the fuel pump, and I had not wired a bypass relay but relied on the oil switch only (stupid oversight) so it would take a while to start (had to build oil pressure at crank, tough to do) and would not always start in cold weather
>  
> 
>  
> -rick
> 
> 
> 
> From: Aaron <aaron at creativeoverdrive.com>
> To: A list for discussing General Motors EFI <gmecm at diy-efi.org>
> Sent: Fri, January 13, 2012 8:41:40 AM
> Subject: Re: [Gmecm] Hard cold starting
> 
> OIL Pressure.
> 
> The first thing that came to my mind was not EFI at all.
> I'm not sure how your fuel pump is wired, but IIRC GM liked to use the ECU
> to fire the fuel pump relay for a short time when the key was turned on.
> Then turn it back on during starting. The ECU would keep the fuel pump relay
> on for another short time after cranking. This second delay was waiting for
> the oil pressure to build up enough for the oil pressure switch to hold the
> fuel pump relay on. 
> To me it sounds like the fuel pump relay is NOT getting held on by oil
> pressure from the oil pressure switch. Thick cold oil or a filter with too
> much material to pump through or a weak oil pump, all could cause this type
> of cold start issue.
> When you added more fuel, stinking rich, I think you covered up the fact the
> fuel pump shut off, by running on leftover fuel in the runners and
> combustion chamber.
> 
> I quick check would be a fuel pressure gauge installed and watched during
> startup or a voltage check on the oil pressure switch to fuel pump relay
> circuit.
> 
> -----Original Message-----
> From: gmecm-bounces at diy-efi.org [mailto:gmecm-bounces at diy-efi.org] On Behalf
> Of Jared Ryan
> Sent: Friday, January 13, 2012 8:41 AM
> To: A list for discussing General Motors EFI
> Subject: [Gmecm] Hard cold starting
> 
> 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
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