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Start up / idle issue

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89GMCK5

1/2 ton status
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Remlap Al.
I know I've read here before that just after start up, like 5 seconds or so, the ignition system performs a "switch" over of sorts. I am thinking that this switch involves bringing the ignition module on line. Can someone confirm?

This is my situation, recent tune up, plugs, wires, cap, rotor, ign module, etc have all failed to resolve a "low" idle condition. When I first start the truck, it idles fine, then about 3-5 seconds after it is running, the idle drops a little bit. This is the issue I was chasing with the tune up, but have yet to resolve.

Completely stock 89 GMC Jimmy, tbi 350, 168,000 miles, been running great, until this little issue.

In addition to the tune up items, I've checked / changed vacuum lines, iac, tps, cts, all to no avail.

I noticed the ign pick up looked terrible when I replaced the ign module, really rusted. I have a feeling that this is the issue, but would like to find out what is going on 3-5 seconds after initial start up.

My other thought is perhaps fuel pressure, but once above idle I do not notice any hesitation or surging. :dunno:

Any input would be greatly appreciated.

Thanks guys.
 
IAC replaced? Low idle on my 90 was the IAC put a new one in and it was a whole new truck.
 
Brand new, no change. Not saying it couldn't be bad out of the box. If that seems to be the consensus easy enough to change again.

Thanks.
 
Sorry, I don't have a tach, but if I had to guess it starts fine and idles at about 750-800 for about 3-5 seconds then drops to about 600 or so. Just for some comparison and the sake of this post.

Also, no codes are being set.

Does anyone know how the ignition system works with the electronic spark timing/ ECM / ignition module on the tbi trucks?

I thought that during cranking the ECM takes the est / esc out of the equation, then after verifying some parameters, turns it over to the esc. I think this is part of the issue, but wanted to verify.

Not to say it couldn't be something else. I am here looking for help.

Thanks for the replies, please keep them coming.
 
Never heard of anything like the computer "switching" ignition control. But what happens when you first start up is the IAC motor opens the throttle slightly to increase the idle at first start. Then it idles down to normal idle speed.
 
Yeah, my normal idle speed is normally a touch higher. Something started recently and it is now lower than "usual", hence the tune up and this thread.

I have noticed, that when it is idleing rough, I can turn the ignition off, and re start, and most often it is back to normal for a short time. Drive a few miles, stop at a light, and it is low again. Not so low to almost stall, but noticably lower than usual.

I need to get my hands on a tach, or at least a good meter with tach ability.

Thanks all for the input.

On the ECM switching the ignition control, I read through quite a bit of stuff last night and apparently at 400 rpm is when the ignition control is turned over to the ign control module. Up to that point, ie start up / cranking, the ecm uses references from the crank sensor, MAP, etc. to send the spark signal. At 400 rpm, it looks for a ground on the bypass circuit of the ign module, if all is okay, it then turns the ign module on and it and the ESC then take control, and feed the reference signal to the ECM.

I may not have worded that correctly, but I can clarify once I get to my manual.

Thanks, and please keep the input coming.
 
It runs pretty decent on the road, but I plan on checking the fuel pressure this weekend. I've been weary of the fuel pump for a while, no real symptoms, just figure it's due.

Injectors both have a decent pattern, checked with a bright led, not with a timing light.

I am also going to check the initial timing and total timing this weekend.

Thanks guys, keep them coming.
 
Spent last night trying to decide on the ALDL cable and Winaldl, or the AutoXray 7000 scanner.

I think that it is going to be most helpful to have more information. There are too many unknowns at this point. I need some data to help me diagnose this.

Thanks,
 
Here are a couple of WinADLD screen shot from my Jimmy.

JiminyData1.jpg


JiminyData2.jpg
 
is it possible you had weak plugs or wires or something you just replaced that could have caused it to idle higher than normal, and the idle you have now is "normal"? dumb question but it does happen
 
I thought the same thing, but.........finally noticed gas smell at the air cleaner the other afternoon, and ir heat gun showed intake much much hotter than temp sender was indicating.

Cat was really restricted....changed out, much better. I feel like something else may be a touch off, but that was 90 percent of the issue. Way too high back pressure, exhaust backing up into the intake.

Wow. That was a first for me. I've seen them when they wil limit a vehicle to about 10 mph with the pedal to the floor. My truck stil ran pretty decent down the road.

Thanks for all the input guys!
 
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