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Old 11-17-2017, 03:07 PM   #1
Sixball
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Intermittent Hard Starting / Rough Idle

I bought a 95 1/2 Ton Silverado a couple months back. 170XXX miles. 350 Motor looks clean, unknown history, but it has a newer intake manifold with a throttle body 1 inch spacer, so some work had been done at one time. I drove it home about 60 miles at highway speeds, no proplems, no complaints. It actually has some pep to it.

But..... I replaced the serpentine belt and it wouldn't start. It would crank but not fire. I must have hit some wirring. It had some sketchy wiring right around the Thermostat, so I wiggled and it started. I then found when running, I could change from idle to rough idle by moving the sketchy wires. I cleaned up the wires, conected / sealed correctly and found a better grounding point. Problem solved. I thought......

Drove about 25 miles, start / stop style driving and at a stop sign it idled rough and died. Would not fire. This has happened twice now this week. Otherwise it starts and runs perfect. No catalytic converter. It has to be the wires. Anyplace to start looking? The wires I worked on were exposed. It must be deeper in the harnes. I know I will have to do some digging, but I was hoping for some things to look for. I'm more into carbs and not EFI.

Speaking of carbs. When this is hard to start at times, you have to pump the pedal luke it's carbureted. That seems like the only way it responds and eventually starts WITHOUT wiggling the wires at all.
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Old 11-17-2017, 05:34 PM   #2
ChevyTech
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Re: Intermittent Hard Starting / Rough Idle

Repair any wiring issue at the front of the intake.

The ECM and engine sensors ground at the front of the intake manifold under the front intake bolts/studs or by the thermostat.

The computer uses the coolant temp to determine how much fuel to inject and how fast the idle speed should be.
The coolant sensor the computer uses is by the thermostat.

Make sure all those connections including the coolant sensor connector are in good condition and tight.

There is also a ground wire between the back of the passenger side cylinder head and the fire wall. It would be a good idea to make sure it is in place.
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Old 11-17-2017, 07:31 PM   #3
Sixball
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Re: Intermittent Hard Starting / Rough Idle

Quote:
Originally Posted by ChevyTech View Post
Repair any wiring issue at the front of the intake.

The ECM and engine sensors ground at the front of the intake manifold under the front intake bolts/studs or by the thermostat.

The computer uses the coolant temp to determine how much fuel to inject and how fast the idle speed should be.
The coolant sensor the computer uses is by the thermostat.

Make sure all those connections including the coolant sensor connector are in good condition and tight.

There is also a ground wire between the back of the passenger side cylinder head and the fire wall. It would be a good idea to make sure it is in place.

Thanks ! I will be looking at it tomorrow. Perhaps the aftermarket intake being aluminum is also contributing to the problem by not having a good ground? The inconsistency baffles me. But a faulty sensor would cause such. The wiring has been repaired. More to follow after I look at it tomorrow
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Old 11-18-2017, 04:08 PM   #4
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Re: Intermittent Hard Starting / Rough Idle

UPDATE: Replaced the coolant temp sensor. So far so good. Wiggled wires and could not duplicate the rough idle syptoms. Run it all week and see what happens. Thanks !
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Old 11-18-2017, 07:58 PM   #5
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Re: Intermittent Hard Starting / Rough Idle

You're welcome!

I will try to remember to check to see if you post any more problems on this thread.
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Old 11-20-2017, 06:52 PM   #6
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Re: Intermittent Hard Starting / Rough Idle

Well I had two good days of easy starting after my Coolant Temp Sensor replacement and now I am back to some rough starting. The temp here has dipped into the 30's. I had imeadiate starts several times, then this morning I had to pump the pedal to get it to fire A rough idle for about 30 seconds then it smoothed out. About 10 minuets later I arived at my destination, shut it off for about 20 minutes and it wouldn't start without pumping the pedal. Same rough idle for 30 seconds, then smoothed out. Drove for about 30 minutes. Shut it off at work, immediately tried to re start and it started right up with smooth idle.

So what else should I look for now ?
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Old 11-20-2017, 07:11 PM   #7
ChevyTech
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Re: Intermittent Hard Starting / Rough Idle

Does the truck have a fast idle when started which slows down as it runs? This is a good indication of how well the IAC is working.

IAC = Idle Air Control

The IAC units get erratic as they get old and stick.

Check and see if there are any trouble code stored in the computer. Here is a link with instructions on retrieving codes without a scan tool.
http://www.chevytalk.org/fusionbb/sh...php?tid/273660

Being you don't have a lot of background on TBI systems you can use my posts in the FAQs thread to get up to speed if you have some reading time.
http://67-72chevytrucks.com/vboard/s...d.php?t=445487
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Old 11-20-2017, 08:28 PM   #8
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Re: Intermittent Hard Starting / Rough Idle

The idle is consistent. No high / slow issues. It just either starts or doesn't when turning the key.

Thanks for the links. I'll read up on them and check out the codes
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Old 11-21-2017, 09:54 AM   #9
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Re: Intermittent Hard Starting / Rough Idle

When the truck is started cold it should have a faster idle speed which is done by the computer opening the IAC. As it runs the computer should slow the idle down. Think of this is being like the choke and fast idle cam on a carburetor.
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Old 11-21-2017, 10:36 AM   #10
Sixball
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Re: Intermittent Hard Starting / Rough Idle

Quote:
Originally Posted by ChevyTech View Post
When the truck is started cold it should have a faster idle speed which is done by the computer opening the IAC. As it runs the computer should slow the idle down. Think of this is being like the choke and fast idle cam on a carburetor.
Makes sense, thanks. Looks like I'll put one in this weekend. Unfortunately I only have weekends available to work on my stuff. I'll also check out those codes before I change it out. They are inexpensive , so it won't hurt to swap em out.
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