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Old 10-31-2016, 08:29 PM   #1
duffs396
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Low Idle, Rough Acceleration, Shakes a bit

93 Silverado, 350 TBI

After replacing the intake manifold gaskets, I've been experiencing a low idle in park, even lower (200 rpms) in drive. Acceleration is poor but eventually when it gets up to cruising speed 40mph+, it doesn't run as bad.

Also, approx 5 mos ago, replaced ign model, plugs, wires, cap, rotor.

I've done the following:
Checked for vacuum leaks everywhere; cigar method, starter fluid, etc.
- REPLACED IAC Valve
- REPLACED PCV
- REPLACED PCV to Carb vac line
- tested EGR vacuum - OK
- tested EGR Solenoid - OK
- tested TPS OK, voltage up/down as throttle is moved (ign on, not engine) - OK
- tested MAP Sensor voltage drops as vacuum (vac pump) is applied (goes from approx 4.7 to 1.9 Volts)

Have set timing to 0* several times with brown/tan wire disconnected. Timing then adjusts itself to about 1/2" off the timing tab.

The last test I did was to completely cover the throttle body with plastic to try and smother it from the top, almost stalled but did not.

I still wonder if I did a good job with the gasket sealer when I replaced the intake manifold gasket.

There's a lot to digest, sorry for the long post but wanted to give as much info as possible.

Thanks,
Duff
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Old 10-31-2016, 10:46 PM   #2
ChevyTech
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Re: Low Idle, Rough Acceleration, Shakes a bit

First thing I would do is check and double check the firing order to make sure you did not get two plug wires crossed.

Another thing I would check is to make sure the EGR is not stuck open a little ways.

Even with problems the system should be opening up the IAC to speed up the idle.

Make sure the ground wire on the back of the passenger side cylinder head is in place. Also make sure the ground wires at the front of the intake are all getting a good connection. The computer and the sensor circuits through the computer connect to ground at the grounds in the thermostat area.
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Old 10-31-2016, 11:06 PM   #3
ChevyTech
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Re: Low Idle, Rough Acceleration, Shakes a bit

The outer ring on the harmonic balancer can slip so the timing line is in the wrong spot.

The timing may not be where you think it is.

This can be verified or ruled out with a piston stop and some work.
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Old 11-01-2016, 07:49 AM   #4
duffs396
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Re: Low Idle, Rough Acceleration, Shakes a bit

Thanks for the suggestions ChevyTech.

Timing and wires double checked. Also did manual TDC check as well.

I sprayed the EGR when running and no change. I've manually pushed it open and the truck stalls and then returns to idle.

I redid the grounds at the firewall (straps and on the frame). Does it matter where the grounds are at the front of the intake?
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Old 11-01-2016, 08:51 PM   #5
ChevyTech
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Re: Low Idle, Rough Acceleration, Shakes a bit

Quote:
Originally Posted by duffs396 View Post
Thanks for the suggestions ChevyTech.

Timing and wires double checked. Also did manual TDC check as well.

I sprayed the EGR when running and no change. I've manually pushed it open and the truck stalls and then returns to idle.

I redid the grounds at the firewall (straps and on the frame). Does it matter where the grounds are at the front of the intake?
You're welcome!

The ground location does not matter as long as they get a good connection to ground. I have run across several that were at the thermostat housing and not getting a good ground. I prefer to put them on a stud on a intake to head hole (intake bolt).
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Old 03-19-2017, 06:25 PM   #6
duffs396
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Re: Low Idle, Rough Acceleration, Shakes a bit

As a continuation of my saga.... I've now discovered a black plug #6, all other plugs normal. I've done 2 engine flushes as the oil was very dirty and I originally though when I changed the intake manifold gasket, something may have fallen into the engine. I also now have a knock, but sounds louder than a valve tap. I thought about doing another engine flush as I really neglected the oil change the last 12 mos.
I short, the engine shakes when idling or accelerating, idle is low.... Have changed out most of the obvious. Any suggestions? 107K miles.Thanks.
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Old 03-27-2017, 07:46 PM   #7
Mikefox
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Re: Low Idle, Rough Acceleration, Shakes a bit

Test compression.
Every body gets caught up chasing computer controlled problem's.
Need to check common motor problems.
Does it run better when at a higher idle.
Could be sticking lifters from the sludged motor.
Test compression the figure out the tap.
Take a tube and put it on the oil pan .
If you hear it pronounced there it's probably a rod.
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