Author Topic: 87 2wd 6 cylinder rough at idle  (Read 1391 times)

Offline 80HD

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87 2wd 6 cylinder rough at idle
« on: February 12, 2019, 07:29:08 AM »
Good morning,
I have a friend that drives a 1987 (C?) 1500 that idles low enough to rattle the dash.  We looked for an idle adjustment but it appears to be “set” with a cap over the adjust screw, if there is one?
If you press on the throttle a little bit, all is well.  Bend the throttle stop on the linkage is the only other way that I can see.  I’m not a motor man at all, need help.


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Offline zieg85

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Re: 87 2wd 6 cylinder rough at idle
« Reply #1 on: February 12, 2019, 10:13:08 AM »
most likely a vacuum leak

1985 C30 Custom Deluxe Ext. cab 5.7L TH400 3.21 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
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Offline MIKE S

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Re: 87 2wd 6 cylinder rough at idle
« Reply #2 on: February 12, 2019, 11:39:57 AM »
That should be a TBI V6. If the idle is always low cold start or hot you probably have an idle air control of IAC motor fault. That would be on the passenger rear side of the throttle body with a square 4 pin connector.

Offline 80HD

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Re: 87 2wd 6 cylinder rough at idle
« Reply #3 on: February 13, 2019, 03:55:52 PM »
That should be a TBI V6. If the idle is always low cold start or hot you probably have an idle air control of IAC motor fault. That would be on the passenger rear side of the throttle body with a square 4 pin connector.
Thanks sir, I read that and told him.  Sorry for not responding.

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Offline 80HD

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Re: 87 2wd 6 cylinder rough at idle
« Reply #4 on: February 13, 2019, 03:56:41 PM »
most likely a vacuum leak
Checked out with no leaks. Thanks for responding.

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Kim Burke

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Re: 87 2wd 6 cylinder rough at idle
« Reply #5 on: February 13, 2019, 04:35:24 PM »
What is the engine idle RPM?
Rich
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Offline 80HD

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Re: 87 2wd 6 cylinder rough at idle
« Reply #6 on: February 13, 2019, 06:56:51 PM »
What is the engine idle RPM?
Not sure, no tach in this truck.  I'd guess around ~500rpm.  For conversation sake, 650-700 rpm is smooth.

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Kim Burke

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Re: 87 2wd 6 cylinder rough at idle
« Reply #7 on: February 13, 2019, 08:01:18 PM »
Is it a V6 or straight 6?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline 80HD

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Re: 87 2wd 6 cylinder rough at idle
« Reply #8 on: February 13, 2019, 08:06:07 PM »
Is it a V6 or straight 6?
TBI V6

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Kim Burke

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Re: 87 2wd 6 cylinder rough at idle
« Reply #9 on: February 13, 2019, 09:01:34 PM »
TBI systems are very responsive and can easily mask mechanical faults and misfires.  Inspect for evidence of dislodged, perforated or collapsing vacuum lines.  Spray around the base of the throttle body and intake manifold with Berrymans B12 aerosol carburetor cleaner or equivalent to verify without question that there are no vacuum leaks.  Place the ESC in bypass mode and check the ignition timing using a proper timing light.  Temporarily remove and plug the vacuum line from the EGR valve to determine whether symptoms disappear.  Perform a cylinder compression check to verify engine condition.  While the spark plugs are out, inspect the insulators for cracks and check electrode wear.  Disconnect the spark plug wires, one at a time, from each spark plug and the distributor cap.  Use a bright flashlight to peer into the insulating boots at both ends of the wires.  Look for evidence of black soot accumulation and/or greenish discoloration.  Remove and inspect the distributor cap for cracks, irregularly shaped black carbon tracks, tower terminal erosion, and the condition of the carbon button protruding from the center of the cap.  Verify proper fuel pressure and make sure the fuel filter is not restricted.

If all of the above checks okay, remove the case hardened steel, base idle speed adjustment screw cover and completely block the IAC (idle air control) air bypass port so that no air can bypass the throttle plate.  Use a proper tachometer and adjust the base idle speed to 600 RPM using the base idle speed adjustment screw.  The idea is to limit minimum idle speed to 600 RPM without taking idle speed control away from the IAC.  The ECM and IAC in conjunction with timing need to continue controlling the idle RPM.  Try this adjustment only as a last resort.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline 80HD

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Re: 87 2wd 6 cylinder rough at idle
« Reply #10 on: February 14, 2019, 01:08:38 PM »
TBI systems are very responsive and can easily mask mechanical faults and misfires.  Inspect for evidence of dislodged, perforated or collapsing vacuum lines.  Spray around the base of the throttle body and intake manifold with Berrymans B12 aerosol carburetor cleaner or equivalent to verify without question that there are no vacuum leaks.  Place the ESC in bypass mode and check the ignition timing using a proper timing light.  Temporarily remove and plug the vacuum line from the EGR valve to determine whether symptoms disappear.  Perform a cylinder compression check to verify engine condition.  While the spark plugs are out, inspect the insulators for cracks and check electrode wear.  Disconnect the spark plug wires, one at a time, from each spark plug and the distributor cap.  Use a bright flashlight to peer into the insulating boots at both ends of the wires.  Look for evidence of black soot accumulation and/or greenish discoloration.  Remove and inspect the distributor cap for cracks, irregularly shaped black carbon tracks, tower terminal erosion, and the condition of the carbon button protruding from the center of the cap.  Verify proper fuel pressure and make sure the fuel filter is not restricted.

If all of the above checks okay, remove the case hardened steel, base idle speed adjustment screw cover and completely block the IAC (idle air control) air bypass port so that no air can bypass the throttle plate.  Use a proper tachometer and adjust the base idle speed to 600 RPM using the base idle speed adjustment screw.  The idea is to limit minimum idle speed to 600 RPM without taking idle speed control away from the IAC.  The ECM and IAC in conjunction with timing need to continue controlling the idle RPM.  Try this adjustment only as a last resort.
Wow, that's great, thanks for the comprehensive diagnostic tree.

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Kim Burke

Offline Irish_Alley

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Re: 87 2wd 6 cylinder rough at idle
« Reply #11 on: February 14, 2019, 07:24:16 PM »
just to put it out there but how old is the fuel? had kind of a similar issue but had some popping through the tbi. added new good fuel to the tank that was low on fuel (only 3 gals) and took the batty out to recharge it and it issue slowly cleared up
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Offline Stewart G Griffin

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Re: 87 2wd 6 cylinder rough at idle
« Reply #12 on: February 18, 2019, 09:19:43 AM »
i'd like to recommend putting a tach(even if only temporary) on it and verifying what the idle rpm is.   i think some timing lights may even have a tach.

These 4.3's before 92-94 VIBRATE at idle.   My whole dash shakes at stoplights and i idle at about between 600-650.   This is something i did not realize when i switched to this engine and it's very nerve racking.

The buick v-6 also had this problem, until they got a balance shaft.   Some 4.3's got a balance shaft in 92 and then ALL 4.3's got it by 95.   All 90 degree v-6's have this problem----i think it has to do with whenever one cylinder is going down, two are going up.  When 2 go down, 1 is coming up etc.?

You say that if you give it a little throttle, all is well.  EXACTLY----after 750-900 rpm on up everything is smooth.   So, everything is "normal" for this engine and........

You may just have to live with it.

If i get another 4.3, it will likely be one of the newer LU3's with a balance shaft.
« Last Edit: February 18, 2019, 09:22:37 AM by Stewart G Griffin »

Offline 80HD

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Re: 87 2wd 6 cylinder rough at idle
« Reply #13 on: February 18, 2019, 12:39:40 PM »
i'd like to recommend putting a tach(even if only temporary) on it and verifying what the idle rpm is.   i think some timing lights may even have a tach.

These 4.3's before 92-94 VIBRATE at idle.   My whole dash shakes at stoplights and i idle at about between 600-650.   This is something i did not realize when i switched to this engine and it's very nerve racking.

The buick v-6 also had this problem, until they got a balance shaft.   Some 4.3's got a balance shaft in 92 and then ALL 4.3's got it by 95.   All 90 degree v-6's have this problem----i think it has to do with whenever one cylinder is going down, two are going up.  When 2 go down, 1 is coming up etc.?

You say that if you give it a little throttle, all is well.  EXACTLY----after 750-900 rpm on up everything is smooth.   So, everything is "normal" for this engine and........

You may just have to live with it.

If i get another 4.3, it will likely be one of the newer LU3's with a balance shaft.

I’ve been checking as much of the diagnostic tree as I’m smart enough to understand.  Nothing seems to be malfunctioning. YOU MY VERY WELL BE RIGHT but living with this will take some getting used to.

Thanks for the info.


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Offline Stewart G Griffin

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Re: 87 2wd 6 cylinder rough at idle
« Reply #14 on: February 23, 2019, 09:20:59 AM »
These paragraphs are taken from Doug Anderson's article regarding the 4.3:

BALANCE SHAFTS

"Any 90° V6 creates some strong, primary imbalance forces, especially in the vertical mode. The 262 is no exception. Chevy originally underbalanced these engines by putting about 46% on the bobweights instead of the usual 50%. This reduced the vertical imbalance that was trying to lift the engine up off the mounts, but created a strong horizontal imbalance that shook the engine from side-to-side instead. So, in order to eliminate a lot of the "noise, vibration and harshness" in the engine and make it into a world-class motor, Chevy added a balance shaft to the premium engines in ’92 and included it in all of them by ’95.

There are two balance shafts, a light one and a heavy one, and two versions of the light one. See photo. The light one is either a 10224542 or a 10172748 casting that comes with or without a metal wear sleeve installed on the back journal, depending on the application. The wear sleeve was used on the lightweight balance shaft when it was installed in a ’92 "first design" engine with the needle bearings in the back, but it wasn’t used when the lightweight shaft was installed in the "second design" engine that had a bushing in the back of the block.

This "first design" shaft should not be used in a "second design" engine because the wear sleeve shortens the surface area needed for the bushing. These lightweight shafts were installed in all of the engines that had the light pistons including the ’92-’98 VIN "W," the ’96-’98 VIN "X" engines and the "first design" VIN "Z" engines in ’95 that were built with the lightweight pistons.

The heavy balance shaft is either a 10224541, a 10105902 or a 12550286 casting. It can be visually identified by the raised identification band around the middle of the shaft. It was used in all the ’93-’94 VIN "Z" balancer engines and in the ’95 "second design" VIN "Z" balancer engines with the heavy pistons. The heavy balance shaft weighs about 125 grams more than the light one, so it shouldn’t be interchanged with the lighter one.

The balance shafts rotate at engine speed and are gear driven off the front of the cam. There are two different gear sets, one with "wide" teeth and one with "narrow" teeth. The ones with the "wide" teeth were used in the "first design" engines along with the needle bearing balance shaft. Some of these early balance shaft engines had a whine to them, so the gears were modified at the same time the block was changed over to the "second design" version with the sleeve bearing in the back. We recommend using only the "second design" gears to help avoid any possible noise problems. "

__________________________________________________________________________________________________________
Not just my dash shakes.  Really, the whole truck shakes and when people ride in my truck at traffic lights, they are like, "Are we going to make it?"  This got old fast.   Fortunately, i don't do a whole lot of city driving.

i think the best way i can describe it is, if you took a regular v-8 and took off one, maybe two spark plug wires and ran it like that;  At idle it's not going to be smooth, but as you increase rpm's it smooths out.

Like i said, you may just have to get use to it........
« Last Edit: February 23, 2019, 09:23:07 AM by Stewart G Griffin »