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Old 07-16-2013, 06:48 AM   #1
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Join Date: Aug 2007
Location: Phoenix az.
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Car: '90 GMC Sierra C1500 2WD
Engine: L05 5.7L GAS 8 CYL (5.7K) V8 TBI
Transmission: 4 SPD THM 700

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Persistent Loping Idle

Friends, I have a loping idle on my '90 GMC C1500 5.7TBI. All engine management parts are new. It's smooth in drive though. I followed the suggestion by Tom Keliher in his article here on ThirdGen at http://www.thirdgen.org/techboard/ne...newthread&f=13.

That procedure did though reduce the loping greatly but there is still a bit of loping. Tom forgot to say when to remove the paper clip but it seemed that after switching the engine off is the right place.

Curios, my idle speed didn't rise after reconnecting the IAC electrical connecter as suggested in the article.

Do anyone have any suggestions that might help me with my loping problem?

I will give my first born male child to who so ever provide a satisfying solution.
SMALL PRINT: delivery of first born male child will be next year immediatly after his being paroled from the criminaly insane ward of California's infamous Pelican Bay supermax prison. He will be heavily sedated and "Shipped in Shackles", that is in a strait jacket, face muzzle, chains everywhere, on a handtruck.
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Old 07-18-2013, 11:49 AM   #2
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Re: Persistent Loping Idle

If the IAC motor is good

Did you check for a vacuum leak? (Map Sensor)

is the timing wire connect to the distributor?

Are the spark plug wires installed in correct firing order?

Rule out the obvious:

What did you change prior to the problem manifesting itself..

you may just have a tired IAC motor and need to replace it
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Old 07-18-2013, 11:01 PM   #3
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Re: Persistent Loping Idle

What is the rpm fluctuation?
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Old 07-19-2013, 01:24 AM   #4
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Re: Persistent Loping Idle

Quote:
Originally Posted by FRMULA88 View Post
If the IAC motor is good
Did you check for a vacuum leak? (Map Sensor)
is the timing wire connect to the distributor?
Are the spark plug wires installed in correct firing order?
Rule out the obvious:
What did you change prior to the problem manifesting itself..
you may just have a tired IAC motor and need to replace it
All engine management/emission parts are new, I checked and found no vac leaks, firing order is correct. The truck was as is when I got it for a project. It runs smooth in drive, all pistons fire. I am going to check fuel pressure and volume. I was doing shaddy cactus diagnostics, replace parts till I replace the problem part, I decided to stop that cause I came to my senses (two pennies)

UPDATE!: I took it to get a smog test knowing it would fail but I will get some information to help diagnose my problem.
Carbon Monoxides are WAY ! WAY ! Off, it is 160.31 where as 20.0 is normal. Hydrocarbons are a little off too, 2.95 when 2.0 is normal.
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Old 07-22-2013, 09:53 AM   #5
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Re: Persistent Loping Idle

Loping idle, as in the engine is "misfiring"?

with the HCs that far off sounds like you are running on 7 cylinders.. or less.

did you check for misfire:

cap / rotor / plugs / wires.
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Old 07-22-2013, 09:20 PM   #6
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Re: Persistent Loping Idle

Quote:
Originally Posted by FRMULA88 View Post
Loping idle, as in the engine is "misfiring"?
with the HCs that far off sounds like you are running on 7 cylinders.. or less.
did you check for misfire:
cap / rotor / plugs / wires.
Loping isn't always misfiring, Worn timing chain and gears cause loping and my lope is similar only I have new chain and gears also new distributor cap & rotor, plugs & wires, even a new ignition module.

In my experience Misfiring have a higher frequency, like every other revolution loping on the other hand is the RPMs going up and down, that is a much lower frequency.

This is some sort of ECM/Fuel problem I think, I need help figuring out exactly what.
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Old 07-23-2013, 08:40 AM   #7
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Re: Persistent Loping Idle

In your experience, did you rule out that it is NOT misfiring before assuming something else.

Check for loose spark plug boots at the cap and plugs

Check the resistance of each spark plug wire for continuity.

just because they are "New" do not assume they are good..

I had a new MSD wire fail once due to exhaust heat... you could not tell by looking at it.. but testing the wires one by one revealed it had no resistance (continuity) since that incident 12 years ago I always use a thermal sleeve & boots to prevent heat damage.
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Old 07-23-2013, 04:46 PM   #8
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Re: Persistent Loping Idle

Quote:
Originally Posted by FRMULA88 View Post
In your experience, did you rule out that it is NOT misfiring before assuming something else.
Check for loose spark plug boots at the cap and plugs
Check the resistance of each spark plug wire for continuity.
just because they are "New" do not assume they are good..
I had a new MSD wire fail once due to exhaust heat... you could not tell by looking at it.. but testing the wires one by one revealed it had no resistance (continuity) since that incident 12 years ago I always use a thermal sleeve & boots to prevent heat damage.
I made sure to route and zip tie plug wires away from heat B U T.. .. .. I will check anyway. About those thermal sleeve & boots, I am going to look into them.
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Old 07-23-2013, 08:40 PM   #9
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Re: Persistent Loping Idle

If all your plugs, wires, ect check out good I'd run a compression test next.
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Old 07-24-2013, 09:34 AM   #10
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Re: Persistent Loping Idle

Quote:
Originally Posted by Chad85T/A View Post
If all your plugs, wires, ect check out good I'd run a compression test next.

/\ THIS !! but first remove and inspect the spark plugs... any that look "oil fouled" is a sign of a problem.

also any white or blue exhaust smoke to go along with that loping idle?
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Old 07-24-2013, 09:53 PM   #11
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Re: Persistent Loping Idle

I just returned from AutoZone renting a kit for testing fuel and one for compression. After those test I will remove inspect and reinstall both the IAC and TPS, then reset IAC idle speed.
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Old 07-29-2013, 08:29 AM   #12
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constriction in return fuel flow

! ! UPDATE ! !

I check the fuel pressure and it was good at 10psi, the specs in the Chilton Manuel said that the pressure should be within 9 to 13.

I did though find something of note, the return line was twisted some, causing some constriction to fuel flow.

It seems someone was trying to remove the fitting with one wrench, that is not using a second wrench to keep the part of the fitting screwed into the TBI from turning.

What problem could a constriction in return fuel flow cause?
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Old 07-29-2013, 08:29 AM
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