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03-02-2019, 09:49 PM
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#1 |
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Registered User
Join Date: Jun 2007
Location: Cape Coral, Florida
Posts: 1,123
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Runs Rough at Idle?
My truck is a 1986 c10 with a stock 305 and 700r4. Recently the truck started idling rough, sometimes feeling like it wanted to stall. As soon as you raise the rpm a few hundred it runs smooth again. The spark plugs had 50k miles so I changed them but still the same problem. I sprayed around the intake with carb cleaner but there are no indications of a vacuum leak. I performed an idle vacuum test, truck hast 17inches of vacuum and the needle is steady. I checked timing and it is at 8 degrees at idle with the vacuum advance disconnected. In the last 5k miles I had a distributor fail and a fuel pump so those are new. I tried upping the idle speed screw a half turn then the truck wanted to diesel on hot shut offs. What could cause it to one day start idling rough with no other concerns?
Also today I drove the truck 225 miles to a show running 75mph and when I exited the highway the truck would not idle at all. I spent a few hours at the show and when I left it idled rough but ran fine, and after the 225 miles home it would idle rough but not stall like in the morning.
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1986 Custom Deluxe 10 1978 Caprice 2 door - sold 1937 Chevrolet 2 door sedan - project |
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03-03-2019, 04:12 AM
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#2 |
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Registered User
Join Date: Dec 2009
Location: Fairbanks, AK
Posts: 1,830
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Re: Runs Rough at Idle?
Electronic spark control motor? If so I would check knock sensors and vacuum sensor.
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-Marcus 1991 V3500 L29 454 4L80E NP205 D60/14 Bolt 4.56's 1984 K30 292 TH400 NP205 D60/14 Bolt 4.56's flat bed 7'6" Meyer Plow 2022 Silverado 3500 L8T Project Daily Driver Project Heavy Hauler Project Plow Truck |
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03-03-2019, 07:54 AM
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#3 |
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Registered User
Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 2,611
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Re: Runs Rough at Idle?
Does it still have any/all emission stuff?
Possible suspects: Stuck EGR valve, leaking PB booster, bad intake gasket - can pull from lifter valley so the standard spray with stuff won't find, transmission modulator vacuum leak. Check for leaks on the suction side of the fuel system. Double check all electrical/vacuum connections around areas where you have repaired/replaced stuff. Amazing how often a stray elbow can knock something loose. |
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03-03-2019, 12:36 PM
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#4 |
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Registered User
Join Date: Jun 2007
Location: Cape Coral, Florida
Posts: 1,123
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Re: Runs Rough at Idle?
It is an ESC truck but I bypassed the system years ago. What is the vacuum sensor?
It still has all emissions equipment, the egr valve is there but no vacuum line goes to it. Truck has manual brakes so no booster. Where is the trans modulator? I will look everything over again for loose connections.
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1986 Custom Deluxe 10 1978 Caprice 2 door - sold 1937 Chevrolet 2 door sedan - project |
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03-03-2019, 12:40 PM
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#5 |
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Registered User
Join Date: Jun 2007
Location: Cape Coral, Florida
Posts: 1,123
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Re: Runs Rough at Idle?
Just did the math for yesterday's drive and my fuel mileage is down too. Two years ago I did that same drive and got 19mpg, yesterday I got 14mpg
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1986 Custom Deluxe 10 1978 Caprice 2 door - sold 1937 Chevrolet 2 door sedan - project |
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03-03-2019, 01:58 PM
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#6 |
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Registered User
Join Date: Dec 2009
Location: Fairbanks, AK
Posts: 1,830
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Re: Runs Rough at Idle?
If ESC is gone no knock sensors or vacuum for that system to worry about.
700r4 doesn’t have a vacuum modulator, just a TV cable.
__________________
-Marcus 1991 V3500 L29 454 4L80E NP205 D60/14 Bolt 4.56's 1984 K30 292 TH400 NP205 D60/14 Bolt 4.56's flat bed 7'6" Meyer Plow 2022 Silverado 3500 L8T Project Daily Driver Project Heavy Hauler Project Plow Truck |
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