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Hard start - Rough idle

Trouble starting and rough low idle, no codes?

Hard start - Rough idle

Postby mjs6f759 [OP] » Sep 24 2010, 9:04pm

My 2002 1500 Sliverado 4.8L all of sudden is hard to start, needs much cranking, and has a rough low idle. I have no engine light so not sure if any codes are in the computer. Seems to be worse when the truck is warmed up? (Seems opposite of what I'd expect for a problem like this?) The truck has 125k miles. I replaced plugs/wires and air filter, no difference. Went to replace the fuel filter but fittings are too badly rusted so will have to most likely need to cut out filter and replace some tubing so was going to put that off. Other than the starting and rough idle the truck runs great! No missing or hesitating at acceleration or constant speed? I've put the key to the ON position for several seconds before starting thinking if it was a fuel pump or blocked filter this would make a difference but it doesn't? Any ideas out there?
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Related Topics  HELP! rough idle, hard shifting, and knocking from engine   Intermittant Low Idle/Hard Start. No CEL, Misfire on scanner   Super Rough Idle on Cold Start   Rough Idle at Start in Extreme Cold Weather   Rough Idle   Rough Idle   rough idle   08 silverado hard start   Hard start but never first of the day   Warm hard start   hard start problem   hard to start after fuel up   HELP.... Hard to start when hot   Rough idle and ticking 
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Re: Hard start - Rough idle

Postby mudman406 » Sep 28 2010, 2:43pm

I have a 2003 with the same engine and it just started doing the exact same thing a week ago. I changed the fuel filter and am running injector cleaner but doesn't seem to be helping. I talked to a few guys last night and these symptoms may be the result of a bad fuel pressure regulator. It is usually on the left side of the engine on the fuel rail. If there is fuel in the vacume line that attaches to it then it could be bad. Another thing to check is the mass airflow sensor. The wires may need cleaned or may be broken.
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Re: Hard start - Rough idle

Postby koolz71 » Sep 28 2010, 3:44pm

could maybe possibly be a bad O2 sensor
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Re: Hard start - Rough idle

Postby trublu » Sep 28 2010, 8:06pm

It really Should have seta code if the O2 sensor was bad. Not arguing with anyone, just saying. And as was mentioned it sure wouldnt hurt to clean that MAF sensor. ;)

:idea: The problem though could be caused by something as simple as a vacuum leak.

Ive personally seen where some vacuum leaks dont show up till things warm up for a bit and the hoses I guess are a tad more pliable causing them to sag, spread open a little and then leak.

You might try and take a look at good look at all vacuum hoses, try and spray a little carb cleaner if you like on the hoses and if the engine momentarily speeds up youve found a leak.

Honestly, Ive seen many rough idle complaints caused by a simple leaking vacuum hose.This would also go along with your "runs great at speed" as vac leaks tend to REALLY rear their ugly heads at idle.

Hey, It's wortha shot, wont cost much and it eliminates a possible problem. ;)



Id really change that fuel filter if its been on that long. It may even be partially plugged. Id definitely change it before I even considered checking fuel pressure as a possible problem.
A dirty filter is also very hard on the fuel pump itself as the pump is trying hard to maintain pressure but it has restriction making it work harder than it should. :(

Just my $.02
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Re: Hard start - Rough idle

Postby HartskiHartski is online! » Sep 28 2010, 9:25pm

I'd say the fuel pressure regulator. Had the same issue on my Intrepid. Same symptoms as your truck. New pressure regulator and she was good as new!
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Re: Hard start - Rough idle

Postby jepics » Sep 28 2010, 9:52pm

This issue has been covered here before - for other suggestions see posts in link below

Rough Low RPM idle
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Re: Hard start - Rough idle

Postby mjs6f759 [OP] » Sep 30 2010, 3:30am

Well, "Service Engine" light finally came on and the idle got so bad I had to take it in to my local shop as I just didn't have time to mess with it myself. Turned out to be the fuel pressure regulator. The regulator diaphram was leaking and making the mixture extremely rich.
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Re: Hard start - Rough idle

Postby trublu » Sep 30 2010, 7:42pm

G OOd DEAL!! At least ya finally found the prob.

Now..change that fuel filter bro! :)
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Re: Hard start - Rough idle

Postby NUDEDOG » May 10 2011, 11:13pm

Wheres the fuel pressure regulator at?
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Re: Hard start - Rough idle

Postby mjs6f759 [OP] » May 11 2011, 3:35am

Final update on my problem. Turns out about a month after I had the fuel pressure req replaced I started having the exact same symptoms again! Took the vehicle back to the same shop as I just assumed it was a bad fuel pressure req that was installed, but NO! Turned out to be a leaking intake manifold gasket and a bad O2 sensor. Was told that the O2 sensor was most likely burned out due to the leaking gasket. After a total of about $1000 in repair bills the truck is finally running ok. To answer the question about where the fuel pressure req is: on my truck it is on the driver side fuel rail directly over the valve cover.
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