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Rough Idle Check List

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Old 05-03-2003, 02:08 AM   #1
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Rough Idle Check List

Hey All;

I have a 350 TBI that's not behaving correctly at idle. I've been doing searches on and off for a week but I have not really come up with a good checklist. What I'm hoping is if it's idle related and YOU WERE ABLE TO FIX IT, please give a quick discription of what it was doing and what fixed it.

So far this is my list of things to check...

1) Vac lines for leaks
2) Burnt / crossed ignition wires
3) Ignition system (distributor, cap., coil, spark plugs, ect)
4) Fuel filter
5) Fuel pressure (regulator/pump)
6) Fuel pump fuse (?? )
7) Alternator AC current bleeding into ESC module (
8) Idle control motor
9) HELP? What else.

What I'm hoping to do is pool some knowledge here in regards to this problem everyone seems to have. Some things like fluctuating rpm by like a 1,000rpm (or more) sure sounds to be more like a vac problem. Where as a "idle runs rough" but no large fluctuations usually is a burnt or underpreforming wire, plugs, distrbuter cap. Also I've had the distinct pleasure to know what a bad fuel filter feels like (no power at the high end, then just gets worse fast!).

With a little luck maybe we'll be able to help a few other people too.

My problem is a idle up and down 150-300 rpm and then just die at idle or at a stop light, I'm thinking that's a fuel pump....

Thanks everyone in advance for contributing!!!!

Last edited by BlackBird89-350; 05-03-2003 at 02:21 AM.
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Old 05-03-2003, 07:14 AM   #2
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Join Date: Jun 2002
Location: Walla Walla Washington
Posts: 751

bad vibration throughout rpm range

FIX: wrong flex plate for motor. The flex plate I had installed did not have a weight and the engine I installed required a counterweight on the flex plate.

backfire through exhaust

fix: reset valve lash and reset timing and idle

backfire through carb problem: retard timing

fix: advanced timing

missing on number 1 cylinder problem broken terminal on the cap.

fix replaced cap and wires wires were leaking dont ask how I found out.
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Old 05-03-2003, 04:03 PM   #3
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Posts: 18,318

Let's back up a step or two, and get some more detail on the symptoms and what you may have done to this point to address those. I'm guessing (hoping) that all the item on your list have already been checked out. What has been done so far?

Also, you need to do some organized troubleshooting, instead of chasing possibilities from system to system.

If you have some notion that you have a fuel pump problem, test the fuel pressure. You should be able to read a constant 11-13 PSIG under all operating conditions. If that is a problem, you'll need to repair it. If not, you can take it off your list of possibilities.

Another item to check is the ignition system. Once you verify a reliable ignition system, you can eliminate that from the list. Test the resistance of the ignition wires, or replace them. Inspect the distributor cap, rotor, and coil for moisture, carbon streaks, pitted contacts, or physical damage. Remove the spark plugs and inspect the electrodes, gap, and insulators for damage, or just install new ones. No "designer" plugs - just the regular old, side electrode, copper cored plugs. While you're there, check the connectors at the distributor, just for grins.

Inspect all the vacuum hoses, including the brake booster hose and PCV system. Clean or replace the PCV valve while you're there. Another vacuum-related item to check is the EGR valve and control system. A lot of mysterious poor idle and low RPM performance problems can be traced to a leaking EGR or faulty EGR control system. Of course, you'll want to check for obvious gasket problems at the IAC, throttle body, and intake. A quick idle vacuum reading might be useful in determining the likelihood of a vacuum or EGR problem.

Once that's done, you should clean the throttle body, throttle plates, and remove, clean, and lubricate the IAC. Set the minimum air positrion and TPS when you do.

By then, you should have a good idea of the problem(s), or at least have done a lot of routine, scheduled maintenance that will eliminate a lot of possibilities.
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