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Idle rough in gear

My 1988 Gmc Sierra 1500 is idling rough and hesitates in gear. We have a new cap and rotor but do I need to change out the spark plugs and wires too?

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Definitely the plugs too. If the wires are many years old, good to change them. If they are the original wires, definitely change them.
Also new gas filter, and new air filter.

Posted on Dec 26, 2013

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  • evitalc31 Dec 27, 2013

    I changed the spark plugs, the wires, the fuel filter and the cap and rotor. It is still idling rough and it hesitates changing gears. Thinking this is a vacuum link. Any thoughts? The fuel pump is new. The truck appears to be running rich.

  • montehammons Dec 27, 2013

    I would look for vacuum leaks. Also the pcv system and that vacuum line. Make sure the egr valve is not sticking open at idle. It should be fully closed at idle, and opening only at higher rev's off idle.//Those old plugs you took out, if all were a light brown or tan color on the bottoms, that is very good. If any had a black, dry sooty appearance, that's a sign of running too rich. But don't confuse that with black wet deposits, which is oil fouling.//Just keep looking, I guess, best advice.

  • montehammons Dec 27, 2013

    Running too lean, like from a vacuum or air leak, will tend to blister the plugs-give them a very white appearance.

  • evitalc31 Dec 28, 2013

    I changed out the spark plugs, the spark plug wires and the fuel filter. The old plugs were black and sooty. It definitely appears to be running too rich. I may be a girl but I grew up watching my dad work on cars. If the truck is running rich what should I be looking for?

  • montehammons Dec 28, 2013

    Okay, have to determine if it's getting too much fuel, or too little air for the mixture. I would be checking the air filter, look for air restrictions. Check fuel pressure, if throttle body injection, pressure should be around 13-20 psi, I believe, but you may need to check the spec for your truck. If pressure is too high, check the pressure regulator-check it even before a fuel pressure test: if the vacuum line to the regulator has any evidence of gas, the regulator has failed and letting extra gas get sucked in through the vacuum line./// Watch the injector spray with the engine cranking over-should see a fine mist cone pattern of fuel sprayed out. If it just drips, the injector is probably clogged and needs cleaning or changing. Also, check for trouble codes by cycling the key. Do you know how to check for codes? Post back if you need help. Maybe the map sensor might need replacing. I have had good luck changing out old, lazy oxygen sensors, with or without a code for it.///Finally, you might consider a compression test-see how healthy your engine is internally.

  • montehammons Dec 28, 2013

    Here's how to check codes, and code descriptions:



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SOURCE: My 1995 S500 Mercedes Benz is idling very rough.

before you spend any more money on your vehicle run a cylinder compression test. I know it seems unlikley with less than 100,000 miles but 119 engines do like to burn valves. the LH fuel injection system was only used from 93-95. LH is a germen abreviation for hot wire mass airflow. The later systems use a hot film. Your vehicle has electronic fuel injection so why did you have the fuel injecters pressure cleaned. unless you have the cis system which means your car is not a 95. Motor mounts are also a very common failure on that vehicle. In any event rule out the worst case and perform a compression test because it really sucks to spend alot on an engine and find out it needs a valve job. I'm assuming that the check engine light is not on. although wireharness issues are another common issue. these early C.A.N. cars are very difficult to troubleshoot electronically and the information provided by the factory is difficult to follow.

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Perform a valve clearance inspection and adjust if necessary. Tight or sticking valves will cause a misfire or rough running concern. Note: Perform valve adjustment when the engine is stone cold. 4cyl: INT 0.26mm EXH: 0.30mm (V6 engine are hydraulic and no adjust necessary.) Also check for a sticking or stuck open EGR valve. For diagnostic purposes only: Remove the EGR valve. Place a business card over the EGR passage and bolt the EGR valve back on. This plugs the EGR ports and if the misfire goes away the EGR valve is partially stuck open and is defective.
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Before replacing the O2 sensors, I would check the fuel pressure. You may also try to have someone pull a trouble code from the powertrain control module to verify why the engine light is on. Do not replace any parts until this is done, otherwise your going to be guessing at the root cause, and it will get expensive with part replacement.

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