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Steve
Steve, Auto Service Technician
Category: Car
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Experience:  25+ yrs experience as a professional working technician; ASE L1 master technician
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1991 oldsmobile silhouette: v..running rough..idling..timing or vaccum

Resolved Question:

I need some advise on a tune up procedure done on my 1991 oldsmobile silhouette. About six weeks ago I brought my van to a local shop to have a tune up done,two days prior I changed the oil myself and was going to change the plugs ,but couldn't because the motor is a v-6 and i wasn't able to reach the plugs positioned at the rear of the motor. Moving on...I brought the van into local shop explained the engine was running rough not idling right and suggested the timing or vaccum adjusting along with replacing the plugs.We agreed to have the van tuned up and left my van overnight because it was around 4pm. The next day approx. 1pm Icalled the shop and the mgr. states that Van is ready for pick-up. I pick up the van and the MGR. states she still runs rough but we changed the plugs. I said what about the timing ? he says you may have an exhaust leak..WHAT.. I SAID SO WHAT WAS DONE? YOU JUST CHANGED THE PLUGS?!! HE SAYS BASICALLY YES. MY VAN STILL DOESN'TRUN RIGHT WHAT DO I DO?
Submitted: 4 years ago.
Category: Car
Expert:  Steve replied 4 years ago.

Hello!

To begin with, the term "tune-up" is kind of out of date; that is a term used to describe a maintenance service that was done on older cars and really is not used any more. On older cars, every mechanic had a different opinion of what was included in a tune-up service, but typically it included replacing spark plugs, air filter, fuel filter, distributor cap and rotor, ignition points, adjusting ignition point dwell and timing, and cleaning and adjusting the carburetor.

On newer engines there are no adjustments possible at all, since the computer controls everything. On yours though it is possible to adjust ignition base timing, although it should never need to be done unless the engine has been disassembled or the timing chain is badly worn enough to change the sync of the distributor. If the distributor has not been rotated, checking ignition timing is not likely to have any effect on the problem you are having.

Replacing spark plugs in this engine is not easy, as you have noticed. According to my flat rate guide, it takes an experienced mechanic 1.3 hours to change the plugs, which in most shops is somewhere around $100 or so in labor depending on their hourly labor rate. If you specifically requested that the shop check your timing, they should have done so, but you do not indicate whether or not you were charged for that particular service.

 

From the description you have, it would appear that the spark plugs are not what is causing the rough running condition youa re experiencing. You may want to take the car to a different shop, if you do not have a good working relationship with your current one or do not trust them.

Most mechanics would want to begin with a compression test on an engine this age, to make sure there are no leaking valves or other mechanical problems before moving on to other areas. Next would be a scope check of the ignition system; this test will show any ignition misfires, vacuum leaks, or fuel problems as these things change the scope pattern in different ways.

ASlso, checking to make sure the EGR valve is not stuck open at idle is something that should be done during the diagnostic process.

If your "check engine" light is on, that indicates there are diagnostic fault codes stored in computer memory that may be useful in diagnosing the cause of the problem.

 

 

 

 

Customer: replied 4 years ago.

DEAR XXXXX,

 

THANKS FOR YOUR HONEST OVERVIEW TO MY SITUATION. IT WAS HELPFUL AND INSIGHTFUL.COULD YOU PLEASE ANSWER 2 MORE RELATED QUESTIONS FOR ME SO I CAN ADDRESS THIS IN A CONFIDENT MANNER...

1st: I WAS ADVISED (from a friend) THAT IT COULD BE THE FUEL FILTER THAT IS CAUSING THE ENGINE TO SHUDDER AND MISS-FIRE and NOT IDLE SMOOTHLY, IS THIS A POSSIBILITY OR NOT?

2nd:ANOTHER FRIEND MENTIONED THAT THE CAUSE FOR THE ENGINE IDLING IMPROPERLY and POSSIBLE MIS-FIRING IS AN "IDLE CONTROL SENSOR or MODULE LOCATED NEAR THE CARB OR DISTRIBUTOR. yes OR no

Expert:  Steve replied 4 years ago.

Hello;

Although changing the fuel filter is probably a good idea if it has not been replaced within the last 20,000 miles or so, the fuel filter is not likely to be the cause of the problem youa re describing. If a fuel filter becomes restricted, it will usually first cause problems under high fuel demand conditions, such as when traveling at highway speed. At idle the engine requires the least amount of fuel, so a filter that is partially restricted will cause a power loss noticeable at high speeds but the engine will usually idle normally. A fuel pressure and volume measurement can identify whether you have any fuel delivery problems.

I am not sure what component your friend is referring to; there is no component called an idle control sensor on your engine. There is a device called an idle air control valve, which controls the idle speed by opening and closing an air bleed passage around the throttle plate. If an IAC valve becomes inoperable, it will cause the computer not be able to control idle speed: the idle will be at a fixed value too high or too low. It will not cause a rough running problem.

Your mechanic should be able to perform a few standard test procedures to systematically check and eliminate the various possible causes for the problem youa re having; no guesswork should be required to diagnose and repair this type of problem. By performing one or more of the routine diagnostic test procedures used to identify the cause of a rough running problem, your mechanic should be able to easily narrow down the possible causes.

To diagnose a rough running condition, your mechanic should recommend one or more of the following test procedures:

(1) Engine compression test to check for internal mechanical problems

(2) Cylinder balance test to identify which cylinder(s) are causing the problem

(3) Scope check of the ignition system

(4) Fuel pressure and volume measurements if a fuel problem of some kind is suspected

(5) Scan the computer for fault codes and live data if the check engines light is on

(6) Manually inspect the EGR valve for contamination if a stuck open EGR valve is suspected

(7) Smoke test the intake manifold and related components if an intake air leak is suspected

(8) Flow test fuel injectors if a faulty fuel injector is suspected

 

Steve, Auto Service Technician
Category: Car
Satisfied Customers: 5494
Experience: 25+ yrs experience as a professional working technician; ASE L1 master technician
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