The engine runs fairly smooth at idle but when you put it into drive it runs rough. We have replaced the fuel filter, fuel pump , fuel regulator, fuel injectors, plugs and wires, and the throttle position sensor as well as the map sensor. The only codes thrown were the tps and the map sensors. The compression checks out great and the engine does not smoke. At an idle if you full throttle it the engine cuts out and surges. We cannot find any vacuum leaks either. Funny, it passed emissions as well earlier in May of this year as well. Now what?????
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Re: 1993 pontiac grand prix 3.1 liter
The 3.1 had some EGR problems, EGR valve stuck open, blown diaphrams, Plugged passages Im not saying that is all your problems and I believe yours has whats called a linear EGR about $350.00 from the dealer (and that is the only place you ever consider buying such a critical part for the emissions system regardless of the cost)
That car is so antiquated as far as computers go that relying on the codes is nearly pointless! Map code can set because of low engine vacuum which you will have with a poor running engine, Check for 5 volt reference voltage at the TPS and MAP with the key on and engine off if no 5 volt reference I suspect a Computer. Computer Ground is on the Thermostat housing and needs to be clean and secure.
There is a legit test in the GM factory manual called the TAP TEST take the ECM (computer) and slap it with an open hand, if the symptoms change the ECM is bad it's very common in the early-mid 90s for the computers to go bad in one way or another the circuit boards come loose and suffer also from cold solder joints.
Make sure the wires are on the correct plugs and hopefuly you used factory plugs and wires and No split fire or bosch plugs only AC delco
Make sure you have good spark on all 6 cylinders.
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Car idle is rough Inspection Service & Cost
A rough idling engine can be caused by a number of issues, some of them are serious while others tend to be minor, but the symptoms remain pretty much the same. The car will feel rough and bouncy when the engine is running. The car will also idle below its regular speed, display inconsistent RPMs and may produce a shaking, skipping or slipping sound when the vehicle is running.
While a rough idling engine may seem to be a simple inconvenience it often indicates a deeper problem within the engine. The vehicle should be inspected and repaired as soon as possible because small problems have a way of turning into expensive repairs.
How this system works:
The idle speed of an engine is basically the rotational speed the engine runs on when it is un-coupled from the drivetrain and the throttle pedal is not being depressed. The idle speed is measured in the revolutions per minute of the crankshaft.
When an engine is running at idle speed it generates enough power to smoothly operate equipment such as the water pump, alternator, and power steering but not enough power to move the vehicle itself. A passenger car will usually idle between 600 RPMs and 1000 RPMs. A properly functioning idle should run smoothly without skipping or slipping.
Common reasons for this to happen:
Dirty Fuel Injectors: The fuel injection system injects fuel into the cylinders, which creates a mix of air, and fuel to ignite and burn. Fuel injectors have tiny nozzles to spray the fuel into the cylinder and they can become clogged over time. clogged or failing fuel injector creates a lack of fuel in the vehicle's engine. This can cause a rough idle, it can also cause symptoms such as slow acceleration or the car not feeling as if it has enough power. If the problem is addressed early, it is possible to clean the injectors, which will restore them to full function. If this condition is not addressed in a timely manner the injectors will need to be replaced. Incorrect Idle Speed: While the average idle speed falls between 600 to 1,000 RPMs, if your vehicle is experiencing a rough idle it could be due to an incorrect idle speed setting. A trained mechanic can easily adjust the idle speed, and it should stay at the proper speed. If an adjusted idle speed becomes inconsistent or changes at random intervals there may be a bigger problem that needs to be explored. Vacuum Leak: If the vacuum system has a leak, it can seriously affect the ability of the vehicle's computer to regulate the air to fuel ratio. This can lead to a rough idle and if the problem is not addressed the car may experience slow acceleration and a lack of power. Vacuum leaks should be inspected and repaired immediately. Incorrectly Installed or Damaged Plugs: Spark plugs are responsible for creating the spark that allows the vehicle to burn fuel. If spark plugs are improperly installed or malfunctioning, the idle speed can be affected. The vehicle's engine may vibrate or there may be slipping or straining sounds coming from the engine. Defective or Clogged Fuel Pump: A rough idle can be related to fuel delivery issues. The fuel pump, which is responsible for pulling fuel from the gas tank to the fuel injectors, can become clogged or defective. If this happens the engine will not get enough fuel, which can cause a rough idle, sputtering, stalling and even slow acceleration. Clogged Fuel Filter: A clogged fuel filter can cause similar problems. The job of the fuel filter is to screen out contaminants in the fuel, over time it will become clogged and need to be replaced. A rough idle is one symptom of a clogged fuel filter. Failing Electrical Components: A problem or failure in the ignition system or various electronic components can cause a rough idle. If this is the case, the problem will usually get worse as RPMs increase. Common culprits include the ignition control module, plug wires, coils, and spark plugs. Defective Airflow Sensor: A defective airflow sensor can be responsible for a rough idle. A mass airflow sensor detects the amount of air coming into the fuel injection system and sends that information to the vehicle's computer. The computer uses that data to deliver the proper amount of fuel to the air in the vehicle. Over time these sensors can malfunction or become dirty. One of the first symptoms of a malfunctioning airflow sensor is a rough idle. The car may also accelerate slowly and even stutter or stall as the problem progresses. Dirty Oxygen Sensor: Oxygen sensors measure how rich or lean the gases are as they exit the combustion chamber. Depending on the results, the amount of fuel entering the engine is adjusted by the vehicle computer. The ultimate goal is to maintain an ideal mixture that produces the lowest emissions. A dirty or failing oxygen sensor will usually trigger the check engine light and can lead to a rough idle, lower fuel efficiency and failed emission test.
It could be a fuel problem, but in most cases the rough running would continue at road speed. You could certainly change the fuel filter and see what happens. A rough idle but no problems on the road would point to a vac problem or maybe a burnt valve. A fuel or ignition problem would be at most speeds including when you put the engine under a load. Its possible to have a faulty injector on one cylinder that is bad enough to cause a miss at idle but smooth out on the road, but that is a rare occurance.
Have the fuel pressure at the fuel rail checked. If you just want to throw parts at it: Fuel filter by the tank first. Fuel pressure regulator next. Order of ascending expense and likely hood. Does the fuel pump hum louder lately? The fuel pump may be on it's way out. It's not getting enough fuel to do anything but idle. Why that is takes some investigation. Somebody like Auto Zone may check your fuel pressure. If the fuel pressure is good, bad spark plug wires are the next guess. Lift the hood and run the engine in the dark (take the light bulb under the hood out if it still works to do this) If you see little lightening bolts traveling over the engine you need new plug wires. If the fuel pressure is good, the wires are good, change the spark plugs. Good Luck!
Also check the vaccum hose from the fuel pressure regulator to the intake manifold, it is right on top, to the front right of the valve cover where the fuel lines attach to the fuel rail, there is a small vaccum hose that runs from the regulator to the intake manifold, I have a 93 grand cherokee limited w/ 4.0L 6cyl, it was hard to start at times, wouldnt start at times without starting fluid spray and had your symptoms at idle, running rough like it wanted to stall like it was out of gas, I had various opinions ranging from fuel pump was bad to the fuel filter to the fuel presure regulator, the fuel pump relay ,etc....after replacing the relay and having the same symptoms, some digging brought me to this vaccum hose, wich had a break in it, when it was replaced it allowed the regulator to have the correct fuel pressure output and solved all the afforementioned problems way cheaper than the suggested alternitives. good luck.
My first guess would be a dirty carb/fuel injector set. Run some fuel cleaner through the tank and see if it gets better. You might have to do it through a couple of tanksful. How long since you've had a tune-up? It might just be that time.