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by Bob Lincoln
If you have a MoPar with one of the rugged 2.2L or 2.5L engines, by now your car is aging enough that you might have to repair certain systems that normally are not part of the typical regimen. But once you reach 150K to 200K miles, you’re beyond the design life of the car and may find that you’re facing one of those once-in-a-lifetime jobs on the car. They’re way too expensive to pay a professional, but it may be hard to find good information on how to do the repair.
This article will deal with servicing the throttle body on normally-aspirated, fuel-injected engines. In this case, it’s a 2.5L TBI engine, using the Holley throttle body design of the late 1980s/early 1990s.
This example is my recently-acquired daily driver, a 1993 Dodge Daytona base model. Purchased in May 2009 with 156,000 miles, the engine has apparently never been apart. Typically, by this mileage, the throttle body is getting gummed up with evaporated gasoline and PCV vapors just like a carburetor does. This can block air passages and cause unusual behavior from the MAP sensor or other engine controls.
Within a few months of purchase, the car started exhibiting symptoms similar to those experienced years ago with my now-retired 1992 Daytona. I was getting idle flare and 'stuck throttle syndrome' from the '93 and it was getting worse with each passing week, to the point that at the end of every morning commute I could expect this behavior. When I put the clutch in, it revved to 2500 RPM or more, and would only drop if I kicked the throttle. The car would sometimes drive itself for over a mile or two at 40 mph on level ground without touching the gas. And it would jump forward slightly when I took my foot off the gas.
From experience with my '92, I knew this was not a stuck throttle, but the throttle body acting up - either sticking AIS (Air Idle Speed motor) pintle or a vacuum leak. And since the body gasket was gummy and there were mouse turds from previous ownership on the top of the TB, it was time to clean.
I had a sunny warm day, ideal. Plan on spending four hours removing, cleaning and reinstalling the throttle body. Yes, you can go faster, but you tend to break vacuum hoses, drop screws, make mistakes, or just not be thorough.
Tools needed include the throttle body rebuild (gasket) kit, a flat screwdriver, 10 and 13mm sockets, a T-25 torx bit, needlenose and slip-joint pliers, an adjustable crescent wrench, a razor knife, an old toothbrush, safety goggles, a torque wrench that measures up to 180 in-lbs or more, some penetrating oil and a full spray can of fuel injection cleaner. And shop towels or paper towels, and ideally an old saucepan over which to spray the cleaner and catch drippings.
You may need to buy the base gasket separately, as the kits usually contain a useless thin one. Autozone has the proper thick one for $7, or for our unlucky Canadian friends outside their delivery area, the dealers have them for about $17. This repair will be about $40-$50, vs. $500+ from a professional.
Remove the air cleaner by unscrewing the lid, removing the air intake tube, the hot air tube at the bottom, the air injection hose if so equipped, the tiny rigid plastic line to the thermostatic vacuum switch (passenger side) and the breather hose to the valve cover. Find a safe haven for all the screws and pieces that you remove, and organize them so you can remember what goes where. [Editor’s note: a digital camera can provide a safe backup to avoid confusion if you photograph as you go.]
Now remove the following, in no set order:
Electrical: Unplug the AIS motor on the front face, the Throttle Position Sensor on the passenger side, and the fuel injector on the right front diagonal. These are latched connectors, so gently lift the latch with a screwdriver tip before pulling. Don’t bend the latch too far, especially in cold weather, or it will snap off. (How do I know?)
For the fuel injector plug, there is a thin wire clip that wraps around the plug. Carefully pry one side of it partially away from the plug using a jeweler’s screwdriver or other fine flat tool, being careful not to let the clip spring away into the engine compartment. (How do I know?) I removed the PCV valve hose from the valve cover and pivoted it to get it out of the way. Remember to reinstall it when done.
Vacuum: There are four hoses to remove – the power brake booster clamped to the left rear, one hose on the right rear above the throttle plate, and two at the front base of the throttle body. Be especially careful of the last two – they are mated to rigid plastic lines that can break easily. Go slow, use patience and a combination of gently twisting the hose with needlenose pliers, and prying outward with a screwdriver tip. For the brake booster, access to the clamp was tough, so I removed it after the TB came off.
Fuel lines: There are two fuel lines, a supply and a return. There will be some residual fuel pressure, but if the car has sat for several hours, it’s minimal. It will only dribble out for a second. Always wear goggles when disconnecting fuel lines, or when using power tools, or when spraying the fuel injection cleaner. Using a crescent wrench, unscrew both lines at the metal hex nuts.
Throttle cable: On the left side, remove the throttle return spring(s), then remove the throttle cable. It will have a slug on the end of it, and to remove, pull some slack in the cable, rotate it toward the firewall and slide to the driver’s side to remove the slug from the cable mount. The cruise control cable comes out after it, also sliding to the driver’s side. Then remove the two 10mm bolts that hold the throttle cable plate on.
Now, using the 13mm socket, remove the four mounting bolts, two at the rear base and two at the top front of the throttle body. Lift the TB off the intake manifold, remove the base gasket, and stuff a clean rag in the intake to prevent any dirt or objects from falling in.
The primary job is to clean and replace the functional part – air passages, fuel injector, gaskets – the exterior is secondary, but worth doing once the important part is done. Note the dirty but not clogged air passages.
This generation of throttle body has an upper and lower half. The fuel injector, supply and return, and fuel pressure regulator are located in the top half, and the throttle plate with air passages and sensors in the bottom half. Spray some penetrating oil, allow to sit a few minutes, then unscrew the top half with a T-25 bit on the two rear torx screws, careful not round them off. The factory does put them in too tight.
Once separated, you'll typically see the leaky air gasket with gasoline all over it. This was a small air and gasoline leak and does affect mileage and performance slightly. Note the AIS plunger visible above the throttle plate. It moves in and out to add or subtract air from the mixture at idle and on deceleration. The air passage can be obstructed, or the motor’s pintle (tip) can stick. On my ’92, the tip was stainless steel; by ’93 Chrysler had cost-reduced it to plastic.
Now, use a razor knife or sharp chisel to scrape the old gasket off. Be careful not to cut yourself, or to gouge the soft aluminum body. Remove all traces of the gasket so the new one will seal properly.
Next, unscrew the TPS and AIS from the lower body with a T-25 bit, set them aside without losing the screws. You do NOT want cleaner on these parts, with the exception of just the tip of the AIS.
Wear goggles, work over an old saucepan with clean shop towels ready, and use the spray cleaner and a toothbrush to clean the gum and residue off the throttle body. Spray through the vacuum passages (use care when it comes out the other side) to ensure that they are not clogged. Spray, wipe, repeat. Work ONLY in a well-ventilated area away from sources of spark or flame. The fumes are noxious in a confined space. Gloves are recommended, as the cleaner can irritate and dry out your skin and nails. I cleaned the bottom half first, and when done, it appeared as below.
Now, on to the top half. The canister on top is the fuel pressure regulator. Remove the three T-25 screws and steadily pull straight up until it comes free. Set it aside for re-use. There is an oval rubber gasket and a small O-ring on the bottom that get replaced.
Next is the fuel injector. Remove the single T-25 screw from the black cap, which is its electrical plug. Pull or pry the cap straight up. Then turn the TB over and pry at the two latches on the electrical plug that’s attached to the side of the throttle body and pull the connector off the TB. This last step is not mandatory, but allows better cleaning access.
Now you can see the fuel injector with its two electrical terminals. To remove, put the tip of needlenose pliers or small screwdrivers into either side of the black part, where you see a hole, hold the tools horizontal, and pry slowly up until the injector comes out. Now you can spray clean and scrub the entire top half of the throttle body.
I found parting lines left from the casting process sticking out into the airstream, so I took a Dremel tool and a tapered grinding tip and ground them off, smoothing out the seam. Since air flow should ideally be straight and smooth (laminar) upstream of where the fuel is added, this will offer a slight, if immeasurable, improvement to performance. It only took a few minutes.
The next step would be to replace the upper and lower O-rings and the mesh fuel screen on the injector body. However, in my rebuild kit, the O-rings were not ‘fat’ enough in diameter to fit properly. And from past experience, it is difficult to slide the new mesh fuel screen on without damaging it.
Since the O-rings were in good condition, I sprayed it all down with cleaner (the mesh screen came clean), re-oiled the O-rings with engine oil, and pressed the fuel injector back in place, with the + terminal closest to the screw hole. Snap the electrical connector onto the edge of the upper throttle body, place the rounded cap plug over the injector and rotate the cap while it’s pressed onto the injector to line up its retaining screw.
Torque to 45 in-lb Put the new gasket and O-ring on the fuel pressure regulator, press it into place, torque the three screws to 45 in-lb.
Now place the thin gasket between the upper and lower body halves and torque the two rear screws to 45 in-lb Replace the O-rings on the AIS and TPS (the TPS might not have one from the factory, but I installed one to prevent attack from gas fumes), and torque the screws only to 20 in-lb. This is plenty tight enough, though the factory overdoes it and probably causes issues with the sensors later in life.
Remove the rag from the intake manifold, place the new thick base gasket down, and mount the throttle body to the manifold. It may be easier to connect the four vacuum lines first, so you can get your fingers in there easily. If you do, don’t flex the lines any more than can be avoided.
Install the 4 mounting bolts partially. Remove the old copper washers off the fuel lines, install new ones and screw them in enough to be sure they line up. Torque the 4 mounting bolts to 175 in-lbs (about 12 ft-lb), then tighten the fuel lines until very snug.
Bolt the throttle cable plate back on, hook up the throttle cable(s) and throttle return spring. Plug in the AIS, TPS and fuel injector. Make sure the PCV valve is re-connected. Check one more time to be sure all connections are re-established and there are no broken vacuum hoses. Install the air cleaner with all its hoses, and you’re done. Start engine and check for fuel leaks. There are no adjustments, so it should idle smoothly and normally. Take your cell phone with you and go for a test drive. There should be no more idle flare or stuck throttle feeling. If there is, scrutinize every vacuum hose carefully and check all harnesses and electrical connectors for damage or corroded contacts or looseness.
If you have any questions on this repair, please feel free to PM me. Happy motoring!
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