It was time to rebuild my power steering pump, and I also wanted to modify it to increase steering response during high-rpm cornering. The rebuild is really simple and inexpensive, and like most things it just requires some patience and attention to detail. Due to the lack of any comprehensive information on the subject other than a few sparse pages, I decided to write this tech article for future reference.
Note: This article assumes the pump is on the bench and ready to be taken apart. Refer to a service manual for information on how to remove the PS pump from the car.
What you will need
Dissasembly
1. Remove the return hose nipple (suction connector), pressure switch, and flow control valve assembly from the pump. Sitting under the pressure switch plug is a small spring and plunger, and a small piston rod. Once the plug is unscrewed (slowly to relieve the spring pressure), carefully tip the pump over and these pieces should fall out. Also note the o-ring behind the suction connector plate, 17.8mm x 2.4mm.
The flow control valve assembly is made up of the upper connector (actually two parts), a spring, and the lower valve itself. We will be separating the upper connector and replacing the o-rings later, but for now the entire assembly can be removed. Once the connector is unscrewed, tip the pump and the spring and valve should slide out.
For some reason, I left the pressure switch plunger out of the picture below. It should be sitting between the spring and small piston rod at the top right. You can see it's actual location in the last assembly pic in the article.
2. With the pump sitting face down on the pulley, remove the four bolts and pull the pump cover straight off, and then remove the cam ring. The cam ring may stick to the cover when you remove it, so be prepared for that. If it does, simply pull it loose from the cover and set them both aside (the cover has two guide pins that the cam ring sits on). If it doesn't stick, it will just be sitting in the housing and can be lifted out. Remove the odd-shaped o-ring between the cover and pump body.
3. Once the cam ring is removed, you will see the rotor and ten small vanes. Some of the vanes may be sitting partially out of the rotor...as the pump spins, these small vanes are thrown outwards by centifugal force against the cam ring, and are what actually pumps the fluid.
Note: If you look closely, the vanes have one end that is flat, and one end that is rounded. This is important, because they must go back in with the rounded end facing outwards towards the cam ring.
The FSM states that there should be no more than .06mm (.0024") of gap between the vanes and rotor slots. According to the manual, Mitsu offers an "oil pump cartridge kit" which includes the cam ring, rotor, vanes, and snap ring. Check with your local dealer for availability and part number for this kit if your's are worn and out of spec. (Apparently, they offer a pulley and shaft kit as well).
The rotor assembly is held in place by a small snap ring on the pump shaft, which needs to be removed. Use some needle nose or circlip pliers to spread the ring enough to get a pick or screwdriver under it, and then carefully pry it off. It will be replaced with a new one from the kit, so it doesn't matter what shape it's in once it's removed.
4. With the snap ring removed, you can lift out the rotor, vanes, and the side plate that sits under the rotor. I cupped my hand over the shaft end, and flipped the pump upside down while holding the vanes in place, but you may want to just slide everything up and off the shaft. Be careful to not lose the vanes...they will just fall out of the rotor slots.
Once the rotor and side plate are out, you can remove the pulley and shaft assembly from the front of the pump housing. You may need to lightly tap the end of the shaft with a plastic or rubber hammer if it doesn't slide out easily.
You'll find two o-rings underneath the side plate; one inside the shaft bore, and a larger one in a groove around the outside edge. (These may have stayed in the pump housing when you removed the side plate). The sizes are 16.8mm x 2.4mm and 47.2mm x 24mm.
The picture below shows the back of the side plate and the outer o-ring; the rotor and vanes are underneath it, resting in my palm.
5. Flip the housing over, and use a flat blade screwdriver to carefully pry out the front seal. Check the rest of the housing for any remaining o-rings, and remove them. Match them up with their new counterparts from the kit as you do so, but don't mix them up with the new ones!
Cleaning and Painting
After removing all the o-rings from the housing, clean it really well with some carb cleaner or something similar and let it dry. Be sure that everything has been removed so no parts get lost, particularly the small piston rod that sits behind the pressure switch. Also be sure to clean and inspect all the individual pieces before reassembly.
Although not really necessary, I decided to blast and paint my PS pump components since it was going back on a brand new build. I first masked all the openings on the housing really well with 3 layers of masking tape and then blasted it with 80 grit glass beads. I sprayed the pulley with gloss back engine paint after covering the shaft with some rubber hose, and painted the return hose braket and rear cover with semi-flat black engine paint.
Flow Control Valve Modification
The spacers mentioned in this section only need to be added if you want to modify the pump to eliminate the high-RPM cut out that these cars have. DO THIS AT YOUR OWN RISK! Otherwise, you can skip to the last step and just replace the four o-rings.
Below is a bit of history about the high-RPM steering cutout issue, taken from the VFAQ article:
So, we are going to preload the spring inside the flow control valve connector.
1. Remove the top part of the connector from the bottom part. I put the connector in a vise and used a phillips screwdriver through the fluid port. The top part unscrews CCW like a normal bolt, but it may be tight.
2. Place the lower part of the connector in a large deep socket (or something similar; the taller the better) supported on its outside diameter, and make sure the small dark colored cap at the very bottom is not hitting anything. Put a long thin phillips screwdriver through the threaded hole in the top until it hits the bottom cap - make sure it goes all the way down and is centered. Tap the screwdriver handle gently with a hammer until the cap pops out. (You may have to tap a little harder, but take your time so as not to knock the cap into your bench).
3. Once the cap is off, you can remove the spring and plunger.
We will be putting our spacer(s) in the base of the plunger, under the spring. This will preload the spring, making it more difficult to compress as the high-RPM pressure increases. For mine, I used two small lock washers that I flattened and then spread open slightly to fit the bore a little better. My total thickness ended up being 2.4mm, compared to the 3.5mm mentioned in the VFAQ. I'll post back with results once they are available.
4. After cleaning and inspecting the parts, put the plunger back into the lower connector, put the spacers into the plunger, and then insert the spring. Be sure that the spring is sitting flush and can be compressed evenly. Find a small socket that will slip over the "nozzle" on the cap, and use a vise to press the cap back into the connector. The cap should bottom out just slightly past the point of being flush with the bottom of the housing.
5. Replace the o-rings on both the upper connector half and the lower part, and screw them back together. Be sure to use some fluid on the o-rings as a lubricant. From top to bottom (outside the pump to inside), the flow control valve o-ring sizes are 11mm x 1.9mm, 13mm x 1.9mm, 17.8mm x 2.4mm, and 13.5mm x 1.5mm.
That's it...you can now set the flow control valve assembly aside.
Note: This article assumes the pump is on the bench and ready to be taken apart. Refer to a service manual for information on how to remove the PS pump from the car.
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What you will need
- A Mitsubishi PS seal kit, part # MR244546 (around $20-$30 from a dealer).
- 10mm, 12mm, 17mm, and 19mm sockets or wrenches. You'll also need a large crescent wrench to remove the flow control valve (it's something around 26mm or larger, which I don't have).
- A torque wrench
- A small diameter phillips screwdriver (3/16" shaft or less).
- A flat blade screwdriver
- A dental pick or similar instrument for removing o-rings
- A can of carb cleaner or similar
- A hammer
- Various sockets to use as make-shift tools (for example, to install the front seal).
- Some Dexron or Dexron II automatic transmission fluid (the factory manual specifies MOPAR ATF PLUS (Automatic Transmission Fluid Type 7176).
- Needle-nose pliers (circlip pliers are helpful, but not absolutely necessary)
- Shop rags or paper towels for cleaning up spilled fluid.
- A factory service manual is highly recommended
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Dissasembly
1. Remove the return hose nipple (suction connector), pressure switch, and flow control valve assembly from the pump. Sitting under the pressure switch plug is a small spring and plunger, and a small piston rod. Once the plug is unscrewed (slowly to relieve the spring pressure), carefully tip the pump over and these pieces should fall out. Also note the o-ring behind the suction connector plate, 17.8mm x 2.4mm.
The flow control valve assembly is made up of the upper connector (actually two parts), a spring, and the lower valve itself. We will be separating the upper connector and replacing the o-rings later, but for now the entire assembly can be removed. Once the connector is unscrewed, tip the pump and the spring and valve should slide out.
For some reason, I left the pressure switch plunger out of the picture below. It should be sitting between the spring and small piston rod at the top right. You can see it's actual location in the last assembly pic in the article.
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2. With the pump sitting face down on the pulley, remove the four bolts and pull the pump cover straight off, and then remove the cam ring. The cam ring may stick to the cover when you remove it, so be prepared for that. If it does, simply pull it loose from the cover and set them both aside (the cover has two guide pins that the cam ring sits on). If it doesn't stick, it will just be sitting in the housing and can be lifted out. Remove the odd-shaped o-ring between the cover and pump body.
You must be logged in to view this image or video.
3. Once the cam ring is removed, you will see the rotor and ten small vanes. Some of the vanes may be sitting partially out of the rotor...as the pump spins, these small vanes are thrown outwards by centifugal force against the cam ring, and are what actually pumps the fluid.
Note: If you look closely, the vanes have one end that is flat, and one end that is rounded. This is important, because they must go back in with the rounded end facing outwards towards the cam ring.
The FSM states that there should be no more than .06mm (.0024") of gap between the vanes and rotor slots. According to the manual, Mitsu offers an "oil pump cartridge kit" which includes the cam ring, rotor, vanes, and snap ring. Check with your local dealer for availability and part number for this kit if your's are worn and out of spec. (Apparently, they offer a pulley and shaft kit as well).
The rotor assembly is held in place by a small snap ring on the pump shaft, which needs to be removed. Use some needle nose or circlip pliers to spread the ring enough to get a pick or screwdriver under it, and then carefully pry it off. It will be replaced with a new one from the kit, so it doesn't matter what shape it's in once it's removed.
You must be logged in to view this image or video.
4. With the snap ring removed, you can lift out the rotor, vanes, and the side plate that sits under the rotor. I cupped my hand over the shaft end, and flipped the pump upside down while holding the vanes in place, but you may want to just slide everything up and off the shaft. Be careful to not lose the vanes...they will just fall out of the rotor slots.
Once the rotor and side plate are out, you can remove the pulley and shaft assembly from the front of the pump housing. You may need to lightly tap the end of the shaft with a plastic or rubber hammer if it doesn't slide out easily.
You'll find two o-rings underneath the side plate; one inside the shaft bore, and a larger one in a groove around the outside edge. (These may have stayed in the pump housing when you removed the side plate). The sizes are 16.8mm x 2.4mm and 47.2mm x 24mm.
The picture below shows the back of the side plate and the outer o-ring; the rotor and vanes are underneath it, resting in my palm.
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5. Flip the housing over, and use a flat blade screwdriver to carefully pry out the front seal. Check the rest of the housing for any remaining o-rings, and remove them. Match them up with their new counterparts from the kit as you do so, but don't mix them up with the new ones!
You must be logged in to view this image or video.
Cleaning and Painting
After removing all the o-rings from the housing, clean it really well with some carb cleaner or something similar and let it dry. Be sure that everything has been removed so no parts get lost, particularly the small piston rod that sits behind the pressure switch. Also be sure to clean and inspect all the individual pieces before reassembly.
Although not really necessary, I decided to blast and paint my PS pump components since it was going back on a brand new build. I first masked all the openings on the housing really well with 3 layers of masking tape and then blasted it with 80 grit glass beads. I sprayed the pulley with gloss back engine paint after covering the shaft with some rubber hose, and painted the return hose braket and rear cover with semi-flat black engine paint.
You must be logged in to view this image or video.
You must be logged in to view this image or video.
Flow Control Valve Modification
The spacers mentioned in this section only need to be added if you want to modify the pump to eliminate the high-RPM cut out that these cars have. DO THIS AT YOUR OWN RISK! Otherwise, you can skip to the last step and just replace the four o-rings.
Below is a bit of history about the high-RPM steering cutout issue, taken from the VFAQ article:
The idea:
Speed sensitive steering is a feature available on many different cars. Its job is to give the car a more stable and heavy feel at highway speeds, and to make the car less prone to sudden changes in steering angle at speeds where such a move would be dangerous. It also allows more steering assist at low speeds for parking maneuvers.
The Problem:
Speed sensitive steering is great, but that's not what our 2L (Turbo & NA) DSMs are equipped with. We have RPM sensitive steering, which acts somewhat the same. It will decrease the steering boost at highway speeds because the engine is turning 3500-4000 RPM, but unfortunately it severely cuts the steering boost in the upper-rev range, regardless of what gear the car is in. 6000 RPM through an auto-x slalom results in minimal steering assist, followed by a sudden lack of any assist. Not exactly what an auto-x driver is looking for.
The Solution:
The steering assist reduction is caused by a valve in the power steering pump itself. We will be adding a spacer to preload the spring inside the small assist varying valve that closes at high RPM. This will stop it from restricting the flow of PS fluid out of the pump at high RPMs, but will not raise the maximum pressure of the fliud leaving the pump.
Some people have in the past shimmed the flow bypass valve in the pump. This is a separate assembly that sits below the assist varying valve. This resulted in more steering assist at high speeds, but accomplished this in a bad way. The flow bypass valve is there to relieve the high pressure created when the pump spins faster (high RPMs) and the flow exceeds that which is needed by the power steering system. If you shim this valve the fluid has nowhere else to go, and its pressure will increase far beyond the factory levels. I tried this, but after a 15 minute drive at moderate speed my fluid boiled and I lost steering assist. Not good.
So, we are going to preload the spring inside the flow control valve connector.
1. Remove the top part of the connector from the bottom part. I put the connector in a vise and used a phillips screwdriver through the fluid port. The top part unscrews CCW like a normal bolt, but it may be tight.
You must be logged in to view this image or video.
You must be logged in to view this image or video.
2. Place the lower part of the connector in a large deep socket (or something similar; the taller the better) supported on its outside diameter, and make sure the small dark colored cap at the very bottom is not hitting anything. Put a long thin phillips screwdriver through the threaded hole in the top until it hits the bottom cap - make sure it goes all the way down and is centered. Tap the screwdriver handle gently with a hammer until the cap pops out. (You may have to tap a little harder, but take your time so as not to knock the cap into your bench).
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3. Once the cap is off, you can remove the spring and plunger.
You must be logged in to view this image or video.
We will be putting our spacer(s) in the base of the plunger, under the spring. This will preload the spring, making it more difficult to compress as the high-RPM pressure increases. For mine, I used two small lock washers that I flattened and then spread open slightly to fit the bore a little better. My total thickness ended up being 2.4mm, compared to the 3.5mm mentioned in the VFAQ. I'll post back with results once they are available.
You must be logged in to view this image or video.
4. After cleaning and inspecting the parts, put the plunger back into the lower connector, put the spacers into the plunger, and then insert the spring. Be sure that the spring is sitting flush and can be compressed evenly. Find a small socket that will slip over the "nozzle" on the cap, and use a vise to press the cap back into the connector. The cap should bottom out just slightly past the point of being flush with the bottom of the housing.
You must be logged in to view this image or video.
5. Replace the o-rings on both the upper connector half and the lower part, and screw them back together. Be sure to use some fluid on the o-rings as a lubricant. From top to bottom (outside the pump to inside), the flow control valve o-ring sizes are 11mm x 1.9mm, 13mm x 1.9mm, 17.8mm x 2.4mm, and 13.5mm x 1.5mm.
That's it...you can now set the flow control valve assembly aside.
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