I decided to make this simple instructional to help those folks who clearly asked the same questions I did, and got no response or the simple "you're out of luck, gotta do it the hard way" answer. Everywhere I looked online, everyone says you basically have to lift the entire engine/transmission in order to replace the rear knock sensor or you have to disconnect the engine and transmission from each other in order to access this part. This is a LOT of work and most people do not have an engine crane or the confidence and experience necessary to disconnect all these components and reconnect them correctly (and safely). The method I used did the trick, it is still not very easy, but it is possible and easier for those with less confidence, experience, and tools. Put in the time and effort and it can be done. A slight modification to a socket is required, this can be done with a metal cutting hand saw, reciprocating saw, chop saw, or Dremel (I used the Dremel, it was what was available to me, just make sure the cut is straight).
Note: I will update with pictures when I can, but I am a busy man so I cannot provide a timeline for when I'll get that done.
2001 Pontiac Grand Prix 3.8L Naturally Aspirated (Instructions also apply to Supercharged Models and all models between 1997 and 2003)
1. Collect your tools
In my case I have a full boat craftsman ratchet set, but bare minimum for just the knock sensors would be:
6" long 3/8" drive extension,
3" long 3/8" drive extension,
3/8" drive ratchet,
3/8" drive 7/8" regular socket,
3/8" drive 7/8" deep socket,
3/8" drive 13mm deep socket,
3/8" drive 10mm deep socket
A long thin flathead screwdriver
An extra long crows foot or extra long needle nose pliers (I highly recommend the crows foot)
5 gallon bucket,
Small extendable magnet,
(3/8" drive torque wrench is recommended but not necessary)
(Rolling Floor Creeper is useful and recommended but not necessary)
2. Jack up the vehicle and support it on the stands on either side just behind the tires underneath the front doors, personally I recommend jacking up all four corners of the vehicle but you can get by with just the front, just jack it up high to give yourself space to work.
3. Buy the parts you need, you will need to drain your coolant/antifreeze and you may need to remove the oil filter to do the front sensor so you might as well put the money down on an oil change.
5 quarts 5w-30 or 10w-30
2 gallons of Dexcool 50/50 (I recommend 70% distilled water to 30% dexcool for hotter climates, however you'd have to do the mixing yourself)
2 knock sensors (they are the same part, front and rear)
2 knock sensor connectors (may not need them but I'm certainly glad I bought them because mine were bad or going bad)
Red or Orange Heat Resistant RTV or New Exhaust Flange Gasket (Check step 10 to see if these parts will apply to you)
Shop towels and rags (I don't see how you could do this job without making a bit of a mess)
Time to go to work:
5. Make sure your engine is cold. After all your tools and parts are collected and the vehicle is supported properly, DRAIN YOUR COOLANT.
I have a buddy with a 1997 3.8L Supercharged model that has a drain valve.
NOTE: When opening the valve, DO NOT TAKE IT OUT!, we made this mistake and a small part of it went missing and upon reinstalling it the valve no longer sealed, so to be safe just open it until it begins to drain and let it finish.
However, I was working on a 2001 Naturally Aspirated model and it did not have a drain valve, in this case you must disconnect the lower radiator hose.
Place your 5 gallon bucket beneath your lower radiator hose. Your lower radiator hose is held on by a clasp that will have a small socket-like head with a slot for a flathead screwdriver. Work your way in with the screwdriver and loosen the clamp until your hose can be pulled off. Let the coolant drain entirely.
6. Drain your oil and remove the oil filter, DO NOT PUT A NEW FILTER ON AND NEW OIL IN JUST YET!
7. Your front knock sensor is located right next to where the oil filter goes.
NOTE: PLACE YOUR 5 GALLON BUCKET BENEATH THE KNOCK SENSOR
The moment you take the old knock sensor out, additional coolant will drain from the motor. When the coolant is finished draining, put your new sensor in and tighten it down firmly (18 ft/lbs for those who care).
8. You may now finish your oil change, replace the oil filter and put 4.5 quarts of 5w-30 in the motor. DO NOT REFILL YOUR COOLANT YET!
The Hard Part, the Rear Knock Sensor:
9. Now is when things get fun, you'll need to modify your 7/8" deep socket for this part. You'll notice when you
check your new knock sensor that there is a plastic connector on it. You don't want to damage this when you install it but a regular deep socket won't fit,it's too long. Measure how long the plastic connector is on the sensor (it should be exactly 1"). Measure the overall length of the deep socket (mine was 2 1/2"). Using a pencil or a screwdriver, check how deep your deep socket goes before you reach the bottom (mine was about 1 3/4" deep on the inside). My deep socket needed to be at least 1" deep, and it was a total of 1 3/4" deep, so I cut 3/4" off of the deep socket. Make sure your cut is straight and doesn't have an angle to it, if it is not straight the socket might not grab the sensor good enough and you'll struggle to put the sensor in.
10. Now that you have your socket, grab all your tools, crawl underneath the vehicle and find the rear knock sensor, you'll be able to see it through a small gap between the engine and transmission. A little further back is your exhaust pipe, for those with bigger hands and arms, you may want to drop the pipes and move them out of the way to allow easier access, the exhaust pipes are only held on by 2 bolts (13mm) to the header and several rubber hangers running down to the muffler. You will either want a new gasket at some point or apply some Red or Orange RTV to make sure the exhaust doesn't leak.
11. Take a look at the back of the header, if your exhaust system is factory original then it will have a heat shield, in my case the heat shield was held on tightly by two clamps (similar to what you'd see on a hand held tool box), I was able to pop these loose with my hands with just a little difficulty, but you may need to use the flathead screwdriver or whatever you think is better to get in there and loosen up the heat shield.
12. Once the heat shield is loose, grab your regular 7/8" socket and carefully drop it behind and underneath the right hand side of the heat shield. It should easily land on that gap (the gap is too small for the socket to fall straight through). Using your long needle nose pliers, long crows foot, or long flathead screwdriver, disconnect the wiring harness from the plastic connector on the knock sensor. Once the sensor is disconnected, use the long needle nose pliers or long crows foot to crush the plastic connector of the old knock sensor, BE SURE NOT TO DAMAGE THE PLASTIC CONNECTOR OF THE WIRING HARNESS! Then squeeze your fingers through the gap and position the socket correctly to seat it onto the sensor, grab your 3" and 6" extensions and connect them together, then connect them into the socket and with your ratchet and loosen the sensor until it comes free.
13. Using the extendable magnet, press in and under the heat shield and grab ahold of the old sensor and pull it out, using this same method, pull out the regular 7/8" socket and place your newly modified 7/8" deep socket and new knock sensor into and under the right hand side of the heat shield and let them fall onto the gap (the gap is too small for them to fall straight through). Then squeeze your fingers through the gap and start the threads of the new knock sensor into the engine. When you have tightened it as much as you can with your fingers, use the long needle nose pliers or long crows foot to continue tightening a few more turns, BE CAREFUL NOT TO SLIP AND DAMAGE THE PLASTIC CONNECTOR ON THE NEW SENSOR!. Keep turning until your newly modified 7/8" deep socket will fit over the new knock sensor. Then connect your ratchet and extensions and tighten the sensor down or torque it down (18 ft/lbs for those who care)
14. If you can, latch the heat shield back together (I didn't.....I was tired and it was 2am.....don't judge me). If you removed your exhaust system then bolt it back to the header after applying RTV or the new gasket to the flange, then reattach the rubber hangers. Make sure the lower radiator hose is attached and the clasp is tightened down, then fill the radiator and top of the reservoir with coolant.
VVVVV ***** VERY IMPORTANT ***** VVVVV
15. Trace your upper radiator hose until you reach a fitting bolted to the engine, on top of that fitting is a small bleeder bolt (10mm). Start your engine and loosen that bolt slowly and carefully DO NOT REMOVE THE BOLT! You may begin to see bubbling and some fluid come from the bleeder bolt, It will make a small mess, don't worry just let it continue to bubble until no more air comes out. Then tighten the bolt back down carefully and look at your cars coolant temperature gauge next to the speedometer. KEEP A CLOSE EYE ON THAT TEMPERATURE GAUGE! Watch the temperature rise carefully, the needle should stop at about half way or maybe a little higher. If it continues to heat up too much TURN THE ENGINE OFF BEFORE IT HITS THE RED LINE! Do not attempt to bleed the system a second time if the heat goes too high. Air in the system may not be the problem. Turn the engine off and wait for it to cool down, then unbolt the fitting from the engine and check the thermostat, if it is clogged and dirty then don't bother cleaning it just replace it. If it is clean you may want to replace it anyway, with a thermostat in place, bolt the fitting back to the engine, top off the radiator and coolant reservoir again and attempt to bleed the system. Then check your temperature gauge again. If it still continues to get too hot your problem may be your water pump. However, replacing that is an entirely different thread.
Rule #1 Too much heat destroys EVERYTHING!, Namely bearings and valves. That is why the cooling system is essential for the life of the vehicle.
^^^^^ ***** VERY IMPORTANT ***** ^^^^^
Wrap it up
16. Collect your tools, clean up any mess, and lower the vehicle off of the stands. Have your codes wiped so you can check if the problem has been fixed. Cheap code readers can go for about $40. Auto parts shops can also check and wipe your vehicles codes for little or no cost, Autozone and Oreillys both check and wipe codes for free. Bad knock sensors often cause a vehicle to run rough so you may notice immediately whether or not the repair was successful.
17. Go for a test drive and keep a close eye on the coolant and oil levels, temperature gauge, and check for leaks and puddles for the next few days. If everything was done correctly then your vehicle is ready for the open road again.
I hope will help and bring comfort to everyone first encountering this problem, folks who have already encountered it with dread of doing it again, and those who desperately want/need their Grand Prix fixed but don't have 100's of dollars to blow at a shop. This method is not entirely "Easy" But definitely works and it's definitely easier than the alternative. I hope to post pictures of what I've just written sometime in the future.