1993 Jeep Grand Cherokee • 6 cylinder Automatic • 160,000 miles

Can I use a 36 psi fuel pump when changing a in tank fuel pump to a fire wall fuel pump.
December 23, 2010.

Fuel pressure is set by the pressure regulator on the fuel rail, not the pump. All you need is a pump that is capable of delivering more pressure than the regulator is set for. The highest pressure is commanded when intake manifold vacuum is lowest which is during heavy acceleration. Thanks to the vacuum hose on the regulator, it will drop fuel pressure when manifold vacuum is high such as during coasting.

Be aware insurance investigators just love to find these kinds of modifications from what came from the factory. Is there a reason you don't want to put the proper pump back in the tank?


Dec 23, 2010.
It's cheaper and will be less effort. In other words what you are saying is I can use a fuel pump that is high in psi then 39 but not lower?

Why would insurance co. Care?

For the same reasons every manufacturer has huge teams of lawyers on their staffs who must approve almost every design detail to protect them from lawsuits. Every mechanic and shop owner loses sleep every night worrying about things they might have done to your car that could make them liable for damages in the event of a lawsuit. Here's just a few examples: A body shop repairs a rusted out section of the car's frame. They weld in steel plates that make it much stronger and more reliable than original. Car is involved in a crash and the driver is paralyzed because the designed-in crumple zone didn't buckle. The driver's seat area crushed instead.

Tire store is out of stock on the tire size you need to replace just the one that got punctured beyond repair. They sell you one that is just one size bigger knowing full well it will not cause a handling problem. You can finish your trip and make it home. They did you a favor. Later that night the anti-lock brake warning light turns on because the computer detected a mismatched wheel speed. It set a diagnostic fault code in the computer and turned the system off. When you get close to home, another fellow runs a red light in front of you. You slam on the brakes but skid into his car, severely injuring him. A week later you find out you are being sued because you knew the safety feature on your car was not working but you continued driving anyway. That feature doesn't even exist on half the cars on the road, but the jury finds you partially at fault because the accident might have been prevented if you had the correct size tire installed earlier. You can't even argue the warning light wasn't on. By connecting the ABS computer to a scanner or the dealer's computer in the service bay, it will list at a minimum, how many times the engine was started since the light turned, and some newer models spell out how many hours and minutes ago of driving time the light turned on. Even after a mechanic erases all fault codes and other diagnostic information, all of that data remains in a different part of the memory that we can't access. Only the manufacturer can get to that information with their computers, and usually as a result of a request from an insurance company or law enforcement agency, or to protect themselves from potential lawsuits.

You need a brake job. The mechanic says you also need two new front rotors because they are worn too thin. How much too thin? They started out 1.200" thick, the published minimum thickness is 1.120" thick. The thickest one on your truck is at 1.117". It is too thin by the thickness of a sheet of paper. You know, the mechanic knows, heck, everyone in the world knows that will not cause a problem, but a good lawyer will convince a jury that the mechanic knowingly modified a part, (by machining it as part of a normal brake job), beyond what was approved by the manufacturer. He will forget to mention that had nothing to do with preventing a crash. All the people on the jury understand is the mechanic did something wrong. You can be guaranteed that shop owner will be sitting in the courtroom as one of the defendants even when the other driver caused the crash. This is one of the first things investigators look for.

They also look for rusted steel brake lines that were replaced by do-it-yourselfers. That pre-manufactured line was 6" too long, so you coiled it up. The extra loop caused it to be too close to the hot exhaust pipe so the fluid boiled creating air in the line and loss of half of the braking system. Lawsuit!

How about the shop that was sued after replacing the engine on a Dodge diesel truck? The engine comes from the factory with lifting loops on the front and back but the rear one was upside down. You flip it over to pull the engine. When you're done, you left it like that. It's not in the way of anything and is in the correct position if it is ever needed again. Years later the truck is involved in a head-on crash. A passenger is killed and you are named in the lawsuit as the primary defendant. Why? You didn't return the truck in the same condition it was when it was new. During crash testing, it was discovered that if the engine was shoved back far enough, the lifting bracket would catch the wiper motor and push it into the center front seat passenger's face. That's why there is a big warning message in bold letters in the service manual to flip that bracket over when you're done using it.

We've all heard the stories about people putting in fuses that are bigger than what was called for. Your insurance policy is worthless after a vehicle fire once the investigator finds that wrong-colored burned-up fuse. It might not even be related to the fire.

For years we've heard the stories of house fires being caused by Ford ignition switches burning up even when the engines are off. Your homeowners policy might bail you out, then go after Ford, but I've started hearing stories about insurance companies refusing to pay because the cause of the fire is a well-documented, known problem, and you continued parking the vehicle in the garage anyway.

There is no better training to be a mechanic than on-the-job training, but you can get the equivalent of five years of on-the-job training in a two-year program in a community college. Every instructor will include many hours worth of discussion on liability issues, just like doctors get. I also got tons of this kind of information from attending various manufacturer's one and two-day classes. Another earlier source was talking with an insurance adjuster for All State when I worked at Sears many years ago. He showed me the trunk of his Chrysler Fifth Ave. It was full of boxes containing a sheet for every car brand and model on the road. Every sheet showed all the places to look for a vehicle ID number, where fuel lines were to be located, how the battery was held in place, brake component sizes, even alignment angles. They even look for things like those ridiculous amplifiers kids put in the trunk. If you're rear-ended and a passenger in your car is injured by that non-standard equipment, better hire a lawyer quick, even though the crash was caused by someone else. Want to add video entertainment equipment, a remote starter toy, floor mats? There is reason all of those things cost much more from the dealer than from Walmart. The manufacturer has tested them for any imaginable kind of liability issue for your specific car model. I'm not saying the dealer's versions of a certain product are a better value for your money, but I AM suggesting there is a lot less chance of a future lawsuit related to that product.

Getting back to your aftermarket fuel pump, there is every possibility it will work fine and never cause a problem, but as I often asked my students, "what if"? It's the "what if" principle we have to always keep in the back of our mind when working on any car. What if I lie and tell the customer his brake rotor is ok to be reused so I look like the good guy and save him a few bucks. He won't know the difference. But if that car is involved in a serious crash and the insurance adjuster learns it had recent brake work, you can be sure he is going to pick the car apart very carefully, and he WILL find that slightly under-size rotor. It won't be the mechanic sitting in the courtroom. It will be his FORMER boss. At most new car dealerships, an under-size rotor, whether intentional or because of incorrect measuring techniques, will get you a strong verbal warning once from the service adviser followed by about an hour-long meeting with the service manager or dealership owner and a written followup in your personnel file. If you're caught a second time, you'll be asked to pack your tools and go look for a job somewhere else. All because of the thickness of a sheet of paper on a brake rotor that is well over an inch thick! Conscientious business owners just can't risk the potential lawsuits over this kind of negligence. They have enough sleepless nights worrying about all the frivolous lawsuits from all the idiots who need someone else to blame.

Now, lets turn the story around a little and look at the pump you're trying to install. If this is a new product you found in a store, it will likely come with a sheet full of all kinds of warnings that you should read. That company is setting itself up to be a defendant in a lawsuit. What they probably have going for them is a lot of safety-related testing, legal advice, and good insurance. One good lawsuit can put a company out of business and their employees out of work. And no one has better lawyers than the manufacturers, and still, look at how many deaths and lawsuits Ford has been involved in since their exploding gas tanks of the 1970s. Now it's Explorer tires and Windstar rear axles. Their lawyers didn't see those things coming and they're way better than anyone you could hope to hire.

I'm glad you asked "why would they care" rather than just dismiss the warning like most people would do. It gave me the chance to share a little background into the professionals' world. Just remember that everything a doctor does or the tests he orders is to cover his butt and prevent lawsuits. We pay the price for other people's lawsuits in the form of higher health care costs and unnecessary tests. Heck, we pay twice as much as necessary for step ladders because too many people sued rather than use common sense or take responsibility for their actions. Getting your car fixed is the same. When you look at the list of shop owner's expenses, you will not be able to figure out how they can stay in business by only charging around $100.00 per hour. There's the free coffee and donuts, shuttle service, cable tv, magazines, parking lot lights and snow plowing, heat, two dozen different taxes, and a dozen different insurance policies. Sure, they have fancy, expensive showrooms and facilities, but those cost a tiny fraction of the total they spend to stay in business.

As a final parting thought, here's a recent lawsuit from a part of the country famous for such things; a New England state. The owner of a GM minivan took it to a small independent shop to have the oil changed and the tires rotated. The owner of a Dodge minivan was sitting at a stoplight, first in line, waiting for it to turn green. Without warning, a tire came off the GM minivan, bounced through the intersection, through the Caravan's windshield, and killed the driver. Who is at fault? Actually, in the lawsuit, there was no dispute over who was at fault. The lawsuit was to determine who to sue!

Can't sue the mechanic who didn't use a torque wrench on the lug nuts. He's protected by being an employee of the business. The business owner didn't have insurance. As soon as he heard of the crash, he went out of business and put his employees out of work. Can't sue GM. Proper procedure wasn't followed when tightening the lug nuts. There was nothing defective in the design of the wheel, vehicle, or lug nuts, and they had no control over what people did to their product later. We have to sue someone and it will always be whoever has the deepest pockets. The GM van owner's insurance company denied responsibility since the owner didn't do anything wrong. For liability, they insured the driver, not the product against defects. The guy who was killed isn't going to sue himself, (well, his estate, actually), because he didn't do anything wrong. That only leaves Chrysler, so the lawyers had to figure out how they could shift the blame to that company. Turns out the windshield was only designed to help protect the occupants in a rollover crash. It was not designed to protect the driver from a flying wheel and tire, and that was the basis of the lawsuit. Thank goodness for lawyers who are experts at making innocent parties the ones who have to pay.

The point of all this is not to admonish you for using a non-standard part. It is to cause you to reflect on the potential problems so you can make an informed decision. You still didn't say why you want to use this pump. The Engine Computer turns the in-tank pump off when the engine stops rotating. That is to prevent a fire hazard if a fuel line becomes broken in a crash. If you wire up a different pump to turn on with a switch on the dash, you could go from being simply unconscious after a crash to being a french fry!


Dec 24, 2010.
I did tell you why I wanted to use this pump. My answer was "It's cheaper and will be less effort"
I have very few funds and I am also disabled so I have to look for the easiest and cheapest way to do things regardless of law suits or insurance companies.
Thanks for explaining law suits to me but you didn't answer my question of.
"In other words what you are saying is I can use a fuel pump that is higher in psi then 39 psi but not lower?"
Can you please help me with this question.
I just want to know if I could change a in tank fuel pump to a fire wall fuel pump and how high the psi could be, I can not find a 39 psi but I can get a 36 psi or a 45 psi and I wanted to know if either of them would work.

Yup. Didn't see your first of the two replies until after I posted my last response, then the 'puter crashed, ... Again!

All you need is a pump with the capacity to develop more pressure than the regulator is asking for. Think of the oil pump in an engine. There is always a pressure relief valve that just dumps the extra oil back into the pan. The pressure is set by the strength of the spring in the relief valve, not by the pump, as long as the pump has the capacity to build enough pressure.

The fuel pressure regulator works the same way. Most in-tank pumps can develop over 60 psi if you block the return hose between the regulator and the tank, but the regulator holds pressure to a very specific value, usually somewhere between 45 and 55 psi for multi-point fuel injection, and closer to 15 psi for throttle body injection.

Besides pressure, you must consider volume too which is rarely a problem since the pump will move way more fuel than is needed. Volume will be the highest during coasting. About the only thing that will prevent sufficient volume is a collapsed pickup screen on the in-tank pump. Your external pump won't solve that problem but a plugged screen is not terribly common. The symptom would be stalling when coasting and it would run fine when accelerating. I know at first that doesn't seem logical. It's a long explanation, but I'll post if you're interested.

The highest pressures occur when accelerating so if the pump was not able to reach the desired pressure, the symptom would be dieing out or sputtering when accelerating.

Unlike GM pumps, Chrysler pumps rarely quit once they're running. If you have an intermittent stalling problem after the engine warms up, a more likely cause is a failing camshaft position sensor or crankshaft position sensor. The clue is you won't have spark either.


Dec 24, 2010.