Okay fuel pressure. I had two tests done, each by a different mechanic. Is the fuel pressure test different from the fuel pressure regulator test ?
Similar conceptually, but In mechanics parlance, a fuel pressure regulator test would normally just make sure the device itself is holding vacuum and no gasoline appears in the vacuum line. A" fuel pressure" test uses a fuel gauge connected to the fuel rail to measure the fuel rail pressure. They’d verify the fuel pressure was correct both at idle and responds in the right direction during a bump in engine rpm when the intake vacuum decreases. They might also do a test at the same time that measures the fuel pump pressure. It’s called a dead-head test I think. Something like that.
My 2nd mechanic did a fuel pressure test. He got a constant reading of 47 psi. That is, it did not change when I hit the gas. He said that was good.
My 3rd mechanic did a test and got a reading of 51 psi. The 51 psi did not change when he disconnected a hose nor when I hit the gas. He says that clearly shows a bad fuel pressure regulator. He called it “deadheading”. I was about to buy a new regulator when a 1989 Sentra owner on another board jumped in. His Sentra measures 47 psi all the time (idle, hitting gas, disconnecting hoses) and says there’s a 5-10% margin of error when two mechanics use two different gauges. He says fuel pressure regulator is probably just fine. Only test he recommends is this one:
“Start the car, and the pressure should go up to 43-47PSi very quickly.
Shut off the car, and watch the pressure, an hour later you should still have at least 40PSI.
Start the car again, and rev up the engine quickly while watching the fuel pressure, does it drop at any time you pressed the pedal, or did the pressure stay steady?”
For a cold engine start and stall and cold engine loss of acceleration what should I try next ?
For the injectors to inject the correct amount of gasoline the fuel pressure between the fuel rail and the intake manifold has to be kept constant. When you step on the gas pedal, the intake manifold pressure rises (the vacuum becomes less), so the fuel pressure should rise briefly when you step on the gas pedal. Some newer cars don’t use a fuel pressure regulator at the end of the fuel rail, but if your Sentra does, you’ll see it has a small diameter hose from the regulator to the intake manifold. That’s how it accomplishes the function above. Make sure that hose doesn’t have any gasoline in it.
Cold start problems could have other causes, for example the engine computer may not realize the coolant is cold b/c of a faulty temp sensor. For loss of acceleration only when low on fuel, a fuel pressure problem is a definite suspect. Your shop may have to connect a fuel pressure gauge and situate it where they can see it as they do a test drive.
Thank you for the reply George. We did test out the coolant temperature sensor and it checked out just fine. I even went so far as to buy a new coolant temperature sensor and plugged it into my cts harness. Car immediately started and failed three times.
The 1989 Sentra does have a regulator. Now I just need to figure out which top hat it is so I can check the hose. I’m guessing of the 3 openings pictured, the top one (small diameter) will be the vacuum hose? Okay to pull that and look for gasoline or will fuel start shooting out of there ?
The 2 larger diameter openings are usually for the fuel, and the smaller for the vacuum hose. Test for fuel in the vacuum hose with the engine off. I’ve never seen fuel shoot out of that hose on my vehicles, but I suppose anything is possible. What you are looking for is any sign of fuel in that hose at all. There shouldn’t be any. Whenever working with the fuel system, common sense rules apply, including eye protection and keeping a big fire extinguisher on hand.
Found the fuel pressure regulator. Pulled the top small diameter vacuum hose off of the regulator and ran my finger over ends of hose and metal port as I saw a guy do in a YouTube video. https://1drv.ms/f/s!AixACD1v4AoNhIdatCPStYctevaSmw
No fuel residue. Hose is in good shape (i.e. soft/pliable). I blew through it and sounded like my air came out around the Mass Air Flow Meter.
Blowing from the other end of the hose I get 100% resistance. Blocked. Also, no fuel trace on the other end (metal port or hose).
Given that test result, the FPR is probably not the problem. You could still have a fuel pressure problem though, due to a faulty pump or fuel line obstruction. Have you asked your shop to connect a fuel pressure gauge in a manner so they can watch how the fuel pressure behaves during a driving situation? Has your s hop checked that the injector pulse length is correct (or is being double pulsed) for cold start and cold engine operation? Have the fuel trims been measured? Is the MAF known to be accurate?
Thanks again George. The first thing we did (months ago) was replace the original fuel pump w/a new one. So it’s possible one of the things we bought (cap, rotor, plugs, wires, ignition coil, remanufactured fuel injector, fuel pump, fuel filter) is defective, but seems unlikely. In other words, that would mean my original must have first failed and then was replaced w/a defective new part.
I do have some guys trying to help me test out the MAF. Mine is not the $20 version on the airbox held in place w/two screws. Mine is 5 wire MAF, difficult to install, and according to the Nissan service manual it cannot be tested except with a “known good replacement part”.
Got ahold of a good MAF and had my mechanic install it. Problem solved. To sum up for the next guy: cold engine start and stall and cold engine loss of acceleration was due to a bad MAF.
Thanks again to George and all the other contributors!