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Fuel Pressure Question, again

Back with the 2002 Grand Prix, 3.1- mild ‘bucking’ at about 3K rpm while driving-

1-Hooked up fuel pressure gage

2-pressure 50 1/2 after pump run

3-pressure drops to 49 when car started and remains steady at idle

4-removed vac line to regulator- pressure rises to 59 1/2

5- do feel vacuum at end of hose (not sure exactly how much- no vac gage)

6- replace hose to reulator- pressure returns to 49

7- rev eng in Park- (wife refuses to ride on or under hood to read gage while car on road) at 4K rpm, eng starts mild ‘bucking’- pressure gage jumps back and forth 49-50

8- turned motor off- no drop in pressure after 5 min

Any ideas on the cause of ‘bucking’ will be appreciated.

What you are describing sounds something like a lean surge. the car is “hunting” around because it is running lean at this RPM for some reason,not a misfire,hence no code.

A scanner that shows fuel trim values could help,driveability was not my department so I don’t know what values to tell you to look for.

I am sure you have eliminated all possibilites of a vacuum leak anywhere.

Haven’t really checked for a vacuum leak- I will now though- thank you-
Used to be easy to look for leaking hose on old carb type motors- taking suggestions one at a time- thanks for vac leak heads up-

My first thought is that it isn’t fuel pressure related. Bucking is between the computer and the engine sensors usually. Were there any codes? Guessignosis won’t work.

Get the car scanned for codes first. AutoZone, Checkers, etc. will do this free and are you sure this car doesn’t have the 3.8 instead of the 3.1?

Some possibilities could be an ignition miss, EGR system fault, or a MAF sensor problem.
If the bucking is worse while you’re trying to gently hold it at a steady speed you might remove the plug wires from the coils and inspect the terminals to see if there is any corrosion in there.

I agree with the others. Fuel pressure is not your problem. The fuel pressure is responding the way it should. Your problem is elsewhere. Is the CEL on? I’m leaning towards ignition problems. What brand of spark plug do you use?

You have fuel pressure; but, that doesn’t mean you have steady, sufficient, fuel injection (into the engine). It could be the engine is running lean at higher rpm and that is causing the misfire. To check this idea, use a spray can of Starting Fluid (with a straw attached, if available), or, use a small bottle of propane with an adjustable nozzle (available at hardware stores). Slide the straw (or, nozzle) past a vacuum hose, or disconnect a small vacuum hose and plug the port (temporarily).
Ask associate to start the engine and, in PARK, raise engine speed to 4,000 rpm. Or, with wheels chocked, foot firmly on brake, automatic transmission in DRIVE, raise engine speed to 2,000rpm, and hold it there. Very carefully (to avoid an engine surge), start a very light spray of the Starting Fluid (or, propane)into the vacuum port, and observe the response of the engine. If the engine smooths, and you can maintain the Fluid, or propane flow, just right to keep the engine running smoothly, this will show that the engine miss isn’t from missing spark; but, misfire could still be from weak spark.

Just woke up- read all replys and suggestions-
I have AC spark plugs in car-
Yes, sure it is 3.1
Already scanned car- no codes set- thought any kind of ign type misfire would set a code so I was looking for something that would cause the problem yet not set a code.
Going to have to look up CEL (learn as you go)and address that.
Problem (bucking) is consistent in that it occurs at the same rpm.Car runs smooth otherwise.
EGR is a thought as I’ve had them crap out on other vehicles, just don’t know if it fits the ‘only at high rpm’ scenerio.
I’m going to try all of your suggestions today and again I thank you all.

ALL engines misfire. The amount of misfire which will cause the CEL to light (DTC to set) is determined by the ‘fuel control strategy’ which is programed into the engine computer.
The DTC (Diagnostic Trouble Code) is set when the emissions rise to 150% of maximum permitted levels. How THAT is determined, by the engine computer, is a good question. I don’t know.

Update- I was told by a tech that the computer has a ‘rev limiter’ to prevent the eng from blowing at excessive rpm’s.
While trying to duplicate the ‘bucking’ (at 3K rpm) I had experienced while driving I was reving the enging, car in Park to 4K rpm at which point the eng would buck mildly, fuel pressure up and down, 49-50 psi.
Was ready to take car in to be diagnosed but decide to test drive one more time- car ran perfect-
Earlier I had taken the advice offered on previous posts and changed the fuel filter- guess that was the problem after all- Always looking for difficult problem and ignoring the obvious.
My thanks to all the posters.

The fuel pressure spec is 52-59 psi. If the pressure is at 49-50 then the pump is defective and needs to be replaced. 49 psi at idle is below specs.

The fuel pressure spec is 52-59 psi. If the pressure is at 49-50 then the pump is defective and needs to be replaced. 49 psi at idle is below specs.

This post might show up twice, I posted this answer and it didn’t show up so I did it again.

The bucking (misfire) isn’t (wasn’t a result of rev limiting. True, the engine computer limits rpm; but, that rpm is higher than the rpm the bucking is taking place. So, there could, still, be a fuel pressure problem, with the engine under load (as in the real world). I have, already, described how to perform a load test on the fuel pump.

Be advised that this car, like some others, limits the revs to a much lower value than normal when the transmission is in park or neutral. This is too reduce damage form “neutral drops” that some people do when street racing. I believe that the OP is correct.