My GP SE 3.1L '97 has 110K on it and my parents have owned it since it had 5K and I bought it from them at 100K. The car is great and been really reliable for my family in the last 8 years. We have done all the maintenance on the vehicle since we've owned and have never had any major problems.

I decided since it had 110K on the clock it would be worthwhile to check the fuel pressure and see how the pump is doing (it sounds normal the whine hasn't changed pitch).

My reading at idle with the FPR vacuum line connected was 38psi. I want to say I read it should be within 45-55psi. I did not unplug the vacuum line to the FPR to see what it would have read after that...

With 110K and that pressure reading is my pump doing okay or should I look at getting a spare to have on hand when it finally bites the bullet? In my experience GM electric pumps usually work fine, start whining for a day and then die the next. I'm not sure though if they start reading lower pressures as they start to wear out since I've never bothered much with pressure readings on previous GM vehicles.

TIA.

You can check the vacuum supply line at the regulator to see if there is fuel present. If so, the diaphragm has a pinhole/tear in it and you need a regulator. If no fuel is present in the vacuum line, then the regulator is fine if pressure spikes up with the engine running when you disconnect vacuum. My new Delco catalog no longer gives fuel pressure specs, but the 97 used two different fuel pumps. Check the last 6 digits of your vin. Cars built prior to vin 238198 used Delco part number 43-918. cars built with or after vin 238198 use MU121. I'll dig around and see if I can come up with a pressure spec for you, if someone else doesn't beat me to it.

Usually you will hear the pump growl as it dies, a fresh ff and not letting the tank get low will help. 40-45 is ok.

Rich is correct in pointing out that the '97 SE 3.1 had two different fuel pumps. The older one reads low. The newer one is supposed to read much higher at 48 - 55 psi like the SE with 3800 or GT. With vacuum connected at the FPR it will read a range of around 45 - 55 psi with engine running. What are you reading at fuel pump prime position that is: Ignition on but engine not running. That reading is supposed to be around 48 - 55 psi.

Forgot to mention that I just did a Wix FF about 2.5K ago.

I will check the prime reading and also check the vacuum line for any raw fuel.

The thing that's getting me to look into all of this is my MPG seems lower than what I figure the car should get. I do mostly HWY driving 25mi each way to work with a bit of stop and go and hill climbing and I'm usually getting 20-22.5mpg. I know I should be getting a bit less with the oxygenated fuel this time of the year, but I would think with 50 mi a day and 90% of it being HWY I would be getting more like 26-27 and then low 30's on road trips. I haven't done a road trip, but I remember my mom getting about 31-32 a few years ago on 300mi road trips.

I will check the VIN and see which pump I have. Thanks for the help.

Forgot to mention that I just did a Wix FF about 2.5K ago.

I will check the prime reading and also check the vacuum line for any raw fuel.

The thing that's getting me to look into all of this is my MPG seems lower than what I figure the car should get. I do mostly HWY driving 25mi each way to work with a bit of stop and go and hill climbing and I'm usually getting 20-22.5mpg. I know I should be getting a bit less with the oxygenated fuel this time of the year, but I would think with 50 mi a day and 90% of it being HWY I would be getting more like 26-27 and then low 30's on road trips. I haven't done a road trip, but I remember my mom getting about 31-32 a few years ago on 300mi road trips.

I will check the VIN and see which pump I have. Thanks for the help.

Great! Just in case I'll post the readings you may get with the older fuel pump once I get home and check my GM Grand Prix service manual. I only remember the readings for the 3800 off the top of my head since that is all I own.

Well, I'm sitting right here and suddenly realize I have my insurance card in my wallet and that has the VIN on it...

The six-digit VIN is 275990, so I guess that gives me the "higher pressure" pump...

Either way, 38psi seems a bit low to me considering it has a new FF.

We also have a '99 Regal 3.8L SC and I got 42psi off of that when I checked it. That car has 60K and we bought it at about 55K so I'm not sure how much life that FF has on it...

Well, I'm sitting right here and suddenly realize I have my insurance card in my wallet and that has the VIN on it...

The six-digit VIN is 275990, so I guess that gives me the "higher pressure" pump...

Either way, 38psi seems a bit low to me considering it has a new FF.

We also have a '99 Regal 3.8L SC and I got 42psi off of that when I checked it. That car has 60K and we bought it at about 55K so I'm not sure how much life that FF has on it...

Just for references purposes the specs for a Grand Prix 3.1 VIN M engine according to the service manual is 41 - 47 psi which appears to be the reading for the older fuel pump.

I am quite sure your readings should be higher if you have the newer fuel pump. The newer fuel pump circa 1997 is the same as the SE or GT with a 3800 engine. So I agree either way 38 psi seems low and I would be concerned about it.

It looks as though the replacement "fuel pump" is the modular pump/sending unit assembly. Can you get a pump without the sending unit tower?

Beyond a torn diaphragm in the FPR and raw fuel in the vacuum port is there any other test procedure to determine if the FPR could be the culprit?

Beyond a torn diaphragm in the FPR and raw fuel in the vacuum port is there any other test procedure to determine if the FPR could be the culprit?


Fuel Pressure Regulator Test:

Install fuel pressure gauge to Schrader valve on FPR or at fuel injector rail depending on engine.

Remove vacuum hose from FPR.

Turn ignition to on (not start) and note fuel pressure reading on gauge.
Record reading.

Start engine and verify you feel vacuum at the hose end.

Reconnect vacuum hose to FPR.

Note fuel pressure reading again and record reading.

Now compare the first and second reading.

Fuel system pressure readings should be in a range of 4 -7 psi of each other with vaccum hose re-connected/installed. Fuel pressure should decrease as vacuum increases.

If above is not obtained (outside of 4 - 7 psi difference) replace fuel pressure regulator.

Alright, I ran the tests today.

The pump spikes the pressure to 46psi cycling the ignition on then drops to about 42psi. With vacuum on the FPR and the engine idling the pressure is 38psi, drops a bit when bringing the RPMs up and then builds back up to 38psi.

I have good vacuum at the FPR and don't see any leakage or smell a strong amount of raw fuel. When removing the vacuum from the FPR the fuel pressure jumps up to 46psi.

So... I have an 8psi differential between FPR on/off and my normal idle reading is 38psi.

I'm thinking the FPR is okay, might not hurt to replace... but the fuel pump might just be a bit tired and wouldn't hurt to replace also.

I'd rather replace the stuff now than having to call a tow truck one of these days since it seems fuel pumps usually die whenever they want.

I think I'll pull the pump tomorrow and just take a look at things and see if the sock is clean and also check the float sending unit since I've had a few times where I'm down on fuel but when I start it up, the needle goes all the way to FULL and stays there for awhile.

Alright, I ran the tests today.

The pump spikes the pressure to 46psi cycling the ignition on then drops to about 42psi. With vacuum on the FPR and the engine idling the pressure is 38psi, drops a bit when bringing the RPMs up and then builds back up to 38psi.

I have good vacuum at the FPR and don't see any leakage or smell a strong amount of raw fuel. When removing the vacuum from the FPR the fuel pressure jumps up to 46psi.

So... I have an 8psi differential between FPR on/off and my normal idle reading is 38psi.

I'm thinking the FPR is okay, might not hurt to replace... but the fuel pump might just be a bit tired and wouldn't hurt to replace also.

I'd rather replace the stuff now than having to call a tow truck one of these days since it seems fuel pumps usually die whenever they want.

I think I'll pull the pump tomorrow and just take a look at things and see if the sock is clean and also check the float sending unit since I've had a few times where I'm down on fuel but when I start it up, the needle goes all the way to FULL and stays there for awhile.


Sounds like a good battle plan. The readings do seem marginal providing that you do have the later fuel pump. The FPR is borderline at 8 psi difference. However, it may be sufficient for a 3.1L V6.

I had a similar problem with erratic fuel level readings and had to replace the fuel level sensor and float assembly. Good luck!