98 Grand Prix - Hard Starting and Higher Than Normal Fuel Consumption

[FCPorto]

Thanks again Bnaylor. I appreciate the help with this. First of all i would agree with ruling out the fuel pump, although the reason I'm focusing on it is because it is one of the pressure relief paths. Really I'm seeing only three paths here either the cylinder through fuel injector or fuel tank through FPR or fuel pump. When the car finally starts after the hard start it runs beautifully. I've had time where it started, then for about 1s or so it seems to want to stall but then immediately stabilizes and runs perfect. Fuel consumption is about 40% higher than normal which may or may not be related to this fuel pressure problem. Not sure. I will try and get some of the Chevron stuff or something comparable (I'm in Canada). I will also look at the DIY fuel injector cleaning procedure. The car has 135,000 kms on it so having the fuel injectors cleaned wouldn't be a bad thing to do just as PM. One thing to note too is that I have run 87 octane in this car for quite some time. Maybe that has something to do with it. Tonight I will also try cleaning the IAT and MAFS with some contact cleaner just as precautionary as well. Thanks again.

P.S. is the fuel pump on these vehicles accessible through the trunk or does the tank need to be dropped for that?

[BNaylor]

I can only recommend using 91 or better premium gas but I don't think the lower octane fuel is related to your problem.

For fuel pump pod access there is a cover with 7 nuts holding the plate right behind the rear passenger seat in the trunk. Easier if you have the rear passenger seat pass-through door. Set aside the trunk carpeting to see it. Good luck and let us know how it turns out.

[Purdue2906]

It might be your Intake Manifold Gasket. I had mine replaced just resently and now it funs great get it checked before its too late

[FCPorto]

Well, everybody it definitely wasn't the FPR. Swapped it out and same thing with same rate of fuel pressure drop. I've just spent the hefty $400 and purchased a fuel module assembly. Will be swapping that out tonight and will post back the results. My first suspicion was fuel injectors, however once the car starts it runs perfectly fine. No stumbling or hesitation during the first few seconds of starting. Therefore I'm ruling out the fuel injector leak. The only other thing I can see here is fuel pump. There's nowhere else for fuel to go. Either back to the tank through the pump or FPR or into the cylinders through the fuel injector. I'm suspecting the check valve in the fuel sender module. Thanks guys.

[tblake]

Yes, I sort of suspected the check valve in the pump also.

[BNaylor]

I don't recall seeing it posted but what were the fuel pressure readings at ignition to ON only which is fuel pump prime? And after about 10 minutes to see if adequate fuel pressure is maintained for starting. The fuel pump is too expensive just to shotgun replace but hopefully it resolves your issue since you already purchased it.

[tblake]

Bob if i remember right, his readings at key on ongine off were 51, but dropped to 0 within a couple seconds after the fuel pump shut off.

I guess he shouldnt rule out a sticky injector, this would jive with the bad gas milage.

A problem like this is hard to diagnose without a very expensive fuel pressure guage.

[BNaylor]

Thanks for the heads up Tim. Too many posts to look at with the limited time I have. The way I look at it is if you are going to do DIY and do it right then diagnostics tool are a must such as an automotive fuel pressure tester and possibly fuel injection system service kits. My Actron gauge wasn't that expensive. I recall around $40 but invaluable especially when I have to adjust my Casper fuel pressure regulator and perform maintenance checks. Also, Actron tech support sent me new hoses and other fittings for free.

Now there are fuel pressure and return line adapters/fittings with turn off valves that you can get to properly troubleshoot fuel system supply problems. I believe the GM dealerships and other specialized auto shops have them where you can determine which direction to head. Simple test with the right tools. For example see below.

Testing for fuel injector or fuel rail leakage: Clamp or turn off the return line. If pressure falls below last known good psi reading, a fuel injector or the fuel rail is leaking. If pressure holds, it is most likely a bad fuel pump check valve.

[FCPorto]

Hey guys, just wanted to close the loop on this one. It was definitely the fuel pump. Popped it in tonight and all is good. Fuel pressure hits 51 psi and holds when priming is done. Sits at about 43 psi when running with FPR connected to vacuum and about 51 psi with FPR disconnected from vacuum. FPR is brand new as well. Thanks for all the help guys.

[tblake]

alright good. Glad to hear you got her running. How hard was the pump to change? Not so bad was it? The easy access to the pump was a selling point of the GP to me.

[FCPorto]

Thanks TBlake. It was not bad at all replacing the fuel module. The access cover just behind the middle swing out armrest made it a snap. The hardest part was putting the new module in because of the new o-ring, but touch of dish soap sure did help that out. Once I did that it went on very nicely. The car is mint now, back to the way it was supposed to be. Fuel mileage is back to normal, so I'm very happy. An expensive fix but i have my GTP back. Thanks to all for the suggestions and help. Very much appreciated.

[FCPorto]

Well, I think the fuel economy stabilization was definitely a mirage in the distance. Fuel economy is back to normal.... shitty. This car always driven around 12.0L/km (average between city and highway driving). It's burning 16.0 L/100 km right now. It's unreal. No SES light or anything indicating a problem. Not too sure what to look for. O2 sensors? PCV? Both? If anyone can just let me know what they think, that would be great. I'm planning on attaching a PC based OBD II scanner to it and logging vehicle data on a drive to work, either tomorrow or the next day and determine whether anything is acting screwy. When I hooked up a few weeks ago, I was getting readings but I don't know what the ideal specs are for all the sensors. Again, some further assistance would be much appreciated. Thanks again guys and sorry for the confusion.

[BNaylor]

Best thing to do is monitor the 02 sensor in real time with a scanner capable of doing that. Alternative with a DMM at the purple wire which is output see if the voltage varies from .1 volt to 1.0 volts.

Usual suspects for unexplained loss of gas mileage is the front 02 sensor, CAT convertor and even the thermostat just to name a few. Other factors are weather and type fuel being used. TCC lock/unlock in 4th - OD.

[FCPorto]

Thanks BNaylor...here's my plan : a) Scan car using PC based OBD II system and determine whether anything is out of the ordinary (MAFS, IAT, Coolant Temp, etc.). b) Clean IAT and MAFS using electrical contact cleaner. c) Replace PCV valve and Thermostat (these have never been done before). If required replace O2 sensors (would it be worth just replacing them both?). What is a good brand of O2 sensor? I've seen Walker, Bosch and Delphi online. Thanks again for the quick responses.

[BNaylor]

For 02 sensor I recommend staying with the AC Delco/Delphi AFS 109 (pre-cat) and AFS 123 (post CAT) for a '98 GTP. Replace both due to the age.