98 GTP: P0171 and MAF

[DrRadar]

Recently started getting P0171 with maybe slightly rougher idle. Scan tool indicated that short-term fuel trim would peg at 20% when I removed my foot from accelerator and long-term fuel trim was always pegged at 16%. Didn't look at mass air flow. Today, I disconnected MAF. Fuel trims are now about zero with no pedal pressure. P0171 gone (but P0102 present...I assume expected with MAF disconnected).

Although I think I know the answers, I'm gonna ask anyway because that's a $200 sensor.

Conclusively the MAF?

Go with new AC-DELCO?

[rkvons]

Originally Posted by DrRadar

Recently started getting P0171 with maybe slightly rougher idle. Scan tool indicated that short-term fuel trim would peg at 20% when I removed my foot from accelerator and long-term fuel trim was always pegged at 16%. Didn't look at mass air flow. Today, I disconnected MAF. Fuel trims are now about zero with no pedal pressure. P0171 gone (but P0102 present...I assume expected with MAF disconnected).

Although I think I know the answers, I'm gonna ask anyway because that's a $200 sensor.

Conclusively the MAF?

Go with new AC-DELCO?

It looks to me like you are experiencing the exact same thing I was experiencing last summer with the exact same car. There is possibility that you have an intake manifold leak that is sucking air where it should not be. I bought the MAF and the O2 sensor and it did not fix it. I sprayed carb cleaner along the top of both rocker arm covers and got a reaction with the idle speed. In my case, it was intermittent. So you have to spray the carb cleaner when it is cold, warm, and hot. I think my leak would close up sometimes. I then had to do my intake gaskets and it fixed it.

http://www.automotiveforums.com/vbul....php?t=1138164

[DrRadar]

I'm glad I asked the question!

But that's disappointing to hear since I replaced the valve cover gaskets, throttle body gasket, upper intake gaskets, supercharger gaskets, and vacuum lines/connectors only a year ago when I replaced the supercharger. The lower intake gasket was a silver (metallic I think) after-market FEL-PRO that looked in good shape, so I didn't replace. (Done by previous owner...I've had the car 1.5 years now.)

The O2 sensor is a common suspect for the P0171 as well as the poor gas mileage problem. I've been thinking the poor gas mileage is due to the catalytic converter because this 98 GTP has more sluggish acceleration than my 99 GTP. (The 99 is currently being refurbished to extend its life.)

I tried to put the 99 MAF sensor into the 98, but they are different...the hole in the throttle body is a different shape and it won't fit. 98 is circular hole but 99 is U-shaped.

Any value in trying the 99 O2 sensor in the 98? Haven't taken the 99 O2 sensor out because I had trouble getting a tool in there to remove it...and that with everything off the engine. It's gotta be a PITA taking it out of an engine that runs.

Is carb cleaner the best solvent to use to check for leaks? It's sure hard on non-metallic material and would prefer to use something less caustic.

Any other ideas out there?

[olopezm]

Have you tried cleaning it with maf cleaner? Dirt and oil will stick to the heating elements and cause it to read rich at idle and lean under load. Try cleaning it before you buy another one.

If you end up getting another sensor, stay away from Cardone remans, the general consensus in the forum is that they are bad out of the box most of the time (myself included).

Oscar

[DrRadar]

Today I sprayed some carb cleaner around intake manifold above valve covers. There was a slight increase in idle in an area above the rear valve cover...noticeable but I don't know if that is causing my problem. Seems to idle fine with or without MAF sensor connected although a bit uneven with it disconnected. This was with a cold engine on a chilly day.

Connected MAF sensor today and cleared codes. No SES light yet but the fuel trim returned to high levels especially at idle. I hope the SES light stays off long enough for a state inspection...I'm overdue as of today.

Considering the fuel trims and normal idle, I'm thinking it is a bad MAF sensor. Any more suggestions to isolate the problem are welcome.

[olopezm]

High fuel trims at idle that go back to normal at higher rpms indicate a vacuum leak, consistent with the higher idle while spraying carb cleaner. I would take a closer look at it; make sure to also check the injector o-rings.

Oscar.

[DrRadar]

New injector o-rings a year ago when I did intake, valve cover, supercharger gaskets.

I'll work on the vacuum leak. Thanks.

[rkvons]

Originally Posted by DrRadar

Today I sprayed some carb cleaner around intake manifold above valve covers. There was a slight increase in idle in an area above the rear valve cover...noticeable but I don't know if that is causing my problem.

I bet it is. That is exactly what mine did. My light has been off ever since I changed my intake gaskets.

[DrRadar]

rkvons, you must be right. I thought I solved the vacuum leak, but the code has come and gone a few times. I'm still getting max fuel trim at idle, whether the code is active or not.

The car did pass inspection, so I've bought time. Looks like I'll need to redo the intake manifold gaskets...bummer since they were done only a year ago. But it'll have to wait a while.

Thanks for the help.

[DrRadar]

Inspection is due this month so time to change the intake manifold gaskets to rid myself of the P0171 (to pass inspection) and solve the rough idle problem.

After changing the gaskets, still had rough idle and got a P0300 (misfire) and I'm sure a P0171 was on its way.

So I carefully reread this thread. Oscar's warning about the fuel injector o-rings made me realize that the fuel injectors went back in very easily when I reassembled. So out come the fuel rail and injectors and sure enough, the lower o-rings were compressed and had lost all resiliency. Replaced them and now smooth idle and no SES light.

Oscar, you're a genius. Thanks.

[olopezm]

Nice work! I'm glad you've finally fixed the problem.

I haven't been in the forum for a long time, as a matter of fact, my GP broke down in March 2015 and remained parked in my garage until about 3 weeks ago when I decided to sell it, I loved that car, but it's time to move on...

Oscar.