1997 GTP getting poor mpg

[ibanez0110]

I have a 97 GTP with a few mods, and about 141k on it. When I first got the car, it averaged around 23-25, and got about 28 on the highway, on a good day anyway. Since I've had the car, it has slowly went downhill. Now it gets 22-24 at best on expressway, and averages around 19. I just recently changed the plugs and wires, put TR6s in, and it has 8.8mm wires. I also recently seafoamed the engine, and changed the oil. There is no SES light, but I also think the bulb might be burnt out. The car doesn't have a cat, so the light has been on for awhile, and in theory, should still be on. Anyway, some help would be greatly appreciated.

[doctorhrdware]

What engine do you have.

[grandprixgtx00]

Originally Posted by doctorhrdware

What engine do you have.

its a '97 GTP, i would assume its a supercharged 3800

anyways...in theory, not having a CAT may drop the MPG a few. have you checked the fuel filter? how long has it been since the air filter has been changed? if you have a CIA...maybe clean and oil it?

[grandprixgtx00]

oops...meant CAI not CIA. you know what i meant...

[CrazyHorst]

If your PCM thinks there is no O2 sensor feedback, then it will default to an open loop condition....which instead of closed-loop operation which is constantly monitoring and tuning injector pulsewidth...simply looks up a pulsewidth from a table based on engine speed and load. The open loop defaults to the rich side of 14.7:1.

Other ways to get into open loop are: lack of coolant temp feedback, very high or very low coolant temps.

There is fuel enrichment strategy for protecting the pistons under wide open throttle operation (power enrichment) and also a catalyst protection strategy which prevents the cat from internal overtemp under high mass-flows and hot flame-temps.

There is also an "LTM" count which deals with long-term integration of the O2 feedback. This is basically a strategy to balance any unmetered air leaks and any fuel injector variation. In simpler terms, if the totalized moving average of O2 sensor values over a long period of time is slightly higher than 14.7:1 then a very slow and gradual change is made to increase the pulsewidth.

In other words, if there's unmetered air entering via a vacuum leak or other gasket breakdown, the PCM steps up fuel (slightly, gradually) to match. If the injectors flow at the higher end of the tolerance band, then gradually it will reduce slightly.

I suggest you need to get in to see if your O2 sensor is toggling and if there are any stored fault codes.

[maney8295]

Which o2 sensor are you referring to..pre cat or after the cat in which you said determines open or closed loop?? Ive been running my grand prix with the 2.5 downpipe from zzp with no o2 sensor, just capped the hole so not to have a leak. Would this cause lack of performance, mpg..etc.?? Thanks in advance.

[BNaylor]

Originally Posted by maney8295

Which o2 sensor are you referring to..pre cat or after the cat in which you said determines open or closed loop?? Ive been running my grand prix with the 2.5 downpipe from zzp with no o2 sensor, just capped the hole so not to have a leak. Would this cause lack of performance, mpg..etc.?? Thanks in advance.

Based on the prior reply (#5) it looks like he was referring to the pre-CAT 02 sensor which is Bank 1, Sensor 1. The post-CAT 02 sensor just monitors CAT converter efficiency. So you saying you are not running a pre-CAT 02 sensor? Is the electrical connector to it hooked up?

By doing that you are defeating the whole purpose of having a pre-CAT 02 sensor which is used in the closed loop mode of operation to establish the proper air/fuel mixture ratio at the ideal 14.7:1 and proper emissions control. It should affect fuel economy by giving poor fuel economy and possibly other driveability issues.

[maney8295]

Oh ok..what i was trying to say was the sensor after the cat is the one im running without. I just have it capped so as not to have a leak in the exhaust. thanks again.

[BNaylor]

Thanks for clarifying that.

No big issue running without a post CAT 02 sensor and it would make sense to plug the hole. Are you running a 02 simulator or is your PCM module programmed to inhibit any DTCs and a SES/CEL light? Is your CAT converter still installed?

[CrazyHorst]

After more thought, a possibility is a bad ESC (knock) sensor or two.

ESC activity over a certain threshold (there's always a little noise from normal combustion events) the engine "pulls out" spark which is to say a retarded timing from MBT is commanded.

You can think of spark advance similar to a pendulum...advance and the heat stays in the combustion chamber, retard, and you throw the heat down the tailpipe, essentially wasting it. The waste isn't necessarily all bad as this is a strategy to recover from ESC activity.

However...it does not help fuel economy...which was the original symptom.

Also I am not aware of any self-diagnosis strategy for the ESC sensor other than perhaps voltage high (short circuit) or voltage low (open circuit).

A quik trip to Autozone would confirm or dispel the notion of the existence of diagnostic codes which are "hidden" by a bad SES bulb.