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Old 08-05-2014, 01:40 PM   #1
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1998 Z28 down on power

Working on a 98 Z28 thats stock as far as I can tell. There are only 2 codes P137 and P157, that appear to be the rear O2 sensors. The car has 167,000 miles on the orignal engine and is starting to drop in power compared to a year or two ago. Even has a hick up once in a while while idling but not all the time. Can the codes cause the performane issues or are they only to monitor the cats? What would you do to check or test, replace of whatever?
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Old 08-10-2014, 06:22 PM   #2
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Re: 1998 Z28 down on power

Did some searching and found almost everyone said the rear 02 sensors have no affect on the cars performance.
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Old 08-11-2014, 02:05 PM   #3
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Re: 1998 Z28 down on power

The PCM does not take any actions that would affect engine power, when those codes set.

But, the 1998 factory manual indicates the codes could be the result of:
-lean injectors
-fuel contamination
-low fuel pressure
-vacuum or crankcase leaks

Those could affect engine power. I'm not sure why the lean condition wouldn't also show up in the pre-cat sensors, but this is what the manual shows.

Have you checked the fuel pressure? Are the long term fuel trims elevated?
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Old 08-17-2014, 12:36 PM   #4
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Re: 1998 Z28 down on power

Quote:
Originally Posted by Injuneer View Post
The PCM does not take any actions that would affect engine power, when those codes set.

But, the 1998 factory manual indicates the codes could be the result of:
-lean injectors
-fuel contamination
-low fuel pressure
-vacuum or crankcase leaks

Those could affect engine power. I'm not sure why the lean condition wouldn't also show up in the pre-cat sensors, but this is what the manual shows.

Have you checked the fuel pressure? Are the long term fuel trims elevated?
The car was parked for a year while he was in Afghanistan. So hoping fresh fuel and driving it will help. Will check fuel pressure also. Is it the same way as my LT1, with the schrader valve that can be hooked up to and ran up thru the cowl? so you can monitor while driving.
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Old 08-17-2014, 08:09 PM   #5
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Re: 1998 Z28 down on power

It's the same.

I think it's interesting to note that the manual also states that the pcm will set these codes when the car runs out of fuel.
Also, you might want to check to see if the O2 sensors have any extensions attached to fool the pcm into thinking the cats are ok. That might keep the outputs low.
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Last edited by GaryDoug; 08-17-2014 at 08:17 PM.
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Old 08-27-2014, 06:52 PM   #6
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Re: 1998 Z28 down on power

Ran fuel through it and it definitely helped a lot but still has a hic up while idling and at triple digits I smelt rotten eggs. I have a cutout so would that keep the cat codes from tripping? I'm going ahead and trying to replace the cats, I want to find some good high flow ones but don't need to pass emissions so I was also thinking just long tube headers and straight pipe with no cats?

I figured cats wouldn't give it a hic up at idle, only higher rpms? For new injectors what would you recommend? Its up there in mileage and I havn't been kind to her on the street. I did the basics (plugs, wires, intake, cleaned maf, etc) What would be some other maintenance needed at this point in the game? fuel filter, injectors, etc?
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Old 08-29-2014, 12:23 PM   #7
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Re: 1998 Z28 down on power

How do you clean a MAF :
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Old 08-29-2014, 02:45 PM   #8
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Re: 1998 Z28 down on power

Quote:
Originally Posted by nikko View Post
How do you clean a MAF :
Click the image to open in full size.

.
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Old 09-08-2014, 08:22 AM   #9
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Re: 1998 Z28 down on power

Down on power.....
Post 02 code.....
GM 100k mile cats......

When cats break down they plug and the car wont go anywhere plus set codes.
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Old 09-08-2014, 08:22 AM
 
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