I need help with a little sputtering problem at idle to 1500Rpms!

[LeeS97Z28]

Hi Gang!

I have a 1997 Z28 with 82K miles on the clock!

Parts Replaced: Optispark, Plugs and wires, EGR valve, IAT sensor, Map Sensor. Cleaned Throttle Body and replace gaskets, New Fuel Pump and Gas Filter. All Parts AC Delco!

No Codes in PCM

Here is some Data Pulled from PCM while car was Idling!

Fuel Cell 16

Long Term Fuel Trim 133Long Term Fuel Trim Right131
Engine Coolant Temp. 175Deg
Map Sensor 27.28HG
IAC Counts 11
Short Term Fuel Left 127Short Term Fuel Left 126
Knock 26 Knock Retard Timming Odegrees
IAT Temp 123
Mass Air Flow 6Gr per second
Map Volts 1.14

O2 Sensors were switching continously from 89 to 900
on both Left and Right sides. However, I noticed that the right side didn't switch as much in the up range of 700-800 milivolts like the left side did.

Spark advance was 26-28 degrees

TPS Volts .65 TPS% =0

Sorry to be so long but I have been tracking this problem for over a year now!

Thanks Lee!

[Dave M]

Lee, I can't help you with your problem but can I ask what you used to get those numbers from the computer and where you got it and how much it cost you?

thanks!

[LeeS97Z28]

I used a Actron Cp9145 which we sell at Autozone for 279.00

I work at autozone so I just used the new scanner we got. It will allow you to view live data!

Thanks Lee!

[Joe 1320]

So what you are stating is a slight miss or stumble at idle or when you touch the throttle?

One possible problem is the TPS sensor. They can develop a bad spot yet the rest of the range of motion in the sensor is fine and no code would be set. I would view the TPS readings as you apply the throttle.

[LeeS97Z28]

Thanks Joe for the information.

Lee!

[LeeS97Z28]

Joe I hooked up the scanner again and checked the response from the Throttle position sensor.

It showed a steady increase without any jumping.

Could an Idle air control valve make it do this.

What do you think about the knock sensor. It showed a number under knock on the scanner but nothing under knock retard in degrees.

What would be the chances of intake leaking air or around a fuel injector.

Thanks Lee!

[Injuneer]

Fuel Cell 16 - NORMAL that is the "idle" cell.

Long Term Fuel Trim 133Long Term Fuel Trim Right131 - Its running a shade lean, about 2-4%. This is within the normal tolerance. long terms are compensating. Keep this in mind though... maybe a little air getting in through a vacuum leak.

Engine Coolant Temp. 175Deg - high enough to put it into closed loop

Map Sensor 27.28HG - Go back and check this one. MAP at idle with a stock cam should be in the range of 9-10"Hg. What you appear to be reporting is the "barometric pressure" (what altitude do you live at?) which the MAP sensor records before the engine starts.

IAC Counts 11 - A shade on the low side. I usually look for a number in the range of 20-40 counts. Maybe a LITTLE air getting in via oither means.

Short Term Fuel Left 127Short Term Fuel Left 126 - means nothing... these are instanteneous, temporary values that "switch" in response to the O2 sensor values.

Knock 26 Knock Retard Timming Odegrees - Again, an error. I believe you may be seeing 26deg ignition timing advance, andf "0" spark retard. What is your idle speed? 26deg advance seems a little high for idle.... normally this will be 18-21deg at idle.

IAT Temp 123 - This is obviously "heat soak" of the sensor. This is why you want to relocate the sensor away from the engine heat if you can. It does pull a little timing, and it can feel like a "stumble" coming off idle and at low RPM.

Mass Air Flow 6Gr per second - Sort of in the normal range 6-9gps at idle, but being on low side swuggest it is POSSIBLE air is getting into the system elsewhere.

Map Volts 1.14 - normal, and corresponds to about 10"Hg..... that's why your MAP reading cited above can't be "MAP", but is "BAR"

For some info on scanner readings, you might want to check out my online scanner wrietup:

http://members.aol.com/InjuneerZZ/ScanMast.htm

O2 Sensors were switching continously from 89 to 900
on both Left and Right sides. However, I noticed that the right side didn't switch as much in the up range of 700-800 milivolts like the left side did.

Spark advance was 26-28 degrees - Seems too high for "normal" idle.

TPS Volts .65 TPS% =0 - Normal and very typical of stock TPS.

Appears there is a POSSIBILTY of a small vacuum leak.... but there are just "hints".... no glaring indications.

You might also want to check the EGR valve. If it is leaking even slightly at idle or at low RPM, very low loads, it will cause roughness. It should stay closed most of the time.

[LeeS97Z28]

Thanks Fred! I really appreciate ya explaining that too me!

I'll Check the EGR valve. I'll also see if I can find an vacuum leak.