Misfire & Rough Idle

[ayppen396454]

98 Chevy K1500, 5.7, AT, 127,000 miles

 

I have been chasing a P0300 misfire (no other codes) for about 2 months. I have been humbled into a "parts changer". I have replaced the following with AC/Delco parts:

 

Cap, Rotor, Plugs & Wires

Fuel Pump & Fuel Filter

Fuel Injector Assy. (did the MFI conversion) with new fuel regulator

Compression Tested good.

10k miles ago, intake gaskets

 

This is tonight's Misfire Data out of 170 Passes :

Cyl 1 - 32

Cyl 2 - 7

Cyl 3 - 1

Cyl 4 - 139

Cyl 5 - 142

Cyl 6 - 634,839

Cyl 7 - 0

Cyl 8 - 0

* If I raise the idle to 1500, the misfire goes away.

 

While reading previous posts, someone stated that Long Term Fuel Trims should be less than 7 and similar between banks. If not, a vacuum leak is suspect.

Tonight at idle,

LT1 was -11

LT2 was 11

* If I raise the idle to 1500 rpms, the LT1 is -3 and the LT2 is -3.

I had sprayed all the hoses and all around the intake previously and noticed nothing. After reading about the fuel trims tonight I tried again. This time when I sprayed around the EGR valve flange the engine rpms went down. Could this be my problem all along?

[Norseman]

Did you make any mods to the truck before this started?

[ayppen396454]

No "real" mods. Cat-back exhaust & remote start over a year ago.

[Norseman]

No "real" mods. Cat-back exhaust & remote start over a year ago.

 

 

I guess you've checked fuel pressure and trim?

[ayppen396454]

Fuel Pressure, the new pump shows 62 psi when keyed and around 55 psi while running. Fuel Trims? I can check them with my scanner, but I'm not sharp enough to know what I'm looking at.

[Norseman]

Fuel Pressure, the new pump shows 62 psi when keyed and around 55 psi while running. Fuel Trims? I can check them with my scanner, but I'm not sharp enough to know what I'm looking at.

 

Man, I hate to say it but, maybe it is time to take it to a good tech and let him take a swing at it.

[ayppen396454]

I know you are right but I am not ready to concede yet. I got the EGR gasket today and will put it on tonight. Maybe it is sucking enough air to upset things. I'm going to check the MAP sensor values before and after I change it.

[tcas1355]

Maybe switch the #6 coil pack to a different cylinder to rule out the coil pack? Not sure on this one.

[ayppen396454]

Maybe switch the #6 coil pack to a different cylinder to rule out the coil pack? Not sure on this one.

Wish I could. My 98 5.7 was still a single coil.

 

Replaced EGR gasket, leak is gone. Cleared code and returned on second key cycle. This is the info I copied off the scanner. Hopefully someone can interpret something, otherwise I'm gonna drive it hard as usual or drop it off at the dealership. (which will kill me) Other than the MIL and a rough idle, off idle and otherwise I have no driveability issues. I just hate knowing something is not right.

Here goes: (at idle, warm)

 

RPM 700-790

 

MAP (V) = 1.25 TO 1.31

MAF (9M/SEC) = 8.41-8.70

COOLANT (V) = 2.11 @ 87 degrees celsius

INTAKE (V) = 2.17 @ 23 degrees celsius

IAC POSITION = 0

SHORT TERM TRIM 1 = -1 TO +1%, 126-128

SHORT TERM TRIM 2 = -2 TO +1%, 126-128

LONG TERM TRIM 1 = -18%, 105

LONG TERM TRIM 2 = 5%, 135

INJ PW B1 = 3.5

INJ PW B2 = 3.5

A/F RATIO = 14.7

 

#6 Cylinder misfire mostly.

 

Thanks for any suggestions.

[wct]

Maybe switch the #6 coil pack to a different cylinder to rule out the coil pack? Not sure on this one.

Wish I could. My 98 5.7 was still a single coil.

 

Replaced EGR gasket, leak is gone. Cleared code and returned on second key cycle. This is the info I copied off the scanner. Hopefully someone can interpret something, otherwise I'm gonna drive it hard as usual or drop it off at the dealership. (which will kill me) Other than the MIL and a rough idle, off idle and otherwise I have no driveability issues. I just hate knowing something is not right.

Here goes: (at idle, warm)

 

RPM 700-790

 

MAP (V) = 1.25 TO 1.31

MAF (9M/SEC) = 8.41-8.70

COOLANT (V) = 2.11 @ 87 degrees celsius

INTAKE (V) = 2.17 @ 23 degrees celsius

IAC POSITION = 0

SHORT TERM TRIM 1 = -1 TO +1%, 126-128

SHORT TERM TRIM 2 = -2 TO +1%, 126-128

LONG TERM TRIM 1 = -18%, 105

LONG TERM TRIM 2 = 5%, 135

INJ PW B1 = 3.5

INJ PW B2 = 3.5

A/F RATIO = 14.7

 

#6 Cylinder misfire mostly.

 

Thanks for any suggestions.

 

Are you having fun yet? looking @ your long term 1 fuel trim seems a little whacked out typical fuel trim readings should be around 0 + or - and around 128 counts. what type of scan tool are you using? can you monitor your o2 sensor readings? Is this happening in open or closed loop? You may want to try to disconnect the o2 sensor located in the pass side exhaust pipe near the manifold to see if this helps your miss disappear. W/ sensor disconnected it should read 450 mvolts. I hope this helps you out. -WCT

[ayppen396454]

Thanks. I disconnected the o2 sensor and sure enough it read 451 mvolts. While disconnected the miss is still present.

 

Snap-On Mt2500

 

Some more info:

Spark Advance is 15-16 degrees.

Cam Retard is -11 degrees.

 

I just pulled #6 spark plug, not wet, did have the start of some grayish/tan deposit around the electrode.

 

I was under the impression that the distributor wasn't to be messed with due to messing up the cam sensor. Since I read that the Cam Retard should be at 0 degrees + or - 2, maybe I could turn it just a hair? 9-11 degrees off does sound like alot to be off but is something else telling it to retard or is the PCM compensating for something else? (I feel like a cam retard...)

[1999 Tahoe 4x4]

Sounds like you found your problem.

 

 

1 With the engine OFF, slightly loosen the distributor hold down bolt.

 

Important The Cam Retard Offset reading will not be accurate below 1,000 RPM.

 

2 Start the engine and raise the engine speed to 1,000 RPM.

3 Use the scan tool in order to monitor the Cam Retard Offset.

4 Rotate the distributor as follows:

 

* To compensate for a negative reading, rotate the distributor in the counterclockwise direction.

* To compensate for a positive reading, rotate the distributor in the clockwise direction.

 

5 Repeat step 4 until 0 degrees is obtained.

6 Turn OFF the ignition.

[ayppen396454]

I will follow the procedure described, but unfortunately I may have another issue. Following another suggestion-

 

"I had a similar problem on a 98' 5.7L Vortec and it ended up being the intake gasket got sucked down into the top of the block and was sucking oil. It would only fire under 1K rpm's. pull the oil dipstick out and put a vacuum gauge into it. Then unplug the PCV and fresh air hoses from the valve covers and plug both holes. If the vacuum gauge shows vacuum, you have an intake leak. You should have positive pressure from the blow-by gases..... Just a thought if nothing else seems to work."

 

I performed the above trick last night. It pulled 10" of vacuum. In denial, I retested after checking all the hose plugs. It did the same thing the 2nd test. While frantically looking for some other component that could leak...I couldn't believe I didn't seal the intake properly when it was off 18 months ago. I guess I'll be buying a gasket set for the weekend and susequently pulling the distributor anyway.

 

signed,

parts-changer

[ayppen396454]

I didn't want to believe it. That port is #6 cylinder.

 

 

 

At the moment I have no SES & runs well.

I also reset the timing. I was a tooth off and now show cam retard at +1.

 

Thanks.

[1999 Tahoe 4x4]

I see it and I still don't get it. How could that happen? Perhaps wrong installation of gasket? Or not torqued/crushed down properly? You seem to have quite some sludge in the engine as well. My 75k engine looks like new inside.