Strange Misfire

[jrsauer64]

99 Tahoe 5.7 4wd with 276k miles on it. Put in a long block at 219k and new cam and crank sensors at the same time.

Lately it feels like it is misfireing but only when you are barely pushing on the accelerator. Happens especially when you accelerate and then let off the accelerator and it shifts into a higher gear then you gently accelerate again. Also when you are in a high gear and barely on the pedal and hit a hill. Often does it when maintaining speed on the highway. Interestingly, it rarely does it when you have the cruise on but turn off the cruise and hold speed with your foot and feels like missing again.

Went ahead and replaced TPS even though it tested ok and it seems to have made no differences.

Any suggestions where to start diagnosing? Forgot to mention that "misfire" not triggering check eng light.

 

[lesterl]

Does it do it only in 3rd or 4th gear (Torque Converter lockup slipping)?

Check fuel pressure?

Ignition tuneup recently? Plugwire loose?

 

[jrsauer64]

Fuel pump was replaced within last 20k and filter was done at 219k. Could prob stand to replace filter again.

Wires and plugs and cap and rotor done first and didn't affect the "misfire" feeling.

Yes it only seems to happen in 4th gear. Can't get it to do it in 1-3rd. If I just accelerate a bit harder it goes away.

How could I test the converter? Why would it be better when cruise is engaged? With cruise on it only seems to do it when you encounter an uphill slope. Without cruise it will miss on flat surface.

 

[samotahoe]

maybe you could use a scan tool to see if there are any pending codes? and if not then you could find a shop with a scope to test with a mode 6 setting to see if there is a misfire but it doesnt occur enough for the computer to recognize it and turn on the check engine light

 

[lesterl]

brake pedal will disengage the torque conv....

Didja check for codes yet?

 

[SouthJerseyTahoe]

Sounds like a worn distributor gear but you have to check for codes. Mine is only 3 degrees out and already causing a surge at steady cruise. They wont set a timing code until they are at least 5 degrees out. Have you changed the spider? Poppets get clogged easily and older GM's dont like setting misfire codes until its a secondary ignition problem.

 

[jrsauer64]

Sorry for the disappearing act

Went to Fla for a week and been way busy since I returned. Drove the Tahoe there and back from Ohio. Pretty much behaved as I described although after driving for hours and stopping I had a couple of instances in gear at a stop that felt like the lock up was trying to engage or remain engaged even though I came to a stop. (Engine chugged and almost stalled a couple times).

Anyway, the idiot light came on yesterday so am going to get to my buddies shop by the weekend and hook it up to the scanner and let you know what I find. Interesting remark on the distributor gear as when I tuned up recently I still found the occasional misfire using the scanner. Will let u know after I check it out.

 

[SouthJerseyTahoe]

See where your fuel trims are. They should average around 0% give or take 5% on a fully warmed engine. Check the cam retard also. should be 0 degrees +/- 2 degrees. If you can access the transmission module you can monitor the torque converter lockup while driving but preferrably with someone else driving. Just for ****s and giggles have you checked around for vacuum leaks?

 

[SouthJerseyTahoe]

Just a note: The cheapo ignition caps tend to crossfire inside brand new right out of the box. Make sure it has brass terminals. You can actually feel the weight difference with the better caps. Did you check the plug gaps? I cant tell you how many I have had bent over right out of the box or even cracked porcelains. Make notes on what cylinders are affected.