My 5.7 is running great, but I've noticed when I'm approaching a stop light and take my foot off the gas, the motor starts surging 200-400rpm. By the time I stop at the light it's idling fine. Any ideas on what could cause this?
Fuel trims are off. Is it bouncing up or down rpm wise?
my guess is a TPS sensor ....throttle position sensor .......when you let go off the gas it goes out of range and the computer dosnt know where its at so it compensates and picks up a good reading at idle.....im assuming that you mean a late model computerized 5.7 litre...
It is bouncing up, Idle is 600, it jumps to probably 800-900. Would that also make it run rich?
Yes its a 2005 LX 5.7, everything is brand new, i was a crate motor with only break in miles on it. I suppose it could still be a bad sensor.
yes. If it is surging up it is because it is too rich at idle. They trims are probably 15% or more off. You need someone to tune it for you to fix that. The TPS almost never goes bad in those hemi throttle bodies. I have seem like 3 in 8 years and I have done 100's of them
This may not be your answer,but it certainly something to watch for.One thing I found while checking spark plug color was that the coil packs are not always sealed to the valve cover very well,water proof. It causes all kinds of weird surging issues, and can eventually cause misfiring. You say " Oh well the coil failed, and the plug is fouled." and that would be true, sort of. The coil hasn't failed, but if you remove the boot from the coil, most of the time they corrode pretty badly inside that boot, and where it contacts with the coil.( it looks like a spring) This is especially true if you wash your engine often, or have a hood scoop. Dip the boot assembled, ( don't remove the spring ) in a zinc phosphate bath over night. Also with the bootless coil laying on a work bench upright, take an eye dropper and put a few drops of zinc in there too. Zinc is a good conductor so I think it will be safe, and the phosphate should remove the corrosion pretty well.