Cadillac LQ-9 - Backfiring at High Speed?

[DDean]

I have an 03 Prostar 197 with a Cadillac LQ-9 motor in it. The motor only has 140 hours on it. It runs perfect at slalom speeds and below (0-36mph). However, when I run it above 36 for barefooting, etc (36-45) it begins to backfire, especially when I hammer the throttle. Does anybody know what may cause this?

Thanks For Your Help!

[rodltg2]

yes i do, get the new spark plugs for it. there was an update.... change them out and you'll be fine...

[Engine Nut]

I have an 03 Prostar 197 with a Cadillac LQ-9 motor in it. The motor only has 140 hours on it. It runs perfect at slalom speeds and below (0-36mph). However, when I run it above 36 for barefooting, etc (36-45) it begins to backfire, especially when I hammer the throttle. Does anybody know what may cause this?

Thanks For Your Help!

When was the last time you changed spark plugs? The plugs in the LQ9 need to be changed every season. We recommend AC Delco 41-985 gapped at .050". The plugs may look fine but trust me, we have fixed numerours backfiring LQ9's with a new set of spark plugs.

If the plugs are new and the problem is still there the next thing I would do is to have the fuel pressure checked. An 03 LQ-9 should be 47-51 PSI at idle, 49-53 at cruise range and 53-56 at WOT. Backfiring may be caused by a lean runniung condiotion related to low fuel pressure.

Engine Nut

[DDean]

Thanks man I appreciate the help!

[6ballsisall]

If you need more help ask MYMC. He knows ALOT about this motor as well. Enginenut is VERY VERY helpful too on this sort of thang.