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2004 Malibu Monsoon runs great for 20 minutes then start misfiring and loses power.

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2004 Monsoon 1074 hours.  Did tune up last summer. Fuel pump. Now having same issues. Runs great for 20 minutes then misfires and cant maintain speed with out having to back throttle down.

Limped back to dock today at 7mph.  If you put in neutral and rev prop to 3000 3500 rpm no issue. Put back in gear and throttle up you start to lose power and misfires.  Stumped any suggestions would be great.

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Check the fuel pressure (even if you already replaced the fuel pump)

check the ICM, when the engine loses power see if the ICM is hot

consider sending the fuel injectors to be cleaned and rebuilt

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Everything above is spot on.  Revving in neutral doesn't put any load on the engine and isn't useful for diagnosis; it basically shows that the engine is running.

The Ignition Control Module (ICM) under the distributor cap has a white paint dot on it.  If it has gotten too hot the white paint will be yellow or brown.  If that is the case, replace the ICM and use plenty of thermal paste when you put it back on.  It is heat sensitive.  An auto parts store will have them, maybe for a 1999 Chevy pickup V8.

If you have never replaced the plug wires, it is time to do that also.

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Thanks we check the anti siphon valve that is fine new fuel filter.  Going to check the ICM we replaced all other parts when we did tune up last summer. Thanks for all tips. Does Malibu service have ability to plug in and check?

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Shameless plug...I have a couple options for scan/test tools for sale that I no longer need (Sold my Response).  Otherwise there are some nice DIY options as noted.





Edited by dtm8119
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Thanks JustGary. I replaced the ICM it had a yellow dot on it. Appeared to be the original one.  Replaced the MAPS sensor too since we didn't have any testing ability. Ran like a champ today. We spent 2+ hours and ran super smooth.

appeared to run smoother at idle too but never had issue at idle. Thanks everyone for the tips!!!

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