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99 tahoe: towing..boat, service engine soon light comes on..rough idle…

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99 tahoe 5.7 while...
99 tahoe 5.7 while towing boat, service engine soon light comes on. rough idle at stop lights. misfire and o2 sensor codes come up on scanner. unhook boat and light goes out , after a couple miles. runs fine. changed 4 -o2 sensors ,mas air sensor , new catlytic converters, new tune up parts,wires and fuel filter. mechanic may have put 170 or 180 degree thermostat in , because guage is reading lower after change. aprox 160 to 170. use to read 190 to 200. please help thank you !
Submitted: 8 years ago.Category: GM
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Answered in 6 minutes by:
7/5/2010
GM Mechanic: Gary, GMC Technician replied 8 years ago
Gary
Gary, GMC Technician
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Satisfied Customers: 18,028
Experience: 15 years gm master tech/15 years ASE master tech
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where the O2 codes for lean conditions?
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Customer reply replied 8 years ago
yes lean
GM Mechanic: Gary, GMC Technician replied 8 years ago
ok more than likely that engine is running lean expecialy when pulling a load.either the fuel pressure is low of the injectors need to be cleaned.you need to do a fuel pressure test key on engine off and see what the fuel PSI is reading should be 60 to 66 PSI key on engine off and the pressure should hold and NOT drop down at all.can you find out what the fuel pressure is?
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Customer reply replied 8 years ago
i can't check pressure here at home. truck has 129000 miles had new fuel pump at 80000 miles. also this is new motor from gm with 200 miles on it. i was towing boat 3 weeks ago, with old engine, same problem then , engine jumped timing , replaced engine
GM Mechanic: Gary, GMC Technician replied 8 years ago
ok but first things first that fuel pressure needs to be checked and make sure the needle doesn't drop down test it key on engine off 60 to 66 PSI
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Customer reply replied 8 years ago
ok check in couple days thank you
GM Mechanic: Gary, GMC Technician replied 8 years ago
no problem 60 to 66 PSI key on engine off and make sure the needle doesn't drop
Gary
Gary, GMC Technician
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Customer reply replied 8 years ago
62psi key off- 55psi running
GM Mechanic: Gary, GMC Technician replied 8 years ago
can you monitor fuel pressure while pulling that boat?
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Customer reply replied 8 years ago

 

-

will do in a couple days.

GM Mechanic: Gary, GMC Technician replied 8 years ago
ok let me know
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