1995 Truck idle surge

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GregGarage28

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Hello my name is Greg.
Would anybody know why a freshly rebuilt 350(5.7L) would surge at idle? That is to say when you open the throttle and let it close the rpm will go up and then come down and almost stall then return up to normal(600-750) rpm level. Also When engine is cold it seems to want to almost stall when rpm drops after gear shift(truck has 4L60E). Could this be related to an oxygen sensor, idle air control valve, coolant temperature sensor? No check engine light has ever come on before or after engine was rebuilt. this also happened before the engine was rebuilt. also the mileage has gone down 150kms per tank since rebuild. Any insight would be greatly appreciated.

Thanks,
Greg
 
Last edited:
GregGarage28 said:
Hello my name is Greg.
Would anybody know why a freshly rebuilt 350(5.7L) would surge at idle? That is to say when you open the throttle and let it close the rpm will go up and then come down and almost stall then return up to normal(600-750) rpm level. Also When engine is cold it seems to want to almost stall when rpm drops after gear shift(truck has 4L60E). Could this be related to an oxygen sensor, idle air control valve, coolant temperature sensor? No check engine light has ever come on before or after engine was rebuilt. this also happened before the engine was rebuilt. also the mileage has gone down 150kms per tank since rebuild. Any insight would be greatly appreciated.

Thanks,
Greg
Click to expand...

Start with the simple and replace the fuel filter. The TBI is fuel pressure sensitive and the injector may not be at 13 PSI and also you may have a IAC issues that needs to be addressed. Start with cleaning the IAC with throttle body cleaner and check your timing.

Good luck and keep us posted. ;)
 
Molson02536 said:
Start with the simple and replace the fuel filter. The TBI is fuel pressure sensitive and the injector may not be at 13 PSI and also you may have a IAC issues that needs to be addressed. Start with cleaning the IAC with throttle body cleaner and check your timing.

Good luck and keep us posted. ;)
Click to expand...

I put a new IAC in and advanced the timing. and it seems to have resolved the problem for the most part. It still hesitates/runs rougly a bit when I give it part throttle,to say get up an approach, and then let off. The fuel filter was changed at the time I reinstalled the engine. What is the best way to check the fuel pressure? Could this be caused by a bad O2 sensor?
 
I have an 04 sierra, that seems to have had a simliar problem, at idle the rpms would be steady after giving it some gas and let off the rpms would drop wayy low and often shut my truck off, even while driving.. after checking all of what you guys have discussed above, i took my throttle body off and there was all kinds of build up witch wasnt allowing the throttle plate to seat properly, after i cleaned it all up. I havent had the problem since.
 
coolant temperature sensor might me the reason, its recommended to get it checked with the showroom once.
 
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