SC400 Runs bad and stalls when warmed up

[JPete3]

Help my 1997 SC400 has started running bad and stalling after coolant temperature is fully warmed up. Car runs fine when first started and drives good but starts bucking when driving and surges and stalls at stop. Check engine light is not on and no codes are pulled. I have done 2 things recently - filled engine fully with coolant as heater did not work and changed valve cover gaskets as oil was leaking. All lines and wires are attached. Would appreciate ideas. Thanks

 

[spf_lexus]

Try disconnecting the MAF sensor and then reevaluate. If driveability improves, this usually says bad MAF. Also, if your ECT sensor (engine coolant temp) is biased (reading low), the computer may assume that the engine is cold when it is not and continue to dump fuel to speed warm up. Once the computer enters closed loop (uses all sensor values), bad oxygen sensors can also cause similar issues. In some instances, a failing or biased sensor can cause driveability issues without reaching the computers "threshold" for setting a code.

In addition, intermittant vacuum leaks can also cause this issue. If the vehicle runs poorly at idle, light cruise or on deceleration, this also indicates possible vacuum leaks as root cause. Hope this helps some.

 

[xspsi6]

Try disconnecting the MAF sensor and then reevaluate. If driveability improves, this usually says bad MAF. Also, if your ECT sensor (engine coolant temp) is biased (reading low), the computer may assume that the engine is cold when it is not and continue to dump fuel to speed warm up. Once the computer enters closed loop (uses all sensor values), bad oxygen sensors can also cause similar issues. In some instances, a failing or biased sensor can cause driveability issues without reaching the computers "threshold" for setting a code.

In addition, intermittant vacuum leaks can also cause this issue. If the vehicle runs poorly at idle, light cruise or on deceleration, this also indicates possible vacuum leaks as root cause. Hope this helps some.

This only works on pre obd2 cars as I once did this in my 97 sc400and it would barely stay running and threw codes.

 

[JPete3]

Thank you for your advice. I had talked to several parts guys and they had many ideas but the MAF wasn't one of them. I didn't really know where to start so thanks for your help. It turned out to be the MAF and everything is working now. Thanks.

 

[spf_lexus]

Hey no problem, glad to help out.