Stalling, stumble off idle on 05 Black Scorpion

[daleatskiworldorlando]

Hi all, I have another hair puller.

2005 Merc 5.7 Black Scorpion with ECM 555

Condition, hard to start when hot, stalls and stumbles off idle. The stalling comes when trying to return to idle from higher speed.

Trouble shooting done so far, ran scan - no codes. Checked the reading of all the sensors - all ok and in range. Checked the manifold pressure with vacuum gauge - ok. Check fuel pressure at rail 25 PSI. Tested the IAC with the scan tool and it has full control of the engine speed. Checked the throttle position sensor for any bad spots - none found; however, as the throttle is moved just above idle and gets to 1.4 percent the engine slows way down. I can hear the air that was rushing into the IAC stop. The engine will start to speed up when the throttle is moved more but there is a dead spot just off idle. If I watch the IAC duty cycle on the scan tool I see around 35% at idle then it drops to 19 % when the TPS hits the 1.4 percent mark.

Thinking it might be a sticky IAC I replaced it with no change to the condtion.

I checked the throttle plate for looseness - ok.

When I'm testing this problem I'm controlling the throttle direct by hand at the throttle body ruling out the perfect pass.

Anybody have any ideas?

Thanks, Dale

 

[newport dave]

Re: Stalling, stumble off idle on 05 Black Scorpion

Have you Checked the cap and rotor? Replaced the IAC muffler?

 

[daleatskiworldorlando]

Re: Stalling, stumble off idle on 05 Black Scorpion

Hi, yes the cap and rotor are new. This engine does not use an IAC muffler.

The new IAC I installed seems to help the stalling problem but the engine still stumbles off idle. Every Black Scorpion I have ever worked on had a little stumble off idle but this one is really bad.

Something must be wrong with it but I sure can't find it.

Later, Dale