CATF800

I have replaced the plugs and checked compression. All cylinders between 162 and 172 PSI. I have replaced the 3-relays up on top as i has a FUEL PUMP RELAY CK code. I checked fuel pressure at the rail with engine off and turning the key on and have 43PSI. I will be replacing the wires and awaiting their arrival. something on the scan is that the IAC duty cycle is all over the place like between 10 and 40 (I guess %?). the "Whistle" of the intake sound is surging up and down and not smooth and steady like the other engine that seems to run fine. on the engine that seems to run fine I scanned it twice. the IAC was first all over the place while watching the first time and then was a steady 0.0% when I checked it the second time. what's this mean? also on the engine that is acting up the problem seems to somewhat come and go about that fast as the tach will move up and down 200-300 rpm with the throttle set at second notch about 1,800 rpm. this is all being done on the trailer in the driveway. I first started all of this because I am only getting 72mph for a top speed and is about 6-8mph off. What's your thoughts?

CATF800

Should I install the fuel pressure gauge on the rail and leave it there while running it on the trailer to check to see if the fuel pressure holds steady? the pressure gauge I have has a release valve on the side so I don't know if this gauge has a check valve and holds the pressure or not kind of like a compressions gauge does.

CATF800

Never looked in the throttle body behind filter/spark arrestor before but the engine that is not running correctly has no muffler. the port engine does. I installed muffler in the star engine and the surging of the intake air whistle is now gone and is constant, but still not running correctly. the idle is low. when moving throttle to first click it seems to struggle to get just above normal idle speed. second notch it struggles to get to about 1,600 rpm and then drops to about 1,000rpm. The other engine will hit 1,500 at first notch and about 2,300rpm at second notch.

CATF800

I exchanged the IAC from the other engine.....same thing. I have approx. 40psi on the fuel rail while engine running. the thing surges like clears up a bit then goes back to stumble then will clear up some and pick up some rpm then drop back and about that fast. it does not sound like one cylinder.

CATF800

engine serial # OW008002. 2005

BUP

iTrader: (9)

Have you rescan the motor - if you do - does it show 100 % available power ? Any other codes / faults. Also check for vacuum leaks especially around the IAC area. Also have you checked for good solid spark from each coil. Use completely sealed inline spark checkers.

IMO you might need your fuel injectors checked out & serviced. 496 engines did have problems with fuel rail corrosion internally. The later ones hold up a lot better as I want to say 2001 - 2005 could be the problem years. When testing your fuel pressure, does your gauge stay steady. These motors really like 42 - 43 steady psi fuel pressure. Have you done an injector balance or dropped test ?

Does your surging happen all the time at all rpms or just @ idle speed ? Can you even reach max rpms ? How old is your fuel ? Run motor on a clean outboard gas tank and new fuel line with fresh fuel to see if you have a fuel delivery problem. With any problems you always want to separate the fuel tank / anti siphon valve out of the mix to pin point at least the problem lies on the motor side or the fuel tank area.

CATF800

Have you rescan the motor - if you do - does it show 100 % available power ?

Always shows 100% power available and no fault codes.

Any other codes / faults. Also check for vacuum leaks especially around the IAC area.
I WILL CHECK FOR VACUUM LEAKS

Also have you checked for good solid spark from each coil. Use completely sealed inline spark checkers.

NO I HAVE NOT AND I DO NOT HAVE ONE OF THESE TOOLS

IMO you might need your fuel injectors checked out & serviced. 496 engines did have problems with fuel rail corrosion internally. The later ones hold up a lot better as I want to say 2001 - 2005 could be the problem years. When testing your fuel pressure, does your gauge stay steady. These motors really like 42 - 43 steady psi fuel pressure.

FUEL PRESSURE IS LOWER ON THIS ENGINE THAN THE OTHER ONE. JUST SITTING ON THE TRAILER N WHACKING THE THROTTLE TO 3,000 RPM I HAVE SEEN THE PRESSURE DROP TO THE AREA OF 37PSI WHEN BACKING OFF THE THROTTLE IS WHEN THE PRESSURE WOULD DROP...OF COURSE THIS IS WITH NO LOAD. I HAVE A NEW SPARE BOOSTER PUMP.

Have you done an injector balance or dropped test ?

NO. I DO SEE THIS ON THE SCANNER BUT DO NOT KNOW HOW IT IS CONDUCTED AND DO NOT HAVE A MANUAL.

Does your surging happen all the time at all rpms or just @ idle speed ?

I DO NOT KNOW FOR CERTAIN. I KNOW WHEN I MOVE THE THROTTLE TO THE SECOND NOTCH IN NEUTRAL THE RPMS WILL RISE TO APPROX 1,800 RPM THEN DROP TO APPROX 1,200 AND SET THERE AND SURGE UP AND DOWN.

Can you even reach max rpms ?

THIS IS A PROBLEM TO ANSWER AS WELL. I LIVE 1 1/2 HOUR'S FROM THE NEAREST LAKE AND WHEN I WAS OUT THE LAST TIME WITH KIDS, TRAFFIC AND CHOP AND SUCH IT WAS A PROBLEM TO EVEN ATTEMPT TO WOT THIS THING LET ALONE STARE AT GAUGES. I NEED TO GO AND TEST IT I KNOW. JUST TRYING EVERYTHING I CAN HERE TO HOPFULLY CURE IT BEFORE I DRIVE ALL THAT WAY AND HAVE TO BRING IT BACK RATHER THAN LEAVING IT IN THE RACK WHERE IT BELONGS.

How old is your fuel ?

I HAVE RUN A FOR CERTAIN FULL TANK OF FUEL THROUGH. I WOULD SAY IT WAS 1/3 FULL FROM LAST YEAR AND WAS WELL STABILIZED AND I ALSO RUN LUCAS TREATMENT ALL OF THE TIME. ALL FUEL THAT WAS PUT IN THIS YEAR WAS MARINA FUEL THAT IS 91 OCTANE AND HAS NO METHANOL/PURE GASOLINE.

Run motor on a clean outboard gas tank and new fuel line with fresh fuel to see if you have a fuel delivery problem. With any problems you always want to separate the fuel tank / anti siphon valve out of the mix to pin point at least the problem lies on the motor side or the fuel tank area.

OK I CAN DO THAT

CATF800

......Also......this weekend I flip flopped the PCM from engine to engine and the IAC. no change. I replaced the items to the good sounding engine on the port side. Now the port side engine is running exactly the same (Bad) as the problem engine on the starboard side. Not certain I have the IAC's correctly placed back and expect the new one from Summit Racing will be here today. The scanner shows the IAC duty cycle to hold at 99.9%. then another time I looked at it, it was moving around between like 14 and 45%, and ran the same as in bad or at least not as it was running. before throttle at second notch it would run at 2,200 RPM now it goes to 1,700 and drops back and surges like the other one.

CATF800

The other thing with the port side engine that is now seeming to run like the star engine that has seemed to be a prob. when I removed the IAC from the engine that seemed to run best it had obviously been replaced at one time and the gasket was spun 90 deg and only one of the bolts was through the IAC. this would for surely cause a vacuum leak and the increase in higher RPM and maybe giving me the idea that this engine was running better than the other, and now with the gasket in correctly curing a possible vacuum leak dropping the RPM down slightly showing me what I have, but there's still the mystery of why I cannot pull what I should for MPH. I am taking it to Chevrolet dealership here in town to have some of the above checked as mentioned by BUP as they have the diagnostic equipment to do this, as he is a friend and can get it in. every where else is out a month......and as mentioned I do need to check fuel pressure while driving it and engine is under load.

CATF800

Thanks much for the help. let me know if you can think of anything else they need to check. are there any links that you would have to a shop manual?