High Idle 454 Crusader XL

[390Express]

Hopefully I don’t cover every issue for these motors, and if the forum would prefer that I use one of my other threads to condense, I will. I figured the issue will be more searchable with a new title, for future reference.

1996 454 Crusader XL, high idle. It won’t idle lower than about 1200 rpm. Intake seems hot to me, but I think that’s just because I’m not used to the closed cooling system. The boat temp stays right around 160 at idle. I’ve idled it at 1,200 rpm for 20+ minutes on multiple occasions, but it doesn’t want to idle below 1200, and it wants to stall if I put it in gear. If I put it in gear and power it up, it’ll stay running. It has power under a load, it just doesn’t want to idle right.

IAF sensor?
MAP sensor?

Are both of these sensors the same for an automotive application as a marine? Other forms suggested going to O’Riely for the parts vs a marine store.

I may still be having an issue with bad gas. I drained the entire system completely at its prior location, but after a 15 mile trip of bouncing around the expressway, it chummed up all sorts of gunk, and with the level of the boat changed, it’s apparent that there was still some bad gas left in the boat. Now my fuel filter clogs up after about 15-20 min of run time. I’m going to temporarily install a 5 gallon can to get it back to the dock, but I’m having the idle issue.

After I get it to the dock I’ll drain the 30 gallons of new gas that I recently put in, after draining the old gas out. I’ll filter it and run it in my truck, and put more new gas in the boat. PITA...

[km1125]

Source of the post
IAF sensor?

Do you mean IAC valve? A bad one could definitely cause a high or low idle. You can block off the air filter or input to it to see if the idle drop considerably, or stalls the engine (a closed IAC should cause an idle of 500 rpm or so). If it drops, then I'd say the IAC is bad but also check the wiring and connections for problems.

[390Express]

Source of the post

Source of the post
IAF sensor?

Do you mean IAC valve? A bad one could definitely cause a high or low idle. You can block off the air filter or input to it to see if the idle drop considerably, or stalls the engine (a closed IAC should cause an idle of 500 rpm or so). If it drops, then I'd say the IAC is bad but also check the wiring and connections for problems.

Thanks KM, can I just stick my finger down there and block it off?

I was also told a vacuum leak could cause a high idle.

[jcoll]

I believe that year has a throttle body injection system. It's a GM system and the components are most all the same - not all are but most. A vacuum leak can cause high idle. A stuck IAC is more likely the cause.

[bud37]

If the idle is up and it stalls going into gear ( load ) chances are the engine is lean for some reason....vac leak, partially plugged injector etc....improperly adjusted throttle, ecu thinks engine at idle and fuels accordingly, IAC open or sticking.....I agree you can get those parts from auto places that can cross the part numbers for you......

[km1125]

Source of the post

Source of the post

Source of the post
IAF sensor?

Do you mean IAC valve? A bad one could definitely cause a high or low idle. You can block off the air filter or input to it to see if the idle drop considerably, or stalls the engine (a closed IAC should cause an idle of 500 rpm or so). If it drops, then I'd say the IAC is bad but also check the wiring and connections for problems.

Thanks KM, can I just stick my finger down there and block it off?

I was also told a vacuum leak could cause a high idle.

If yours is the type that has a foam filter on the back, then yes. Some just route the air from one side of the throttle plate to the other, so there's no filter on the unit itself.

[390Express]

Mine doesn't have a foam filter, but it does have a little pocket for the IAC. I put my thumb over the port that it goes to, and nothing. Didn't change the idle at all. It seems like it's starting to "learn" the idle. It's idling better now, but still a little rough. Now the idle is back down from 1200 to around 500-600, but it's a bit rough. It doesn't stall when I put it in gear, but it's so sketchy that I'm generally afraid it will. I'm hoping it corrects itself on its own at this point. It's done well so far. I'll try some on/off cycles, and let it idle at the marina for a while. It won't be leaving the dock anytime soon. I have some bad gas to correct in both tanks, and I need a new ECU for the port motor.

Yesterday, when moving it back to the dock, it idled kind of rough, then running at 1,000 rpm it started sputtering and stalled. Wouldn't start for 2-3 min, as I was getting blown toward some rocks. It was a bit dicey, but I got it straight. Always an adventure...

[bud37]

If you blocked the IAC port and nothing happened then the IAC was already closed and the engine is getting air from around the throttle plates or elsewhere ....at idle the generally accepted spec for the IAC is around 40-50 counts..closed is 0 counts, full open is around 150 counts......you will need a scan tool to get on top of that stuff.

[390Express]

Source of the post If you blocked the IAC port and nothing happened then the IAC was already closed and the engine is getting air from around the throttle plates or elsewhere ....at idle the generally accepted spec for the IAC is around 40-50 counts..closed is 0 counts, full open is around 150 counts......you will need a scan tool to get on top of that stuff.

I'm not too familiar with how the IAC works. Are you saying that I likely have a vacuum leak?

I just put my thumb over the majority of the hole. Air could still get around it, but it was sucking pretty hard. Blocking the majority of the hole didn't effect the idle RPM at all. Not sure what that means, or what to look for next?...

[pepmyster]

Do you have a photo of your throttle body? If so, we can see the type you have. See my post on deceleration and the cause.