5.8 Penta idles high

ctfortner

Cadet
Joined
Feb 26, 2018
Messages
13
My (new to me) 5.8 EFI Volvo is idling high, around 1500 rpm, even after it is up to temp. I was thinking the IAC valve may be bad or previous owner may have adjusted the TB idle screw. It idles good, although high, and ran great on the water. Shifted slightly hard in and out of gear because of idle.

Today I ran it to temp and then disconnected the IAC, it stuttered briefly and died. Would not crank with it disconnected. Plug it back in and it is right back to idling 1500 rpm. Should it idle (even poorly) with IAC disconnected? I am wondering if someone before me adjusted that idle screw, or should I just get a new IAC vave and swap out and try that first?

I have not disconnected the throttle cable yet to see if that could be the issue, but this may sound dumb, but If the boat is in neutral, I have tried pushing on the throttle itself to see if it would idle down...but the throttle cable will not move forward or backwards when trying to do so at the TB by hand., while boat is sitting in neutral.

I made a video of it running and then dying, see if this link will work
https://photos.app.goo.gl/ewfrZQMDkfJDx4502
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
43,216
Should it idle (even poorly) with IAC disconnected?

No

I am wondering if someone before me adjusted that idle screw, or should I just get a new IAC vave and swap out and try that first?

Maybe but first try disconnecting the throttle linkage from the TB. Then with it idling, push the TB closed

If does not change anything, see if there is a vacuum leak. Get some WD40 and spray around intake and TB
 

ctfortner

Cadet
Joined
Feb 26, 2018
Messages
13
No



Maybe but first try disconnecting the throttle linkage from the TB. Then with it idling, push the TB closed

If does not change anything, see if there is a vacuum leak. Get some WD40 and spray around intake and TB

Ok, I will give that a try first and if no luck, I will remove the IAC and clean it and see what happens then.
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
43,216
Cleaning the IAC would be good, just don't use anything caustic like carb cleaner on it.
 

Lightwin 3

Petty Officer 1st Class
Joined
May 18, 2010
Messages
300
What would you recommend using? Seems most info online is carb cleaner type of stuff

There are specific products just for cleaning Throttle bodies. I just saw some at NAPA the other day. Any auto parts store should have it. I believe it was made by CRC.

Just found it: CRC# CRC05078 There are other brands also.
 
Last edited:

ctfortner

Cadet
Joined
Feb 26, 2018
Messages
13
Ok gotcha thanks. I have used TB cleaner before, I was thinking you meant not to use that either (on the valve at least). I will grab some today and work through these things and report back.
 

ctfortner

Cadet
Joined
Feb 26, 2018
Messages
13
Alright, I disconnected the throttle cable, removed IAC valve and cleaned with TB cleaner, cleaned inside TB with spray and a brush. Put IAC back on, fired it up and she just purrs at 1500 rpm without missing a beat...so, no change.

Engine was hot by then so spraying solvent wasn't in my want to do list, will try on cold engine later today. I'm still thinking hard about that dam idle screw though. What now ? If it was a vac leak would the rpm bounce around? If it is a vac leak, would it be noticeable in running WOT on the water. I drove it before I bought it last month, performed flawless for 30 minutes, just idled high. No stutter, hesitation, stalling, nothing. It would be awesome to know what and how to adjust or check if the idle screw is in the factory position. Is there any manual that discusses that?

Is there any harm in noting the amount of turns, and then adjusting idle screw and testing?
 
Last edited:

kenny nunez

Captain
Joined
Jun 20, 2017
Messages
3,443
If the problem is a vacuum leak the gasket sealing the plenum to the lower manifold is usually where the leak will be.
 

ctfortner

Cadet
Joined
Feb 26, 2018
Messages
13
I went home at lunch and checked under the dash and the tach did have options for 4, 6, and 8 cyl. It was set on 8 already. As suggested by several in Cobalt forum, I turned it off of 8 cyl and then put back on 8. Assuming that would do no good, I got the ol benzomatic propane torch out and rigged up a hose to it so I could go vacuum hunting just to see. Fired the engine up and what the #&$%$&, she started idling right on 700 where it should be! Dilly Dilly! Either it was the tach acting up or it re-learned after the cleaning I did of everything, but I think it is the tach because the engine sounds and looks the same as before while running.
 
Top