5.8l efi idles too fast

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lvhdude

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I have a 1996 220 Horizon that idles around 1200 rpm. Somewhat erratic and slow to return to idle after revving or running at speed. So far I've changed the IAC valve, the TPS adjusted the stop screw to just the point that the butterfly does not stick, and resealed the two throttle body gaskets to no improvement. What should I look for or test? What else to try? Thanks.
 
Scott Danforth

Scott Danforth

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look for intake leak

if you need hard parts for the EFI, most are NLA.
 
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lvhdude

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Anyone know of a service manual for this boat/engine available? What do you guys use to look for intake leaks?
 
alldodge

alldodge

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Have you check the throttle linkage?
Remove throttle cable, with TB fully closed see if it just slides back on. If it does remove and adjust one more turn so it apples a slight pressure to help hold it closed
 
ChrisinPHX

ChrisinPHX

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lvhdude said:
Anyone know of a service manual for this boat/engine available? What do you guys use to look for intake leaks?
Click to expand...
Carb cleaner or brake cleaner. There are a number of vacuum lines on the underside of the intake plenum that can be hard to see that can have a cracked vacuum hose or dry rotted vacuum caps. You don't need to spray tons, but if you shoot it in a certain area and your idle changes, you have found your spot to look further in. Most parts are still easily obtainable if you look at 1995 Ford Lightning pickups with the 5.8L. Those are the 5.8's with the GT40 heads. Also, have you bothered to pull codes?
 
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lvhdude

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ChrisinPHX said:
Carb cleaner or brake cleaner. There are a number of vacuum lines on the underside of the intake plenum that can be hard to see that can have a cracked vacuum hose or dry rotted vacuum caps. You don't need to spray tons, but if you shoot it in a certain area and your idle changes, you have found your spot to look further in. Most parts are still easily obtainable if you look at 1995 Ford Lightning pickups with the 5.8L. Those are the 5.8's with the GT40 heads. Also, have you bothered to pull codes?
Click to expand...
Are you talking about under the Upper Manifold? Under the Throttle Body? I will get some brake cleaner, (easier on paint) and spray selectively around that area as well as the base of the upper intake. I'm familiar with the OBD II test ports on autos, but what do I need, and how do I connect to check codes on the boat? I have had it to the local boat shop, they put me on to a suspected leak at the throttle body gaskets, but the ones I found were too small, so I cleaned up the surfaces and applied a thin coat of silicon sealer. I'm confident there are no leaks there, but it did not improve performance.
 
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lvhdude

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alldodge said:
Have you check the throttle linkage?
Remove throttle cable, with TB fully closed see if it just slides back on. If it does remove and adjust one more turn so it apples a slight pressure to help hold it closed
Click to expand...
There is a stop screw that keeps the air valve from sticking closed, the valve closes well against the stop, but the idle is still erratic and around 1200 rpm. Even if I back it out, with the valve closed tight. Shifts easily in and out of gear out of the water, but very had to get out of gear when pushing the boat at 5 - 6 MPH.
 
ChrisinPHX

ChrisinPHX

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lvhdude said:
Are you talking about under the Upper Manifold? Under the Throttle Body? I will get some brake cleaner, (easier on paint) and spray selectively around that area as well as the base of the upper intake. I'm familiar with the OBD II test ports on autos, but what do I need, and =how do I connect to check codes on the boat? I have had it to the local boat shop, they put me on to a suspected leak at the throttle body gaskets, but the ones I found were too small, so I cleaned up the surfaces and applied a thin coat of silicon sealer. I'm confident there are no leaks there, but it did not improve performance.
Click to expand...
Under the upper plenum (manifold).

Just need an OBD tester. They are pretty cheap on Amazon. Here's the one I use. Is your boat OBD 1 or OBD 2? My 95 is OBD 1 and OBD 2 was already around in the automotive world, but the boating industry has a tendency to lag behind.
https://www.amazon.com/gp/product/B000EW0KHW/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Alldodge is right in asking you to check the throttle with the cable disconnected. Scott is also correct that a vacuum leak is the most common culprit. You might get a rolling idle with the IAC or the MAP, but typically not a solid consistant raised idle.

Further down the rabbit hole - If it's not throttle body or vacuum related the ECM is what controls and sets your idle RPM. The ECM's have been known to have capacitors that can leak, and being vertically mounted, can leak down the board causing all sorts of issues. I've taken mine apart just to inspect and also make note of the capacitors used.

One final thought, is that the 5.8Fsi's used a fuel cooler to keep the fuel from boiling and creating a vapor lock. The float can stick on those and excess fuel will make it's way into the system, and more fuel flow can easily = higher RPM. You can tap on the top of the cooler to try to free the float, or you can take it apart and service it, but the parts are small, brittle, and worst of all virtually non-existant. I was able to scoop up a rebuild kit on eBay a few years ago and haven't seen another one on there since.
 
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lvhdude

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ChrisinPHX said:
This guide was put together by the PCM guys, but 99% of the info is relevant to you. I've got all the VP hard books for our engines, but this is more handy. Here's my dropbox link. https://www.dropbox.com/s/vf59bqelcsjfefz/Pro_BOss_GT-40_Service_Manual.pdf?dl=0
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I didn't see this until now, Thank you so much! Still having issues with the idle. Was unable to find more vacuume leaks, but have the gaskets for the lower intake manifold. Was intending to swap out to an Edelbrock with a Holley, but if I find issues under the lower section maybe I'll see if I can save the injection. I like the way it starts and runs but for the idle issues.
 
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lvhdude

Seaman Apprentice
Joined
Dec 23, 2022
Messages
36
ChrisinPHX said:
Under the upper plenum (manifold).

Just need an OBD tester. They are pretty cheap on Amazon. Here's the one I use. Is your boat OBD 1 or OBD 2? My 95 is OBD 1 and OBD 2 was already around in the automotive world, but the boating industry has a tendency to lag behind.
https://www.amazon.com/gp/product/B000EW0KHW/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Alldodge is right in asking you to check the throttle with the cable disconnected. Scott is also correct that a vacuum leak is the most common culprit. You might get a rolling idle with the IAC or the MAP, but typically not a solid consistant raised idle.

Further down the rabbit hole - If it's not throttle body or vacuum related the ECM is what controls and sets your idle RPM. The ECM's have been known to have capacitors that can leak, and being vertically mounted, can leak down the board causing all sorts of issues. I've taken mine apart just to inspect and also make note of the capacitors used.

One final thought, is that the 5.8Fsi's used a fuel cooler to keep the fuel from boiling and creating a vapor lock. The float can stick on those and excess fuel will make it's way into the system, and more fuel flow can easily = higher RPM. You can tap on the top of the cooler to try to free the float, or you can take it apart and service it, but the parts are small, brittle, and worst of all virtually non-existant. I was able to scoop up a rebuild kit on eBay a few years ago and haven't seen another one on there since.
Click to expand...
Hey Chris, can you tell me where the test port is? And what the OBD I looks like? Thanks. I did get the manual to download.
 
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