TBI 260 Wont Idle When Warm - Will Run other RPM

RTonelli

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Feb 2, 2019
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Hey everyone,
On the last trip last season my 1999 Sunsport with twin 5.7 mercruiser EFI (TBI) motors had an issue. Port motor runs great. Starboard motor starts right up when cold, will idle great, can advance the throttle and all goes well. Once it warms up for about 5 minutes it cuts out, almost like I pull in a choke or something and just stalls right out. It will start right back up no problem but then die right out if at idle. It will run if I advance the throttle to like 1500-2000 rpm but once I back off it stalls again.

I just did all the good regular maintenance and the problem still remains. Both motors got new cap and rotor, fuel filter, oil change and filter, plugs, cleaned spark arrestor and both drives serviced. Problem is still there.

Anyone have this issue before or know where I should look first? My plan is to dig into it a little more this weekend. Fuel pressure test first? It does have the cool fuel system, not sure what that really does? From what I can see the previous owner replaced the IAC already, but I can try it again. Port motor runs great so I don't think its a tank vent issue but I could be wrong. MAP or TPS?

thanks!
 

alldodge

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Mar 8, 2009
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32,353
My plan is to dig into it a little more this weekend. Fuel pressure test first? It does have the cool fuel system, not sure what that really does? From what I can see the previous owner replaced the IAC already, but I can try it again. Port motor runs great so I don't think its a tank vent issue but I could be wrong. MAP or TPS?

Agree, start with fuel pressure test

Don't start replacing parts until we know what is causing the issue

What's your motor serial number?
 

RTonelli

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Feb 2, 2019
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Fuel pressure is 30 psi. The motor will not idle at normal range, even when cold and it's idling its about 1000 rpm. When it tries to stall it starts making a sucking type hissing sound from the intake. Both injectors spray good, it will basically stall but then catch itself and shoot up in rpm and slowly come back down to 1000 rpm then do it all again.
 

alldodge

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Mar 8, 2009
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Use a small propane torch (don't light it) and move it around the intake a TB base plate. If it starts to change the idle then you found the leak. Can also use WD40 of other things, even water squiring on places
 

RTonelli

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Feb 2, 2019
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Well I ran into a bigger probelm now.

my STBD motor (same stalling motor) is not flowing water as it should. Both drives were just serviced with gimbals, impellers, bellows, cables, etc. PORT motor runs great and the risers run cool (95 degrees at idle). STBD risers will shoot up to about 160 sometimes just idling on the hose. The STBD drive flows a lot of water out of the prop, cold water so it’s not being circulated, as well as through the weep hole on the drive. The PORT drive flows well out of the idle relief and not as much out of the prop which I think is right. What could be the blockage or issue?

Replaced/Checked
- water circulation pump on the motor
- inspected risers and verified no blockage, installed new gaskets (they are dry joint risers)
- pulled all hoses one at a time from the T-Stat housing and started the motor to verify water flow from each hose/port - GOOD (including intake)
- new T stat because I had the housing off figured I’d change it
- exhaust flappers on both exhausts still in place and functioning correctly
- dropped the bottom half of the drive again to verify impeller was correct and everything checked out

the exhaust coming out of the prop is steaming something isn’t right. Manifolds themselves are cool, risers are hot. Any ideas? I need to tackle this before the stalling. If I unplug the ECT sensor also it will idle to problem.
 

alldodge

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Need the pump redone again or there is a restriction between the drive and cool fuel. The water coming around the muffs is more on the starboard is more then the port. Could be at the transom, or hose kink from bell housing to transom
 

RTonelli

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Feb 2, 2019
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Need the pump redone again or there is a restriction between the drive and cool fuel. The water coming around the muffs is more on the starboard is more then the port. Could be at the transom, or hose kink from bell housing to transom
Did you see the video of the water intake hose? Seems to have good flow off the leg?

I am thinking a possible exhaust blockage. Something is restricting flow. I hooked the hose to the water intake hose when I had the lower unit off to verify water would flow good from the cool fuel unit to the intake tube in the upper and it was flowing strong. The risers are only 2 years old, I am thinking soaking in muratic acid and see if the blockage is there.
 

alldodge

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I didn't see water coming out of the cooler with any force. It should be moving faster with the motor running
 

RTonelli

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Feb 2, 2019
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I didn't see water coming out of the cooler with any force. It should be moving faster with the motor running
I will verify again, but I popped the port side off to compare yesterday and it seemed to have the same flow - cools much better though.

What I think I should try is when the motor is cold, take the intake hose off the inlet to the cool fuel, and start the motor to spin the impeller. While it is off, check the drive to see if it is sucking more water which will verify if there is a blockage before the cool fuel.

If it sucks more efficiently, replace the hose on the inlet to the cool fuel module and remove the outlet end and recheck. If it is not sucking water efficiently then I know it is in the cool fuel module.

If it still is sucking efficiently when not connected to the TSTAT housing, then its a blockage somewhere in the engine.
 

Scott06

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Apr 20, 2014
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1,608
Spray starting fluid at the gasket surfaces by the intake, throttle body etc. If there is a vacum leak the starting fluid will get sucked in and pick up some rpm.
 

RTonelli

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Feb 2, 2019
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Quick update:
I started the boat with the inlet hose disconnected from the power steering cooler to rule out any blockages on inside the engine compartment. Still water pouring out of the prop. This leads me to believe the issue lies between the water pickup, and the thru Hull fitting.

I ordered a new impeller kit again (quicksilver), but what else should I look into? The pickup hose from the bell housing to thru Hull fitting is brand new. I will also backflush it to see if anything comes out.

thanks for all your help so far!
 

alldodge

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Might be the pump seals. Get the gaskets for the entire pump if not a new pump housing

Might be a misalignment with the pump tube in the drive
 

RTonelli

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Feb 2, 2019
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Water issue fixed! Intake hose was misaligned in the lower unit. Runs cool now.

still will only idle at a high idle. Also found if I give it WOT from a idle it will bog and sometimes backfire. Compression checked all 8 and they were good and all within the same number of each other. Spoke with the previous owner and he said he’s been chasing it for 5 years and have fun. I guess he had a shop try and diag it and they couldn’t figure it out. They changed the IAC already too.
I read the codes and there are none. Not sure what it can be
 
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