1998 mercruiser 7.4l horizon

[jake88]

Port engine, turn key on fuel pump pressures up to only 10psi after start goes to 30 and fluctuates down to maybe 28psi and up to 32psi. at idle motor runs smooth, push throttle stick up to say 1700 rpm and then rpms go up to 25, 2600 rpms and back down up and down. changed fuel pump and regulator, plugs and fuel filters. scanned and everything looks within parameters. had a code for a map sensor, cleared code and it didn't come back. swapped map sensor's from other engine anyways and still continues. Any ideas ?? bad tps bad iac bad injector?

 

[scoflaw]

Is your fuel pressure back to normal?

 

[jake88]

this is after a new pump and regulator was installed.

 

[scoflaw]

this is after a new pump and regulator was installed.

Is that a Yes ? Pressure good at the previous failure point? IAC and TPS values would show on your scanner
Sure, 22 year old injectors could certainly be your issue.

 

[jake88]

Im sorry no fuel pressure with new pump and regulator was 10psi with key turned on, went to 30 after engine start. dropped alitttle when rpms were pushed up to 1700 rpm. manual says 34 to 38 psi. wouldnt the Iac only be an issue at idle not when your over say 100rpms? Tps looks ok on scanner.

 

[scoflaw]

Obviously fuel pressure i the problem. Probably a collapsing hose, clogged anti syphon. I would replace everyone of those old hoses and check your tank pickup. Maybe try running on remote fuel cell to confirm

 

[jake88]

I used a gas can plumbed to the fuel pump and same issue.

 

[scoflaw]

The 2 gas lines from the filter to the pump/regulator and pump to fuel rail are suspect

 

[jake88]

kinda what i was thinking. thanks