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PCM GT40 5.8L EFI abrubtly barely runs.

phkemp

New member
290 hours on this 1996 engine. Was running this weekend perfectly fine. Tied up for about three hours, when I started it up to head to the ramp...the idle is very erratic, and when throttled up, there is a delay in revving up, it starts to rev, then threatens to quit unless I throttle back; I have to throttle up and down to get the revs up at all, but the don't stay. There is also an erratic, (not regular) knock every 5 or 10 seconds, sounds like the port valve cover area, but I can't feel anything if I put my hand on it. If I try to work it up to higher revs, it stalls out, and also when I return back to idle it'll stall, unless I give it little pokes above idle speed. And it knocks.

Today I replaced the plugs (first time, but others were fine despite their age due to low hours; drained the fuel and replaced with fresh; removed and cleaned the flame arrestor, which was pretty dirty (and then promptly lost it when I put it on the bumper of my tahoe and went to run an errand...trying to find, but need a new one). I also drained the fuel cell, removed it. The filter didn't stay on the high pressure pump, it was stuck pretty good in the bottom of the fuel cell. There also was some corrosion around the bottom of the high pressure pump, but the bottom/inside looked clean. The filter did not stay on the high pressure pump like they say it should, I had to work it out of the bottom of the fuel cell. Thinking that maybe the filter was really clogged and causing my problem, I replaced the fuel cell and tried running the engine briefly with no filter...problem didn't improve any.

So...that's about as far as I can go I think, unless anyone can give me some ideas based on my vague description:

1. Not a gradual decline...it was running great, then on start up three hours later, was a dog.
2. Idle is erratic
3. Quickly throttling up stalls it out
4. Working the throttle up from idle back and forth can increase the revs, but it's not smooth at all, and won't maintain high revs.
5. When I was returning to the dock on Sunday, working in and out of idle speed to keep moving slowly, when engaged with the prop the engine shook quite a bit as well. Not as much when in the driveway in neutral, but still a shaky.

Any ideas before I tow down to a shop and hope they can make a quick diagnosis? Also, I believe that a properly equipped shop can run some diagnostics and get error codes to figure things out quickly? If that's the case I hope to find a shop that can do that, rather than have a mechanic try to diagnose manually?

Thanks in advance.
 
error codes = computer controlled, which means you need a scan tool to read them. If that's the case, the investment now may be of long term benefit if you plan to keep the boat or upgrade. There are serveral posting of GT40 service manuals on the www - search and you shall find.

The other key item is to verify adequate fuel pressure with a gauge. The standard auto gauges work; some places will 'loan' them to you.
 
error codes = computer controlled, which means you need a scan tool to read them. If that's the case, the investment now may be of long term benefit if you plan to keep the boat or upgrade. There are serveral posting of GT40 service manuals on the www - search and you shall find.

The other key item is to verify adequate fuel pressure with a gauge. The standard auto gauges work; some places will 'loan' them to you.

Thank you for your feedback. Going to test the fuel pressure. Also ordered new plug wires, new high pressure fuel pump hose inside the FCC (heard that this $1 part can crack or get a pinhole in it), and a new relays...all items that could probably do with replacement after 15 years.

Would a fuel pressure issue cause the knocking sound, possibly? I was thinking also ignition or arcing, which is why I order new ignition wires.

Thanks again
 
lack of fuel pressure can drive a lean condition...depending upon the load, you can hear all kinds of "new noises" in that condition.
 
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