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Brand new 5.7L Volvo Penta idle issue

Regal1973

New member
Some back history of my idle issues. After finding that my new to me 2008 boat I I purchased had an engine that was taking in water via the exhaust, I purchased a brand new 2008 spec 5.7L Volvo Penta GI-G replacement short block motor ( not a remanufactured motor).

A local VP dealer installed motor and since day one, it has had an idle issue. Over 1 year now and the VP dealer has basically thrown up their hands saying they've done everything that they can do to try and figure it out. Once warmed up, motors idle lops up and down about 150-200rpm. Makes it hard to shift smoothly but lopping goes away as soon as you advance the throttle the slightest bit.

The compression, vacuum, fuel pressures high & low all within spec. Crank trigger sensor replaced, IAC valve replaced, throttle position sensor operating correctly, map sensor appears to be operating correctly, thermostat is a 160*, injectors all cleaned and flowed when engine installed, spark plugs look good, timing correct etc.

VP dealer put laptop on motor and no issues or codes showing up. I rented a scanner diagnostic tool from company I purchased motor from and all feedback info seems within specs. The company I purchased motor from has done more to try and diagnose over the phone the issues than I feel the VP dealer has done. The VP dealer has given me so many excuses and no resolve. Their last option was for me to send my computer out to California to get it reprogrammed because they said the new motors fuel burn rates are not compatible with the new engine.....BUT ITS THE EXACT SAME ENGINE WITH EXACT SAME SPECS AS ORIGINAL that came in boat.....so I'm super confused at this point.

The idle issue I think I've narrowed down to a temp related issue but please, I'm looking for ANY FEEDBACK. As soon as motor warms up, it will begin lopping. Temps will rise up to around 170-173* & then the temps drop quickly when thermostat has opened up. As soon as temps drop below 160*, the engine idles perfectly smooth. As soon as temps rise above 160*, it begins lopping again. So it appears that ( if temp guage is correct) that between 160* & 170ish, the idle issue rears its ugly head. The motor runs perfect without a hiccup, it's just at idle that it has any issues.

I'm in west central Florida where water temps range depending on time of year, but it does it on the hose too when on trailer, and I'm pulling fairly cold water from our well. At this point I have no idea what to do next. VP dealer had written me off saying they csnt help, motor company has tried to help over phone but no success and VP corporate office has been a dead end for the most part. I hate looking at this expensive beautiful boat on its trailer and thinking that it has become a piece of yard art....sighhhh :confused::mad:
Any help would be greatly appreciated!!!!!
 
Sandkicker thank you for the reply, no I haven't tried another t stat. Thinking maybe its hanging up?
I'd thought of drilling a couple relief holes in the orig t stat and see what that would do for it. Was told they do not offer a 150* stat for the gi-g 5.7L. The company I purchased the motor from was kinda wondering why it would be approaching 175* with a 160* t stat. I pulled the t stat the other day just to verify that it did have a 160* installed & it did ( I was initially wondering if the VP dealer installed the wrong temp stat but they didnt).
Any concerns that I'm reaching 170-175*? I would think that you would surpass the 160* threshold in order for the t stat to function & respond quickly & as designed correct? Once the t stat opens, the temps quickly plummet to 100* which is where my gauge starts, then slowly the temps rise up again, repeating the cycle. Once it's in the low 160 and under range, it idles perfectly.
 
Another tricky problem. Well having an OMC with a 160 stat for many years (and V/P used this same exact thermostat housing design) I can say that it would usually idle from slightly under 160 to right at 160, the only time it would hit 175 would be momentarily after coming off plane, then after running slow for a few minutes it would again drop to 160. So it was never at 175 for any sustained period of time.
Getting to your problem, for the EFI system to work there has to be a water temp input to the computer, and I think a different sendor is used for this than for the temp gauge. For the temp gauge the sender is often screwed into the front right side of the intake, but on the later model engines it could be also in the thermostat housing. Go back and see if you can find the ohm reading specs for the temp sending unit for the EFI system.
Also, you may need someone to scan it while the engine is running, to get live stream data like fuel trims, etc. I was able to troubleshoot a lazy O2 sensor on our '98 Jeep this way. The upstream sensor was getting stuck or not reading as it should and this caused a near stall at idle speed after the engine went from warm up to normal operation. No codes were set but it was a known problem with older O2 sensors on the Chrysler efi system. New sensor and 100% back to normal. And was able to read the live data while driving it for the proof.

If you can put a good engine scanner on it, look at fuel trims when its running and when it acts up at idle.
 
Louche, thanks for the info. I had the VP dealer put their laptop on it and look at all the data and they felt all was within spec ( I dont have any faith in this dealer at this point though). I also obtained a scanner from the company I purchased the motor from and took some screen shots of different data screens. I passed the info on to the VP dealer and the motor company and received no red flags back from them....but.....I didn't do all this while on the water, only on the trailer at home.

It did cross my mind on whether or not the temp sensor was giving an accurate reading. What is the most accurate way to obtain a true real temp reading in the motor? And where to get this reading? This way I can compare the temp guage to the "true temp". Should I use a temp gun shooting at the heads, exhaust or some other way?
 
get a hand held laser temp gun and compare the reading at the sender to the gun. Some can be off as much as 15 degrees on either side .
 
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