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Issues with 99 74 MPI

300_ray

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"I hope you have some time if

"I hope you have some time if ur lookin at this post!!!, gonna take a while! Start out by saying that I am a GM dealership performance Tech. that does alot of marine work on the side. Recently took a job trying to fix a very poor running 1999 Cruisers 33 with twin 310HP 7.4 MPI's. Start at the beginning by the boat was bought in RhodeIsle last fall and delivered to NY and never run till this spring in the Finger Lakes of NY. STBD engine ran very poorly and Marina Techs were having a hard time figuring out. Heres what has been figured out at this point. Upon taking the job we started at step 1 and found only 10lbs fuel pressure. Had the marina replace pump and they called to say that it now had 90 lbs. of pressure, we then replaced the pump mounted regulator that was clogged with rust. We then had the normal 45 PSI. Engine ran but very poorly. We then removed the fuel rail and tested the injectors with a factory injector testing tool at the shop. Only 1 of the injectors held pressure and had a normal spray pattern. I ordered 8 Delphi injectors ( 99 Chev. 7.4 mpi truck engine.) Engine still runs but poorly. Digging deeper mechanically, We found no compression on #3 cyl and very low comp. on #4 and #6 like 80lbs. Found several valves cold froze and a bent rod on cyl#4 from what appears to be water intrusion. Motor was removed toredown rebuilt .030 over and valve job done, both manifolds and elbows also replaced. Now heres the latest issue. At 2500 engine seems to run good but will not idle. With a scanner installed everything looks normal eventhough engine acts like it has a vacuum leak, just dies when you release the throttle even backfires when you try to throttle back up. Timing has been set in service mode to 8degrees. If we unplug the MAP sensor engine will idle but we have tried plugging the MAP sensor from STBD Eng. in and same result. We were hired by the marina because their in house techs. were struggling and we have made strides with their help but we are now all at kind of a loss of words. All parameters look normal TP is .45 IAC normal when MAP unplugged at 37. The one thing I am questioning is the fact that I put GM truck injectors in it. They fit the same model year engine, same electronics except that marine engines run a fixed fuel trim program due to the lack of oxygen sensors, to tell rich and lean. Any help in this matter would be greatly appreciated. Dan"
 
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