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  1. #1

    Default 1994 PCM 5.8 Pro Boss Starting Issues

    I have a 1994 Supra Sunsport with a PCM 5.8 Pro Boss Engine. I have been having starting issues for the past three years. First start off of the trailer is hard and requires a lot of cranking. After it warms up, on the second start it fires and dies and then will refuse to start for up to five minutes. No gas coming out of throttle body injector. After a while gas begins to come out of injector and engine will fire. I have had both fuel pumps, fuel filter and fuel pump relay replaced. Both fuel pumps come on when ignition is turned on. This issue has stumped two mechanics. I have seen one other post on this forum but no solution. Anyone have any thoughts???

  2. #2
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    Default Re: 1994 PCM 5.8 Pro Boss Starting Issues

    Have you monitored the voltage on the pump power leads during operation? There's a safety requirement for controlling the electric fuel pump and many installations use an oil pressure switch for this function....sadly they have a fairly frequent failure rate....the volt meter will provide insight into the pump portion.

    On the initial hard start, assuming the pump primes the injectors, it could be due to one or more of the sensors being out of tolerance....I'll have to see if I can dig up my manual on the pro boss stuff to get specifics....

  3. #3

    Default Re: 1994 PCM 5.8 Pro Boss Starting Issues

    I haven't monitored voltage on either pump during starting. My hands are pretty full just getting things running as we float around in the middle of the lake. But the pumps do cycle on when the key is turned on. We did have a bad oil pressure sender that caused intermittent alarming even though oil pressures were fine. Original mechanic did replace that sender. They also checked fuel pressure in the lines and report 40 lbs. The boat runs fine one we get it started.

    You may be onto something with a bad sensor. Getting one may be problematic though. It has also been suggested that I may have some corrosion on a ground or connector somewhere also.

    Let me know what else you find. This problem has pretty much ruined two boating seasons and I don't want to go for three.

    THANKS!!

  4. #4
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    Default Re: 1994 PCM 5.8 Pro Boss Starting Issues

    ok....my memory ain't quite what it used to be.....there's an oil pressure switch but I'm 99% sure it doesn't effect the fuel pump control logic....

    Do you have the coil pack igniton setup?? also known as Po-Tec?

  5. #5

    Default Re: 1994 PCM 5.8 Pro Boss Starting Issues

    I'm fairly certain that my 94 Pro Boss 5.8 has the Pro-Tec ignition.

  6. #6
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    Default Re: 1994 PCM 5.8 Pro Boss Starting Issues

    Then I guess you know they work great until something breaks down and parts are hard to find....

    Just make sure you have a fuel issue and not an ignition issue....regarding the hard starts....

  7. #7

    Default Re: 1994 PCM 5.8 Pro Boss Starting Issues

    Good call on the oil pressure switch. My new mechanic tracked down the problem to an issue in this switch - different than the sender that was replaced. This switch allows the fuel pumps to run for 15 seconds without pressure. If no pressure - indicating no starting - the switch shuts down the fuel pumps preventing burnout. My switch was intermittent, keeping fuel from reaching the engine during starting. For some reason, once it did start, it worked normally. He bypassed the switch and we are going to further test this weekend. I'm hopeful my starting problems are behind me. Thanks again for the help!!

  8. #8
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    Default Re: 1994 PCM 5.8 Pro Boss Starting Issues

    That's the USG mandated safety feature....bypassing for test is ok but that function needs to be there......though not necessarily under computer control.

    FWIW, there are many www sites where you can find the GT-40 service manual....I think most of it is applicable but for the fuel injector portion (Throttle body vs port injection)....the controls (sensors) are likely to be similar but haven't don't the comparison.

    Good luck with the upcoming test!!

  9. #9

    Default Re: 1994 PCM 5.8 Pro Boss Starting Issues

    Good point on the safety feature. I have already requested a quote on a replacement from Vince at SkiDim.com. They seem to have a good supply of parts and good knowledge on these older engines. I think I have some wiring diagrams on the for the Pro Boss in the miscellaneous documentation I got when we bought the boat. I have some users guides from before 1994 and after as well. I got those from Supra. I couldn't get the exact year because of a fire at the factory causing loss of some of the records. Go figure.......

  10. #10
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    Default Re: 1994 PCM 5.8 Pro Boss Starting Issues

    then you are coupled in to the 'experts' for that vintage....and the Supra sites have lots of knowledge too!!

  11. #11

    Default Re: 1994 PCM 5.8 Pro Boss Starting Issues

    Problems continue. Boat now starts but as soon as you begin to accelerate - around 1800-2000 RPM, the engine starts to miss badly and will go no further. My mechanic thinks the ignition has finally gone out. Our boat is 200 miles away at the lake and impossible for me to work on. Does anyone else have any thoughts?? I think replacing the ignition if I can keep the throttle body injection would be a good choice. Replacing both would be expensive and I would hate to do it if there was another way.

    Does anyone know if the Protec unit can be rebuilt??

  12. #12
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    Default Re: 1994 PCM 5.8 Pro Boss Starting Issues

    I don't know anybody that does it.....I'd suggest you check with somebody that rebuilds EEC modules (Ford stuff).....

    You can check around but most people replace the protec with a good old distributor.....I'm sure Vince can tell you what they have found....

  13. #13

    Default Re: 1994 PCM 5.8 Pro Boss Starting Issues

    I am checking around for rebuilders. In the meantime, I did check with Vince. Because I have TBI, to go to a distributor, I would also have to replace the TBI with a carburetor. Not as bad as a whole engine swap but still somewhat expensive, especially if I don't do it myself.

    My thinking now is that the boat is going into "limp mode" due to my mechanic grounding the input to the oil pressure switch. While that seemed to fix the starting issue, it prevented it from running normally. A new oil pressure switch might fix everything. But testing will have to wait until next year.

  14. #14
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    Default Re: 1994 PCM 5.8 Pro Boss Starting Issues

    if he grounded the oil pressure switch, its highly likely it will stay in limp mode (aka SLOW mode) and never get over 2500 (?) RPM...

    there's nothing magic (or marine) about the pressure switch....so there's no point in over paying for one....

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