454 mag won't rev over 4k - fuel pressure?

[Needafunboat]

Hi Guys,

Just back from an absolutely great week at Watts Bar - boat ran great for the most part. We had the bimini up 95% of the time since it was about 95 degrees and sunny the whole trip - so very few high speed runs. Under about 50 mph, it was the same boat - lots of power out of the hole, good fuel economy, ran smooth, still hasn't used any oil (that I can see on the dip stick) in the 30 hrs I've put on the boat.

Boat previously hit the limiter around 4900 rpm / 65.5 gps quite a few times, even with a medium load in the boat, around 80 degrees max air temp.

Towards the end of our trip, I ran the tank down to about 4 gallons of gas. The next day, I put the bimini down and tell the wife I just need to run the boat a little. It hit 56 mph on the first blast and then wouldn't run over about 50 mph / 4K rpm. I figured it was a clogged pickup or even fuel filter/separator (which was replaced 30 hrs ago). So I didn't push the boat much (assuming it was running lean) and took it easy for the last day of the trip, kept the bimini up and running below 35mph. I did do a few fast WOT starts and got what sounded like a little backfire, so I let up. I'd think a backfire would be a rich condition, which seems contrary to the lean / low fuel pressure condition.

I'll replace the fuel filter, drain the tank and clean the fuel pickup. It has a cheap plastic tank and 15 yr old fuel hoses (remember this is a stingray - not a check).

Also, my mechanic confessed to me that he ran the tank dead dry AFTER he replaced that fuel filter. So I'm hoping clogged fuel screen/filter is the issue.

Any other thoughts on what to it could be, anything to be careful of when taking this stuff apart? I assume the fuel system (MPI) won't be pressurized on the low side of the pump. BTW - motor has 480 hrs on it now and the P/O did very little maintenance, so my bet is that tank has a bunch of gunk in it.

Thanks for any help!

 

[boatn70]

yep start with all the fuel options your talking about, check timing too......just so you know. moisture in the distributer cap??? just thinking outloud.....fouled plugs, pull em all and look them over.
plug wire loose or grounded somehow??

 

[Needafunboat]

So the fuel pickup screen was clean, I changed the fuel filter too. No change at all. It will still only run to about 50mph / 4K rpm. It seems to backfire more often today though.

Kevin - I think you're right - could be something electrical. The boat didn't have much moisture in it - other than being in the water for a week, but it just seems a little low on power overall and very low at higher rpms. I'll pull the plugs and see if there are any clues there.

Any help is appreciated!! I seem to remember hearing similar posts about this but couldn't find any when I searched. I'm getting a little nervous.

 

[jazzy]

did you change anything from the last time you used it is the hold down on the distributor loose

 

[Big Red]

cam lobes missing in action

 

[Needafunboat]

OK, now you have me thinking. I didn't check the distributor at all, but I will. I did notice that one plug wire seemed a little loose on the plug. But the motor never ran rough and it wasn't THAT loose.

The only thing different is that my PERKO battery switch went so I wired the batteries together - and for a brief second, I connected the positive of one battery to the negative of the other battery - just dark and in a hurry, stupid mistake. The ignition was off, but maybe 24V for about 2 seconds fried something?

I'll check the distributor and the grounds next, and see what the plugs look like.

Maybe this is the excuse to get that new recurved distributor from MK??

The more I think about the backfires at low speed, I'm thinking about a rich mix or a weak spark letting unburned fuel out?

 

[jazzy]

sounds like intermittent mis with a load on motor maybe it needs a tune up?

 

[Needafunboat]

cam lobes missing in action

Ughh - I guess that's the worst case scenario, but exhaust lobes going away is looking like a possible cause. It would explain the intake backfire...

Would it come on within 10 hours of running though? Early in the trip, the boat was making full power (65mph) and less than 2 tanks later, it's down to 50mph.

I pulled all the plugs today and they all looked fine. I checked compression and there was no change from 30 hrs ago. # 7 is still the lowest - around 120, the rest are 140 to 150.

Idling on muffs, I can consistently get a small backfire through the K&N if I quickly go from idle to about 1/3 throttle.

I'm going to check the fuel pressure tomorrow (need to borrow a gauge from work). There's a vacuum fuel regulator that should pull pressure down just off of idle. If there's a vacuum leak, I think the pressure could stay high dumping some extra fuel an causing that backfire?

Any other ideas? Would a valve spring do this?

If it looks like a cam lobe, the motor's coming out and getting freshened - it will kill a big chunk of the summer, but not much choice!

Arrrggghhh!!

 

[Needafunboat]

Looks like Vacuum pressure

So I've checked out the fuel pressure and it looks good - 38psi with motor off and about 42 with the motor running. Regulator (runs off of vacuum) seems to work fine. Pressure builds a little with RPM - tested up to around 3500 rpm - and was around 45-50 psi up there.

I put a new cap and rotor on which helped slightly with the backfire - but didn't solve the issue.

The thing I found was my manifold vacuum pressure was only around 12psi at idle. It was very steady though, which pretty much rules out a camshaft or valve issue. Good news there - no major issues and a good lead on the issue.

So I'll spray water around the plenum and see what I can find.

I guess the other possible cause is timing that's off - which sends me right back to checking sensors and needing the Merc. scan tool. Timing does advance with load, but not with a slow RPM increase or at steady state. Not sure what this means

My mechanic buddy comes back from vacation on Sunday. I'll probably get him to take my boat to work on Monday and pay the extra labor for him to diagnose it during the day - unless he's willing to work on it Monday evening.

The other thing I notice is if I ease into the throttle, there is some hesitation around 1/4 throttle.

I put the extra MAP sensor the PO gave me on the boat and it behaves the exact same - but worse. I'm sure it was a bad sensor. It actually stalls easing into the throttle at 1/4 throttle. That suggests maybe a bad MAP sensor. I'd put a new one in just to try it, but they're $120!

At the very least, I'm learning about these motors that sit in FRONT of the transom... Just need it running well within a week for Torch lake!!

 

[hotbeek]

I'd spray some starter fluid on/around intake instead of water. If rpms increase you know you have an intake leak.

 

[ezstriper]

12" vac at idle real low...look into that..

 

[Needafunboat]

Sprayed starter fluid ALL over - no rpm increase anywhere.

I did toss a new MAP sensor on it and the hesitation is gone. However the intake backfire is still there if I really quickly stab the throttle. Not sure if that backfire was always there or not.

I'm draining the gas, checking the TPS setting and heading to the lake to try it out. Is suspect the low power issue is still there, but it'e 90deg and we only burn a gallon or two on that little local lake so it should be fine.

Do any of your BBC's backfire under a no-load very quick/aggressive throttle tip in? Thanks!

 

[PA-Checkmate]

12" vac at idle real low...look into that..

I agree, did you check compression yet?

 

[Needafunboat]

Yeah, the compression was exactly the same as 30 hrs ago: #7 was still low at about 125psi - the others were all 140-145 psi.

The vac pressure was dead steady at 12". Not sure how accurate my mity mite vac pump + gage is, but yeah - it sounds low. If there are no leaks, any other ideas what could cause a steady but low vac. pressure? Idle ignition timing hunts from about 4 to 12 degrees BTDC with the ECU functioning so I don't think it's ignition timing.

I also checked the TPS voltage and it is about 0 at idle and 4.25 at WOT. I think that's about half a volt low on both ends - so maybe there's something there. Probably not enough to cause the type of power drop I'm seeing. I have to double check the shop manual specs.

 

[PayCheck]

Do any of your BBC's backfire under a no-load very quick/aggressive throttle tip in? Thanks!

Yes - I had a sticky intake valve that caused a backfire when you punched it, but that was in an aluminum head that had overheated from a lean condition. I was able to fix it with some emery paper on the carbon burned onto the valve stem.

The motor has 480 hours and I imagine the springs are a little tired - are you planning to rebuild or just maintain?

 

[Needafunboat]

60 PSI fuel pressure at idle

So I threw in the towel and sent it to my merc. marine tech buddy. He's busy this week so he is doing the work at his marina (on the clock and labor rate!).

He said his vac. gauge read 15 to 16" of vacuum, so maybe my mity mite gauge isn't too accurate.

He also saw SIXTY PSI of fuel pressure at idle. Just about double the spec. Boat came with an aftermarket adjustable regulator that he said is no longer adjustable.

I may be reaching a little here, but the first time I checked fuel pressure - it was 42psi at idle - and the boat was not making full power, but otherwise was OK at the lake in TN. The next time I checked, it was about 46 and was on muffs. On Sunday it was just over 50 PSI and it ran the worst it ever has. Many intake backfires and barely able to get on plane. One moving it would get up to 45mph - but it is one sick motor. (note, I ran the boat very little to do this since I figured I was probably hurting the motor a bit - assuming a lean condition).

Today, my buddy sees 60 PSI - very high.

So could high fuel pressure cause my woes on an injected motor? Intake backfires and big power losses? I was assuming it was lean - which it sounds like isn't the case.

He's going to check for fuel restrictions - which could cause the lean condition and the fuel pressure. Should know more tomorrow!

 

[jazzy]

i dont know a whole lot about the fuel injected setups but sounds like too much pressure did you check the oil if you persistently flood an engine you can push gas down past the rings you may have been a few quarts high did the motor start to get a little noisy as well as backfiring

 

[Needafunboat]

Thanks Jazzy - No change in oil level, actually ever. Boat has used very little oil since the last oil change (30 hrs ago). It's never been flooded - there's always a spark - sometimes the wrong way, but a spark and a bang just about every time!

Hopefully it's a silly restriction or just that regulator. ughhhh.

 

[Needafunboat]

Update

So we changed the FP regulator and it's back down in the low 30's. BUT, engine still has the lean intake backfire. So all 8 injectors are being flowed, cleaned and checked at my work. They are keihn 5IDD's. An old style knock off of the original Bosch EV1 injectors. Bosch is now up to EV17's.

It's great how many people I have at work going after this issue. Everyone wants to help. The keihn's are just a few mm's wider on the intake connection, so I can't use our EV1's. Funny thing is that even in 1995, this was a VERY outdated design. They are now $200 each!! Hopefully a few are very clogged and all are usable.

So next week, I'll have them back in and hopefully all will be well.

I can't believe no one else has had injectors issues like this.

 

[yellowfiero88]

I had this happen on a old chevy truck. The timing chain was stretched and started jumping teeth. It would still idle but coughed and studered when it accelerated. 3 weeks after fixing it, the gear broke off the dist but it was still spinning just at a different rate. Talk about a mug trying to figure that one out. every so often the motor would kick then die. Only 2x the truck broke down with almost 280k miles when sold.

The chain might be way off but an ideal. Good luck