High speed/RPM miss on 350 mag 22zx

[jrockin08]

Took the boat out a couple times after the winter and noticed when I get to around 55-60 mph or around 5 k I'm getting a lugging or a missing like its not getting fuel? But never any problems cruising below that speed or idling along. Put sta-bil in a full tank last October before backing it in the barn. Took distributor cap off and noticed its kind go chalky looking? The gasket is all hard and cracked as well. Could that just need cleaning? If so what to use?

otherwise I thought maybe the plugs, so I took them out and will replace just cuz I'm not sure when the guy I bought it from did it. What plug brand is recommended?

Thanks all

[BUIZILLA]

your probably bumping the rev limiter...

[jrockin08]

Never did it last summer,and at higher speeds than that. It's not going above 5k when it happens.

[GeneD]

I just did a motor for a guy.
Motor was spitting and coughing at 4 bbl operation, but started right up! Idled like a dream!
Ran great right up to half throttle...
Had me a bit confused there for a bit...until I replaced the plugs and it went away.
Go get yourself a new cap, and a set of AC MR43T's and call it good.

[Tidbart]

If you don't think it is the rev limiter, then do what Gene said. Replace the cap, rotor, and plugs. I would even go one step further and replace the wire along with them. You would then know the condition and age of all of these parts as well as where not to look if your problem is still there.

I would also put in either the AC Delco or NGK marine plugs. Verify you get the right plug via your serial number for you engine. Some of the later 350s take different plugs than the standard MR43T's. And make sure you check the gap.:rlol:

B

[MOP]

+1 on the plugs, over time the gap widens or they carbon up.

[MOP]

If your plug wires are more than 3 years old change them, they live in a very damp atmosphere!

[jrockin08]

Ok..so I changed plugs,cap, rotor,filter and still doing this. Lets talk rev limiter...anyone know what it would be on a 350 mag mpi?

if it is the rev limiter what's that mean ? Prop bad?

[jrockin08]

Should say I have a 4 blade 24p bravo 1 prop ... And a 22p sitting in the barn brand new.

Alpha one drive, but idk gear ratio

[Donzi_Dude]

if you dont have a limter (5K?) it sounds like a plug wire to me. it will miss under load, a coil can do this as well..

[biggiefl]

Could be the cam but that would be more of a POP than a miss. Hell...vaporlock? Would only happen when hot.

[Phil S]

Also, check or replace your fuel filters...if you haven't done so already.

[jrockin08]

I'm still working on this ! Work on it during the week, test on weekend. I ordered a new fuel regulator , as with a fuel pressure gauge on it read 20-30 psi at idle on water, and when it started surging it went to 15-20 psi. Never got to the 38-42 range. So regulator first then pump I guess.

Poseibly the fuel pickup tube but I'm not about to try and get that off...last thing I need is a busted tank. I tried and the square angled valve on top wont budge.

[BUIZILLA]

does it have a fuel cooler?

[jrockin08]

Yes...fuel cooler and pump are in the same assembly with the regulator. The pump and cooler come as a kit...$500 ...so I'm praying its just the regulator.

[BUIZILLA]

when you remove that fuel cooler regulator there should be a tiny mesh screen down inside the block, under the regulator, take a small pick and remove it...

[jrockin08]

No mesh screen. Just a O ring.

[BUIZILLA]

look INSIDE the aluminum block, down about 3/8-1/2" deep... not inside the regulator..

[jrockin08]

Ok...found the screen. Replaced it with a new one and new regulator. Started up in barn with muffs...let her run ..shut it down did it again for a few times. Runs fine, sounds fine. Again tho my fuel pressure is at 25. But I was told the 38-42 it needs isn't constant. Heading out on water to test.

Check for leaks in the lines I had off too. Everything seems good. Will post results. If its not the cure, it's on to a fuel pump...even tho the pump sound fine when I turn the key...

wish me luck...Boyne thunder coming up soon, would like to be there!!

[BUIZILLA]

with the vacuum reference hose disconnected from the regulator it should be about 42-43#, if not you still have an issue somewhere..

[jrockin08]

Ya so I took it out and it's worse now. Can't get on plane now without it stalling and losing all fuel pressure and even backfires now. Wait a minute and fire it up and it idles fine but when given juice it dies. My guess is either wrong regulator or fuel pump is shot.

Is the fuel pressure supposed to be constant 40 psi? Or only at wot?

I did put a new coil on also...not sure if its wrong one and that causes the backfire?

So frustrated now. I can change fuel pump, but beyond that it might be time to take it in and let the repair shop have it for a few weeks.

[donzidon]

You could try to blow some compressed air down the fuel line into the tank. If the pickup is clogged that might clean it up for the time being, and you would know the origin of the problem.

[BUIZILLA]

let me be clear on the fuel pressure, at idle with the vacuum hose connected it should be a solid 37-38#, as the load and rpm increases, or you disconnect the line, it will go up to 42-43# and should stay solid there at WOT, no exceptions

anything other than that, and it's a supply problem to the rail

period.

[jrockin08]

Ya I'm only getting 20-23 psi at idle...with vacuum line disconnected it goes to 30ish.

It's just odd that before the new regulator I could get on plane, up to 45 mph, and then it'd misfire etc...now it can't get on plane! Doesn't make sense to me or anyone else here why it'd be worse

[BUIZILLA]

if you have to replace the fuel pump, I have found the Sierra pump to be better than the factory Merc pump, just my .03

also, if that little hidden mickey mouse screen was even slightly clogged or restricted it would give more than normal fuel pressure, since it is in the return circuit back to the filter head, and any restriction in the return side will elevate the fuel pressure... very few mechanics that even know about that screen, smart mechanics won't leave it in there.. I left mine out.. the mere fact that you lost pressure with a new regulator there tells me it was slightly clogged..

does your engine have another fuel regulator on the back side of the fuel rail?

[jrockin08]

There is no other regulator on the fuel rail or elsewhere I see. I ran it on muffs and pinched the return line-from regulator to filter housing- and pressure didn't change. Stayed at 23-26

Thoughts now is that the pump works cuz when the vac line disconnected I get a spike in psi...30-35 ish?

You seem pretty knowledgable abt my engine. Feel free to call! 616-540-4561

[BUIZILLA]

your pump is dying or there is a restriction before the pump

if it didn't increase pressure when you pinched the return line against the flow that confirms the pump is dying and unable to support a deadhead flow, or there is an inbound restriction

the only way to eliminate the inbound restriction thought is to go from another source (hose in a can) eliminating the tank hooking in right before the fuel pump inlet, if the pressure rises then it's not the pump

very simple, it's either A or B, there is no C

[jrockin08]

Ok...so I put the hose in a gas can...no added pressure. Still at 22psi at idle. So no restriction to the pump.

so it's not B...it's gotta be A! I'll order the new pump assembly on Monday. Comes as a kit with the fuel cooler. Pricy but at least it's easy to get to

no inbound restriction, low psi,new cap,rotor,coil,plugs,wires...anything else I can change while I'm at it!!

ill let u know how it turns out with the new pump

[BUIZILLA]

don't order any thing yet, I may have something stashed... I think you can get the pump by itself...

FWIW, are you SURE you have an accurate gauge?

[jrockin08]

Nobody makes the pump itself anymore due to a service bulletin that Mercury put out stating it only comes as a kit with the cooler. Not sure if the design has changed or what?

My serial number for engine is OM064392
the pump number is like this on the pump. 861156-1
This number below the top 2510 A

the gauge is brand new...so im going with it works!?

[Donzi_Dude]

ya its working. the fact thats its going up under idle indicates that.

the reg. will blow it off around 40 anyway. u just got low pressure and if it causes a lean condition you will definately miss.

[BUIZILLA]

FWIW, I didn't ask if the gauge WORKED, I asked if it was ACCURATE..

there are 3 pump/cooler numbers available, I think the pump itself is the same on all 3 setups, the only difference is the cooler length and whether it has a lower draincock or not

FWIW, the 496, 454 MPI and EFI, and small block Black and Blue Scorp's, all used a higher volume pump with, once again, the same 3 cooler bodies

last time I checked, a single Sierra part number covered all of the V8 engine models, except 4 and 6 cyl models

if you have the center mount throttle body MPI then you have the exact engine I have, there IS an MPI 350 with a front mount throttle body and it does have a regulator on the rail as well as on the fuel cooler manifold, mine does NOT have a rail regulator..

hope all this helps

[jrockin08]

I hope it's accurate...it's brand new!

Ive found a few pumps on eBay and such for a lot less than the whole kit. 180$ compared to 474$ is a big savings!

Its the center mount throttle body MPI. If we have the same engine has your pump gone bad?

once I get this fixed I'd like to do some upgrades for a little more power..but first I need fuel lol

[BUIZILLA]

this thread forced me to have a look at my engine just now for reference info, AND because i've been chasing a sporadic miss since I bought it 18 months ago...

low and behold, upon CLOSE inspection I just found a cracked injector connector on the #3 injector body :banghead:

back out in the 95* sun to take it apart...

damm

[jrockin08]

Lol your welcome! How'd you find the crack injector?

[Donzi_Dude]

low and behold, upon CLOSE inspection I just found a cracked injector connector on the #3 injector body :banghead:



JRock.

heres the rub. your pump may be good and you have a wiring issue.


A,B,C?

:wink:

[jrockin08]

Ya I checked all the wiring by hand and it all looks good...I have a feeling the pump is good also, but I'm no mechanic. Where would be the best place to start with wiring?

[Donzi_Dude]

the pump leads with a meter.

[BUIZILLA]

my harness connector did not break

the connector on the injector itself had a crack at the body, and subsequently seperated from the body

I am no longer able to upload any pic's since this last board update, so I can't share any pic's of this

Jrock > I have a complete pump/cooler assy if you need it, PM me for a price

[jrockin08]

Some reason I can't PM. Email me jrmayrand@gmail.com or call 616-540-4561. I'm interested but want to make sure it's right. I'd like it by Wednesday night so get with me soon. Thanks

[BUIZILLA]

email sent

[jrockin08]

Installed new pump, started on muffs and still only 20-25 psi. Ill water test it later this evening but I'm not to confident it's fixed. Ive heard that psi is low til under load and ive heard its constant so idk who to believe but a water test will tell. If it dont work its for sale!! lol Pretty much give up on boating and donzi's lol. 2nd Donzi I've owned...another season of no use. Best off sticking to a dinghy!

[Just Say N20]

my harness connector did not break

the connector on the injector itself had a crack at the body, and subsequently seperated from the body

I am no longer able to upload any pic's since this last board update, so I can't share any pic's of this.

I believe this is what you wanted to post.

[Donzi_Dude]

Installed new pump, started on muffs and still only 20-25 psi. Ill water test it later this evening but I'm not to confident it's fixed. Ive heard that psi is low til under load and ive heard its constant so idk who to believe but a water test will tell. If it dont work its for sale!! lol Pretty much give up on boating and donzi's lol. 2nd Donzi I've owned...another season of no use. Best off sticking to a dinghy!

did you check your voltage?


:frown:

[jrockin08]

Yes..forgot that part. Voltage was fine at the pump.

I took the batteries out during the winter is it possible the engine lost codes or has to re learn the system? In waiting to hear from a service shop as to when they can put a comp to it

[BUIZILLA]

ECM default or run program doesn't affect fuel pressure...

[dsparis]

Have you verified the accuracy of the pressure gauge ?

[jrockin08]

FYI the volts at the pump was 10-12. In guessing that's right where I need to be.

Not sure how to test the gauge for accuracy but it is new

[BUIZILLA]

pump must have battery voltage to run properly... if it reads 10v at the pump you have a big problem somewhere else

[Donzi_Dude]

14.2 is optimum with the engine running.

12.3 or higher from the batt. alone. you found your problem.

lol!

[jrockin08]

Think we solved it. Volts were fine... I didn't check it while engine turning over, which then gave me 14v.

I put the new pump on with my old regulator , and boom. Planes out, 62mph, no miss, no cough runs great.
But here's the weird thing, fuel pressure still don't get above 32! I used another gauge from the marina not just mine.

?? Problem down the road?