502 surging and loading up?
 

I have a mercruiser 502 serial im guessing is OK173702. It is in a 98 powerquest. Cold it runs like a champ, good oil pressure, no surging. After the engine is up to operating temp, guage shows 160 and I come into a no wake zone the engine will hunt. I usually try to pump it slightly off throttle just for sake of thinking its gonna stall. It isnt enough to make the tach fluctuate, but you can definelty hear the engine varying in rpm. If the no wake zone takes awhile to get through the engine almost seems loaded up, trying to take off it feels flooded, can tell its not responsive and sounds loaded. Usually giving it more throttle or coming off the throttle and then back on will clear it up within 5-10 seconds but doesnt seem right. I have noticed alot of black on the transom and under swim platform after every use. Any ideas what to look at, or being fuel injected am i stuck to having it scanned?

As with just abt any electronically controlled fuel injected eng, the scanner is the way to go. Having said that, no harm in looking for obvious items like unplugged sensors or damaged wiring. Another thing to look for would be a cracked vacuum hose for the MAP sensor if so equipped.

you say its a 502 ? is it a stock mpi ? if so my guess would the IAC and yes a scanner is your friend

Yes stock 502, hasnt been touched. Ive only put about 4 hours on it after purchasing so I dont know if it was a previous issue or not. Unfortunately all the shops around here are booked a month out for scanning :/

Fuel regulator is on cool fuel module hanging off the left side of the engine down by the oil pan. There should be a vacumm line connected to it with manifold vac. Dont remember exactly where it connects to the intake but verify you have good vac supply to the regulator. Checking your fuel pressure would be next logical step to me.

How old is the fuel?

Thanks for the SN, cool fuel MPI and MEFI 1
Pressure should be 43 psi at the rail. Schrader valve located under the plenum just behind the thermostat

I concur, double check the vacuum hoses. One of mine was plugged with gunk.

IAC motor is relatively inexpensive to try out.

Might be time to have the fuel injectors cleaned.

Idle hunting, from what I’ve seen, is usually timing or sensor related unless you have a bad vacuum leak. To diagnose it correctly, you need a Merc Techmate or Rinda diagnostic terminal. The IAC “may” cause some hunting if it’s stuck, but they really don’t go bad.

Things to try...

Pull the throttle body and IAC, and clean the chamber that the plunger sits in. If it’s full of gunk, vacuum can get goofy and throw everything off.

Put the system in timing/service mode and verify timing - 8 degrees BTDC if it’s off, it will surge (you can do with a paperclip at the diagnostic connector... sorry... you’ll need to look-up).

Fuel pressure usually makes it run soggy until
upper RPM. If you’re running around with it, more than likely it’s all good

If doing the above doesn’t net anything, you really need the diagnostic tool to see what the ECM is seeing.

Fresh, Ive put fuel in it 3 times this spring already

Check the yellow looking lines coming off the mechanical fuel pump and the VST system. They usually 'T' together and are connected to the intake either near the TB or the rear of the intake. They should be dry inside. If not it is likely fuel venting into your intake when causing your issue. Either the mechanical pump diaphragm is bad (one source of the leaking fuel) or the internal VST diaphragm is bad.

AD said serial # is cool fuel

Same here
 

I'm having the exact same issue with the exact same engine I have the mercruiser 502 MPI in my formula boat my problem is very similar in that let's say the boat's been sitting awhile because we've been fishing for a few hours when I try to drive run it or cold started I have to throttle it up literally get it to start it just turns over over and over unless I throttle it up with these solutions help me as well as the original poster

Thanks everyone for the replies. This weekend I will be looking into a vacuum leak, and checking the timing. I am kinda leaning toward IAC/throttle body being dirty because when i snap the throttle and come back to idle it will act like its gonna die and takes 5 seconds or so to recover.

On another note I noticed when running through hull exhaust the starboard exhaust will spray the water out, but on the port side its just running out. Is this an issue or something to look into? Does have a new sea pump, never gets over 160-170 temp wise. It does have alot of volume coming out so I dont think flow is an issue.

Water flows the path of least resistance, so no issue at idle. Higher rpms they should start to even out some more because of more volume

you need a scanner to check the timing, timing must be set in base mode

EDIT, Ive had a few 502 mag mpi like yours and the water flow was always very equal out of both pipes

I was told you could put it in base mode with a paper clip and check it, this not true? I did talk to a marine shop who thinks its due to the fuel system having paint on the inside and clogging injectors. Which may be true, but most everything ive experienced with clogged injectors ran like crap all the time, not just warm and at idle. Of course they cant get it in until after the fourth. As far as the water I understand the path of least resistance, put I never seen a through hull not spraying. Repair shop wondered about silent choice baffles being hung up so I will look into that as well.

Dirty to the point of clogged flame arrestor.

Normal Exhaust pulsing causes the water to spray. So yeh, if the water is just running out, sonething is eliminating or avoiding the pulsing .

Yes Ive heard of this, I use a scan tool.......not something from the teachers desk.......:riding:

Yes, paper clip pins A and B
Can also read codes (if any) with a resistor type LED or using a digital meter with tone mode

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Well Im guessing I found alot of my problem. Ive been reading alot of topics on the injectors in these and cleaning them should be routine maintenance, so I pulled them out lastnight and sent them to smitty. In the process found my throttle body extremely gunked up, iac and iac port where full of gunk. So cleaned throttle body out, replaced iac with new, now waiting on the injector report. Fingers crossed
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https://cimg1.ibsrv.net/gimg/www.off...dc8ecf6c70.jpg
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You should always have a scanner.

Well unfortunately my repairs didnt change anything. I had the injectors sent to smitty, cleaned the throttle body as it was nasty, replaced the iac and relearned, replaced most of the vent tubing going to intake. We did find a fitting in the upper plenum that was clogged, I believe it was the vent coming from the VST. Took the boat out yesterday and it did the same thing, wierd thing is it will run at idle all day long in gear with no surge, its always bt 900 to 1200 rpms where it will start surging. And then when I take off after putzing around for awhile it fill cough and pop for a couple of seconds before cleaning up. I might try to pull the plugs this week and see what they look like. Other wise the I have an apointment July 6 to see that they come up with.

I dont know if a faulty TPS will cause this or not, fill us in when you get it scanned

What would cause the IAC to get so gunked up?

Padraig

Crank case vapor. Happens in all motors but the 502s have a low tension oil ring on pistons from the factory so even fresh it has more blow by than Other motors. Multi port injection put injectors below the intake me and fold so none of that stuff gets naturally washed off

So, clean the IAC yearly???

Padraig

If your flame arrestor gets black easy from motor blow-by then I would move the hose away from the arrestor and place in a bottle to catch the oil vapors

I didn't have any issues with my VST 502 IAC (several years, forget how many), nor my current cool fuel 509 IAC (4 years)

Have you replaced the ignition components? Plugs, cap, rotor, wires? There's a module under the cap that was pretty ugly when I replaced all of mine.

Just got off the phone with the repair shop, they said the computer has no fault codes, said all sensors are functioning as should. Timing is set at 8 degrees, he did say the fuel pressure was a little low at 26psi so they are going to look into that. He did also say that 502mpi's are known for surging between 1000-1200rpms, of which Im not so sure what to think off.

My old VST 502 never did

Fuel pressure regulators notorious for going bad on these. Mine was surging at idle a lot and was difficult to hot start. All of that went right away when I changed the regulator and had proper fuel pressure again.

Honestly pretty frustrated with the repair shop right now. Repair shop scheduled in for July 6, this was scheduled a month out. I finally called them at the end of the week and they said they did not have time to get to it and didnt think they would this week. I asked if I could pull it out and use it this weekend since summer is short and not loose my spot, his response is, "we will see what we can do". After further convo he agrees it will be right back in line, I dropped if back of sunday night, never heard anything. Called them tuesday the 13th and found out they had looked at it and they found the low fuel pressure but would have to figurie out what to do about that. I said thats fine, I am leaving for vacation tomorrow (the 14th) so you will have to leave me a voicemail. Almost another week goes by and I still havent heard from them, so on the 20th I call them wondering whats up, they respond with "its ready to be picked up, the low fuel pressure isnt anything that can be addressed and it is what it is". At this point Im frustrated and say fine whats my bill. He didnt have it completed yet but will call later with it. Later that same day a lady calls and tells me its $215. The very next morning i get a call wondering when I am picking it up because they are limited on space, then again that after noon reminding me to pick up. Like really, this things been there for weeks and you have yet once to call me and I told you I was on vacation.

So all in all i guessed I have learned that all 502s surge, and there is nothing to be done about low fuel pressure.

Crazy.

Where are you located ?

Tell em to choke on their $215!
low fuel pressure can be fixed!
Think it should be in the 38-43 range depending on app.

Grand Rapids, MI

Well I took the boat out yesterday after picking it up from the marina, after cleaning all the grease stains of my brand new interior that is. Boat ran rough as usual, there was a couple instances going through no wake that it would stutter and I wondered if it was gonna recover. I took a video of it doing it but I cant upload it as it isnt the correct format. Another thing I noticed is after a 30 minute run, mostly no wake I shut it down for 10 minutes or so, after refiring it was steaming a ton from starboard side only. I havent noticed this before, it followed as we accelerated to the no wake zone and then for a few minutes into the no wake. Boat was running at 160 degrees, normally I would think riser/manifold gaskets but I just did those this spring as they were leaking externally. I would think if they were leaking internally it would also cause runability issues, however it isnt ever steaming when its running rough. Ive got a call into a couple other marinas now, but being the heat of the summer Im sure Ill be waiting until fall.

Sure can with an adjustable fuel pressure regulator that bolts in place of the OE unit. I have one on my 454 MPI with surge issues that haven't been solved in almost 4 years.

https://www.offshoreonly.com/forums/...le-please.html