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502 MPI troubleshooting help

T

ThGdThBdThUgly

New member
So just over a week ago I took my boat out and the engine was idling high and surging. I jumped online and found that I should probably check IAC. Went ahead and installed a new one. Problem fixed. Took it out last Sunday and it seemed to run good for most of the day then it started to not fire on all cylinders a couple times but would snap out of it. So since I forgot to check timing last weekend I went ahead and checked it yesterday. It was not at 8 degrees BTDC so I set it and the boat seemed to run good again. Took it out yesterday and about half hour in, the engine started only running on 6 cylinders. I shut it down and checked wiring connections. Nothing seemed to be out of place so I fired it up and attempted to head for the dock which was a couple miles away. I slowly got it up on plane and it would only rev to 3900 rpm's since it was not firing on all 8. Today I replaced the map sensor, tps, spark plugs, cap, rotor, & fuel filter. Boat starts and runs but not good. Cylinders 1,3,5,& 7 fire great and the exhaust water is warm. Unfortunetly the other exhaust is cold and the water smells like gas when it is running. When changing the plugs I put the camera down each cylinder and cylinders 4 & 6 are clean/washed. Not to mention both of those plugs are covered in gas. I plan on pulling the injectors to have them cleaned and tested. I am not positive that will fix the problem so I am asking if anyone has some suggestions? Is it possible the injectors could flood those cylinders to the point they won't fire?
 
Thanks for the info on the IAC, my 454 mpi mag was surging at start up and had an unsteady idle. just ordered a new merc IAC should be in before next weekend:cheers:
 
So compression test didn't turn out in my favor. What is weird is it is only on the one side LoL (I have to laugh because it is better than being frustrated)
Cylinders 1,3,5,7 = 139-145 psi
Cylinders 2, 4, 6, 8 = 45 psi
 
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ThGdThBdThUgly said:
So compression test didn't turn out in my favor. What is weird is it is only on the one side LoL (I have to laugh because it is better than being frustrated)
Cylinders 1,3,5,7 = 139-145 psi
Cylinders 2, 4, 6, 8 = 45 psi
Click to expand...

Yea not knowing will drive you crazy. Hopefully just a head gasket.
 
ThGdThBdThUgly said:
What did yours do exactly before you replaced the coil? My scenario or the surging idle?
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It would do something really similar to what yours is doing. I would think I have it fixed, run good till it got warmed up, then fall on its face. Wouldn't rev, had no power and would start missing like crazy. Even made the motor sound like it had something blown. The only way I finally found it was when it decided to go completely out and wouldn't start anymore. A $30 coil later, it ran great the rest of the season!! Took me 2 or 3 trips out to finally get it figured out.
 
Injectors are a great place to start. Make sure you label them what cylinder they came out of when you do. I just burnt a piston badly in my 2000 502 MPI due to a faulty injector. Luckily didn't hurt much more than the piston and exhaust valve. My injectors are being flow tested now. Culprit injector flowed 24% low and bad spray pattern.

I have a hard time believing that an entire bank of cylinders all tested 45 psi. I would repeat that test. No variation at all between them? Something funky going on.

What did you discover the timing was set at? Higher or lower than 8* BTDC? If higher, you may have done damage as the midrange/cruise timing is pretty aggressive at 40* but top end timing is very lacking on that program. Did you set timing with a jumper wire/base timing tool? If you set it at idle with the PCM not in base timing mode, it still is probably not set right as it can be jumpy.
 
Gimme Fuel said:
Injectors are a great place to start. Make sure you label them what cylinder they came out of when you do. I just burnt a piston badly in my 2000 502 MPI due to a faulty injector. Luckily didn't hurt much more than the piston and exhaust valve. My injectors are being flow tested now. Culprit injector flowed 24% low and bad spray pattern.

I have a hard time believing that an entire bank of cylinders all tested 45 psi. I would repeat that test. No variation at all between them? Something funky going on.

What did you discover the timing was set at? Higher or lower than 8* BTDC? If higher, you may have done damage as the midrange/cruise timing is pretty aggressive at 40* but top end timing is very lacking on that program. Did you set timing with a jumper wire/base timing tool? If you set it at idle with the PCM not in base timing mode, it still is probably not set right as it can be jumpy.
Click to expand...

So, today was informational. I am going to get new injectors as one of them is stuck open and another is sticky. Not to mention they are almost 20 years old.

After doing another compression test (without a helper) cylinders 2 & 8 were 35 & 44 psi. Cylinders 4 & 6 were 5 & 15 psi. My helper yesterday must have not known what he was doing.

I set timing with PCM in base timing mode. It was advanced a couple degrees ATDC.

I did a leak test and confirmed the problem when air started coming out the adjacent cylinders tonight. Head gasket was blown between each cylinder. Good thing I ordered Fel-Pro gaskets and ARP bolts today just in case.
Probably should do the other side while I am at it. :-/
 
Also hoping it wasnt ran long with blown gasket causing the crossfiring to erode the block and head surfaces. I don't think a gasket and some new injectors will fix this one. As EZ said, may be some valve issues and head should be disassembled and checked at minimum. Chances are the cylinders where injector stuck open washed down and took the hone out. Also if you were getting a lot of fuel dilution into the crank case, your engine bearings may be shot too.

In other words, I think you are gonna be farther ahead to refresh whole motor.

On another note, those injectors aren't cheap. and whatever you do, don't order them from eBay or a non-reputable injector supplier. There are Chinese copy's out there. We flow tested a set of Chinese version from supplier who claimed "flow matched and tested set". Not good, injectors actually has such weak coils they shut down completely at 40 psi......which your system runs 43.5 psi so you guess what happens. There are a few different injectors that will fit into that intake and rail, however none that will be a drop in equivalent. Anything but the stock Italian made injectors will need a re-tune of the PCM. A friend has been working on finding a good drop in replacement for these Merc injectors and has been unsuccessful so far.
 
So after inspection my block needs, bored, decked, etc. I am either going to buy a crate engine or try to find a low hour used one to get me through the season. Thanks for the input. :rof:
 
ThGdThBdThUgly said:
So after inspection my block needs, bored, decked, etc. I am either going to buy a crate engine or try to find a low hour used one to get me through the season. Thanks for the input. :rof:
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A low hour, used (newer mefi3) 502 is going to cost as much as a re-manufactured long block assembly. Might just look at one of those. Can have the new motor that will last for many years that way.
 
Gimme Fuel said:
A low hour, used (newer mefi3) 502 is going to cost as much as a re-manufactured long block assembly. Might just look at one of those. Can have the new motor that will last for many years that way.
Click to expand...
.

Long block will be here tomorrow, mine was a good core. :cheers: :) Will be putting it in first of next week. Going to install A1000 fuel pump and regulator too. :D
 
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