502 MPI problems, help
 

1999 502 MPI. Engine looses power at around 3000 rpm. rpm according to tach goes down 1000 rpm. after a few seconds goes back up to 3000 pulls strong up to 4500 then backs down to 3500. The time it takes for the tach to move up or down 1000 rpm is only half a second so I wonder if the actual engine rpm can change that quick. If I hammer the throttle at 2500 rpm, the tach goes down to 2000 but the engine slowly accelerates the boat???
Is it fuel or ignition related?? It feels like I loose some cylinders . All the plugs look OK.

Re: 502 MPI problems, help
 

You need to hook a diagnostic computer to it. The engine is trying to tell you something. If the computer finds an issue such as knock it will pull timing away and go into a "limp" mode. You could have the problems that you are thinking about, but a diagnostic computer could save you a lot of money and time trying to find the issues.

Re: 502 MPI problems, help
 

Do you think it goes into knock retard? Long story short; Is it possible that using a 30 psi fuel pressure regulator instead of a 43 psi regulator can cause it to behave like this?

Re: 502 MPI problems, help
 

Check your fuel pressure, you are due for a fuel pump ! also change your water seperator filter.

Re: 502 MPI problems, help
 

You betcha !!!!

Re: 502 MPI problems, help
 

that sounds like where your problem is....or as stated above.....You Betcha

Re: 502 MPI problems, help
 

Thanks for your input. The story is that the engine was retrofitted with an adjustable regulator. I then decided to put an OEM regulator on it but we were not sure which one to take since the engine visually looks like an 1998 (1998 injectors, ECM, etc) allthough the serial number indicates 1999. The fuel pressure changed between the years.

Re: 502 MPI problems, help
 

Changed the fuel pressure regulator to a 43 psi. I think this is the correct one. The plugs look somewhat more normal but no big difference. However, the problem persists!? The engine intermittently looses rpm at around 3000. No trouble codes. What could it be?? I will try to get hold of a scantool and a fuel pressure guage.

Re: 502 MPI problems, help
 

Not to through a monkey wrench at you, but is the exhaust original?
The problem you are having sounds a lot like I've experienced with a leaky riser. Water ingestion can make an engine run like what you are describing.

Re: 502 MPI problems, help
 

The exhaust is Gil. However it may of course leak. Please describe what happens if I have a leak. Please observe that all the plugs look OK.

Re: 502 MPI problems, help
 

Hook up a fuel pressure gauge and run it. I have replaced the pump on my 99 502 MPI. The symptoms for mine were all was goood until I went over 3800 RPM then the pressure wopuld drop, I'd back off and the pressure would rise up (a bit low) then hammer down and the pressure would drop again. Replaced the pump with a new one and haven't had a problem since. I have a fuel pressure gauge in my dash by the way. And I'm a mechanic so I checked it with a fuel pressure gauge I have at work. Good Luck

Re: 502 MPI problems, help
 

When I had the leaking problem a couple of years back it was on a 1999 7.4MPI with stock cast iron exhaust.
Toward the end of the season the engine would cough and sputter at about 3500rpm up. It would loose rpm then run OK for a few minutes then loose rpm. I checked the fuel pressure and changed the fuel filter 3 times thinking it was fuel related.
I pulled the plugs and they looked fine. I did a compression test, all good.
After the season was over I pulled the engine to rebuild it with a little more kick.
When I pulled the exhaust off the inner 4 cylinders had rusty exhaust ports. Then I pulled the intake and there was all kinds of gray sludge on the bottom and in the lifter valley. Also the same sludge in the valve covers.
The exhaust riser to exhaust manifold joints had a vary small leaks and water was going into the engine. The harder I pushed the engine, the more water would go in. That is why she seemed to run OK at low rpm and die out at higher rpm.
I'm not saying this is your problem, just something to think about if you do not find a fuel related problem.
Good Luck

Re: 502 MPI problems, help
 

R Addiction, I have an electrical fuel pressure guage that I have not installed yet. Where did you hook your sender up? Somewhere back where the fuel line goes into the rail? Did you use some special fittings?

Re: 502 MPI problems, help
 

its possible your anti siphen valve on your fuel tank isnt working correctly if you have exhausted all other measures try replacing it and see if that works[ very inexpensive too]

Re: 502 MPI problems, help
 

OK. I hooked up a mechanical fule pressure guage and a scan tool. The fuelpressure was 43 psi throughout the rpm range. It took 15 minutes of driving before the rpm loss showed up. For a while it was difficult to get the boat on plane. When I hammer the throttle it feels like a loss of power and the rpm goes down according to the tach but somhow the boat slowly comes up on plane. Still the fuel pressure was rock solid, 43 psi. The scan tool showed nothing. No trouble codes, no knock retard , nothing. The plugs look OK. Still not sure if the pressure should be 43 psi or 30. The scan tool says it is MEFI 1 SW in the ECM.
So.... where to go from here? I am completely at loss..

Re: 502 MPI problems, help
 

Mats, I was told when I had my problem to check the fuel pick up in the tank. Sounds as if there may be something in the tank getting sucked onto the pickup.

Re: 502 MPI problems, help
 

If you are not losing fuel pressure then the fuel system is good. I suspect you may have a module going bad in the dist, are you getting tach bounce when or before it starts to screw up??

Re: 502 MPI problems, help
 

R Addiction, I actually tried one thing more, and that was to run the engine off a separate plastic gas tank filled up today. Same result.....
Also emptied the water separating fuel filter in a transparant bottle. No water.
So what is left?

Re: 502 MPI problems, help
 

The tach bounce follows the loss of power exactly. The whole distributor assembly is new. Using a timing light at idle on the individual plug wires as well as on the high tension lead to the distributor shows very consistent flashes. But I have not been able to check with a timing light at the rpm loss situation.
A funny thing is that when hammering the throttle the tach goes down and the engine feels sluggish, but somehow the speed increases slowly!? The boat does eventually get on plane. It is like I loose a number of cylinders which is reflected in the tach going down but the cylinders that work makes the boat accelerate. Is that possible?

Re: 502 MPI problems, help
 

Could it be some intermittent fault in the MAP or TP sensor. That the ECM does not realize that I am hammering and thus it does not provide enough fuel to fire all the cylinders or at least that the combustion becomes insufficient?

Re: 502 MPI problems, help
 

Mats, You said you hooked up a scan tool.....Was it your tool or did you have a Merc certified tech do it? Do they show codes? I'd love to have one but was told that you have to be a certified Merc Tech to get one :(

Could be a ignition coil heating up and like Insptech said possible module heating up and breaking down.

Re: 502 MPI problems, help
 

Hi. I borrowed the scan tool from a local merc shop here. It is a very simple tool, basically a processor that communicates with the ECM, a display and a few buttons. It shows trouble codes if any as well as some 20 parameter values (rpm, vaues from all the sensors, ignition advance, knock retard, etc) The tool is self explanatory. I would say that a wrist watch is more complicated. If you want one I think the main problem is the outrageous price the will most likely charge you. http://216.230.203.183/marine/mtspec.htm

Re: 502 MPI problems, help
 

Thats not too bad....Considering the cost of a Snap On Modis, or a GM Tech 2 :D Thanks for the link. I'm headin' out to the bay. I'll get some pics of the fittings for the fuel pressure. :D

Re: 502 MPI problems, help
 

R Addiction,
Did you know you can get trouble codes with just a 12 volt LED and a paperclip? I can give you the details if you want.

mats,
I think we have been over this before, but if you have a MEFI1 computer and Keihin (1993-98) injectors, you should be running a 36PSI regulator (860349A1), not a 43PSI.
Dave

Re: 502 MPI problems, help
 

Hi Dave. Yes, we have been over this one before. Based on what we concluded the last time I put in the 860349 A1 (which Mercury claims is 30 psi). But then the engine started to misbehave as described in this thread and I thought for a while that it might have been the wrong regulator. I then threw in a 861126A1 (43 psi). However the problem persists. Now I know for sure that it is MEFI-1 SW and the regulator should most likely be the 860349A1. But the question remains , what makes the engine lose power? I beginning to think that it is ignition related. When I hammer, the tach shows that the rpm is going down while the engine and boat slowly picks up speed. It feels like the engine is running on maybe 4 cylinders during the acceleration?! What drives the tach, is it the ECM ? can the ECM supress eg each other pulse to the primary side of the coil and tach in certain situations?

Re: 502 MPI problems, help
 

One thing you might check it you have not already, is check for Knock Sensor wire or Ignition Control circuit wire touching a plug wire. Also, have you tried it with the tach disconnected?

Dave

Re: 502 MPI problems, help
 

I have tried with the tach disconnected. I will take a look at the Ignition Control circuit. I guess you suggest that the plugwire could induce a current/voltage in the Ignition Control circuit.

Thanks for all your input. This was a tricky one......

Re: 502 MPI problems, help
 

Mats, Here are the pics....Hope they help

Re: 502 MPI problems, help
 

Thanks for the pictures R Addiction. Now I know how it has been done. Great. :D

Re: 502 MPI problems, help
 

This is a perfect example of the downside to EFI, if this were a 1980 motor with points and a carb you could diagnose and fix it in about an hour.

Re: 502 MPI problems, help
 

True..... :rolleyes:

Re: 502 MPI problems, help
 

Yes ,I had a 7.4 L carb for 6 years, spent only a few hours on it, have had 502 MPIs for 6 years and I must have spent more than 700 hours on troubleshooting and the like. Actually more troubleshooting than enginehours... :mad:

Re: 502 MPI problems, help
 

Dave N- you said to make sure the knock sensor or IC wires are not touching a plug wire. Is this only a problem with cheap or old plug wires. Could a good set of 8mm MSD wires create the same problem?? Do they actually have to touch or just be close? thanks-

Re: 502 MPI problems, help
 

Dave , you are my hero. :D . A plug wire was touching the ignition control wire. Put some tape on, extending the protective plastic cover and the problems appears to be gone! Thank you all for your help.
Tried to attach a picture of two identical plugs both run in the No1 cylider, the left using the "30 psi" regulator and the right using the 43 psi regulator. Which one should it be?

Re: 502 MPI problems, help
 

mats,
Glad to hear you found it, and glad to hear I was actually helpful for once :drink:

Looking at those plugs, the one on the left (30PSI) does look pretty white. I think the 43PSI one looks better. No soot or anything with the 43PSI regulator?

Dave

Re: 502 MPI problems, help
 

Mats, is there a performance difference between the different regulators now that you have it running good again? Was anything else done to the boat, performance wise? :cool:

Re: 502 MPI problems, help
 

By the way.....What model Baja do you have?

Re: 502 MPI problems, help
 

The boat is a Baja 235 Force. The only performance mod is a Rex(GIL) exhaust.

Yes, I also think that the 43 psi plug looks more normal. It has only been running for about an hour but there is no soot on it so far. The 30 psi plug has been running for more than 10 hours and I think it is whiter than it should be. Still, the difference is smaller than you would think. I read somewhere that the flow through the injectors are proportionate to the square root of the fuel pressure. That would mean that the 43 psi would flow 20% more which is quite a lot. The boatspeed at WOT is about the same. Well, I continue with the 43 psi regulator and see if the soot builds up.....

Thanks again guys.

Re: 502 MPI problems, help
 

mats glad to hear you found the problems


dave great call.......now you have to keep up the great work !!!!!!!! :evilb: :evilb:

Re: 502 MPI problems, help
 

On the scan tool, the TP sensor only reads 73% at WOT. Is this of any sigificance and can it be adjusted? the throttle plates open fully.