Fuel Pressure Problem

[winstonwilliams]

I have a fuel injected 502 Mercruiser 8.2 and I cannot get it started... I hooked a pressure gauge up to it and turned the key (to turn on just the fuel pump) and only go 22 lbs of pressure. Is this correct? It seems low to me but I have never checked the fuel pressure when the engine is running ok. I have been looking thru the manuals that you guys posted, but I cannot seem to find typical pressure

Thanks

Winston

 

[Fun Times]

Re: Fuel Pressure Problem

I have a fuel injected 502 Mercruiser 8.2 and I cannot get it started... I hooked a pressure gauge up to it and turned the key (to turn on just the fuel pump) and only go 22 lbs of pressure. Is this correct? It seems low to me but I have never checked the fuel pressure when the engine is running ok. I have been looking thru the manuals that you guys posted, but I cannot seem to find typical pressure

Thanks

Winston

What year is your engine And do you know the engine serial number you have???
Yes 22 psi is to low, But some times you need to cycle the key a few times to build pressure up. They had a few Fuel Pressure spec's, Some where around 36ish psi, and other's where around 43ish psi.

 

[winstonwilliams]

Re: Fuel Pressure Problem

I think the engine is around a 95 but I am not sure... cannot find the serial number anywhere

 

[Bondo]

Re: Fuel Pressure Problem

cannot find the serial number anywhere

Ayuh,... There's an aluminum tag riveted to the block, just ahead of the starter...

 

[45Auto]

Re: Fuel Pressure Problem

Have you checked to see if the engine is getting spark, before you start beating on the fuel system?

It would probably be worth the money to have the engine scanned and read the error codes before you start randomly replacing components. Lots of sensors that could be causing your problem.

From Mercruiser Manual #16, 1993-1997 GM V8 454 & 502, page 1B-4, the fuel rail pressure with your engine running should be 37 PSI.

If you can't find the fuel pressure in the manuals, you may want to consider having a shop diagnose your problem.

 

[winstonwilliams]

Re: Fuel Pressure Problem

Thanks for the info!

 

[winstonwilliams]

Re: Fuel Pressure Problem

I got the engine running today, but the fuel pressure sits at 28 when the engine is running. There is also a knock coming from the engine that gets louder as the rpms increase. Do you guys think that there is a spark knock because the fuel pressure is so low?

Thanks

Winston

 

[45Auto]

Re: Fuel Pressure Problem

It's possible that low fuel pressure could cause detonation in your engine. The amount of fuel injected is controlled by the time the injector is open. The engine computer is calculating how long to open the injector based on engine load and the designed fuel pressure. There is no fuel pressure sensor in the system so it doesn't know what the real fuel pressure is. Running with low fuel pressure on a marine injected system (like a car in open-loop mode, no oxygen sensor feedback) would be the same as running a carbureted system with too lean jets in the carb. It's delivering too little fuel for the amount of air flow.

What's the history on the thing? Was it running fine then one day suddenly would not start? Did it overheat? Has it been running bad for a while then became difficult to start? A little background on the problems might provide for more insight.

 

[Bondo]

Re: Fuel Pressure Problem

There is also a knock coming from the engine that gets louder as the rpms increase. Do you guys think that there is a spark knock because the fuel pressure is so low?

Nope,.... Probably deeper Mechanical issues...

 

[winstonwilliams]

Re: Fuel Pressure Problem

The noise is coming from the top... almost like a valve or lifter knock, or even an exhaust leak... but I need to get a stethoscope and listen to find out exactly. Checked the oil level and it is fine and the oil pressure gauge on the dash says it is ok too. Checked all the cylinders and they all have spark. Checked the voltage going to the fuel pump and it is ok. The only thing that seems to be off is the fuel pressure. I wish I had checked the pressure on this engine before I had these problems so I would have a base line to compare the fuel pressure too. The engine did have a hard time starting when it was warm and it surged a bit in low RPMs

 

[45Auto]

Re: Fuel Pressure Problem

A scan tool is your friend ..........

 

[winstonwilliams]

Re: Fuel Pressure Problem

I was thinking about buying a scan tool... Can anyone point me in the right direction? I am afraid to see how much it would be for the closest mechanic (30miles away) to come hook their scan tool up to my motor

Thanks for the help

Winston

 

[Glastron_V210]

Re: Fuel Pressure Problem

I don't think a scan took will do you a bit of good....what will it tell you??? If it does have codes, they will probably be related to a lean condition, which you already know about. There is no o2 sensor, so no real time fuel correction, so at 22psi you will be way lean!

Fix what you have identified as wrong, then go from there.


The low pressure is either a dying fuel pump, blockage at the filter or kinked line or blocked tank vent (Drawing a vaccuum) or a bad fuel pressure regulator on the fuel rail...could also be a few stuck open injectors, but that's rare.


One thing at a time eh!

C

 

[winstonwilliams]

Re: Fuel Pressure Problem

Glaston v210

I understand that the scan tool might not help me that much with this problem, but I figured it would be a good tool to have in the toolbox for future use since this is an older motor.

As for your suggestions:

It could be the fuel pump, but I want to check everything else before I spend the money on a new fuel pump.

I have recently (in the last couple weeks) replaced the fuel filter. I guess I could go ahead and replace it again just to be sure.

I checked all the lines for kinks and did not see any.

I opened the fuel fill cap to make sure there was no vacuum on the tank

With the engine running and the fuel pressure at 28 psi I kinked the return line coming out of the fuel pressure regulator. From my understanding the pressure should increase when I do this if the fuel pressure regulator is bad... Is this correct?

If the injectors were stuck open the fuel pressure would drop as soon as the motor was shut off right? It does not do that... The pressure stays right at around 28 after the engine is shut off.

I was thinking about disconnecting the fuel line from the fuel filter and attaching it to another small tank of gas. This would rule out all vacuum or fuel filter theories. Correct?

Thanks for all your help you guys

Winston

 

[Glastron_V210]

Re: Fuel Pressure Problem

Kinking the return line removes the ability of the regultor to regulate, and should show an increase of pressure at the fuel rail to the max pressure ability of the fuel pump. So, short answer, yes, you're right on there. If it's port injected I would expect a reading of 60-90 psi with a blocked return line.

What happens if you double or tripple prime? Turn the key on and off to do this. Still max 22 psi?

You're right about the stuck injectors. If the fuel rail is not bleeding pressure after turning the engine off this possibilty is eliminated. I think all the injectors would have to be open to cause a low pressure issue so that was a long shot anyway.

If you connect the fuel line to a small tank after after the filter but before the fuel pump you can isolate tank/filter issues as well as bad gas issues.

Good luck with it!

Chay

 

[winstonwilliams]

Re: Fuel Pressure Problem

If I double and triple prime the pump the highest I get is 28psi. The reading goes up higher for a second while the pump is on but settles at 28psi. and again once the engine it running the pressure sits at 28psi.

Think I will try to connect it to a remote gas tank tomorrow

thanks

Winston

 

[Fun Times]

Re: Fuel Pressure Problem

If I double and triple prime the pump the highest I get is 28psi. The reading goes up higher for a second while the pump is on but settles at 28psi. and again once the engine it running the pressure sits at 28psi.

Think I will try to connect it to a remote gas tank tomorrow

thanks

Winston

Good idea on the remote fuel tank that will help give you an idea if your problem is the boat or the engine it's self.

Glaston v210 is giving you varry good info and he is dead on
Just to be help full, If one injector was stuck open it would still lose fuel pressure at the rail, It would just take a little longer to go down is all.

What did the pressure go up to when you kinked the return line?

Have you unhooked the vaccum line of the fuel pressure regulator with the engine running to see if the fuel pressure goes up to around 4-6 more psi like it should?

 

[winstonwilliams]

Re: Fuel Pressure Problem

I have some good news! I disconnected both fuel lines (the main and the return) from the fuel filter and put them into a remote gas tank. I turn on the fuel pump and the gage read the same. Which means the problem is not in my fuel filter or my gas tank. I went ahead and started the engine and the gage still read the same. I did notice that return line was pumping a lot of gas back into the bucket, so I went ahead and put my thumb over the return line and the pressure jumped way up.. To me this means that the engine has a bad fuel pressure regulator.. Is this correct?

I told you guys that I pinched the fuel pressure regulator line to see if the pressure jumped up. But I don?t think I understood exactly how the system worked. From looking at the picture I have attached the pressure regulator from the fuel rail (picture on the left) is connected to the fuel pressure regulator. This is the line that I pinched. From the attached drawing (on the right) I thought this line was the return to the fuel filter... but this is just a vacuum line that tells the fuel pressure regulator when to pull more fuel out of the fuel cooler? is this correct? I guess if there was a picture of how the fuel cooler worked I would have understood that I was pinching the wrong line.. Guess you live and you learn.

How do I know if the fuel pressure regulator (next to the fuel pump) is bad or the pressure regulator (on the fuel rail) is bad? because it seems like to me the pressure regulator is telling the fuel pressure regulator what to do..

Thanks for everyones help!

Winston

 

[Glastron_V210]

Re: Fuel Pressure Problem

Good question...I've never run into a system which has dual regulation. In car the pressure is regulated by a single regulator at the throttle body, or at the fuel rail, but never by the fuel pump.

Ok, looking at it closer, this is a returnless system....meaning there is no fuel return from the fuel rail. Perhaps that's the reason for the dual regulation.

So, you must have blocked 'k' on the drawing, or the return line from the fuel pump regulator, and the pressure went up. This suggests that is was the fuel pump regulator holding the pressure down and that the problem is the fuel pump pressure regulator.


The vaccuum line operates a diaphram which allows the fuel pressure to go higher as the vaccuum goes lower. So, when the engine is under high load (Low vaccuum) the engine will get more fuel. This helps to reduce the amount of time the injectors are open (Duty cycle) to deliver the same amount of fuel. It's pretty standard, and should net 4-6 psi as already stated by boat teck when you remove the vaccuum line (I.E: engine looks to the regulator to be at full load, 0 vaccuum).


Chay

 

[winstonwilliams]

Re: Fuel Pressure Problem

Well I replaced the fuel pressure regulator by the fuel pump and the fuel pressure went up to 34 psi (engine running). I removed the vac line to the fuel pressure regulator on the fuel rail and the pressure jumped up to 40 psi. so all these numbers are better but i still have the knock in the engine. so the low pressure was not the cause.