Would high fuel pressure cause misfire

[lonewolf41]

Fuel pressure is high at 70-72 psi. Can even smell gas, so it is running rich. It idles OK, but around 2500 RPM it develops a miss. This is all in the driveway, BTW. The plugs, cap, rotor, and wires all have under 5 hours (probably closer to 2) on them since replacement. While I wait for my new regulator, I am wondering if the high fuel pressure would also cause the misfire or if I need to start looking at other things causing that. Everything that I read seems to indicate that high fuel pressure would cause misfires at idle that would go away at higher RPM. Mine is the exact opposite.

 

[lonewolf41]

So I cleaned the screen on the regulator even though it didn't look bad. A few specs of stuff, but not clogged for sure. Now it is running at ~46-47 psi. So it is low now, but I still have the misfire at higher RPM and it doesn't want to start unless I give it a little throttle, whereas it used to just fire right up without touching the throttle. I can change the cap again, but i am starting to throw parts at it. I have the diacom cable and software that I haven't hooked up to it yet, but I don't know that it will tell anything on a 5.0GXi-B motor about a misfire. Starting to rain so I am not able to do any more tonight, but welcome any thoughts.

 

[alldodge]

5.0 GXI-B
Fuel over pressure might cause a miss due to excess fuel, but would more expect a backfire

Have you checked timing?

If timing is good I would suggest a compression test

 

[Fun Times]

Because the engine was running rich in fuel and seemingly repaired, you’ll still want to pull, inspect and probably clean or possibly replace the spark plugs even if they are still new as mentioned as they could be compromised on a short timeframe with that much extra fuel pressure going on as a fouled spark plug could cause misfire type symptoms.

If you’re having to open the throttle to help start the engine then possibly the throttle body housing/plate needs cleaning along with the idle air control valve if the valve doesn’t need replacing…. That or your in the process of removing any air in the fuel supply system and it’s a one or two off situation and may correct itself in time.

Is there a vacuum line connected to the fuel regulator? If yes, try pulling the line off with the engine running and see if it’s flowing fuel out as it shouldn’t be if working properly and also see if the fuel pressure goes up roughly 5 psi.

Bear in mind that being on the trailer is not putting a load on the engine to get proper fuel pressure readings at power should there be any possible restrictions elsewhere but with today’s technology, not being in gear and free engine revving in neutral would potentially put you up against the rev limiter extremely easy, which could simulate a engine misfire which the limit varies per engine manufacturer but generally starts around 2500 RPM in neutra … So you’ll want to be sure there’s a true misfire happening when in gear and under load vs revving up in neutral.

 

[lonewolf41]

5.0 GXI-B
Fuel over pressure might cause a miss due to excess fuel, but would more expect a backfire

Have you checked timing?
View attachment 408931
If timing is good I would suggest a compression test

When I installed the distributor, the rotor was pointing right at the protrusion on the distributor as shown in the pic. Beyond that, I can't find any info on how to adjust the timing. The distributor won't turn but maybe 10-15 degrees total before hitting the intake. How do you check/adjust timing on this motor?

 

[lonewolf41]

Because the engine was running rich in fuel and seemingly repaired, you’ll still want to pull, inspect and probably clean or possibly replace the spark plugs even if they are still new as mentioned as they could be compromised on a short timeframe with that much extra fuel pressure going on as a fouled spark plug could cause misfire type symptoms.

If you’re having to open the throttle to help start the engine then possibly the throttle body housing/plate needs cleaning along with the idle air control valve if the valve doesn’t need replacing…. That or your in the process of removing any air in the fuel supply system and it’s a one or two off situation and may correct itself in time.

Is there a vacuum line connected to the fuel regulator? If yes, try pulling the line off with the engine running and see if it’s flowing fuel out as it shouldn’t be if working properly and also see if the fuel pressure goes up roughly 5 psi.

Bear in mind that being on the trailer is not putting a load on the engine to get proper fuel pressure readings at power should there be any possible restrictions elsewhere but with today’s technology, not being in gear and free engine revving in neutral would potentially put you up against the rev limiter extremely easy, which could simulate a engine misfire which the limit varies per engine manufacturer but generally starts around 2500 RPM in neutra … So you’ll want to be sure there’s a true misfire happening when in gear and under load vs revving up in neutral.

I never knew that the ECM would limit the engine speed in neutral. I don't remember it ever doing that. This is a rebuilt motor, but all the sensors and such are the same so I wouldn't think there would be much "learning" involved with the new motor. I have a new cap, rotor, and regulator coming so we'll see if that fixes it. I ordered an AC Delco brand cap this time as it got good reviews for fixing misfires and the rotor is Delphi so these should be good quality parts.

Thanks for the feedback.

 

[alldodge]

When I installed the distributor, the rotor was pointing right at the protrusion on the distributor as shown in the pic. Beyond that, I can't find any info on how to adjust the timing. The distributor won't turn but maybe 10-15 degrees total before hitting the intake. How do you check/adjust timing on this motor?

There isn't anyway to adjust timing with motor running. The motor is brought to TDC on #1 cylinder and dizzy is set in place. If the motor was not exactly at TDC then the timing is off

 

[lonewolf41]

There isn't anyway to adjust timing with motor running. The motor is brought to TDC on #1 cylinder and dizzy is set in place. If the motor was not exactly at TDC then the timing is off

Thank you for the input. I put the new ACDelco cap and Delphi rotor on and no difference. Here is my setup in the pics. I tried and couldn't get the rotor to point any closer to the mark than what I have it. I may be a degree or 2 short of TDC, but that is not much at the distributor. I could rotate the distributor maybe 2-3 degrees before hitting the intake, but I think I am within the limits shown. Setting the distributor one tooth either side seemed to put it way off. Does it look off? Thanks.

 

[alldodge]

Don't know if it's off from the pic because it's from an angle. Would need to see it looking straight down

The miss your hearing is still on the trailer and not under load, correct?

If this is the case and as FT mentioned it could be that it's running lean because of no load. A boat ECM is programed differently than a auto. The boat needs a lot more fuel than a auto so the ECM doesn't really know how manage.

Another issue you can run into later is there is no garden hose that can supply enough water to the impeller, and with that the impeller can be damaged. Make hose rpm is around 1800

 

[lonewolf41]

Thanks for the info. Here is a picture taken looking down about as straight as I can. Yes, this is still on the trailer. I just hate to get to the water and have the same miss, but if that is the only way to test it accuratley, then I guess I will have to do that. I never knew the ECM would throttle or limit the RPM when not under load. Learn something new everyday I guess. BTW, fuel pressure is now 56-57psi with vacuum on the regulator and goes to about 65psi when disconnecting the vacuum to the regulator. 10 psi on the low pressure pump so I think my rebuilt fuel cell is working well.

 

[alldodge]

I took what FunTimes mentioned is that your motor "might" limit. Merc has a in-gear switch and limits rpm when not in gear

Reading thru your posting I didn't see anything about actually having a miss under load. Have you had the boat out prior and did it miss?

 

[lonewolf41]

I took what FunTimes mentioned is that your motor "might" limit. Merc has a in-gear switch and limits rpm when not in gear

Reading thru your posting I didn't see anything about actually having a miss under load. Have you had the boat out prior and did it miss?

I have not had it out since putting in the new motor. Trying to make sure everything is good to go before hitting the water. I have plug wires on the way and I have some plugs with ~ 15 minutes on them that i could swap in to try. Weird thing is, even when the old motor was knocking due to the bad rod, I don't remember the motor missing like it is now....even when racing it up to hear the knock (approx. 3000 RPM). Hopefully we'll have a break in the rain to take it to the water to try.

Thanks again.

 

[lonewolf41]

I pulled all the plugs. They all looked like the example pictured here. The gas is old so I drained the fuel cell and the fuel filter and put new gas in them and ran off an external tank. No change. Something else that is strange is it will die after running for a few seconds. It sounds like it is sucking air when it does it. I have attached a link to a video of it doing this at about 15 seconds into the video and it dies. I wasn't touching the throttle, it was just idling. The second time it does it, I am trying to give it a little throttle. The IAC valve was replaced a little bit ago, but I am wondering if it is having issues based on the way it is behaving. I took it out and turned on the key and the pintle moved, so it is not locked up or anything. I left the spark suppressor off to be sure it was not restricting ait flow. Does any of this behavior raise any flags? It seems like after it dies once, it runs fine after that. Here is the link to the video: https://drive.google.com/file/d/1YS3lgpR2C9SXGf2aD_4E5B6_XlWpQqb4/view?usp=drivesdk

 

[alldodge]

Your running rich for sure
Need a scanner on the motor to actually see what is going on

Tee in a vacuum gauge with fuel pressure regulator.

 

[cyclops222]

This is still more problems of your Brand New Volvo ?
You should have the dealer doing all this work.

 

[lonewolf41]

This is still more problems of your Brand New Volvo ?
You should have the dealer doing all this work.

Not brand new. Motor is a 2003 with brand new long block.

 

[lonewolf41]

So to put a pin in this...I took the boat out and to my complete surprise, it ran great. Fuel pressure stayed 57-58 psi. Temp was good. Oil pressure well within spec. Ran like a champ. Weird thing is while sitting in the water waiting for the trailer, I revved it up in neutral to the same RPM as before on the trailer and it didn't miss then either. The only thing I can figure is that I hadn't let the temperature come up far enough while running on the hose and maybe that caused the misfire or maybe the ECM intentionally limits performance until the temp comes up. At any rate, I'll take the win and finally get to enjoy the boat. Thanks for all the help and feedback.