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