7.4 mph misfire when throttle up

[CKG-Regal]

My 1999 Regal 7.4 mph misfires under load above 1400 rpm. The fuel pressure is 38 at idle and 36 during throttle up. It has been sitting for a year. Idles fine under load and can putt around fine but starts misfire when hitting the throttle. Back fires if try full throttle. I have changed fuel filter, put new gas in and sea foam. Any ideas?

 

[Scott Danforth]

have you checked timing?
your fuel pressure is low. have you checked the low pressure and high pressure pumps?
new fuel or year old varnish?

 

[CKG-Regal]

Low pressure is 8lb and high 36-38, new fuel

 

[QBhoy]

If you’ve put fresh fuel in on top of bad old fuel…you probably haven’t fixed anything to be honest. Sounds typical of bad fuel to me and running lean.

 

[alldodge]

What's your serial number?

 

[CKG-Regal]

What's your serial number?

I will get the serial # tomorrow

 

[CKG-Regal]

RGMNA544H899

 

[alldodge]

The number comes up as invalid, most are numbers

 

[CKG-Regal]

This is what’s on my boat

 

[alldodge]

I'm sorry, need the motor serial number

 

[Tail_Gunner]

Low pressure is 8lb and high 36-38, new fuel

This must be a Volvo engine? What type of fuel system. Carbed, tbi or multiport?
This misfire does it clean up and smooth out say after 2500 rpm. Does this misfire occur during throttling up and then smoothing out?

Can you hold a constant 4000 rpm once you get the engine spun up.

 

[ripjmk]

If this has a flat crab distributor cap I'd change that first. Has the distributor been disturbed recently? If yes then the phasing of the rotor to plug lead pins in the cap may be off and timing the distributor may be all that's needed (you say it runs ok at idle and low rpm but not at higher rpm when the ECU advances the spark timing) .

 

[CKG-Regal]

Can’t find serial number. It’s MPI. I was able to get it up on plane and it would sputter and than run strong than sputter than good again. It’s almost like a switch is turning on and off

 

[alldodge]

It should be a Gi something, and fuel pressure is good "IF" it stays around 38 when throttling up. Does the pressure stay up under load?

Should have the EST ignition, and has the distributor cap been looked at to see if contacts are in good shape?

The backfiring is from the motor running lean or bad gas. As Q mentioned, if it had old gas in the tank and adding some fresh just gave you more bad gas. Use a gas can and place the fuel line in the can and connect it to the fuel filter. If motor runs good on the can then you have the answer, if not then need to look further

 

[CKG-Regal]

I’m starting to think it is bad gas. I will work on that next week. Thanks for all the advice everyone has given me

 

[CKG-Regal]

Todays test was installing a small outboard style tank with fresh gas. This was to eliminate the old gas in the fixed tank. This didn’t change anything. I am wondering if it could be dirty injectors

 

[alldodge]

Does the pressure stay up when under load ?

If so they may very well be injectors. Being an MPI it could be one or more of them.

 

[Tail_Gunner]

I believe you have a ignition problem, if I were in your shoes I would pull the dist cap with the plug wires in place. Turn it upside down, take a well insulated screw diver and place the tip in between the center node and the outside node and turn the engine over. You need to check for cracks and every node to node ark.. the spark will bridge from node to node or will cross fire into the cap itself or jump to the wrong node. If it cross fires there is your issue.

Dust caps need a smooth glassy like finish in the node upper area, if the surface is porous or ruff toss the cap.

Below is video of a crab style dust cross firing, old Delcos while different style can do the very same. Note here the old style one arcs the jump from center node to outer node.

Just for the sake of spitballing..

 

[CKG-Regal]

Ok. I will check that

 

[Scott Danforth]

in high school, we used to take a pencil and draw a line on the inside of a distributor cap as one of the troubleshooting tests in Autos class. that little carbon trail was enough to cause a motor not to start