Still has a misfire

[Bagged90]

Bought the truck a little over a month ago with a misfire and can't seem to get rid of it. Truck is a 90 2.3 stick shift I have replaced all 8 plugs and wires and both coils. I checked fuel pressure and everything seems alright. Mostly misses under a load. It seems to run fine at 2000 rpm at about 55 mph but any load it misses pretty bad. Any info or advice would be great I was thinking maybe an injector issue? Truck does have a cone style filter not sure if that will make any difference.

 

[RonD]

You can test the injectors with an OHM meter.
You need to test all 4
Allowed range is 11-18 ohms
They should be about 14.5 ohms

This only tests the solenoid in the injector, not if it is sticking or clogged, but you would usually get a CEL and OBD code if one injector was acting up.

When you tested fuel pressure did you unplug the vacuum line on the FPR(fuel pressure regulator)?

At idle fuel pressure should be about 35psi, and FPR is open(high vacuum) to send fuel back to the fuel tank.
When engine is under load, using more fuel, the vacuum pressure drops so FPR closes which keeps the fuel pressure stable.
If it is not closing then fuel pressure might be dropping to the point where engine is stumbling.

So with engine at idle and fuel pressure gauge hooked up, pull off the FPR's vacuum line(put finger over the end so engine doesn't see a vacuum leak), this is just like engine was under load, fuel pressure should jump up, 5 to 8 psi if I remember correctly.
If fuel pressure doesn't change replace FPR.

MAF sensors need to be cleaned now and then, this sensor tells the computer how much fuel to use based on the air coming into the intake manifold.

If there is a leak in the air plenum(big plastic tube from MAF location to intake), then the MAF is telling the computer the wrong information, extra air is coming in from the leak so computer is running engine leaner than it should.

 

[Bagged90]

Ya fuel pressure looked good when I checked everything. Haven't looked into cleaning Maf. It does have a cone style filter could it be getting to much air for the Injectors? Was thinking about getting bigger injectors for it this weekend for more hp once the miss is gone. Also kinda figured kill two birds with one stone. It would rule ou injector issue and get bigger at the same time. My only question is what injectors would work. I thought like 88 mustang 5.0 19lb ones work. Correct me if I'm wrong. I looked on orileys website and it doesn't say if there basic Injectors are 19lb or not

 

[HAPPY+RANGER+GUY]

Would not waste money on injectors unless you have no kidding 'bad' ones. The cone style air intake should not be an issue and stock injectors can keep up. You might tackle the chore of cleaning the MAF however.

 

[Bagged90]

I was going to go with bigger injectors as an upgrade to do a cam swap and other things later. Just figured now would be as good time as any to swap them

 

[Bagged90]

Well it's not the maf! I stopped by the parts store and bought one. Still has a miss. Starting to get aggravating!

 

[HAPPY+RANGER+GUY]

Have you watched the fuel pressure under load?
Any trouble codes in the computer? Check engine light?
Could throw a new fuel filter at it pretty easily. Maybe previous owners got the timing a little off when doing timing belt.

 

[Bagged90]

New fuel filter and no codes I guess I will hook up my fuel gauge and run it up and down the road

 

[JC 97]

Have you verified that the miss is not mechanical in nature? I would do a compression check and go from there.

 

[killj0y]

I would also check for weak coils, I've had one coil seem to work but cause misfires under load.