why does my 4.7 run well on 19lb injectors?

[xjdoug]

ok ill start out with the motor its a 4.7 that i bought used and im not sure of the compression numbers and cam specs, i know its running a set of dished aluminum pistons, a 12wt cj crank and very healthy cam. The head is just a cleaned up 94 stock unit thats been refurbed and a 94 intake and tb. injection system is all stock 87 renix stuff in a cherokee. when i first put it together i ran a set of ford 24lb injectors and it ran so rich the motor would barely run and it fouled a set of plugs out so bad that it wouldnt even start up after a day of riding. on a hunch i hapened to bring the stock injectors with me and swapped them back in. it ran great, and after a tuneup and proper tps calibration it runs even better. It makes a lot more power and turns 37 inch tires very persuasively on or off the road. my question is why is it running so well on a set of injectors its not supposed to run on? its not pinging or showing any signs of leaning out throughout the rpm range, and its making a lot of reliable power at this point and im still baffled why its running on stock injectors..... im not gonna screw with a winning system but everything ive been told says it wont run the way ive been running it for 6 months

[Bodo]

Renix fuel injection system makes the diff I guess? I know that the stock injectors did not do the job on my 96 4.6L

[SilverXJ]

Just because it runs well does not mean you have the proper air fuel ratio and it isn't running lean.

[xjdoug]

Just because it runs well does not mean you have the proper air fuel ratio and it isn't running lean.

i worried about that but then why is it running wayyyyy to rich with the ford 24's? tailpipe smelled like fuel and it soaked a brand new set of plugs in a few days so bad it wouldnt run

[Exos]

are you sure it's a 4.7L?

[xjdoug]

are you sure it's a 4.7L?

yes quite sure.... like i said up top its running a long snout cj crank and aftermarket pistons and cam.... besides the obvious hard parts in the motor it makes a lot more power than your average 4.0 motor when you push the go pedal

[Exos]

Is there such a thing as "overpressure" from the fuel pressure regulator/pump? I don't know why it would run too rich like that on 24lbs. Maybe the computer is toast. Do you have an EGR valve on this? Check it out, just to be sure it's not plugged.

[SilverXJ]

Yes, the fuel pressure regulator could be fubar and putting too much pressure to the fuel rail. I would question the MAP and O2 sensor if it was running rich enough to foul the plugs. Even with 24s it should still be able to pull enough fuel out to run at 14.7:1 in closed loop. What is the pressure at the fuel rail? You don't day what year your XJ is, but I'll assume its a 94, so your fuel pressure should be 39 psi. When you put the 24s in did you reset the PCM so it could relearn the fuel trims for the new injectors?

[xjdoug]

Yes, the fuel pressure regulator could be fubar and putting too much pressure to the fuel rail. I would question the MAP and O2 sensor if it was running rich enough to foul the plugs. Even with 24s it should still be able to pull enough fuel out to run at 14.7:1 in closed loop. What is the pressure at the fuel rail? You don't day what year your XJ is, but I'll assume its a 94, so your fuel pressure should be 39 psi. When you put the 24s in did you reset the PCM so it could relearn the fuel trims for the new injectors?

its running renix injection in an 87, so no learning. ill have to get a fuel psi gauge and check the rail. it is a brand new fpr and map but the 02 may be the culprit in the system... im unsure of whether its working right or not.


edit: also no egr so i know thats not an issue

[xjdoug]

ok so heres the plan.... im gonna replace the 02 as well as the coil (1987 original part, figure it may be getting weak which isnt letting it play nice with the 24's) any thoughts on this?