I have a '94 XJ 4.0L that has starting pinging more and more. I have a CAI and the '99+ intake and injectors. Now, I was reading up on fuel injectors, and I have the 99+ injectors in right now. What I read was that Chrysler changed the fuel pressure system from 39 psi to 49psi in the late 90's. If this is true, would running the 49psi injectors in a 39psi system be cause for the pinging?

I'm just trying to do the normal trouble shooting before I go nuts changing everything out.

And there is no EGR, and no knock sensor on my 4.0L. I run the stock plugs, with 8mm wire's and hot ignition. 3" lift with 31" tires, but still have the stock gearing till I get the cash for a set of 3.73's.

This is my DD.

Any thought's or comments?

Thanks,
J

 

Yeah, the injectors might be your problem. I'm not sure but I think they are designed to flow the same amount as the older injectors but at a higher pressure, so if you drop the pressure they don't flow enough fuel. If you still have your old injectors I'd try swapping those in since its free. Ifnot just buy some Dodge Neon or Ford MS injectors. <- if you decide to go that route PM me for part numbers.

 

Thank you much. I did just score some ford yellow injectors off ebay. They are the type III 19lbs ford injectors. I got rid of my old one's. Would using a colder plug be a good or bad thing for the 4.0L?

Hope those are right.

Thanks again,
J

 

I'd just use whatever is recommended stock. as far as injectors go the for 19lb ones are right on the money

 

Bad... unless you're running some really high CR or have a turbo.
Why don't you pull your plugs and see what's happening in there?
As far as I know the only differences between injectors are flow rating, impedance, and spray pattern. I don't think that injectors are designed with a particular fuel pressure in mind. If you change the fuel pressure you'll change the duty cycle of the injectors.
You probably know this but detonation is usually caused by too much compression or a lean condition in the combustion chamber. Do you have a wideband and gauge or are you just kinda taking your chances on adding more fuel and air?

 

Yes, and the newer cherokees have a higher fuel pressure set at the rail. As far as I know they flow the same amount but at a higher pressure(I suspect it has something to do with better atomization of the fuel due to the higher pressure) Thus those same injectors have a lower flow rating, and at a lower pressure(39 instead of 49psi) they wouldn't flow adequate fuel to prevent pinging.

 

No wideband or gauge. I haven't really changed to much. I just wasn't sure if running the 99+ injectors that are a 49psi injector might lean everything out due to not having enough pressure behind it to work correctly, since my 94 XJ is running at 39 psi. I've done the sea foam, and water injection to clean out the combustion cambers, so I don't think that it would be carbon build up. Has any one ever heard of the gears on the distributor wearing down and cause a loose effect that would change the timing just enough to cause such pinging?

Thanks again,
J

 

Thanks Zach. What I was getting at with J was that you have to keep the air/fuel ratio correct. If you have too much fuel you lose performance and shorten the life of other parts like the o2 sensor and the cat. When you have too much air you get detonation. Anybody here use adjustable FPR's when they start changing injector sizes and adding more fuel and air? How much learning ability do the later ecu's have?

 

Actually an adjustable FRP is a required mod when you install a stroker motor. because you need bigger injectors and higher fuel pressure to keep up with the bigger motor.

 

So, I did put $20 of the 89 octane in my jeep and she love's it's. No more pinging. I havn't changed the fuel injectors out yet. But, the 89 octane should only be a bandaid fix for the underlieing problem right? I'm going to put the 19lbs ford injectors in and see if I can go back to running 87 octane, if not then I'm guessing that it's not the injectors.

Thanks,
J

 

If your exhaust is not smelling rich or if you don't have black smoke coming out the tail pipe, I doubt this is your problem. First off, any CEL or codes? You have a knock sensor that senses pinging that is supposed to alert the ECM of predetonation. My first guess is that it is not working. If it is not, your timing will be advanced too far and pinging will be the natural result. I would replace the knock sensor first, or check wiring and connections to it and go from there.

 

The exhaust is only rich during the warm-up. Once at temp it leans right out. As for my XJ, it's a '94 and does not have a knock sensor, (I thought that also). As for the timing, that all ECU controled and can't be adjusted. No ECU codes. I have read and heard about the dist. gears getting worn down and it throwing the timing off, but after thinking about that, not sure if it would advance the timing. I think it would more likely retard it being a worn gear.

So, like I said, I'm going to try the new injectors, if that doesn't work, I'm going to make my own adjustable MAP sensor to adjust the Air / fuel mixture.

Thanks all,
J