MAP Adjuster Setting??

[jimmydaux]

I have a 98 cherokee with a 99 intake, 2.5 inch downpipe and cat, dynomax catback, k&n drop in Filter, bored throttle body and most recently, '96 lt1 injectors. I believe I'm running really rich at the moment, or at least a drop from 15 mpg to 8 mpg would lend to that conclusion. Does anyone out there have any suggestions on what voltage I should set my map sensor at? My current voltage is 5.07 volts.

Thanks!
Jeremy

 

[Talyn]

What LT1 injectors do you have. They run at different PSI and LB depending on what they came out of.

Get a correct sizes injector and ditch the map sensor. With your mods there is no need for larger injectors nor a map adjuster.

 

[jimmydaux]

I have the '96 injectors out of a caprice, grey top I believe. Im planning my stroker, but got a good deal on them and was hoping to make them work with the use of the map adjuster because of the 6 hole pintle, hoping that would possibly help with throttle response and fuel economoy. It obviously isnt helping economy thought Would I not be able to adjust the map voltage enough to make them work?

 

[blue95xj]

If anything I would think you would need an adjustable fuel pressure regulator to make them work properly. But I could be mistaken.

 

[gradon]

Since you have the map and the lt1s installed already, try voltages lower than 5, in the mid-high 4s. If it pings when you floor it then you've turned it down too low. Ideally you'd have a wideband o2 and adjust it until you got 12.5-12.8:1 afr on full throttle(I bet the pcm is doing all it can to pull fuel from those lt1s). You should also reset the pcm when you start to tweak.

 

[Talyn]

I'm not sure but I think they are 24lb @ 43.5 psi, which means they would flow even more on the Jeep fuel rail. I doubt the PCM can pull enough fuel out. I'm also suprised it hasn't thrown a MIL.

Did you but them used? Another possibility is that they sat around too long with fuel in them and gummed up, leaving one or several stuck open.

 

[jimmydaux]

yeah they were purchased used. I tested each one with the rail out, and even though I cant tell too much from just looking, they all seemed to have similar spray patterns. None of them were completely stuck open, and none were plugged. I'm going to pull the rail and do a leakdown test to see if any of them drip at all. How hard would it be to add an inline adjustable fpr on a '98 to dial down the factory 49 psi in tank fpr?

 

[Talyn]

I think hesco has an adjustable fuel pressure setup for the 97+ setup. IIRC its around $400.

 

[jimmydaux]

ouch....

 

[old_man]

The best way to adjust the MAP adjuster is with a tailpipe sniffer (and a dyno if you can afford it). If you run too lean for any length of time you risk serious engine damage.

 

[Talyn]

Too rich and you can wash the cylinder walls, but you have to be very rich to do that.

 

[jimmydaux]

Well I turned it down to 4.5 volts and will drive it for a while to see if that helps out at all. Ill have to see if I can find someone who will let me use a tailpipe sniffer. Thanks for the suggestion old_man

 

[Talyn]

After you installed the injectors did you reset the PCM? It could still be running on the old fuel trims.

The PCM should be able to adjust closed loop operation using the O2 sensors. Unless they are just to big for it to pull fuel out.

 

[hubs97xj]

How much can the PCM trim the injectors back? Is it limited to a few percent, or capable of trimming 20, 30, 40% etc?

 

[Talyn]

I think 20-25% is the max.

 

[old_man]

I am running the #21's on my 4.7L stroker here at 5000ft. It was running too rich to pass emissions without the MAP adjuster. I dropped it about .5v and it cleaned right up, so my answer is that the O2 control loop doesn't compensate that much.

 

[Talyn]

old_man.. what is the rest of your setup, and where/when was it running too rich?

 

[Dr. Dyno]

I'm not sure but I think they are 24lb @ 43.5 psi, which means they would flow even more on the Jeep fuel rail.

You are correct sir and with the OBD II 4.0L Jeeps running a 49psi fuel pressure, the LT1 injectors would flow 25.5lb/hr which is definitely too much. Jeremy's already made a good start by dropping the MAP voltage down to 4.5v but I suspect he might need to go down a bit further to optimize the A/F mixture.

 

[jimmydaux]

Dr. Dyno, will I be able to pull enough voltage to get the A/F ratio within spec with these injectors or am I chasin ghosts with that idea?

 

[Talyn]

Personally, I don't think you will be able to, as that is a heck of a lot of fuel to pull out. You may want to pull them out until you get your stroker built. But thats just me.