MAP Adjuster & A/F ratio Q's

I've recently installed brand new 24lb Ford Racing injectors in my rig due to the motor running lean (upgrades listed below). The engine idles very rough since installing them. It will idle around 700 like normal, but about every 20 seconds or so it drops down to around 400-500 for about 10 seconds and then goes back up. It seems to act normal during city driving (around 1500-2500 RPM), however.
I built a MAP voltage input adjuster to fine tune the a/f ratio, hoping that the engine was simply running too rich now. When I turn the voltage below 5v the engine gets even worse and dies. If I raise the voltage to about 5.5+ the engine runs much better and idles smoother.

My question is this:

Is my engine really starving for fuel even with 24lb injectors, or could it be something else? From what I've read, only stroked engines need more than what 24lb'ers can offer.
Has anyone run into the same situation with the types of upgrades I've installed?
I took the injectors to a local shop to pressure test them and they were running absolutely perfect - in case you were wondering.
I still want to take my rig to a local shop to read the exact a/f ratio so I can try to fine tune it even more with the MAP adjuster, but until then I'd like to hear your input.

-Ron



Current engine modifications (91 HO engine):

- High flow K&N style air filter
- 62mm bored TB
- 99+ intake manifold
- Port/Polished HO head w/ 5 angle valve job
- Compression ratio increased to about 9.1:1
- Mike Leech header
- 2.5" exaust system with Carsound cat and Magnaflow muff (2" tail pipe)
- Jacob's ignition kit/Jacob's plug wires

Are you sure that you've added Ford 24lb injectors (blue top) and not 19lb injectors (orange top) by mistake? A modified HO 4.0 like yours should run rich with 24lb injectors (my old HO 4.0 with similar mods did and I reduced the MAP to 4.15v to correct it), not lean. My 4.6 HO stroker still has Ford 24lb injectors and I only have the MAP set at 5.15v (2% enrichment).

Dr. Dyno said:

Are you sure that you've added Ford 24lb injectors (blue top) and not 19lb injectors (orange top) by mistake? A modified HO 4.0 like yours should run rich with 24lb injectors (my old HO 4.0 with similar mods did and I reduced the MAP to 4.15v to correct it), not lean. My 4.6 HO stroker still has Ford 24lb injectors and I only have the MAP set at 5.15v (2% enrichment).

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I was told they're 24lb when I purchased them, and they're definitely blue. Could it possibly be anything else?

In that case, I don't think there's any question that you do have Ford 24lb injectors. What surprises me is that your engine seems to be running lean with them. Check the fuel pressure at the injector rail. It should be ~39psi with the vacuum disconnected from the regulator.

Good idea, I'll check that this weekend.
Thanks for the advice Dr. D.

Another thing you can try is taking out your distributor and cut of the two ears that align it on the block. Since it is a crank trigger motor you can adjust the timing of the injector pulse by rotating the distributor. You would be surprised at the difference. I believe the distributors sold by Hesco come with the ears removed for just this purpose.

After stroking my motor I just replace the fuel pressure reg instead of the injectors and I have never had a problem with it running lean ( as it does stock ).

4.6
60mm tb
Borla header
gibson cat back
tb spacer
cold cone conversion

INI SLDR said:

Another thing you can try is taking out your distributor and cut of the two ears that align it on the block. Since it is a crank trigger motor you can adjust the timing of the injector pulse by rotating the distributor. You would be surprised at the difference. I believe the distributors sold by Hesco come with the ears removed for just this purpose.

After stroking my motor I just replace the fuel pressure reg instead of the injectors and I have never had a problem with it running lean ( as it does stock ).

4.6
60mm tb
Borla header
gibson cat back
tb spacer
cold cone conversion

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howwly crap i can't believe you have stock injectors get 24 lb and i guarentee you will like them better they compliment the adj.press. reg. so much better.

xjvenom-get the a/f ratio and vacume checked at the shop. if it looks good there, i would try running it w/o the jacobs ignition, i've heard that they are kinda unreliable. sounds to me like timing, and the injector pulse control is tied into that through the cps and cam sensors. do not screw with the distributer unless you think you did something to set it off from where it's supposed to be. if you think you did, time it from scratch! i had a problem that started like that, and it was <stock>ignition.

I do not want to sound like an ass but why are you running 24lb injectors? I do not think you list of mods constitutes an injector swap. As instructed check the fuel pressure, have you gone through a standard tune-up. Is the fuel filter okay, and the the fuel pump?

katarn

katarn444 said:

I do not want to sound like an ass but why are you running 24lb injectors? I do not think you list of mods constitutes an injector swap. As instructed check the fuel pressure, have you gone through a standard tune-up. Is the fuel filter okay, and the the fuel pump?

katarn

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I upgraded to 24lb because it was running lean - the spark plugs were showing white and the engine ran quite a bit hotter than prior to the mods. Not only that, I decided the original injectors with 170k+ miles on them were due for retirement anyway.

I've already done a complete tune-up and the fuel filter has been recently replaced as well. I'm not sure about the fuel pump though, and I've had my suspicions. I guess I won't know if the fuel pump and/or FPR is bad until I test the fuel pressure.