Idling Issue After Injector Upgrade

Hey folks,

My 91 4.0 idles at about 750rpm and suddenly begins to stumble and drop to about 400rpm every 30 seconds or so, then gradually goes back up within another 15 seconds before repeating the cycle. This only happens during idle in both closed and open loop mode. This problem occured after I replaced the stock injectors w/ brand new late-model Ford 24lb injectors (they have four pin holes instead of one for better atomization). I'm using a MAP voltage adjuster, but my engine stumbles even more when I set the voltage under 5.0v, (around 4.25v according to what most people use with the same mods) and seems to stumble a little less when I set it higher (around 5.75v). I tested the coil resistance of the injectors and they were within spec (about 15ohms). I had the injectors tested for pressure and spray pattern to make sure they weren't bad, and they passed with flying colors. I'm not sure what is causing the issue, as I have read of plenty of other people installing Ford racing 24lb injectors after making these types of modifications without running into problems. I'm stumped, which is why I have come to you. Any suggestions would be greatly appreciated! (BTW the engine was rebuilt about 4 years ago).


This is what I've checked so far:

- o2 Sensor (replaced)
- IAC Motor (replaced)
- MAP Sensor (tested w/ new one, but no change)
- TPS (replaced)
- CTS (tested and it's within ohm specs)
- Checked torque on intake/exhaust bolts
- Compression test (all cylinders at about 158psi)
- Checked vacuum lines for leaks
- Checked all ground connections to engine block and frame
- Checked wiring to injectors from PCM (tested resistance across the wiring harness and all the wires were under 0.1 ohm.)


Engine modifcations are as follows:

- Mike Leach/Mopar Performance exhaust header
- 3 inch exhaust upgrade (2.5 inch cat-back) with straight through Magnaflow Muffler
- K&N Cone style high flow air filter
- Bored throttle body
- 99+ intake manifold w/ matched ports on gasket
- Port/Polished & 5 angle valve job on head
- Raised compression ratio by shaving head and using thinner gasket (about 9.0:1 CR)

My mods are just like yours but different parts, borla, bored tb, high flow cat, ported, matched, polished head, I am only running the 21 lb 4 hole fords. Yellow top. I would take your adjustable map out of the mix and see if that cures the problem. But lots of people have more experiance than me. Good Luck

Re:

Thanks for your suggestion, XJING. Currently the adjustable MAP is completely bypassed and using the stock setup anyway. I also forgot to mention that once in a blue moon the issue dissappears for a short while (like a day or so) and then comes back again. I thought it could have been a bad connection to an injector somewhere which is why I tested the resistance in the wiring harness, but as I mentioned above everything there tested OK, too.

Ghost in the machine..... TTT, For the late night crowd.

Sounds like you copied most of the mods that I had on my old 4.0 before I swapped in my home built stroker.
Check the fuel pressure at the injector rail. That stumbling could be an intermittent fuel delivery problem e.g. fuel pump.

Dr. Dyno said:

Sounds like you copied most of the mods that I had on my old 4.0 before I swapped in my home built stroker.
Check the fuel pressure at the injector rail. That stumbling could be an intermittent fuel delivery problem e.g. fuel pump.

Click to expand...

Oops, sorry I forgot to mention that was another thing I had already checked. The pressure at the rail was at a constant 34-35psi and would raise up to about 37 at higher RPM. The pressure wouldn't drop when the engine would begin to stumble. With the FPR vacuum line disconnected it was about 42psi. From what I've read these numbers are within spec, but I could be wrong. There are a few other fuel system tests that I did according to the FSM. It passed all tests except the Fuel System Rest Pressure.


Fuel System Pressure Test - Pressure @ fuel rail with engine running. According to FSM the pressure should be 39psi w/ vacuum line disconnected and 31psi w/ vacuum line connected.

Deadhead Pressure - Manually run fuel pump w/ engine off by jumping connectors on the fuel pump relay (terminals 30 and 87), and checking pressure @ fuel rail by momentarily pinching off return line. Psi should raise up to at least 53psi (Don't let it raise above 60psi).

Fuel System Rest Pressure - Start engine and measure fuel pressure at rail. Turn off engine and wait 30 minutes. The pressure should not drop below 20psi. (mine lost almost all pressure within 30 min)

Capacity (Volume) Test - Disconnect fuel line and place into container. Manually run fuel pump w/ engine off by jumping connectors on the fuel pump relay (terminals 30 and 87). Volume of fuel in container should be 1.05 qts (1.0L) in one minute.

Fuel System Rest Pressure - Start engine and measure fuel pressure at rail. Turn off engine and wait 30 minutes. The pressure should not drop below 20psi. (mine lost almost all pressure within 30 min)
Looks like you need a new check valve, the great cheap fix for this is a bmw check valve, used to have the link, lost it, do a search, you will find it. Its an easy fix.
dont know if that will fix you problem, but while I was down there I would replace your fuel filter too, lots of people forget to replace them cus they are out of sight.
Good Luck

Check valve

I didn't bother replacing the check valve cuz I figured it wouldn't be causing this problem, but it's definitely worth a shot now that I think about it. Are they expensive to replace? I'll look into replacing it and post an update after i've installed it. Until then, anyone else have any other ideas?