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 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)
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