4.0L head swap + Mopar MPI issues - Pirate4x4.Com : 4x4 and Off-Road Forum
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Old 11-15-2008, 03:09 PM   #1 (permalink)
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4.0L head swap + Mopar MPI issues

Well guys, I rebuilt my engine and in the process I decided to try to get some more power out of the engine by performing the 4.0L head swap and installing the Mopar MPI kit. As usual, I ran into some problems.

When I had the machine work done on the engine I told the machinist to bore the cylinders 0.030" over, order and install some parts and order a new cam. Once he got done with it I took it home and finished building it myself. Once I got the engine reinstalled and a couple of kinks worked out I noticed it still wasn't running right. I took it to the shop and they couldn't find anything wrong so I told them to try a compression test. They found that it was only getting up to about 75PSI. They did a leak down test and they said it wasn't leaking. I researched the part numbers of every part I put in there and it turned out that the machinist ordered the 4.0L cam. It's actually not a bad thing, except that it doesn't work with the 1981 timing set, which is the year of my engine. I told the shop to try to figure something out with the timing and they said they could machine the sprocket if they had to.

They called me back saying they got the compression up to 118PSI and they put it all back together. I was pretty annoyed since I should be getting more than that since I swapped the head (does anyone happen to know how much, much less the compression ratio?). They also said they didn't modify the sprocket or anything, but before I just go and buy the 1994 Jeep 4.0L timing set does anyone have a better suggestion? I don't want to peak it up for high or low RPM, I just want it to run right and I've been told that the 4.0L cam is actually better in this case since it was made for fuel injection.

Since I'm going to have to go back into the engine anyway, should I keep that cam in there or should I try to get a custom one since I've swapped the head and bored the cylinders 0.030" over?

Also, if my engine has been running for a while, I can feel it start shaking. The check engine light comes on and it doesn't feel like it's running on all 6 cylinders. I've checked the error code and it's code 27. The kit that I installed is the newer one with the '96 and up fuel pressure regulator which means that it came with 23.2 (or 23.3)lb/hr fuel injectors instead of 21lb/hr injectors. Am I still going to need 24lb/hr injectors because of the swapped head to fix this or is there some other way? Thanks in advance.
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Old 11-16-2008, 02:05 AM   #2 (permalink)
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If you want your engine to run right, you need to have a .000" deck height. Then it needs to have a .040-.050" thick gasket. This will give you the required quench height. This is common for the stroker guys, some shave the head instead of the block and with the quench out (the 4.0 head is a quench head) they get ping with high compressions. I used the 753901 crane cam. You also need to use an oil that has a lot of ZDDP in it or you can wipe a cam in as little as 4-6000 miles. You dont need the larger 24lb injectors, that is mostly for the 4.6's. Check the burn color on your plugs, it may give a clue that you are loading up being too rich or its burning too lean...hope this helps. Tim
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