2000 jeep cherokee rebuilt 4.0 with cam, Low Vacuum at idle

I have a 2000 jeep cherokee i bought with a seized engine. I built a new mainly stock engine with a block that was bored .30 over. I used a new casting 331 head and mildly ported it and assembled it with new stock valves and performance springs. I installed a comp cams 252h grind cam (68-200-4) port matched the intake and am running an earlier style header. running 4 hole injectors and a 62mm throttle body. The jeep is rolling on 33's with 4.10 gears and a AW4 trans. I have replaced cam and crank sensors and both o2's with ngk's. I have properly synced the cam sensor and am running champion plugs.

So my issue that I am trying to figure out is 1. it seems to be under powered. Driving on the highway i don't have enough torque to maintain 70 on a 4% grade unless my rpm's are above 3000.

Issue 2, my vacuum at idle is only at 14.7 in hg.

I beleive the two issues are related but I am trying to determine what i should do.

In researching I have found an old article discussing a custom grind that has a greater lift and duration on the exhaust side to overcome the "restrictive" exhaust port on the 331 head.

So if this is the source of my problem then would replacing the cam again correct my problem, or would a performance head from titan that has chevy swirl valves installed and greater flow on the intake and exhaust correct my performance.

Or is there some other issue that i am not seeing. I was hoping that someone may have some insight on my issue and possibly some experience with camming with the 331 head.

thank you

Low vacuum can be due to a much longer duration cam, but more likely is a vacuum leak or cam indexing.

My 1999 with +284,000 miles is a dragster compared to my 2000 with 140,000 miles. The 0331 head is the problem.

Since you have an exhaust manifold already, swap in a 0630/7120 head.

Did you read the cam specs saying "not for fuel injection use"? That grind is for carbureted models!
http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=8&sb=2

Not ruling out a vacuum leak, but this camshaft is the one you should have selected for your EFI 4.0 build: http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=12&sb=2

Ok, in regards to the vacuum leak, I have tried checking for a leak using propane and watching engine speed and o2 with no change seen. I am also running the remflex manifold gasket so that should correct for any slight variation and seal the intake properly.

As far as the cam indexing, That thought cam to my mind as well. I did not degree the cam when I installed it. I have thought about degreeing it now, but I don't want to tear it down twice if I decide to just end up going with another cam.

As far as the comment to the head goes, I was thinking about swapping it out as well, and i was looking at either a remanned 0630 or the 0331 from Titan with the gm valves. According to the specs I could find, it flows better than the stock 0630 head. I haven't quite decided what to do though, wanted to see what advise I got from here.


So in regards to the grind, in researching I found many good reviews on this cam in the 4.0, unfortunately they weren't with this head. I was contemplating the 232 cam due to the lift and duel plane, and am thinking that I will probably go that way but yet again I wanted the get some advise before pulling the trigger.

Most cams seem to run pretty straight up as far as degreeing goes. The head isn't the problem either, I'm running a 0331 Russ P+P head with big valves on mine with the 68-232-4 cam and it's a major hp/trq producer.
Did you match the intake to the TB?
What injectors are you using(part#/color)?
What are your fuel trim numbers?

Looks very nice. I did match the intake to the tb. I will say I put the 62mm after running the stock tp for some time. I didn't notice any change to vacuum or performance.
Injectors are the 784's and are black.
my fuel trims are negative typically. At idle short and long term add to to 10 to 14 negative or so.

Tim_MN said:

My 1999 with +284,000 miles is a dragster compared to my 2000 with 140,000 miles. The 0331 head is the problem.

Click to expand...

Thank you. I could never understand why my 99 XJ ran so much stronger than the 2000 I have now. You solved a long standing mystery for me.

RCP Phx said:

Did you read the cam specs saying "not for fuel injection use"? That grind is for carbureted models!
http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=8&sb=2

Click to expand...

That cam is fine for fuel injection.
I pull over 19hg at idle with the same cam in my 96

Green XJ Jeep - thanks for the input, I didn't feel like aurguing that point as I have read plenty of posts and reviews where guys have used that grind on their fuel injected XJ. And thank you for letting me know what vacuum it pulls, I couldn't find that mentioned anywhere. Not to go off topic, but how does that cam perform in your XJ?

Well those injectors are to big, verified by your fuel trims.

RCP Phx: I may be wrong, but I attributed the negative fuel trims to the low vacuum. Being a speed density system, the lower the vacuum the higher the load and it would increase fuel. or so I figure

Your numbers show it try to lean the injectors out. Flooding the cylinders will definitely decrease the power.

Yes, flooding the engine would decrease the power, and also cause misfires/rough running among other issues. The computer is compensating so its not being "flooded". the fuel maps would increase the injector pulse length due to calculating a higher load with the low vacuum. being that the engine doesn't actually have an increased load, the computer would have to pull from the fuel trims to not run rich.

I'm currently running the equal Ford injector in my build(which iay more than yours) and it's a little to fat. I'm going to try 715's and I used to run 007's when I was about where you are.

My org build thread, a little long but it's got some good reading regarding ECU tuning, Dyno's, and injectors!
https://naxja.org/forum/showthread.php?t=1140839