New 4.6 build with bad miss on #1

[Bdorcheus]

I am a newbie to this forum but not to the combustion engine.

I have a 1999 jeep Cherokee 4.0 just overhauled to 4.6. It has a significant miss on cylinder #1. break in was 30 minutes at 1500 to seat rings and cam followers then drove for a few minutes 30mph to 50mph repeated 10 times.
This was built by professionals and they now have it and can not solve the problem.

Things I have tried:
swapping injector locations; I can hear/feel the injector fire
swap plugs and wires number; one plug is obviously rich and discolored compared to the others
checked for vacuum leaks with starting fluid spray
distributor is placed by aligning the inner rotor to the housing and then the ignition rotor is positioned at #1 wire all with the crank on 0 degrees to marks

Things the shop has tried:
smoke test for vacuum leaks
valve lift on 1 is the same as number 2
computer diagnostics shows some miss on #4 as well
crank position sensor shows some variation; new mopar part swapped in with no improvement
compression tests shows 135 +/- 5 psi across all 6
leak down test was good on all
They reported a reduced vacuum signal


Questions i have:
Could the cam be bad as far as lobe position for #1
Could the distributor need to be modded to get more adjustment for computer?
I matched the gasket to the ports since it was covering nearly 0.25" on the bottom of the exhaust ports, could that be a problem?

I am at a total loss and beginning to get frustrated.

Thanks for anything that I can try or at least leads me in the right direction!!!

Brian

[jsawduste]

http://www.jeepstrokers.com/forum/viewt ... =15&t=5009

Try here for starters

[Bdorcheus]

Thanks I had already read that thread and though informative and I might try scoping the injectors, mine has a definite noticeable miss.

I will further investigate on aa1car thanks

[Bdorcheus]

Still can not get this resolved! anyone else have any suggestions

my next step is the junk yard to start getting harnesses and ECU

Thanks

[Bdorcheus]

UPDATE:

I drove this back home and although it seems to be getting better there is still a significant miss.
I re-installed the old cap, rotor, wires, and set the distributor (tabs have been cut off) with the trailing edge of the rotor just past (i was told 0.020" but that is hard to measure when in the car) the number 1 position of the cap.

The shop reported vacuum of 11 at idle and 16 at high idle.
The timing advance is 6-7 at idle and 27 @ 2000rpm unloaded. According to an online vacuum chart this is way off.


Which port is best for measuring vacuum?
Could this be a bad MAP sensor reporting incorrect vacuum?

Could this be a valve not seating correctly and still show good compression and leak down?
I know there is a very small exhaust leak on a number of cylinders right on the manifold flange, could this cause a miss?

Thanks for any help

BD

[NickInTimeFilms]

Well, you swapped your injectors and spark plugs around so those seem to check out. Cam lift was measured so they should be alright.

What kind of compression/quench/octane are you running? Maybe you are getting some pinging? Quick test would be to run some higher octane through it and see if that helps. Any carbon build up on the pistons? Are you getting good spark on number one, messed up dizzy cap?

[Bdorcheus]

FYI update,

after 200 miles the miss is nearly gone cold and completely gone when warm. this is by ear and hand not via miss counts.

I am thinking a valve seat wasn't perfect or something but thanks for the help guys.

BD

[Retlaw01XJ]

.....The shop reported vacuum of 11 at idle and 16 at high idle......

.....Could this be a bad MAP sensor reporting incorrect vacuum?.....

I think the MAP sensor uses a different scale than a vacuum gauge would.
I noticed my scanner showed 12 at idle but it increased under acceleration..... the opposite of what a vacuum gauge on the intake manifold would read.

I had a vacuum gauge on it this summer and it was 19 at idle, about where it should be.

Great to hear your problem cleared up!