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dist. indexing/ too much cam/ or both- need suggestions & opinions

TJS93xj

NAXJA Forum User
Recently finished up the rebuild of a 4.0. Stock rebuild with the exception of a Borla header, 2.5" cat back/flowmaster, air tube/K&N, and a Crane cam P/N 753901, (260 deg. intake/272 deg. exhaust duration).

Upon the initial start up it didn't want to start, backfiring through the exhaust etc. Did some research and figured that with the aftermarket cam i needed to index the dist. Used the indexing info found on the Strokers website but still no start. Did some more research and found another site that talked about indexing but instead of the rotor leading the the #1 plugwire pickup by .020 @ TDC it showed the rotor lagging the #1 pickup by .020. Tried that and voila it started and ran. Any info on the discrepancies between the 2? Has anyone used the indexing info off the strokers website with good results?

It seems to run really well on flat ground when up at speed (2000-2500 rpm range) but there are still some things that make me think something is still not right.

1. At start up it catches on the 2nd crank and idles fine for about 25-30 seconds then begins to stumble and miss, almost like pulling plugwires one at a time. After 30-45 seconds of this it clears out and idles fine. When warmed up it starts and idles fine with no stumbling.

2. When climbing moderate grades at freeway speeds there seems to be a loss of manifold vaccum, ie. the a/c defaults to the defroster vents until I top out over the grade. And when climbing these same grades it seems like it dosent have the low end grunt it should. On flat ground it will go from 60-80 while passing with a good seat of the pants feel, but off the line (1000-2000 rpm) it feels sluggish.

I guess I'm wondering if the problems are centered around the dist. indexing, or if the cam I chose is too much cam for the engine,or is there something else that I am totally missing/not thinking about.

Any info, ideas, insights, money for a stroker, etc. would be great.

Tim
 
The loss of vacuum and the loss of low end leads me to believe you may have selected to much cam. I don't have much experience building 4.0's, but the same thing occurs to V-8's with to large a cam. I had to install a HUGE vacuum canistor on my Chevelle to allow the brakes enough vacuum to operate. The idle problem may have something to do with your ecm, hopefully one of these guys more knowledgeable on 4.0 will chime in and help you out.


Good Luck,

JJ
 
The index procedure on the strokers files section is the factory procedure ... and it is incorrect (as are a number of the factory TSBs). The best way to index the distributor is to set the static index as you have done, and then after a few miles pull the rotor and inspect the tip for a clean and centered burn marks. Centered symmetrical marks indicate the rotor tip is utilizing the full length of the rotor tip for advance & retard (without tracking off the end of the tip). if the marks are not centered, file the locating tab open a little and adjust (rotate) the distributor accordingly.

You fail to mention if the engine is non-HO Renix or one of the HO variants?

If it's Renix the stumble may be the EGR valve opening and seating (or sticking).

The vacuum loss is likely to be a hole in the vacuum line from the engine to the canister in the front bumper, or the tee to the axle disconnect.

The engine is new, and will need some time to break-in and seat the rings for better low rpm compression sealing. I would suggest an exhaust gas analysis after a few thousand miles to determine the possible need for larger injectors (or a chassis dyno run to test the same).
 
With all the induction and exhaust mods you have, I see no mention of larger injectors. My guess is you're ruuning lean.
 
update @ more info

Just some more info on the problems I'm having. I failed to mention in my original post that the jeep is a '93 with the H.O.. I have around 1500 miles since the rebuild.

Went out this morning to check for bad or cracked vacuum lines and was able to rule that out. I pulled the line off from the vacuum resevoir and it drew in air for a couple seconds so the vacuum held for 8-10 hours with the jeep off. This leads to another question, is there a one way check valve in the vacuum system somewhere?, otherwise the system would not hold a vacuum when the engine is off, please correct me if I'm wrong.

I'm still wondering about the indexing. Can the dealer or a shop with the diagnostic equip actually see the relationship between the cam position sensor and the crank position sensor.

I am also wondering about the cam as well at this point. What should vacuum be at idle, 1500rpm, 2000 rpm.

Neither the block or head were decked during the rebuild. Both were checked and said to be good so I didn't have it done. The pistons are the stock comp. ratio and the head gasket is the standard fel-pro gasket that come in the master kit. Would going to a thinner head gasket to bump up the c.r. possibly help if it is indeed a cam related issue? If so any recomendations on which head gasket to use.
 
You may be on the right track with the dist indexing. RV or lower torque type cams often are made with a few degrees advance built into them, this will throw off the indexing, I second Eds advice on grinding the distributor's locating tab to properly align the housing. There is a check valve on some at least, I think it's on the line just above the int manifold near the IAT sensor on '93's IIRC. Did you change the dist gear also? It can easily be installed 180 deg off, which may be an option instead of grinding the tab. HTH TC
 
Re: update @ more info

TJS93xj said:

Went out this morning to check for bad or cracked vacuum lines and was able to rule that out. I pulled the line off from the vacuum resevoir and it drew in air for a couple seconds so the vacuum held for 8-10 hours with the jeep off. This leads to another question, is there a one way check valve in the vacuum system somewhere?, otherwise the system would not hold a vacuum when the engine is off, please correct me if I'm wrong.

I'm still wondering about the indexing. Can the dealer or a shop with the diagnostic equip actually see the relationship between the cam position sensor and the crank position sensor.

I am also wondering about the cam as well at this point. What should vacuum be at idle, 1500rpm, 2000 rpm.



The vacuum check valve is in the vacuum canister, so a vacuum leak in the tubing between the manifold and the bumper can still exist, even if the canister holds vacuum. A hole, rubbed into the tubing leading to the bumper, is a very common problem where the tubing is routed through the radiator support. The test you made will not rule out a vacuum leak (I would add a new tube between the manifold and canister to rule out a leak in the factory tubing).

The indexing problem may be possible to see on a chassis dyno or engine analyzer, by comparing the ignition signature on a scope at various distributor positions (rotate to find the best location based on the scope reading).

This cam should not alter the idle significantly, with a good smooth 17 to 19 inHg idle at 500-700 rpm, and 21 inHg at 1500 or more rpm. The vacuum should be checked from a tee fitting added to the MAP sensor tube (if possible).
 
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