Cutting out at 4200rpm

[tucsonxj]

OK heres the deal

What I'm getting seems almost like hitting a rev limiter at 4200 which from what I have heard is too low. It's like the ignition is cutting out and won't go higher. It used to. Here's my specs:

4.6 built with a 97 block and head (0630) in a 91 cherokee w/4.10's and 33's.
About 4500 miles on the stroker.

JE forged pistons, 3.905 bore, 1.370 comp height (zero deck) .270 inverted dome (30cc dish)
Eagle SIR rods w/ floating pins & wave lock bolts
89 Yj crank 9short snout)
Crane 753905 cam/lifters
Crower pushrods
Yella Terra roller rockers
MP 1.97 intake 1.56 exhaust valves and MP springs retainers locks
Home prot job
HV oil pump
dbl roller timing chain
99 intake w/ ford 24# injectors stock fuel pressure
stock tb and exhaust manifold (for now, plan on the 65mm mustang tb and header later)
2.5" cat back
cone intake
MAP adjuster at 5.10V

Uses no oil (moble1 with half bottle EOS). After break in of 1000 or so miles it seemed to do this at about 5000rpm but only occasionally. i put in new plugs and new wires which helped some, now it is always at 4200. I just added the MAP adjuster today with no difference except a smoother idle. I had a mechanic friend hook up his new 10,000 dollar snap on scanner last week and confirmed it was running lead at WOT while under load. He said all the sensors looked good, it was doing the rich lean thing at idle.

It seems like ignition to me but I don't know. The plug looked good last check (champion rc12 IIRC gapped at .035") I have tried another coil. Can the spark gap be widened? Don't know it that would help tho. Plug wires are new. No arcing in the dark. It does pop slightly out the intake when this cutting out is happening.

Anyone have anything similar? Any Ideas? I'm unemployed now so I can't just throw money at it but I will spend to make it right when I know what it is.

Thanks

[gradon]

I like the part list--esp the pistons comp height. How much were they? Could that snap-on scanner verify that the cam sensor and crank sensor were in sync(check bratcop's build in the project section and there is an article pdf that explains this and shows what the oscilloscope sees)?

[tucsonxj]

We sisn't really look at that. I did do the pin through the sensor to the distributor housing thing to set it up initially. I can look at that next week, don't have access till wednesday.

Thanks for the idea

[tucsonxj]

OPPS that should say we didn't look at that. And the pistons were 516 ordered through a local performance machine shop. That pin height is based on the amount they decked the block. Sorry I don't remember the amount but it was very little, just to true it up. It was done in 2004 so the price is old. I took a LONG time to do this thing.

[SilverXJ]

I would definitely check the cam position sensor. I know of one person on here and a few on another site that had a similar issue and it was the cam sensor... there is also a good PDF in one of those threads.. might want to search for "toothpick"

[Cheromaniac]

I had a mechanic friend hook up his new 10,000 dollar snap on scanner last week and confirmed it was running lean at WOT while under load.

It does pop slightly out the intake when this cutting out is happening.

You might wanna hook up a fuel pressure gauge onto the injector rail Schrader valve and check the fuel pressure when you rev the engine up. I can't help wondering if you either have a clogged fuel filter or a weak fuel pump.

[Jeep-Power]

I have the same thing happen to mine-- except at a lower RPM. It shows here on the dyno sheet:



As can be seen, the A/F ratio drops sharply and then falls to it's lowest point after 3500 RPM...

I guess the pump is working well (it has done this with two different pumps). I wonder about that filter...

[tucsonxj]

I did check the fuel pressure at the rail and it stays steady as the cutting out happens. I did that parked and not under load. I wll have to see if my buddies scanner will show the scope info like the pdf mentioned. I know he said it can watch like 16 sensors at a time and record so many seconds if neccessary. I won't be able to mess with it till wednesday due to school and my friends work schedule.

I agree with the cam sensor being likely the cause. I think I am hitting an rpm where the spark advance puts the rotor position too far from the cap's terminal and am losing spark at that rpm. Thats what my logic is thinking now. We'll see.

Thanks guys I'll post back what I find.

[Mgardiner1]

Click on this link and follow the instructions to the "T". I mean it too, follow exactly where the line is "scribed" on the distributor and notice what part of the rotor gets indexed to it.

I had this problem that occured @ 2700 RPM and this is what my problem was. Compliments of Dave AKA dwg86

[SilverXJ]

Click on this link and follow the instructions to the "T". I mean it too, follow exactly where the line is "scribed" on the distributor and notice what part of the rotor gets indexed to it.

I had this problem that occured @ 2700 RPM and this is what my problem was. Compliments of Dave AKA dwg86

What link?

I think if your friend has a 10k snap on tool he should be able to find a faulty sensor.

[Mgardiner1]

Sorry, rough day yesterday...

http://www.geocities.com/fosgategod/34849806.htm

[tucsonxj]

That Alldata info looks like it is copied from the FSM. I will look at that closely too. I do believe the scanner in question will be able to look at the sync signals to verify that the cam sensor signal is seen in the right place. This isn't a bad sensor so to speak but a time sequence thing. I don't get a cam sensor code, but don't know if the 91 system gives those anyway unless the signal is missing totally.

I'll report back tomorrow.

[Mgardiner1]

Trust me, it doesn't. At least my 91 ECU didn't....

And to actually test both sensors and their timing ACCURATLY, i'm pretty sure you need an oscilliscope

[tucsonxj]

OK my friend didn't have the right scanner at home yesterday dangit. So I was not able to check the sync of the sensors yet. Now it will have to wait till saturday. I did pull the dist. and cut off the ears. I put it back in and tried it advanced about the half width of the hold down bolt, and retarded the same. It made absolutely no difference. I set it back to normal (with a allen wrench in the holes) I will still check it on sat, but I am starting to rule that out. I need to check the fuel pressure under load, I did check it parked and it was fine and it does this cutting out parked without load, same RPM. I doubt it is fuel pressure related. I have tried a different coil, not new but a spare with no change. I still think it is spark related. I do get some intake backfire when it cuts out. I pulled the plugs and wire brushed them. They had a reddish deposit on the insulator which I am told is the anti knock stuff put in here in winter. There was a bit of black carbon on the body but no other bad signs that I know of. I re gapped them to .040. They are Autolite 984 IIRC. I will likely swap them out soon to the NGK's or Champion's. which one should I get for the 91? I did break in the engine with RY12 Champions.

This is ticking me off now. I am fearing the worst, like a cam failure. What would the signs be? It seems to run fine otherwise. I do remember that the machine shop said the flexplate ring had alot of runout. Could the CPS lose signal on part of the revolution and cause this? Just a wild thought. The dist. is new as with the cam sensor, they are from a 94 not a 91 so the rotor and cam sensor are different. I'm kinda out of ideas on this.

Thanks everyone for the ideas.

[John]

A damaged,warped flex plate could affect the cps signal and cause problems. Do you have a record of what "a lot of run out" was?
John