Another 4200 RPM stumbling/cutting out question...

[mustangfrank]

This forum has been a great source of info during my build. I've been through all the posts related to this question but it seems that none have been followed up on with the final solution. This thing falls on its face at 4200 and stumbles or sometimes even stalls completely. Sounds like ignition miss. Up to that point it is great, part throttle great, just WOT at 4200.

98 Cherokee
98 block, 99-up head, (new casting), 1.94/1.50, Chev. springs.
Comp 232-4
.030 944s (22 dish).
Ford 24lb
01 Intake
Stock exhaust, Dynomax catback.
Stock throttle body temporarily, (sending core out for boring).

Fuel pressure is steady at 45-46 (my gauge?) idle and load.
Advance of around 10-36, has peaked at 41, ODBII port.
Cam not degreed.
Plugs look awesome.

Any ideas, especially some follow-up solutions?

Thanks.

[mustangfrank]

Replaced the fuel pump and both strainers yesterday...no change.

I'm thinking it is valve float now, the head is the only assembly work I did not perform or check parts content myself.

[SilverXJ]

Did you replace just the fuel pump or the whole assembly? The fuel pressure regulator is the part on the very top of the pump assembly that the fuel line attaches to. Also, when you say under load, is that while driving?

[mustangfrank]

Just the pump and filters/screens. Yes it happens while loaded/driving although I am able to get it to kind of happen just before redline (5000) while parked. Nowhere near as bad but it starts to stumble.

[mustangfrank]

I appreciate your opinion...got me thinking in another direction. I'm going to go to storage and check my guage on another XJ, a Mustang and whatever else I can. If my guage is correct I'm running on the bottom of the range, (46-51psi). I'm also going to tape the balancer with a full set of degree marks and make sure I'm not advancing out of the range of the rotor contact(s). Not sure how accurate the OBDII port and my Nemesys is but I've recorded peaks of 44 degrees advance.

[racing89]

Miss, under a load, on any vehicle most commonly is a plug or fuel contamination

[mustangfrank]

It is more of a wall it hits than a miss. At 4200 using 3/4 throttle to WOT it will hesitate/stumble and if you stay in it will stall the engine. If I let out of the throttle it will clear up and stay running.

All new NGK plugs, (gapped .035), Accel wires, cap and rotor, original coil.

I'm heading out the door now to verify my fuel pressure guage.

Whatever I find I'll make sure to follow through on this forum for future reference on this common issue.

[Cheromaniac]

A friend of mine is having exactly the same problem with the engine in his TJ and he also has the same CompCams 68-232-4. I tried to troubleshoot it but without success. The engine runs great until it hits a "wall" at 4200rpm both on free revving in neutral and under load in gear. Fuel pressure was OK. Checked the computer for codes and the only one that came up was 11 (crank position sensor, cam position sensor). I tried clocking the distributor but that didn't help at all; it became worse so I rotated it back to the original position. I even tried swapping in a spare cam position sensor that I know to be good but that didn't help either.

My friend talked to this German guy in Dubai who's apparently an expert at tuning the Jeep OBD II computer. The tuner said that the engine was running extremely lean at the 4200rpm "wall" as if there was a fuel cutoff, but hasn't yet figured out what's causing it. The engine has stock valve springs.

I'm beginning to wonder if the TJ computer doesn't like the 232 cam and is triggering fault code 11. My friend still has his original stock cam as a spare so I suggested to him that he could swap it back in if all else fails to see if that cures the problem.

[mustangfrank]

I get a P1391 crankshaft sensor error if I try to retard the dizzy (block side ear cut off) too much.

I went with the 68-232-4 because it is supposed to be a FI friendly cam and the overlap kept my DCR just under 8.0:1.

Checked my fuel pressure guage on a 2001 XJ, same reading 45-46psi. I'm thinking this is not the issue, seems if there isn't quite enough fuel pressure or volume up top it would start leaning out, ping, something, not fall on its face.

I'm trying to find some timing tape for this balancer size, the local shops only have SBC 6-3/4" and 8". I'd like to see the rotor position at 40-44 degrees BTDC just to make sure it can make spark.

[dwg86]

Have you taken it to the dealer to have cam position sensor checked with the DRB scanning tool? I ran a [email protected] duration cam with a 112 lobe center advanced 3 degrees in my stroker with no problems.

[mustangfrank]

No, never considered the dealer as an option, maybe I should.

Rain finally stopped here so I'm going to play with the dizzy/cam sensor first thing this morning. I kept the outside ear on the dizzy to give me a point to return to but I'm curious if "half a tooth" the other direction would help. Other than the problem it runs great with the ears in the factory position. ANY adjustment to retard it lights up the P1391 error.

I'm kicking myself for not degreeing the cam now to verify the grind is to spec.

[Cheromaniac]

Well, my friend Majid e-mailed CompCams to get their opinion on this issue and this is what they said. The CompCams cams have fast valve opening/closing ramp rates so if you use stock valve springs, the valves will float at ~4200rpm and this seems to be a common issue. CompCams said that the cure is stiffer valve springs.
Majid's ordered a set of Mopar 5249464 springs with matching retainers/locks and after he's installed them, he'll let me know if that cured the problem.

[mustangfrank]

Thank you for updating. I should take a couple springs off today or tomorrow and have them tested.

[dwg86]

Well, my friend Majid e-mailed CompCams to get their opinion on this issue and this is what they said. The CompCams cams have fast valve opening/closing ramp rates so if you use stock valve springs, the valves will float at ~4200rpm and this seems to be a common issue. CompCams said that the cure is stiffer valve springs.
Majid's ordered a set of Mopar 5249464 springs with matching retainers/locks and after he's installed them, he'll let me know if that cured the problem.

I thought the stock valve springs were only good up to .430 lift?

[Shark]

I thought the stock valve springs were only good up to .430 lift?

Seems as if its not because the amount of lift but what comp says is the quick open/close rate of the valves at higher RPM's.