misfire on renix 4.0 on hot startup

I have tried several cps, including high altitude sensor, added additional grounds to tps harness, intake and firewall. If I run my flex a lite fan for 5+ minutes it starts and runs fine. weird eh, defininate temperature/connection somewhere, its an 89 so no c101 connector

Have you ever checked the output voltage of the CPSs?

You might try using a hair dryer on various parts like the HV coil, or each injector to see if they are temperature sensitive.

Ever try this?

Renix Jeep ICU/Coil contact refreshing
　
　
The contacts between the coil and the ICU on your Renix Jeep can become corroded and loose causing a complete or intermittent no-start condition. I recommend the following procedure as a maintenance precaution to insure this is eliminated as a possible cause now and in the future.
The coil is attached to the ICU by two T20 Torx bolts. Remove these two bolts and lift the coil up off the ICU. You will see 2 pins and 2 sets of contacts. Clean both the pins and springy contact pieces with a good electronics cleaner.
Squeeze the springy contacts closer together with some needlenose pliers. Apply some dielectric grease to the contacts and bolt the coil back on to the ICU.
While you’re right there unplug the connectors from the ICU and inspect the pins in the harness connector. Make sure the pins are not retracted into the connector. Spray out the connector and the receptacle of the ICU with the same good electronics cleaner you used earlier. Apply dielectric grease to the connectors and plug them back in.
I feel this procedure should be performed at least once in the lifetime of a Renix Jeep.
　
Revised 11-29-2011

X2, good idea! Heat can open any contacts especially those. I have never had a Renix do a heat soak restart misfire.

No I havent checked voltage on cps, I'll do this in the next day or two,one reading cold and one after hot soak. I tried the coil and icu contact thing, then replaced both with new and made sure the connection was tight between the two. I'm mounting a 10" electric fan on my hood under my hood vents this weekend just for diagnostic purposes.

Friggen years later still having exact issue but will now occurs start missing when pushed hard on hot days or as per usual after every restart on hot soak. Eg. Jeeps up to temp and I go refuel, restart and misfires till I hold the gas down and it clears up. If I stop on the trail I either leave it running fans on or shutoff for 10m let my hood mounted 10 inch fan above the intake ducted through the hood cool it off. Temp probe is zip tied to the intake runner. Worse these days as I'm on 35s so extra taxing on the engine. I just added front and rear dana 60s so I'm trying to remedy this now. The extra weight will only make it worse. When it began way back back I had stock injectors, since upgraded to Ford 5.0 five motorsports injectors. Motor, head, intake, eng harness, has been swapped with no avail. Thinking rerouting the fuel line to the rail away from the tunnel, possibly upgrading to a plastic injector that doesn't conduct or hold heat, any recommendations? Or input overall on my issue. I'm a auto mechanic since 99,i specialize in diag, electrical, high end high output eng builds, everything turbo so I'm no fool. Sucks my own heep is my biggest mystery

Injector change didn't change the issue?
Ever monitored the fuel pressure during that time?
Sounds like fuel pressure is bleeding off when hot and flooding the engine through an injector that isn't closing completely.

If you pull off the vac line to the FPR, is there gas in it? .

cruiser54 said:

Injector change didn't change the issue?
Ever monitored the fuel pressure during that time?
Sounds like fuel pressure is bleeding off when hot and flooding the engine through an injector that isn't closing completely.

If you pull off the vac line to the FPR, is there gas in it? .

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Still all these years later... still dealing with this.Ive gone through tested , replaced , upgraded and added grounds. Sensors, even swapped them all off my spare renix longblock. It doesnt skip a beat until its driven up to temp and shut off for even 2 minutes. Restarts like its lost a cylinder. Clears out with throttle held. Fuel press, pump, ignition components replaced. Whatever it is ..its heat related. I mounted a small fan above center of intake, temp controlled to turn on, and stay on after shutdown until temp cooled. Running it fulltime doesnt help. If your really working the jeep, putting the coolant system to its safe limits it will start the miss as well. The problem just began one day years ago while snow wheeling, that engine ended up with a sticky lifter, i swapped it with a fully rebuilt engine, different head even. And it continued to do exact same. Im to the point...of thinking about a full harness swap. Injectors also replaced , and dei thermal boots on them.also did a poormans under rail heat sheild, with dei heat tape on both sides. Id suspect a sticky valve or slow coolant intrusion into a cylinder but zero coolant loss, fully rebuilt head. And fact its exact issue with last engine. Any fresh ideas?
I run dedicated coolant, and tranny temp gauges, oem tstat. Csf rad. Hd fanclutch.shroud, hood spacers, hood vents, 2 through hood 6 inch puller fans to help evacuate underhood heat to try and help with my saga. 23 years ive had this same 1989 xj now! Shes on 05+ sd one tons , beadlocked 37s. The engine i built was to suit and support a small turbo project . But not even going to entertain that until i can come to some fix on hot soak miss.

Any noises associated with the miss? Clattering lifter?

cruiser54 said:

Any noises associated with the miss? Clattering lifter?

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no noise , no clicking or ticking. Very much like pulling a plug wire every time its restarted after up to temp.worsens when taxed

can you narrow the miss down to one particular cylinder when this occurs?

Or a fuel injector? It would help to narrow down which cylinder. Just restart it and when it skips, disconnect/reconnect each FI connector. You'll detect no change in RPM when you find the culprit. Then need to determine if it's a fuel issue or ignition.

I had a intermittent dead #3 cylinder. Turned out to a fuel issue. I swapped injectors around and the problem was still #3.

My issue ended up being a bad splice on the ground leads for the injectors. On Renix, the ECU sends power to each injector to fire them, all of them come to a common ground splice. The splice is in the harness along the vc just where it turns to run along the fire wall.

While I'm not sure this fits your symptoms, you mentioned a harness change so I decided to mention this to you.