Surging, No Power Warmed Up (Closed Loop)

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Neuner
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Surging, No Power Warmed Up (Closed Loop)

Post by Neuner »

I've tried to search and read to my wits end. Tried a lot of different phrases and search terms. Hoping you can lead me in the right direction before I throw money and parts at it.
What I have;
- '98 ZJ 4.0 OBDII
- '98 manual XJ PCM
- OEM wire harness and PDC

How it's reacting;
- Starts, idles and runs great in Open Loop.
- When it just starts getting warm, it begins to surge/fall back a little, searching for an idle. This gradually gets worse as time goes on, to the point where it's revving to around 1500 rpm then dropping to almost a complete stall but catching itself from quitting completely.
- The more it warms up, the less load it will take. Eventually it won't take even the smallest amount unless it's under WOT (Open Loop).
- When it's at it's worst, it won't react to throttle except for full throttle.
- Has a new TPS.
- New plugs and wires
- Has a new upstream O2 on just a downpipe with no exhaust pipe or muffler yet.
- Has a new CPS on the harmonic dampener (Hesco).
- Has original MAP sensor.

What I've tried/researched;
- Cleaned the IAC and it helped the idle.
- Removed O2 sensor and it still reacts the same.
- Fuel pressure at the rail is 75psi. I need to get a regulator to get it to 49 but from what I've read, says this should only give bad MPG's, not hamper the operation?
- Pulled the MAP sensor and it has a little 90-degree rubber elbow for connection. Seemed fine but going to look for a new one.
- Bought a MAP sensor but reluctant to try it until I get the new rubber connection tube.
- Cleaned the PCM connections.

Since it runs well in Open Loop does that mean I can eliminate the PCM, CPS, CMS, distributor, spark, IAC, and high fuel pressure as culprits?

Does Open Loop use the MAP Sensor and therefore can I eliminate it as a culprit?
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SilverXJ
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Re: Surging, No Power Warmed Up (Closed Loop)

Post by SilverXJ »

Neuner wrote:- Has a new CPS on the harmonic dampener (Hesco).
I assume this is because you are using a manual transmission and couldn't get a correct fly wheel for it?
- Fuel pressure at the rail is 75psi. I need to get a regulator to get it to 49 but from what I've read, says this should only give bad MPG's, not hamper the operation?
Holy hell that is high. Why?
Since it runs well in Open Loop does that mean I can eliminate the PCM, CPS, CMS, distributor, spark, IAC, and high fuel pressure as culprits?
No way.
Does Open Loop use the MAP Sensor and therefore can I eliminate it as a culprit?
Yes, and no.

What injectors are you running?

Have you tried any diagnostics, or are parts just being thrown at it? Checked fuel trims?

While the high fuel pressure/injector selection may not be the culprit its not helping anything.

Even if you are running stock injectors (23.2 lb/hr @49psi) you will still be pushing too much fuel running 75psi (29 lb/hr!!), which the PCM will not be able to dial back.

Also, what are the specs of the engine? 4.0L? 4.6L stroker? Cam?
Neuner
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Re: Surging, No Power Warmed Up (Closed Loop)

Post by Neuner »

Neuner wrote: What I have;
- '98 ZJ 4.0 OBDII
- '98 manual XJ PCM
- OEM wire harness and PDC
A 4.0 oem.

I haven't thrown parts at it yet. The reason some are new are because they didn't come with the engine when I purchased it.

I'm using an e2000 rail pump but just need to get a fuel regulator installed.

Have not had the opportunity to try diagnostics. Hoping a friend is able to bring over his phancy computer to perform that this weekend.
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SilverXJ
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Re: Surging, No Power Warmed Up (Closed Loop)

Post by SilverXJ »

Neuner wrote: Have not had the opportunity to try diagnostics. Hoping a friend is able to bring over his phancy computer to perform that this weekend.
I wouldn't waste time with that yet. Get the Fuel pressure down to 49 psi first. Reset the PCM and see what it does. That will probably clear up 90% of your problems if not all.

Has the PCM thrown any codes yet?
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Re: Surging, No Power Warmed Up (Closed Loop)

Post by Neuner »

Just now, in trying to find a pressure regulator, I came across a post where someone had the same dilemna as mine with the same setup. They didn't have a regulator installed either. He ended his post by saying that he was going to try it that night. Since he didn't report back, I'm assuming that was the culprit. Hope to get one soon and hope that works.

Didn't mention but this is in a CJ7. I have the data link wires capped off and in a bundle but haven't found the connector yet. Been struggling to find one. Will hopefully be able to swing by a junkyard this weekend.

Does the PCM reset by just disconnecting the battery?
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Neuner
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Re: Surging, No Power Warmed Up (Closed Loop)

Post by Neuner »

Welp, that was it. Got a bypass fuel filter for a WJ to set it at 49. Took it for a nice cruise around the neighborhood with no issues.
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Re: Surging, No Power Warmed Up (Closed Loop)

Post by Neuner »

Still doing it but not to the extreme that it was. I think the higher pressure was making it react faster and harder but it still acts this way a little more than 50% of the time when at idle.

Everything was smooth but then started acting up again. Accelerating and cruising down the highway at 65 is easy with no hiccups. Idle and initial take off from a stop are sometimes rough. The longer it idles, the worse it gets.

Thinking either IAC or MAP sensor at this point?
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Re: Surging, No Power Warmed Up (Closed Loop)

Post by SilverXJ »

Have you verified that the pressure is holding at 49 psi?

The MAP is easy enough to test. I suggest starting there. As well as testing the TPS. Before replacing the IAC give it a cleaning.
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Re: Surging, No Power Warmed Up (Closed Loop)

Post by Neuner »

The return line is too small and restrictive and it's building pressure back up over time.

The correct way would be to drop the tank and swap in a new larger return line but my family has run out of patience for me to spend anymore time in the garage. I have a whole dropping of the tank and redoing everything major project planned for next winter. Trying to see if there is a temporary solution until them.

I've read of some diesel engines, swaps and upgrades installing a fuel return system with a "T". I'd install a new larger line going to the "T". One branch would then go to the tank and the other go to the fuel pump inlet. I'd have to include for a bubble/air trap but wondering if that would work for now? Attached an image of what I was thinking and found.
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Re: Surging, No Power Warmed Up (Closed Loop)

Post by jeepxj3 »

If your return is to the top of the tank and not the bottom like the supply line, your fuel pump will just suck air from the top of your tank.
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Re: Surging, No Power Warmed Up (Closed Loop)

Post by Neuner »

Understood and a good point. Was thinking about using another fuel regulator/filter for the split in the return line. I haven't checked one of them to see if it works like a check valve and preventing reverse flow?
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Re: Surging, No Power Warmed Up (Closed Loop)

Post by Neuner »

Not confident that the smaller line was the problem I got a different pressure tester that didn't leak. The pressure stays constant at 52 psi whether revving or idling for quite some time but the engine still acts up. It only acts up at idle so I'm going to replace the IAC.
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Neuner
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Re: Surging, No Power Warmed Up (Closed Loop)

Post by Neuner »

Replaced IAC and it didn't fix it. Old may have been working fine but it looked bad even after cleaning so good to replace it anyways.

So it only stutters/surges while idling and warmed up. Performs great accelerating quickly or slowly and cruising. Has to be something in the Idle Closed Loop that is causing it. MAP?
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Re: Surging, No Power Warmed Up (Closed Loop)

Post by Neuner »

MAP tests out fine. At a loss.

I've searched all around and found just a couple of others that had the same problem. All sensors were new or checked out but it appears the PCM was bad. That was the last they were going to replace or change and they never reported back. Is there anyway to test if the PCM is bad?

Getting the pressure down helped the most. Although I'm using a rated 49psi fuel filter regulator from a WJ, I believe I should try and get it down more. Can't find one that notes in the specs a lower psi rating to try out. Read that some Ford Rangers use 39psi?
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Re: Surging, No Power Warmed Up (Closed Loop)

Post by jeepxj3 »

I would maybe try some new NTK up and down stream O2 sensors. They control closed loop.
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