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is it my computer or engine?

onelow

NAXJA Forum User
Location
oregon
Okay well as I said before I have a wierd low idle lope and a shuder well driving. I have checked almost all sensors. My question is there a way to seperate the electronics from engine (such as what it just takes to run spark fuel combustion.) of the truck (87 mj 4.0l 5spd.)? I know you can with other rigs but this jeep thing is killing me to understand like no check light and what not so there is no way to check the computer correct?
 
I didn't read your other post, but how old is the Cap/rotor/plugs/wires and the fuel filter
 
The O2 sensor, TPS, CTS and IAC all can be disconnected while the engine is running. If the idle stablizes it is the TPS or O2 sensor that is bad. It will not accelerate properly with the TPS sensor disconnected, but you can very slowly raise the rpms with the TPS disconnected. If you push the acelerator rapidly with the TPS disconnected it will run poorly and even back fire.

It will also run and start fine with out the CTS connected. (ask me how I know). The IAT sensor and the MAP sensor are a must. It will not start with out the CPS, much less run.

onelow said:
Okay well as I said before I have a wierd low idle lope and a shuder well driving. I have checked almost all sensors. My question is there a way to seperate the electronics from engine (such as what it just takes to run spark fuel combustion.) of the truck (87 mj 4.0l 5spd.)? I know you can with other rigs but this jeep thing is killing me to understand like no check light and what not so there is no way to check the computer correct?
 
I would like to think cps everything else checks out fine, but 02 couldnt get a clear read out on it was kinda bouncy. tried every thing else. plugs are also rich? could this be the start of where to look? cap plugs rotor ect... just changed ragular tune up. but i know some one else had a problem close to this and it was the coil?
 
Ecomike said:
. It will not start with out the CPS, much less run.
If you mean CPS disconnected, I agree 100 percent. If you mean CPS defective, not.
I had two different CPS scenarios, both allowed starting and run but were bad.
One would barely run at all, (way out of time?), and the other was a consistant stumble at 2800 rpm which was otherwise reliable for over a month that way.

This may not be the problem here but I respectfully disagree from my own expererience EcoMike..
I also had and EGR problem that killed response from a stop sign, red light. It would stall very quickly.
MAP would also be a good suggestion as stated.
 
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onelow said:
I would like to think cps everything else checks out fine, but 02 couldnt get a clear read out on it was kinda bouncy. tried every thing else. plugs are also rich? could this be the start of where to look? cap plugs rotor ect... just changed ragular tune up. but i know some one else had a problem close to this and it was the coil?

O2 sensor output test and the noise test on a TPS need to be done with an analog meter to see the cyclic movement. Digital MMs just don't cut for those tests. The O2 sensor should swing back and forth quickly from about .5 volts to about 4.5 volts. At higher RPMs it should tighten up to more like 2 to 3 volts crossing the 2.5 volt mark at least once a second if not fatser. Noise test on the TPS is done my slowly moving the TPS from idle to WOT while watching the analog needle for any rapid volatge swings, which if you find any indicates a failing TPS.

My only comment on the CPS was that it wil not run or start without a working, or partially working CPS attached. He had asked what he could disconnect that was not critical to the running and idling of the engine for diagnostic purposes. The CPS can not be disconnect for diagnostic purposes (engine running diagnostic purposes).

Not sure I understand your disagreeing on EGR. I never mentioned the EGR. But you are right, it should be considered a possible source of his problem. Easy to test, just pull and cap both ends of the vaccum line to it.
 
I had a weird low idle lope, which turned out to be loose intake manifold bolts. If you check the bolts, spray them down well beforehand, and be careful of the end studs can snap off easy. The rest of the bolts you also have to be careful with. There is often a fine line between too tight and way loose :). It wasn't leaking enough to significantly affect my idle (though it was just a touch high). I finally caught on when I replaced my plugs, some were rich, some weren't. My cylinders were firing with an assortment of fuel/air ratios.
As far as the shudder goes, a few possibilities.
Marginal CPS will often mess with your starts, cranks for a long time etc. How they function is often related to motor temperature. They can act much like a miss or even on occasion a disconnected MAP.
My tranny would shudder when the dealer put ATF in there (once).
Flex plates are know to come loose and/or crack. This almost has to mess with the CPS signal somewhat. And may cause a shudder.
 
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