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Tach drop or bucking over bumps

rancor

NAXJA Forum User
Location
Colorado
So I replaced the cap and rotor on my 89 4.0, also beefed up the ground by the dipstick and ran a better vacuum line to the throttle body.

Since then she runs great, unless on a bumpy road. After a good bump the tach drops to zero then power returns.

It only happens on bumpy roads so it's hard to troubleshoot where I am. So far I checked the coil pack and all the plug wires, checked and reinstalled the rotor cap with no change. Any ideas? Summer is coming I need my jeep
 
Being a renix I'm not sure where to look next. Relays ratting lose, a wire in engine harness or computer are just wild guesses I'm sure the renix guys will have a better ideas.
My 85 had a slightly similar problem like this but it was ignition wire from the distributor to ignition coil, GM engine. The PO were electrical butchers and screwed up those wires, electrical tape and a few twists were all that held it ever so often a bump or turning with speed would jar it lose.



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your crank sensor is loose or the wire is melted and shorting on the exhaust. if the crank signal is lost the ecu stops firing spark and injectors. and the tack drops almost instantly.
 
your crank sensor is loose or the wire is melted and shorting on the exhaust. if the crank signal is lost the ecu stops firing spark and injectors. and the tack drops almost instantly.
X2
 
Thanks! It was bucking pretty bad across a few hundred feet of smooth dirt road the other day totally makes sense if dust has something to do with it.
 
dust normally wouldn't do anything to it. but even the smoothest of dirt roads have vibration. i found if you support the transmission and transfer case and remove the cross member you can slowly drop the back down and get a little more room to swap the sensor.
 
Could also be a loose or incomplete SENSOR ground. My truck wouldnt' run at all, new Mopar CPS until Cruiser told me to check sensor grounds downstream of the C101 (if you have) or in that cluster of wires.

What do you mean a "better" vac line to TB?
 
I'll check the cam sensor. I streched that cluster of wires to a different spot and relocated the vacuum canister before I noticed this so I'm going to check grounds and reinstall all the wires.

I mean i tapped the throttle body to the map sensor per cruiser54
 
I'll check the cam sensor. I streched that cluster of wires to a different spot and relocated the vacuum canister before I noticed this so I'm going to check grounds and reinstall all the wires.

I mean i tapped the throttle body to the map sensor per cruiser54

It's not a cam sensor on Renix. You can unplug it and still drive fine.

I would look at the CPS wiring and plug.

Also, do Tip3 and 4 at www.cruiser54.com.

As for MAP sensor hose, see this:
http://cruiser54.com/?p=280
 
So I went through and cleaned every connector and reinstalled with dielectric grease. Now I have a no start condition no spark to the coil fuel/starter is fine.

Something arced while I was doing the starter relay is there anything downwind from this that controls spark?

I'm going to put in a new cps tomorrow, the rubber on the wires is worn on both ends that has to be it. Is a napa cps acceptable?
 
I swapped in a new cps today and she purrs now fixed my phantom misfire too.

I never thought about that with the dielectric, I've never had any issues just filling the connectors with it. Searching around it seems to be a controversial issue.

Thanks for the help guys
 
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