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Rough idle driving me nuts!

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I am at my wits end with my mj!  I have this rough idle/rich condition that i cannot get rid of.  To go with this, am getting 12 mpg.  When i first start the truck and try to take off, it just wants to die and spits and sputters at low rpms.  High rpms seem to run alright, but not quite the amount of power that i feel it should have either.  When it gets up to operating temp, it idles better, but still noticeably rough, and i can feel it jiggling the truck.  


Here is a list of what i have changed in the past 3 months:

IAC, TPS, Map sensor, coolant temp sensor, crank position sensor, oxygen sensor, Vacuum harnesses, Fuel pump ballast resistor, fuel pump, ground for fuel pump, fuel filter, spark plugs, intake gasket.

I am just totally puzzled and don't know where to look next.  The motor is a remanufactured engine with roughly 20k miles on it(the guy before me installed it though).  I have checked fuel pressure and i have 31 psi, and when i unhook vacuum line from regulator there is 39 psi.


I don't know what to replace next.  If anyone has anything that could lead me in the right direction that would be great.  Thanks in advance!


P.S. my truck is a 1988 Renix.

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  • 1 year later...

Since you have already gone through most of the likely sensor culprits, i second the egr system as likely. When either the egr solenoid or egr are not engaging, you feel engine hesitation or even stall when you press the gas pedal. When your trying to drive calmly in the grocery store parking lot it sounds like your contantly reving the engine and never a nice sooth idle as you cruise. May stall when you try to accelerate hard. I. Not sure about the poor hiway mileage.


Its easy to trouble shoot under the hood. If you look carefully you should see the egr valve engage when you quickly throttle the engine. If you don't see this then check the egr solenoid by disconnecting the output tube and putting your thumb over the nipple. You should feel the solenoind suck on your thumb when you throttle and should probablyfeel the solenoid clicking when you thrittle hard. If not clean the egr solenoid with some wd40. These egr solenoudshardly ever fail. Just get dirty. On some years like 1989 the egr valve has a tranducer valve at the input to the egr. These often fail. Unfortunately you have to buy the whole egr valve to get it. On the 87 it doesnt have this problem.


Actually since its already running rough at idle when you first start up, and it gets better when warm its probably not the egr. Check it anyway. Map sensor or vacuum line leak going to sensor would be my first bet.


The jiggling at idle also makes me think it could be ignition or timing related. Coil, distributor, cables plugs. Also, the firing is suposed to retard and advance based on rpm. This is checked with a strobe light on the crrack shaft pully i believe. Ive done it but i forget. Some degrees before or after top dead center. This is an area where a rebuild can be done wrong. The timing chain and alignment can't be off.

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When warming up it is common for rough idling since the O2 sensor is not warmed up yet. The O2 sensor is in open loop mode. When the sensor is warm enough the ECU then goes into closed loop mode fluctuating the air to fuel ratio. The renix does not do a great job at this so the engine will kind of "hunt" for a stable idle no matter what... But I also did about everything in my power to get rid of this. If you click on the link in my signature youll find my solutions, tips, and see what helped me.

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I here what your saying. I was interpreting rough at idle more as misfiring. A little rough idle when the engine is cold is business as usual. I will definitely check out your info. Thanks!


I kind of like that rough idle sound first thing in the morning while i sip on my coffee.. Reminds me that I'm not drivin a hownda...


I get about 17 mpg on the highway, and 19mpg when I'm lieing.

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