Topic: Comanche won't accelerate; tachometer reads double actual engine RPM's

[RustBucket]

1986 Comanche 4x4 2.5L TBI, 187,000 mi. w/o AC; Renix engine management system (Renault-Bendix)
Upon first starting, it idles rough.  Once heated up, it smooths out some, but wants to stop.  Idle actuator motor keeps trying to adjust the idle speed to keep it going.  On the road, I can get it up to about 35 mph, but then it starts to chug, lope, buck and stall.  It usually won't accelerate beyond 40 mph.  I did get it up to 55 mph once -- the tach showed 4,000 RPM.  A mechanic tested the engine speed and concluded the tach is showing exactly twice what the RPM's actually are.  Could something be 180 degrees out of alignment?  He thinks it is either the crankcase sensor, the ECM or the ignition control unit.  The crankcase sensor puts out a square wave pulse ... it's either on or off, so I think if it were not working correctly, the engine would not run at all.  I installed a new ignition control unit and a new HEI coil.  Plug wires are new.  I changed all the vacuum hoses and the TBI gasket, which was leaking air.  The O2 sensor and coolant temperature sensors are new.  The throttle position sensor is adjusted to spec.  The MAP sensor seems to be okay, too.  It's putting out the correct voltage.  I believe I have eliminated all vacuum leaks, but when I drive it, it won't accelerate.  The EGR valve can be squeezed by hand, but with more effort than a new one requires.  Fuel delivery seems to run rich.  Fuel pressure seems adequate but probably should be tested.  Air filter is new.  Exhaust system should be providing EGR enough back pressure to operate it at higher RPM's.  I put in a new clutch and made sure I installed the resurfaced flywheel oriented the same way as I removed it
Is a sensor or the ECM telling the Tach to double it's RPM's?  Is the ECM also telling the new fuel injector to put out twice the amount of gas as if it were running at twice the RPM's than it actually is?  There is no check engine light on this truck.  I have not been able to get engine codes out of it.  If the ECM is bad, how can I test it?
Any thoughts???

[Mr_Random]

Have you tested the MAP across various pressures? That and the EGR system seem to be your most likely culprits for jumpiness/stalling. Now as for the tach, I have no clue??? There are some writeups on various forums (maybe here too) on how to remove the cluster and clean it.

OH WAIT... The ICU seems like a problem too, as it determines timing and gives the output for the tach (pretty sure). In any case, if you haven't replaced the CPS for a while it would be good to do so now, just to make sure everything is in good working order.

[RustBucket]

I have successfully fixed the problem !!!!!  It required two fixes.  I installed a new crankshaft position sensor for about $38 from Napa.  The old one was reading resistance that was too high, about 500 ohms when it should be around 200 +/-.  The result is that the idle smoothed out nicely.  The second fix was to replace an unlikely candidate:  the thermostat.  I noticed the engine coolant temperature never getting above 160 degrees.  The thermostat was stuck open.  The computer thought the low coolant temperature demanded more fuel, so at driving speed, the computer was telling the injector to put too much gas into the intake manifold.  It was running way too rich.  I installed a 195 degree thermostat with gasket ($5 from Autozone) and now my Jeep Comanche runs smoothly at all driving speeds.  Total cost - about $43 bucks to solve a two year old problem !!!  The one problem that has not been corrected is that the tachometer still reads exactly twice as many RPM's as the engine is actually producing ... but I can live with that for now.  It's nice to have my Jeep back.  Thanks, Mr_Random for encouraging me to replace the CPS ... great advice ... it solved the idle problem.
Thanks,
RustBucket

[Oilsmoke]

If you ECU thinks it running twice as fast in rpm would it add more fuel? This a question Becuase i am not sure