Me, well I got a 1990 4.0 liter xj, Renix injection. my coolant temp sensor reads only 180F per my scan tool, while my dash gage reads 210 F. This is with my rig at idle immediatly following a run being warmed up cruising at 65 to 70 on freeway, 70F air temp, sunny day, even after 20 miles of driving.
The sensor showing only 180F when my jeep is warmed up is what I suspect is causing my ecu to ignore the Oxygen sensor and stay in open loop mode as per my scan tool, the O2 sensor value also indicates it is not being used for control, and thus in open loop mode all the time , as I understand the renix ecu only goes into closed loop, Oxygen sensor control mode if warmed up, which I assume means at least 195F as that is the stock and my thermostat setting.

So do you reckon my ecu coolant sensor is going bad, giving false low readings?

I have read up on subject, but have not yet found at what temperature of the coolant sensor does the ecu allow closed loop control, can someone tell me please? At what temperature is closed loop operation allowed?


Thanks!

 

Sounds plausible. Check that it's making good contact with the block.

 

My 90 Renix goes into closed loop almost immediately after being started. My coolant temp sensor runs usually at 187 or while my temp gauge shows 178 or so. Could be just a bad o2 sensor.
My o2 sensor heater voltage was 4.5 with no variance. When I changed the o2 sensor it went into closed loop as soon as the temp gauge moved. No further problems.

I just now saw your other thread on this. My VAC is low also (7.5) but the engine runs and idles well.
What should the vac reading be at idle?

 

My MAP voltage is 1.7 at idle
MAP is 42 in kPa, Man. Vac. is 61 kPa, Baro Press is 102 KPa

that 7.5 value, is that inches Hg? that is too low for idle vacuum if so.


Got the new coolant temp sensor installed. also tested both new and old sensor in boiling water.

new sensor shows 199F, old sensor 180F when dash gage is at 200F

New sensor reads 180 ohms, old sensor shows 198 ohms in boiling water, specification is 185 ohms at 212F

Old sensor output was near 20F reading too low.

Test drive shows me going into open loop mode now! New sensor did the trick.

 

Nice! Closd loop should improve your outlook on life considerably
Yeah, 7.5 inches of Mercury is what it's reading for vacuum. I'll have to do some poking around I guess. I'm thinking 16-18 at idle would be reasonable.

 

My idle eep at 61kPa is 18 in.Hg

I had a huge bad vacuum leak, my egr valve seat came loose, kept the egr wide open all the time. i put a vacuum gage on manifold, vacuum was bad like yours, sniffed for leaks with aerosol carb cleaner, tried isolating the brake booster, the vacuum hoses, no luck, but the leak was huge! then borrowed a smoke machine and filled the intake with smoke. It poured out the tailpipe! Golly is a pair of valves overlapped open? no biggie, stuff some rags in the tailpipe to build up backpressure, and I search for smoke, no smoke, pull a vacuum hose, smoke pours out. i keep looking, use a light, look under the manifold, and then I see it, a faint wisp of smoke, there it is again, coming from the valve stem of the EGR! I remove EGR and find its steel valve seat had come out of the aluminum EGR body. the EGR was always dumping rather huge amounts of exhaust into my motor, all the time. No wonder my vacuum was so poor.

if your vacuum is bad, your fuel mixture will be all screwed up, as the engine brain uses the manifold vacuum sensor as a primary control signal.

good luck

 

I have a friend that has a smoke tester. I'll have to buy him some beer and see what we find.Thanks for the info.

 

My friends smoke machine is inop currently so I pulled the Egr and cleaned the passages. The diagphram moves when you apply suction but had a odd very slight hiss before it went solid.
i cleaned up the gasket surfaces and reinstalled the egr valve with a sheet metal blocker plate.
My readjusted throttle body blade produced a 810 idle rpm in park/neutral. It was 710ish slightly hunting in drive.
With the blocker plate installed warm idle in park/neutral is 1000 and 750 in drive. Vacuum on the Rem Ii is now 18.3 "HG of mercury. The cruise control previously had a surging issue below 40 mph that appears to be gone as well.
The blood gods were also appeased!

 

Good job BTW!!!! On the Jeep.

 

Oh, and here's something for you guys.

 

And a few others in case you need them.