robsjeep

Hi I have a 1990 4.0 liter Renix and am having issues that are perplexing. I have a snap on MT2500 scan tool hooked up on the diagnositc port.

Sometimes the jeep lacks power, other times it is ok, sometimes the idle is rough, surging to maybe 1500 rpm then back down, this happens randomly, other times she idles fine, regardless of engine temperature.

I have a fairly recently installed O2 sensor that I used to pass smog last May.

Today I hooked up the scan tool. I Allowed the jeep to warm up, and this is what I get at idle...

800 rpm, Injector pulse width 6.3 msec, O2 volts is steady at 2.11 to 2.13 V. The system is indicated to be in "closed loop", and the scanner mixture cycles between "lean" and "rich" about once per second. HOWEVER WHY IS IT CYCLING BETWEEN LEAN AND RICH IF THE O2 VOLTAGE IS PRETTY MUCH CONSTANT?
the scanner also shows... ST (short term ) trim to be 201 to 206, LT (long term) Trim to be steady at 128, MAP sensor at 1.6 Volt, MAP Press is 12.1 Hg
Barometric pressure is 18.2 Hg, TPS is at 0.68 Volt, throttle is 14%. Throttle switch is "closed", Fuel sync signal cycles about once per second between + and -, Coolant is 168F, charge temp is 122F, spark is 14 degrees Before TDC, EGR commamd is off

Now when I rev to 2100 rpm and hold it there the O2 voltage goes to 2.42 volts, the mixture goes to lean, and it indicates open loop operation

I then drop back to idle (812 rpm) and now the O2 voltage is 2.44 with no cycling, system indicates it is now in closed loop and indicates "lean /rich" cycling once per second, the injector pulse width changed to 5.6 msec.

So what is happening?

Why is it in closed loop and cycling lean to rich when the O2 voltage is pretty much steady?

Also isnt the idle a bit high, If so how to adjust?

at anyrate, whatmis going on here?

Is it normal for throttle to be at 14% when at idle?

I am totally confused.

ps last week when I stuck the scan tool on the scanner read that the system stayed in open loop, and mixture remained "lean", so something is different now than then. during that time I replaced the water pump do to a leak.

Thanks!

robsjeep

ps the factory manual says that the thottle position switch volts is approx 1 volt at minimum opening (idle) and a max of 5 volts at wide open, yet I get 0.68 at idle. is this an issue

lawsoncl

That .68 looks about right, 14% is spec I think.

cruiser54

My website. www.cruiser54.com

robsjeep

yes, already posted on your site (black1990jeep)

I did the dip stick ground repair a while ago, added a big ground cable to chassis from battery, cleaned relay contacts on the panel near battery. I think I may need to go and ohm all the splices next.

The wiring is fairly convoluted on the jeep, some signal go thru mutlible connectors, the O2 relay also triggers fuel pump ballast bypass, and such, a very strange way of wiring for sure. Not what I am used too i.e. 1970s D-Jet or 1980s Digifant injection systems. Oh and gawd awful connectors with brittle locking tabs! I am beginning to hate modern cars, and mine is a 1990! hahaha. My old 1960s VW Bug, heck I can draw the entire electrical system by memory on a small piece of paper, simple, logical.

robsjeep

where is the ecu ground point, which pin in which connector supplies the ecu ground?