Question about 1990 Isuzu Trooper

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Isuzu Trooper, 1990, 2.8L - 6V, Manual Transmisson. Milage 140.000 miles.  Problem: Engine starts when cold and climb to high rpm and gradually goes down to 800 rpm as it should. When it?s hot stalls on idle and than it?s hard to start. Can?t hold idle. Immediately stops.. Will not start when is hot. I have replaced head gaskets, inlet manifold gaskets and all exhaust gaskets. Also new thermostat. New spark plugs. New parts in ignition distributor. (3000 miles ago). Fuel pump gives right pressure 11 PSI (Manual requires 9 - 13 PSI). Fuel filter clean. PCV valve and EGR are working properly. Vacuum lines seems to be OK. Oil engine and filter recently changed. Valves are not burnt - nice and clean. I?m getting code 51 - Make sure MEM-CAL or PROM is properly installed in the ECM or replace them. How to fix ECM problem? Can I get good used ECM and where and for how much? What should be resistance (Ohms) of Coolant Temp Sensor at 25 Celsius and at 100 Celsius? Maybe it?s faulty? Please advise what to do? Thanks, Mafet, mafetar@yahoo.com

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About the only items I see you haven't checked which could cause this condition according to my books is the O2 sensor, Map sensor and Throttle position sensor. Occasionally the Crank position sensor will throw a bad code at you but not often. I would check or replace the O2 sensor, AND check all the wiring harnesses for a short which is most likely causing your problem. A used ECM from a Junk Yard will run from $400 to $600 in most cases depending on the make model and year and how many where built. A new one in contrast will cost twice that. The only reference I can give you for testing any of these and getting the resistance reading you asked about is the Haynes Repair manual on your vehicle.

Posted on Aug 03, 2010

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  • Mafetar55 Aug 05, 2010

    Thank you for answering. The problem is that I am getting only code 51 (check MEM-CAL and PROM), no other code was ever set.. It looks like sensors are OK. How to check O2 sensor? Where it is located? I can't find O2 sensor. It's not located on exhaust manifolds. On right manifold I've got only some kind of valve. I can not replace all sensors. It;s too costly solution.
    Where am I suppose to check harnesses for short?? What area? Any other solution available? Regards, Mark

  • James Young Aug 05, 2010

    The O2 sensor is normally located on the exhaust manifold right side but occasionally it will be on the exhaust piping just past the catylatic convert usually on the drivers side underneath the vehicle. Again the Haynes Repair manual on your vehicle will have the specifications for testing it, since this would vary by vehicle.

  • Mafetar55 Aug 06, 2010

    OK, thank you very much. I will try to to replace O2 sensor. I hope it will help. What indicates that this sensor is bad? How practically check TPS sensor if it's good or bad? I can not find any technical specification regarding sensors in Haynes Manual. They don't give any technical parameters for sensors used on Isuzu. Can I find them on Internet? Maybe you know the links? Where can I get these information from? Still I don't know what means Code 51: "Make sure MEM-CAL or PROM is properly installed in the ECM or replace them" - how practically approach this problem? Can I do it by myself? What this problem may cause? I've called few diagnostic garages and they couldn't answer me. Also they are helpless in case if I bring them my ECM. Can't check or repair. Where should I call with this problem and what could be setting this code? It sets each time after I reset ECM by disconnecting battery. No other codes are ever set. Thank you in advance, Mark

  • James Young Aug 06, 2010

    If your resetting the ECM and you have good connections at the plugs and no shorts on the wires going to the ECM your going to end up replacing the ECM. Those ECM's, once you do get them open, are actually packed in a glue gel to protect them against impacts and shorts from the outer case being crushed down on top of them. Unless you have a heat station that can melt that away you are not going to be able to get to the actual board and as such will not be able to get at the Eproms and such. That was the case when I last took one apart I had here at the house. In most cases when it throws that Loose EPROM code it's due to a short or bad ECM or combination of the two. Occasionally the O2, MAP or EGR sensors will throw a false code. The code you are seeing is a generic code that shows when the system detects a fault but isn't able to identify it. I always get sensor testing info from my Haynes Manual. It's in Section 6 in the 3 manuals I have here by me. Although it maybe listed in the wiring diagram section near the index.

  • James Young Aug 06, 2010

    I forgot to answer one of your questions in your previous reply. As far as having some one do the repair on the ECM goes I wouldn't hold my breath. Out of all the repair and service spots here in my area, about 100 of them dealing with ECM's, only 1 will even attempt to do an ECM repair and then he will only try it on Chrysler and Toyota models. He won't even look at any other make. It's pretty tough to get through that goo. Now I was thinking about your question about the testing voltages. I have a shop I go to for certain jobs. They are the best shop in town as far as I'm concerned, They have been known to let me have voltage testing schematics on occasion. No major diagnostic shop will do this for you. But for a little time to get to know them and maybe a Six pack you might be able to find a small shop somewhere that might do that for you. They have special software that provides this information to use in Manual Diagnostics. Again it's going to be one of the smaller shops off the main area of town most likely.

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