2004 Rodeo with 3.5L DI CEL ALERT

[Rmasters3]

My 2004 3.5L Direct Injection was showing an intermittant CEL, codes were P0116, P0125, P0300, and P01287. Truck has 50K on it. The basic code shown was "ECT Temp out of Range". Replaced the Engine Coolant Temperature sensor, it's connector and engine thermostat, to no avail.

Took it to my Isuzu dealer who had seen this before. Apparently this DI engine is prone to carboning up on the intake and valves (the Trooper 3.5 doesn't do this). His fix cost me $450 and included a $16 PCV valve (mine was recently replaced and a $30 can of injector cleaner (this stuff better be miraculous for $30 a can). Otherwise the intake manifold was removed valves cleaned, etc.

My wife drives this to work, maybe 15-20 minutes each way and around town so the carbon build up was, at least in part, due to this.

I asked what can be done to prevent this and he said "Not much" although a can of fuel injector cleaner regularly wouldn't hurt. We also plan to get it out on the highway more often.

My preventative fix also will now include having a local garage use his engine cleaner service (about $60) periodically.

So, if your 2004 with the 3.5 is idling a bit rough and showing an on & off CEL with odd codes, be aware.

[Rmasters3]

After two trips to the same Isuzu dealer and two de-carbon clean outs, I still get a CEL. After the second trip the dealers called Isuzu (he tells me) and they advised this sometimes happens and to repeat the de-carbon clean-up. Within 50 miles the CEL is back.

Anyone got some insight or experience wiht this problem?

[Cat Fuzz]

I don't know much about the 3.5L DI but I just can't comprehend how a certain engine would be more prone the carbon build-up than any other engine. Especially when someone says that the Trooper 3.5L DI engine doesn't do this. They are exactly the same engine.

The p0116 code definition is "Engine Coolant Temperature Circuit Range/ Performance" and the p0125 is related to that being, "Insufficient Coolant Temperature For Closed Loop Fuel Control".

This tells me that the PCM isn't seeing that the engine is warming up to operating temperature. When this happens, the PCM uses default values to run the engine and these values are not the most efficient. The result is that you're burning more fuel than you should be and this could be causing the carbon build-up. Since you already replaced the ECTS and connector the next thing I would be looking at is replacing the thermostat. If it's stuck wide open, the engine will have a very hard time warming up properly and the PCM will not go into closed loop.

The p0300 is random or multiple cylinder misfire. This may or may not be related to the previously mentioned codes. You might simply need a new set of spark plugs.

The p1287 is a manufacturer specific code. I can't seem to find a definition for that one.

[Rmasters3]

Thanks for the reply. I sort of suspected this might be a bass/ackward issue where the carboning was being caused by not reaching proper operating temperature. When I first drove it into the dealer service, they talked like this was a problem they had previously encountered. If they called Isuzu as they said on the second visit (does Isuzu have service support any more in the US?), Isuzu backed them up in their diagnosis, or so they told me. I have also heard these engines are somewhat different from those in the Trooper and that carboning up was an issue.

I did have the thermostat replaced. It was the second thing tried after a new ECT sensor and before I went to the dealer. One thing I have not done is check to see what spark plug is in it. I bought it with about 22K on it and assumed the factory original Iridiums would not have been changed, but I may have to give that a look. What plug do you recommend?

[Cat Fuzz]

I do believe that these engines need a very specific plug. I don't usually get picky about spark plugs except in this case.

I do believe that the factory plug is a Nippondenso Iridium. If that's the case, those specific plugs must be in there or it will cause problems. Something about "Ion Sensing" which I'm not familiar with.

A scan tool would be very handy is this situation. It could tell you what temperature the PCM is seeing and then you could get a close actual measurement by using an IR thermometer and scanning near where the ECTS is placed. If there is a huge difference in the scan tool reading and the IR thermometer reading, something is up with the sensor circuit.