V6 VN Series 2 - Cold starts fine, after 3 mins it runs rough and erratic, CEL flickers

[WeeGee]

Don't quite know what's going on but a couple weeks ago cold started engine and only drove about 2 mins up the street and at the lights the idle surged up to 1300 +/- 200 all by itself. However, being a manual I was able to drive it in slowly for another min until it went away by itself. For the next week no more problems. I just thought the throttle cable was a bit sticky.

Then, this week similar thing happened but this time much worse. After 3 mins the idle was very rough and the CEL was flickering very fast while the engine surged and then almost died over and over and then had practically no power and so on...

Pulled over shut it off and ran the diagnostic check and got the "1-2" all good result. 15 mins later, it ran normally (briefly). I then thought it was gone however it suddenly came back - could barely drive as any attempt to get into second gear was rejected with erratic power surges (exactly like symptoms of running out of fuel - yet tank is 90% full). It did idle again but fast while I was trying to drive in first. Eventually after about 3 more mins of this I pulled into a nearby carpark and the problem seemed to go away as quickly as it came. Again running the ASDL diagnostic test came up all clear. Then engine then seemed to run normally even though I wasn't driving anywhere.

The next day I pulled the leads plugs off the 3 coils (note: its the Series 2 type) and measured the secondary windings, the result was good as all 3 pairs were: 5.5k, 5.5k & 5.7k Ohms (this is fully within specs). Also Leads are Bosch personally fitted about 2 years ago (done only 10,000km since). Plugs are the Bosch platinum of same age. In 30 years I have several leads go bad at over 40K however never like this dramatic. I guess I have to rule out plugs and leads.

So I tried driving again the next day from cold and it happened again like the previous time.

It seems to me to be either:
1) Bad ECU and/or
2) Bad DFI module and/or Coil pack.
3) (Potentially) a bad O2 sensor (car is 1991 and 250,000Km on factory original O2 sensor)

Has anyone had this surging higher than normal idle with limited or no power and flickering CEL and basically "running out of fuel" symptoms?

Any ideas what else to check or replace...?

 

[lowandslow]

When my o2 sensor(s) died on my LS1 it would do exactly as you described.

 

[WeeGee]

But shouldn't a bad O2 sensor throw a DTC code? I want to believe its something that is heat related as it only happens once its warming up somewhat from cold (after 2-3 mins).

 

[lowandslow]

Mine threw a code only sometimes. 2-3 minutes is probably long enough for it to enter open loop and one or both sensors is out of range or lazy.

 

[WeeGee]

OK Will try to swap it (there is only 1 on the right side bank) with an NGK or Delphi brand replacement. They are only about $50

 

[WeeGee]

Problem solved with a replacment ECU. The problems were isolated to the ECU after the diagnostic link ALDL pins A & B were shorted and tge engine run. Rather than flashing the CEL light at 2.5Hz during warm up, it flashed even while the motor was off. It was even attempting to switch the radiator cooling fan in sync with the CEL light. This confirmed that it was not O2 sensor or oil switch or anything else.

 

[losh1971]

When I use the paperclip my thermo fan switches on during diagnostic mode. Does this mean I have a faulty PCM??

 

[delcowizzid]

It' meant to turn on

 

[losh1971]

It' meant to turn on

phew i was hoping this was the case. When I read the OPs post it had me second guessing.