89 NF V6 3.0 Running rough when warm

[iggy]

G'day, I've just bought a 1/89 NF station wagon for (AU)$1500. It has 437,000km on the chassis and about 40,000km on the engine, new tyres, battery, timing belt, distributor, leads, plugs. I have changed the oil and filter, fuel filter, coil, power transistor, capacitor and thermostat.

Whether cold or hot it starts immediately. When cold it runs very smoothly and accelerates well with no funny noises etc. Once it warms up a little (like after 8km) it starts running very rough, missing and coughing if you try to accelerate. I've connected a light to the diagnostic output of the ECU and it says there is nothing wrong.

The ticking of the injectors on the drivers side seems more noticable when it's running rough. I've tested the throttle position sensor,idle switch and the resistance of the injectors is in spec.

Faultfinding this is driving me crazy! Any suggestions would be appreciated.

Ingmar

[Begbie]

Can be a symptom of cracked head - might be worth doing a chem test to rule this out?

[-Scott-]

Engine temp feedback? If the ECU continually thinks the engine is cold it will make the mixture rich? Does your temperature gauge work? Or the O2 sensor is stuffed, so the ECU is receiving the wrong feedback?

Cheers,

Scott

[iggy]

Hi Scott,

The coolant sensor is fine, I even put a 2k7 resistor across the terminal to make the ECU think it was set to 20C. This kept the idle high (as expected) and it ran like a dream until it warmed a bit. The temperature gauge never gets more than about 5mm from cold with an 85c thermostat in it. Also, my oil pressure sits just over 2bar (?). I pulled the O2 sensor off and it's still the same. I'll measure the voltage coming from it tonight. I also called the service manager at the local Mitsu dealership. Told him all the things I had checked. I still have a feeling it's an injector on the drivers side but I guess I'm trying to avoid paying someone else $400+ to service them ! I'm very good with electrons but a fair amateur at serious mechanicery !

Ingmar

[iggy]

Woo Hoo !!! I fixed it.. On Wednesday it started running particularly rough so I pulled up on the side of the road and on a hunch pulled the distributor cap and rotor off (both are new), under these is a cover plate which when removed exposes the optical encoder wheel and sensors. The very fine slots in the encoder wheel were partially blocked by fine dust. I used electronic cleaning solvent, cleaned up everything and voila I have an extremely cheap, and brilliantly smoothly running V6 ! My guess is that as the optical sensor heated up it's sensitivity decreased and the dust was just thick enough to cause misfiring etc.

The bouncing tacho was the clue that gave it away, something the Mitsu dealer completely ignored whilst he was telling me it was serious and deep in the belly of the beast.

Thanks to those that offered suggestions.

Ingmar