I have just finnished doing a engine rebuild on my 1988 XJ6,put hole in # 2 piston due to pluged feul filter,MY NEGLECT !! Now on cold start up she misses for about 30 secons and then runs perfect when hot. also the imfamous oil preasure hitting the red of and on at idle,new sender and also modified oil pump when out, Kiwi

 

Hi Kiwi,

A few possibilities come to mind with your rough idle at cold startup:

Engine Coolant Temperature Sensor (ECT) - if the sensor is providing an incorrect signal or there is a problem with its electrical connector or harness, the Engine Control Module (ECM) may not know it needs to apply cold start fuel enrichment, so you might be running lean when cold.

If your carbon canister purge valve is stuck open (if fitted), that could also contribute to a lean condition when cold.

A problem with the idle air control valve (IACV) or its stepper motor could be the cause.

If the fuel pressure regulator (FPR) and/or fuel check valve are allowing the pressure in the fuel rail to leak down when the engine is not running, it may be taking 30 seconds for the fuel pump to overcome the leaking FPR to keep a steady enough supply of fuel in the rail. Also, if the diapragm in the FPR has failed, raw, unmetered fuel is inhaled into the intake manifold, causing a slightly rich running condition. If you disconnect the vacuum hose from the FPR and find wet fuel, the diaphragm has failed.

I think I recall that the ECM ignores the O2 sensor for 20 or 30 seconds until the O2S has reached operating temperature. During that startup time, it uses programmed maps. If that is correct, then we may be able to assume your problem is not related to a stuck or lazy O2S, since it should not be in circuit while your problems are occurring.

As far as the oil pressure sender goes, the circuit relies on a good ground connection between the body of the sender and the engine block, so any oil or anti-sieze compound will cause problems, and the engine ground strap and other engine bay grounds all need to be nice and clean.

Hopefully some of our other members will offer some ideas.

Cheers,

Don

 

Thx for your help Don,I will follow your quot step by step and give you a finnal report,Kiwi