3.9 v8i Rough Idle When Hot

I am having some trouble with my v8i - it starts beautifully when it is cold, but after the engine warms up it starts to idle rough, and if I stop the engine then restart it while it still hot it will start OK but then the revs will drop right off, and sometimes it will conk out.

A few weeks ago I replaced the head gaskets (new inlet manifold gasket as well). After the rebuild the engine has run fine except for this rough idle problem. I had moved the distributor in the rebuild, so I adjusted the timing as per the manual (with AC off, vacuum advance hose off and plugged).

To avoid conking out at the lights I have temporarily increased the idle speed by moving the thumbscrew on the throttle cable, but I don't want to leave it like this.

Any suggestions on where I should start to sort this out?

Have you checked whats happening at the exhaust end , i.e. is it sooty eg very rich when it cuts out , as its most likely a mixture problem , have you checked the pipe that goes into the manifold from the engine breather?

Check coolant thermistor.

simple as you have removed and refitted the inlet manifold things have changed you will need to reset the base idle.
heres how to do it.

disconnect the battery for half and hour to clear faults.
replace the injection temp sensor as a precaution
ensure the air flow meter plug is clean and fitted good
remove the stepper motor and clean the crud off and the housing inside (do not twist the plunger as it will break!)
connect an external tacho and reconnect the battery start and allow the engine to reach full temp ensure there is no air con on or and other electrical load on!
use a brake pipe clamp and clamp the hose to the stepper motor housing at the rear of the plenum.

the idle will fall and should settle to 520 rpm =/- 20 rpm if not use the allen key bolt in the top of the plenum turn in to lower and turn out to increase idle.
once set remove the clamp and allow the idle to settle to around 720+/- 50 rpm then road test

vougese39 said:

simple as you have removed and refitted the inlet manifold things have changed you will need to reset the base idle.
heres how to do it.

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Thanks VogueSE39....I take it that this is not the first time you have seen this problem?

I was starting to think that I'd need to take it to a garage, but I'll give this a try first.

tacr2man said:

Have you checked whats happening at the exhaust end , i.e. is it sooty eg very rich when it cuts out , as its most likely a mixture problem , have you checked the pipe that goes into the manifold from the engine breather?

Click to expand...

I have eye-balled the breather pipe it seems intact, but I will pull it off today and see if it is blocked.

thanks.

classic kev said:

Check coolant thermistor.

Click to expand...

Its connected, but maybe I have a bad connection - will take a look.

this is the most basic setting on the rover v8 with 14cux injection system and the most over looked does need checking once every few years and after major work on the engine

ozlandy said:

Its connected, but maybe I have a bad connection - will take a look.

Click to expand...

If you followed the instructions for base idle you would have replaced this, but to check/test you need to measure the resistance when cold, when warm and when properly hot then check the results are correct. Values can be found in Haynes book of lies.