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Title sounds like an adult movie but…..
2001 4.6.145,650 miles.
Brief recent maintenance history:
Changed the plugs, leads, oil, filter, cabin filters, air filter about 500 miles ago.
About 200 miles ago after starting when warm she starts fine, idles fine for about 5-10 seconds then idles rough (about 600-850 RPM) for about 15-25 seconds then smooths out and no issues at idle or driving. There are no engine warnings and the NANOCOM is not showing any faults. I swapped a brand new MAF out with my other restored P38 to no avail. I’m running the highest octane fuel and did a Seafoam additive at last service (145,000 miles).
She starts without this issue when stone cold….
Thoughts?

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Is this an NAS spec, or UK, or???
If you are here, where we have the wheel on the correct side (as opposed to the right side) then I would suspect
it is emissions related, like a sticky PCV valve perhaps?
My first go to was the Maf, but you beat me to it!
Have you reset adaptive values with Nano to see if it goes away?

I am sure Gilbert, when he wakes up, will have some insight as well as a dig at LHD vehicles and their owners!

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@Bolt…. NAS

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Too early for digs.... I'd suspect a sticky IACV as that is what controls the fuelling at idle. If sticking closed it will give a rich mixture until it warms up and unsticks itself.

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Gilbertd; I probably didn’t describe the issue correctly.
When I start the vehicle and it’s stone cold it has no issues, no rough idle …. All is normal. The roughness only occurs after I turn engine off, leave it off for anywhere more than 3 minutes, then restart it. Then she will idle normally for a few seconds, then idle rough for 15-25 seconds then smooth out again. After this she will operate fine with no roughness at any stage.
If I rev the vehicle a bit during this rough idle she will run smooth…. It’s the initial roughness after a warm start that’s the issue….
That being said, I will swap the IACV valves with my now fast becoming “donor” and see what’s what then report back.
Thanks

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So, being NAS, it has all of the emissions equipment....
I am wondering if the fuel vapor canister purge is the culprit as I believe it purges fumes for about 20 seconds, but not sure how often it does this, or in what part of the drive cycle.
As a simple test, leave the fuel cap loose and try it a few times. That will throw an check engine light, but ~may~ keep it from pumping additional flammable vapor into the intake since it will not be able to build pressure in the vent side.
Worth a shot?

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Oh, the purge valve sits under the expansion tank and has a 2 wire plug going to it.

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Bolt wrote:

Is this an NAS spec, or UK, or???
If you are here, where we have the wheel on the correct side (as opposed to the right side) then I would suspect

I am sure Gilbert, when he wakes up, will have some insight as well as a dig at LHD vehicles and their owners!

It all started with the Romans. Gotta keep that sword handy in the right hand !!

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As Pete says, we drive on the left so the right hand is free to use the sword on someone coming the other way. Unfortunately Napoleon was left handed, and didn't much care for us Brits, so he insisted that the French drove their horses on the right. Rather than keep swapping from one side of the road to the other when you crossed the border into another country, the rest had to follow France. Quite how the US ended up having the wrong hand free is anybody's guess.

This does beg another question though. What side does the groom stand on at a wedding in the US? Here, the bride will always be on her father's left arm as they walk down the aisle and she stands on the groom's left so in both cases the father and groom have their right arm free to use their sword to protect the bride. Is it the same in the US or has that been reversed too? (What makes you think I used to do wedding photography?)

Now back to the original question but the purge valve opening and flooding the manifold with petrol vapour does sound like a good possibility. If loosening the fuel filler cap doesn't cure it, try unplugging the electrical connection to the purge valve so it doesn't open and see if that makes a difference.

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Gilbertd wrote:

As Pete says, we drive on the left so the right hand is free to use the sword on someone coming the other way.

I've heard that somewhere, also heard it's because if your whip (right) hand is nearest the middle of a road you're less likely to whip someone at the side of the road. Makes sense if driving a manual car too as you keep your right hand on the steering wheel when changing gear.

On the main topic....Not sure about P38's but on most evap equipped vehicles evap purge valves are usually pulsed (rather than being just held open for long periods) and the PWM of the valve wouldn't be enough to upset fuelling (false air to lean / petrol vapour to rich) enough to cause a problem even if there were a problem with it, except for really picky systems like Jag engine'd L322's and certain Volvo's. But if the evap system is in doubt you should be able to confirm/disprove an issue by blocking the pipe to the manifold.

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She finally threw codes…

P0413 Secondary Air Injection
P0300 Multiple Misfires
P0308 Cylinder 8 Misfire Detected
P0304 Cylinder 4 Misfire Detected

Thoughts?

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Misfires will be as a result of running rough, so the only one that points to the actual fault will be the SAI code. It decodes to P0413 OBD-II Trouble Code: Secondary Air Injection System Switching Valve A Circuit Open so that is where you need to be looking.

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Thanks Gilbert……

I’ve ordered a Secondary Air Injection Solenoid and will report back…
Cheers

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From what I've found online, it could be down to a dead SAI pump but I don't really know enough about it as it is an NAS only feature.

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I’ll also swap the SAI pump out with my “donor” at the same time….

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From what I've seen about SAI, it's a bug, not a feature...

But are cylinders 4 and 8 using the same ignition coil? Or is that only GEMS.

th.

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@Thorst

Cylinders 4 and 8 use different coil packs.

Swapped the SAI pump……. No change…. Issue still present.

SAI solenoid next up…..

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Wits end….

MAF not the issue
SAI pump not the issue
New SAI Solenoid not the issue
When she was doing this I removed the hose from the SAI Solenoid leaving to the manifold then covered the pipe with my thumb, the engine idled correctly. When I released my thumb she went lumpy again. I then attached the hose and she had the same lumpiness.
There is vacuum…..
Where does the hose from the SAI solenoid that runs towards the firewall go to? IF this hose was disconnected along the route from Solenoid to wherever it goes would it cause this lumpiness? I cant for the life of me see where it leads to and the RR Maine-tenancy manual doesn’t help…

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Can't you just plug the pipe and leave it at that? The rest of the world survives quite well without any of the SAI stuff.

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I suppose I could but then I’d have nothing to do …….
FWIW, I put a multimeter at the plug for the solenoid and it reads 7.3V! Whats that all about?