Engine running rough

Hi all,
I am having issues with my xj6 series 3 which I have owned for 2 months.
The engine is running rough and chugging slightly.I had to get towed home as I could not drive it.

Since this episode I have flushed both fuel tanks several times and the fuel came out brown and lots of flakes also,this was done over a 4 hour period until the fuel came out clean.
I also replaced fuel filter which was full of crap and flushed the hoses to the pump,I do not know how the fuel was getting through.

Now the car is sometimes running great and then starts to run rough again,the tacho occassionaly fluctuates between 750 and 850 with either tank selected.
When I open the fuel tank caps I get pressure releasing,is this normal?

The timing is set as per decal on bonnet to 4 deg BTDC and I have adjusted the RPM to 800 but the allen screw seems to be in too far.

The fuel return to the tanks is working correctly ie left selected fuel returns to left tank and visa versa.

I thought it may be the cannister in the right front guard as the vacuum valve was disconnected when I bought the car,but it makes no difference if its connected or not.

I think the car had been sitting for about 4 years before I got it.

Has anyone got any more ideas of what I could look at.

Regards Felix

1982 XJ6 Series 3

So far you have done very OK, Felix…

It is likely that the fuel delivery is still impeded by the grunge. You really need to access the drain plugs on the tanks to remove residual gunk in the tanks. There is a small plug (for draining petrol) in a bigger one for cleaning access. Just cleaning the lines and filter is insufficient - gunk is quickly replenished from the tanks…:slight_smile:

Since the car periodically runs OK; this may be all there is to it - the pumps chews through the gunk, but larger pieces may intermittently clog up the feed. However, some vacuum normally builds up in the tanks as fuel is consumed, but a faulty charcoal canister (right front guard) may rapidly apply engine engine vacuum to the tanks. And vacuum may build up to a level which may impede fuel flow - and implode the tanks. Disconnecting the vacuum line to the canister should really prevent that - and you an also drive with the filler lids ajar, for testing the possibility…however unlikely…

Finer particles are meant to be caught in the main filter - coarser particles are stopped by the in-tank filters. Which may be intermittently clogged, or may be missing - they can be inspected/changed by removing the large drain plug.

Others have fitted external filters in the tank to changeover valve (which may also intermittently clog up) as an extra security. However, this is no substitute at this stage for tank inspection/cleaning as a first step in this case. I don’t think anything else is amiss…

xj6 85 Sov Europe (UK/NZ)

Found the purge valve near charcoal canister was not getting any vacuum to open the valve and the EGR valve was also not working due on same tube,
1 of the vacuum outlets on the throttle body was blocked,now cleared and both valves are working.

I removed the check valve near canister to stop the pressure building up in the tank,I thought the check valve was supposed to allow fuel vapour into canister when engine not running.
The check valve is only allowing vapour back into the tank,the orientation of the valve is correct as it is marked Tank and Canister.

The engine is still running rough so I removed the fuel filter and the colour of the fuel was dark brown with fine sediment at the bottom of the bottle.

I am going to order the rubber and alloy seals for the tank drains and drain the tanks again.

The check valve is actually 2-way, Felix - the closed tank venting is somewhat devious…

The charcoal canister is open to air on one side of the charcoal - the other side is connected to vacuum and separately to the tanks via the check valve. However, there is no vacuum to the tanks - ambient air pressure in the canister is maintained by air passing through the charcoal.

With the engine running, petrol is consumed, and vacuum builds up in the tanks - reaching some 1,5(?) psi the check valve opens, letting (charcoal filtered) air into the tanks, to prevent tank collapse. When the engine is stopped; vacuum (1,5 psi) is retained in the tanks (unless lids are opened). If petrol in the tanks heats up and expands - the check valve will open the other way and vent fumes through the canister charcoal to open air. The fumes will be adsorbed by the charcoal - and only clean air vents out…

When the engine is restarted, outside air is drawn through the charcoal, freeing the petrol fumes from the charcoal - to be burnt in the engine. Be aware that the vacuum hose connection is restricted by an annular ring - to prevent excessive air flow.

As an aside; the vent line from the tanks is routed via fuel separators in the C-pillars - liquid fuel is returned to the tanks, only vapour should enter the canister. However, if the charcoal has solidified; vacuum may build up in the canister, sucking fuel from the tanks via the separators - and vacuum will be applied to the tanks. Liquid fuel will destroy the properties of the charcoal, which will deteriorate over time anyway - so changing the canister/charcoal is a routine maintenance procedure…

The canister vacuum line is usually connected to the crankcase ventilation duct - which, together with the restrictor, reduces vacuum applied. The canister vacuum line should not be connected directly to manifold vacuum…

But cleaning the tanks is certainly a first step…

xj6 85 Sov Europe (UK/NZ)

You said it, you have pressure releasing from the cap, have you replaced
some items for the gas flow?

No I have not changed any items for the gas flow.
Disconnecting the check valve at canister has stopped presure build up in tanks.
Waiting for the new drain seals so I can clean tanks again as I have found dark brown coloured fuel and fine sediment in filter again.

Thanks For the info above Frank.I will try and locate where that vacuum hose is supposed to be connected to when I find this crankcase vent duct.
Do you know where on the tank is the fuel outlet to the pump,you would think its slightly higher than the drain plug,I cannot find any photos of it.

The tanks are shaped to form a sludge sump forward, Felix - forming the lowest part of the tanks, where the drainplugs are placed. The sump is where water and debris collects - and the pump intake is in that vicinity, but well above. An in-tank filter is fitted to the intake, which effectively prevents large debris bits into the fuel system, but it may itself be blocked or fail - and it will not block water. Particularly with low tank levels there is considerable agitation of the petrol - washing debris out of the sump…

The debris, except water is usually bits of the tank linings peeling off - it is there to protect the tanks against rusting. And large amounts of such debris warrant a tank inspection and possibly refurbishing. Cleaning out the fuel system outside the tanks is, as said, insufficient - there may be plenty of debris for resupply…

You’ll get a better idea of what is going on with the drain plugs out. Just remember to have a substantial container for the fuel - which may come out with some force eve with only the small drainplug open…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Hi Frank,
I cannot see where the vacuum hose for the purge valve on canister connects to engine,I have left it connected on the throttle valve for now.Would you be able to pin point exactly where this hose is supposed to be connected to.
Thanks Felix

It may vary with model year, Felix…?

In any case; make sure the ‘strangler’ is in place in the hose - to restrict air flow through the canister. However, there should be no vacuum in the canister if the charcold is filtering properly. To check; disconnect canister vacuum line at the check valve, and run the engine - there should be no vacuum at this hose.

If it is; this vacuum will be applied to the tanks - which is plain wrong, and may implode the tanks. The reason is likely blocked charcoal, or possibly excessive air flow - but whatever; disconnect the vent line until remedied…

xj6 85 Sov Europe (UK/NZ)

I finally got the new drain plug seals and sender seals.I removed the senders and drain plugs and the sludge was still coming out.The filters on the pickup pipe in the tanks were completely covered in crap,no wonder I am having all these issues.I tried vacuming the tank and using compressed air to clean the tanks but that did not work.I did not want to really remove the tanks so I disconnected the fuel lines to the tanks in the boot and flushed both tanks out with a garden hose through the sender unit hole.The amount of sludge that came out the drain was incredible.The tanks are now clean and I have blown them out with compressed air.I will let the tanks dry a bit longer before assembling it back together.I will keep you updated of the results.

On the XJ-S I have long recommended removal of the screen on the pickup pipe and installation of an inline fuel filter in the line between pickup and fuel pump. Hence, any crud in the tank gets sucked into the filter, and then the filter can be completely removed and replaced with new, getting the crud completely out of the system. I’m pretty sure some have done something similar on the two-tank scheme in the saloons, removing both pickup screens within the tanks and installing two inline filters instead. You are now in a position to decide for yourself whether this is a good idea.

If you remove the big drain plug the screen filters can come out and you can clean them.

You are doing exactly right, Felix…

However, the amount of sludge does indicate large delamination of the tanks’ lining - exposing the tanks to rusting - they are sheet iron. So do inspect carefully for rust - and if serious; the tanks should be taken out and refurbished, or you would have downstream issues…

If you can clean the in-tank filters, fine - otherwise replace. Adding external filters as mentioned by Kirbert, on each tank to changeover valve hose for ‘our’ two-tank system, does no harm. And with rusty tanks it’s not a bad idea…

xj6 85 Sov Europe (UK/NZ)

I have removed the screen filters,they were both blocked.I am amazed any fuel was getting through.

Hi Frank,Looking inside,the tanks look OK and the screens are now clean.I will put everything back together next week and give it a run with the old filter still fitted,then replace the filter.

Unless you leave the pickup screens in! I don’t recommend both, it adds too much resistance on the suction side of the pump. One or the other. If you’ve cleaned the tank it might be clean now, but down the road I still think inline filters are a better idea because they actually remove the crud from the tanks.

I put everything back together yesterday including the in tank filters.I have back flushed the old filter with clean fuel
and re used it temporarily until I get another one.
At least now I have clean fuel going to the injectors.
I also drilled through the fuel tank check valve at the canister with a 1/8 inch drill as pressure was not going back to canister via this valve.
The engine now has a slight chugging at the exhaust pipes,I will see if it settles down this week.
The timing and idle speed is set correctly.
Will report back next week.

Well, you know that ain’t right, don’t you? It’s possible the engine is now pulling vapor from the fuel tank, resulting in a rich condition. Why don’t you replace that drilled valve with a new one?

Thanks Kirbert,Yes you are right,I need to get a new valve.
It most probably is sucking fumes into the engine now,I suppose what I should have done temporarily is plug the vent hose which goes into the canister and leave the hose that goes to the tank open.Thanks for your help.