Rough idle. Under 1500rpm only

Allright, to start off my 1988 3.6 Xj40 sovereign ran out of gas the other week.

It was running smooth before it ran out gas. The fuel cap was under water as the drain pipe was blocked at the time. I did mange to get most of the water out all tho some may have entered (It was night time )

I put fresh petrol in and it started running rough straight away. But only under 1500rpm. Anything above that is fine, runs normal.

Fuel fail 4 has poped up on the dash at this time.

Has the odd backfire in the intake and exhaust.
Intake also has a loud squeel could this be a air leak ?

So far ive replaced ;
Fuel filter
Cleaned and drain the fuel tank twice.
Spark plugs
Spark plug leads
Distributor cap and rotor
Cleaned intake mani but i didnt take it off just cleaned around the throttle body & replaced the gasket for it.
Cleaned iacv or steeper motor
Cleaned hoses
Cleaned afm

Would anyone know what would cause it to idle rough only under 1500rpm ? ? ?

First, since you’ve drained and cleaned your fuel tank and are still have the idling problem I don’t believe I’d worry about the small amount of water that you might have gotten in your gas.

Next, since you ran the tank empty I’m more inclined to believe that somehow you sucked up some debris lurking in the bottom of the tank. Common sense would tell me that this should have been collected in the fuel filter but maybe some managed to bypass it and worked it’s way down the system to either your fuel pressure regulator or your fuel injectors. If the problem was the pressure regulator I would think you would just have trouble starting the car and keeping it running. Each injector however has it’s own small filter and if one or more of these got clogged or partially clogged that could explain the rough idle. At higher RPM the effect would be much less noticeable.

Just a guess, but quite frankly I can’t think of any other correlation between the events and your problem.

Yea thanks mate

ill take the fuel rail off tomorrow and get the injectors tested.
It drives fine as ive been around the block a few times in it.
Its just when i stop i have to keep the throttle on

I also think you probably had debris go through the fuel system when it ran out of gas. However, in my mind, the first suspects would be the fuel pressure regulator or fuel damper. Replacing the screens in the fuel injectors is quite easy, so I’d recommend replacing those as good measure.

Could someone explain this: Assuming fuel tank debris settles to the low point in the tank and since the fuel pickup is always at the same point in the tank whether the tank is full, near empty, or completely empty why is there a greater propensity to pick up debris when the fuel level is low? Just wondering.

Yep will check those out in the weekend. Fuel pressure reg was replaced maybe 1 or 2 years ago

John , That sir is a very good question !

You asked … “Could someone explain this: Assuming fuel tank debris settles to the low point in the tank and since the fuel pickup is always at the same point in the tank whether the tank is full, near empty, or completely empty why is there a greater propensity to pick up debris when the fuel level is low? Just wondering.”

Urban myth or reality?

Your question really piqued my interest so I did a bit of research on the subject and I’m still not 100% sure what the answer is. Common sense says that since the pickup tube is generally located in the very lowest part of the tank so as to have access to all the fuel that, you are indeed correct.

I did however read an article on how the pickup creates a type of vortex so that the fuel isn’t really being picked up from the bottom of the tank, Not being a fluid dynamicist (but I do play one on TV) this also seems to make sense.

Perhaps we have a jaguar owner who is also an expert on this subject that could add some expertise.

Dennis,

And neither am I, but I remember being told ages ago that in normal operation with plenty of fuel in the tank, the particles that could adversely effect the fuel flow are floating around in suspension in the fuel. As the fuel level drops, the density of particles ‘per pint’ so to speak increases until, when the last few drops of fuel remain, the particle count is very high which causes the pick-up filter to clog and/or allows an unmanageable amount of debris to be pumped in to the fuel filter/rail.

I believed that advice and for many years have never let the fuel level to drop below a quarter full and have never experienced clogged filters, carb jets or injectors.

1 Like

Although running low or out of fuel completely can cause debris problems with some cars, I don’t think it’s an issue with XJ40s.

When I had the fuel tank out of my Sovereign to replace the internal pump, I was astonished how clean it was inside.
I’d owned the car at least five years and I’m fairly sure the tank had never been removed before that.
There was about 5mm of petrol left in the bottom of the tank and it looked absolutely clean.

I think Bryan’s theory is correct in that the petrol is constantly sloshing around in the tank as you drive and also each time you fill up, so any particles are bound to remain in suspension rather than settle to the lowest point, but if that is the case, I think the particle count per pint would remain more or less constant regardless how low the fuel level becomes.
I’ve found its usually the type of fuel tank that has a small sump, like on an E.type that cause problems running with low fuel levels. The sump is about the size of a baked bean can fastened to the tank bottom on an E.type and any particles that sink into it overnight are not disturbed by the sloshing of normal driving so tend to accumulate in there.
The fuel pick up pipe ends just above this sump and the sump contents get drawn up as the pump sucks the last few inches of fuel from the tank bottom.

Spent the day cleaning eveything .
Tried removing the iacv plug but did nothing
Cleaned it and hoses to it, nothing again.

Ended up playing with the idle screw . Holds idle now but still rough just doesn’t cut out.

The acceleration on it is still good. Picks up to 120kmh easy enough and still continues to pull. Just wants to keep going…
Its just this idle problem is giving me a headache.

It sounds like you’re really being aggressive in working this problem. We all know how frustrating something like this can be and a rough idle ranks right up there at the top.

Lets start eliminating things. Have you done a compression check? This will eliminate the “mechanical”
part of the equation. If you’ve burnt an exhaust valve that could certainly account for the rough idle. If that checks out OK then that leaves you with fuel, air, and spark. Most people skip the compression check and on this engine it is SO-O-O easy to do.

Since the XJ40 uses a distributor rather then individual coils at each plug (like most modern cars do) the spark is rather easy to troubleshoot. Normally a problem here will occur at higher RPM not at idle. So if you’re getting a good solid spark on each plug, great. If not work your way backwards through the plug wires, the distributor cap, the rotor, and the coil.

Checking out the fuel is very straight forward. Install a fitting in the fuel pressure line and use a fuel pressure gauge. If your pressure is good (and I’m betting that it is because your engine runs fine at higher RPM’s) then the pump and filter can be eliminated. A clogged fuel injector could be faulty though.

The last thing is the air, or really the fuel air mixture. You should do a smoke test on the intake part of the engine. These old cars have a lot of rubber and bellows that can split or crack. Plenty of good advice on the web about how to do this on the cheap.

Good luck Luke and may the horse be with you !

Yep will do .

I haven’t compression tested it yet as it was running smooth before it ran out of petrol.
Allready replaced spark plugs and leads, distributor cap and rotor. Cleaned the coil.New fuel filter, clean the tank twice .

Pulled spark plugs out the are white witch means its running lean.Im guessing it has a air leak somewhere will do the smoke test tho.

Thanks Bryan, that is the best explanation I’ve heard. And I too never let the fuel level get below 1/4 tank and have never had filter, injector, or carb issues due to contamination.

Update had a massive air leak in 1 of the gaskets not sure what one as i replaced them all idles stable enough, but i played with the idle screw before i did this, any 1 know how to set it. Fuel fail 4 still pops up on the dash tho???

Does this help?

http://www.jag-lovers.org/ebooks/view.php?Vbook=xj40&Vsection=10.6

Jdox_1 …

I’m curious, did you discover the air leak with a smoke test ?

Have you cleared the code by disconnecting the battery for a minute? The code won’t clear by itself even if you correct the problem.

Yes smoke test. All though smoke was everywhere.
Yea battery was disconnect the whole time i was replacing all the gaskets on the intake mani.
Will try that today thanks bryan